Et Tunc Nulla Erat IX (Dinos 2)

Et tunc nulla erat IX 

(And Once There Was)

Dinos: Part 2 of 3

Theropoda:

Theropods’ temporal range is from 231.4-0.0 mya first appearing in the Late Triassic all the way to the present as birds. The term ‘theropod’ means “beast footed” but the term ‘bird footed’ would have been more befitting since the pes (foot) are more comparable and the theropodal end result is birds. This dinosaur group is mainly characterized in having three-toed limbs, hollow bones and derived from a carnivorous ancestry. Most theropods were carnivorous with recurved sharp teeth, but some evolved into herbivores, scavengers, insectivores and primary piscivores.  

Theropods are a very extensive and diverse bipedal group of dinosaurs and in the innumerable cladistic structuring’ and phylogenetic rearranging. It can become confusing, as in the very simplified clade list of divisions and subdivisions below. So, as revisions continue to grow due to more fossil finds of new theropods, we’ll be presenting clades in a much more facilitating manner.

Theropoda divisions

In the Middle Triassic, theropods evolved from lagosuchian ornithodirans. If you might recall, lagosuchians were small lightly built archosaurs notable for slender legs and well developed feet which was a prelude for primitive theropod features. The hind limbs were larger than the forelimbs, but as facultative quadrupeds, could move bipedally with induced speed.

Theropodal archosaurian ancestries were ectotherms (cold blooded), but as active hunting terrestrial predators, (opposed to lying in wait ambush semi aquatic predators as in extant crocodilians), paleontological evidence and crocodilian oxidative metabolism points to the fact that these early terrestrial archosaurs had the potential for improving body heat. The theropod line and indeed dinosaurians in general took advantage of this. For sure the avian bird-line theropods had developed endothermic metabolism. Unlike mammalian endothermic down to the cellular in burning fuel for generating heat, dinosaurs, like birds produced internal heat through increased oxidative capacities. Other theropods were at least mesothermic (not quite ectothermic but not quite endothermic either). This is evident from the high muscular tissue content in theropods and other dinosaur lines. With multiple strong and effective muscles, dinosaurs were active making them the dominant animals during the Jurassic and Cretaceous. 

The transformation from an ectothermic to an endothermic metabolic state occurs by the interaction between the increased basal uncoupling of aerobic metabolism from energy production demand. Increased oxidative capacity and increased uncoupling of mitochondria witnessed in developing bird embryos is a process controlled by tyrosine-based thyroid hormones via its effect on PGC 1alpha (key regulator of energy metabolism) and adenine nucleotide translocase (ANT) gene expression. Most likely, this was the same molecular mechanisms for theropod endothermic capabilities.

Eoraptor skeletal anatomy

It appears that theropods originated in what is now southern South America. Living 231-228 mya ago, Eoraptor (E-oh-rap-tor) was one of the earliest lines of basal theropods and was found in Argentina’s ‘Ischigualasto Formation’ composed of muddy siltstones. At 1m/3.3ft long, this animal was slender and as lightly built running digitigrade (moving on its digits rather than the whole foot) in an obligate bipedally upright position, it was designed for speed.

Credit: KN3  Eoraptor

Obligate or habitual bipedalism today is a rarity as seen only in humans and kangaroos, but theropods were virtually all bipeds. The only one I know of that could choose to walk as an occasional quadruped was, Spinosaurus as long as the pelvis could align and the manus wrist bones wouldn’t be crushed in the overbearing weight. Eoraptor’s long tubular bones, such as the femur were hollow.

Dentition was heterodont consisting of serrated, recurved teeth in the upper jaw, as later theropods would possess and leaf-shaped teeth in the lower jaw, like the teeth of basal sauropodomorphs. Eoraptor with its speed and agility could easily overtake any of its small prey then immediately immobilize them with its forelimbs’ formidable claws. But, due to its leaf-shaped herbivorous teeth in the lower jaw, it is considered to be more as an omnivore.

Eoraptor was close in relations to the 231.4-225 mya family, Herrerasauridae (Huh-ree-ah-ruh-sawr-ruh-day) with multiple shared features such as the saurischian lizard pelvis, but it is now phylogenetically been determined that herrerasaurids were not ancestral to any later dinosaurian groups. It currently is being proposed that Eoraptor may be the common ancestor to all dinosaur groups with more emphasis towards the sauropodomorpha line. Eoraptor went extinct during the Triassic/Jurassic extinction.

Artist: Paul Sereno  Eodromaeus

A basal theropod was Eodromaeus (E-o-dro-may-us). Occurring 231.4-229 mya during the Late Triassic of Argentina, Eodromaeus was relatively small at 1.2m/3.9ft in total length. The main features to this primitive theropod were: it being a biped carnivore with smaller forelimbs ending in hands that were transitioning from five to three digits, as digits IV and V were shortening to the point of becoming vestigial.

Artist: Jorge Gonzalez  Tawa

Theropodal cavity air sacs 

Tawa (Tah-wah) occurred 215 mya during the Late Triassic and was one of the first N. America early theropods to have lived in what is now New Mexico. At 2.5m/8.2ft long, it had a very slender build as compared to other earlier theropods, but was a trait to be characteristic of its descendants. The femur being longer than the lower tibia and fibula leg bones was a unique feature, but the main characteristic was that the neck vertebrae contained air sacs which were almost identical to the air sacs of birds.

Artist: apsaravis  Daemonosaurus

The name, Daemonosaurus (Day-mow-no-sawr-us) literally termed means, ‘prominent tooth’, but loosely termed means ‘buck tooth’ and for good reason. At 1.5m/5ft long, this theropod may have been small, but considering its bite, you would not want to tangle with it. What makes this theropod unique among theropods is that its short skull bore large and sharp conical maxillary and premaxillary teeth that protruded out from the tip of the mouth. There was a lack of fusion between the braincase bones, but the fossil remains were of an adult as the neurocentral sutures of the vertebrae were fused. Perhaps with the teeth projecting outwards as they did, it was necessary to have unsown suturing and expandable skull bones to accept the biting force of this animal. With its unique skull traits, Daemonosaurus was an early theropod that went extinct during the Triassic/Jurassic extinction period and left no genetic lineage.

Artist: Maurillo Oliveira  Tachiraptor

Tachiraptor (Tack-he-rap-tor) occurring in the Early Jurassic 200 mya was not a raptor, as the name might imply to readers. It preceded the raptor family, Dromaeosauridae (Drom-me-ah-o-sawr-ah day) by 74 million years and is not directly related. However, it represents a very primitive basal member to Neotheropoda, therefore, it is an unequivocal stem group to averostrans, the ‘bird snout dinosaurs’, in which we’ll dwell on a bit later.

At 1.5m/4.9ft long, Tachiraptor ’s fossil remains were found in Venezuela and it is considered a ‘ghost linage’ to birds. This shows that theropods were radiating out from their place of origin in southern South America. Early theropod species also began appearing in N. America during the connection between the two present continents of N. and S. America. Also, Pangea at the Triassic/Jurassic border connected these two continents to Asia and Africa. Once land bridges were cut off by continental plate tectonics further into the Jurassic and Cretaceous, theropod species as isolated evolved into newer forms. 

Postulating the spread from the earliest dinosaurs originating from what is now Argentina and later into what is now N. America, aids greatly in the understanding of the geological evolutionary role played on dinosaurs by confirming the equatorial zone of the supercontinent Pangea’s influence over the development of early dinosaurs through ecological and geographical isolation via natural selection.

Neotheropoda simple Cladogram

Neotheropoda: This is a major clade that includes all the theropod groups that survived the Triassic/Jurassic extinction, namely the ceratosaurs, noasaurids, abelisauroids, and the coelophysoids. Neotheropods’ revealed traits were size increases of ilium surface attachments and enlarged pelvic muscles, while reducing pedal (foot) digits I and V making the foot tridactyl while walking on three toes. Also, there was a further reduction in manus (hand) digits IV and V making the digits either vestigial or obsolete in later forms. These distal limb features would now be the prevailing trait of theropods.

Coelophysoidea Cladogram

Coelophysoidea ~ (See-luh-phi-soy-dee-uh) is an older sister group to averostrans and were small slender dinosaurs that had radiated out globally.  Coelophysoids appeared 220-183 mya during the Late Triassic and Early Jurassic. Coelophysoids were direct descendants of the earliest dinosaurs but with smaller slender builds, dentition for capturing insects/small reptilian prey, definitive feathering and hollow bones. As a predecessor, they passed these genetic traits onto coelurosaurs.

Artist: Alain Beneteau  Procompsognathus

Procompsognathus (Pro-comp-sog-nath-us) had a temporal range of 210 mya during the Late Triassic and was a small and slender theropod at 1m/3.3ft long. It was a very refined and lightly built dainty theropod as the name, taken from the Greek words, ‘kompsos’ and ‘gnathos’ attest to in meaning, ‘elegant jaw’ when put together respectively. Long hind limbs, short forelimbs ending in large clawed hands, a long slender snout filled with many tiny teeth and a stiffened tail are key features of Procompsognathus. This theropod dieted on insects and small animals like lizards and the young of other smaller dinosaurs. The fossil remains come from what is now Germany when the country was a relatively dry and inland environment. The name Procompsognathus does not mean it is a direct descendent of Compsognathus. It only implies that with some similarities, the former came long before the future one.

Dilophosaurus fossil

Dilophosaurus (Di-lo-fo-sawr-us), once considered a ceratosaur, then a basal coelophysoid and now is under dilophosaurids which are considered as a sister group to averostrans. It had two cranial crests that sat atop the skull just below the neck and measured 7m/23ft long. The crests were formed from extensions of the lacrimal and nasal bones and would’ve been only useful as displays. There was no sexual dimorphism so both sexes supported crests. With a temporal range of 193 mya in the Early Jurassic, its fossil remains were discovered in Arizona, U.S. Dilophosaurus had a weak bite and it was incapable of biting through bone as most theropods easily could. It most likely, in conjunction with its manus distal claws used its teeth to immobilize and kill smaller prey, then pick flesh off the bones.

Averostra simple Cladogram

Averostra (Ay-vee-ros-truh): With the term standing for ‘bird snouts’, averostrans were composed primarily of two main groups and one subgroup in being the ceratosaurians and tetanurans. The tetanuran line gave rise to the orionideans, the largest of theropod groups. Through biomechanical studies, it’s been shown that in the later and larger averostrans, the skull narrowed laterally but was deep vertically. This shows averostrans were more dependent on strong repeated bites in the skull withstanding the up and down pressures and forces from repeatedly biting down hard into larger prey. But, in being narrower, the skull was not equipped in resisting side to side pressures and forces due to possessing a weak muscular and fragile jaw bone not capable to grip for the shaking of prey.

Ceratosauria Cladogram

Ceratosauria (Seh-rat-o-sawr-ee-ah) ~ This is a clade of theropods that shared a common ancestry to all the non-avian dinosaur groups, but possessed hollow bones as did the maniraptoriforms, which includes birds. Diverging from the dinosaur bird groups in the Early Jurassic, ceratosaurians arose 185 mya while dying out 66 mya at the end of the Late Cretaceous. An original ceratosaurian feature was the complete fusion of the astragalus and calcaneum ankle bones. Known as the tibiotarsus, this astragalus and calcaneum fusion to the tibia would survive through the latter theropod line all the way to birds. Three main groups of ceratosaurians were in the families: Ceratosauridae, Noasauridae and the more derived Abelisauridae (Eh-bel-ah-sawr-ah-day).

Artist: karkemish00  Berberosaurus

A basal ceratosaurian was the 185 mya Early Jurassic, Berberosaurus (Ber-ber-o-sawr-us) and it may be the earliest of ceratosaurians. At 5m/16.4ft, its fossil site was found in the continental series ‘Toundoute Group’ of Morocco. It was a biped carnivore giving distinction to the later form theropod morphologies and osteological anatomy in its vertebrae arrangement, metacarpals and hind limbs.

Artist: Yu Chen  Limusaurus

Ceratosaurians were primitive predatory theropods with the exception of the 161-157 million year old noasaurid, Limusaurus (Le-moo-sawr-us); it was an herbivore in its adult stage. Isolated as the only known Asian ceratosaurian, Limusaurus through the process of aging underwent a drastic transformational morphology. Juvenile teeth were lost and replaced with a beak in the adult stage changing the diet as an omnivore to an herbivore.

Artist: vaderxl  Ceratosaurus

Two ceratosaurians were the ceratosaurid, Ceratosaurus (in which Ceratosauria derives its name) and Genyodectes (Jin-yo-decks) where both supported head crests, although it’s only purported that Genyodectes had crests. In addition, Ceratosaurus sported a nose horn with large nasal passages and small osteoderm armor running down its spine. Similar to birds, the sacrum and pelvic bones were fused together. Ceratosaurs at 5.3m/17.4ft long had radiated out into Utah/Colorado U.S., Tanzania and Portugal. The temporal range during the Late Jurassic was 153-148 mya. It was ancestral to abelisaurids and Carnotaurus (Car-no-tar-us), which also possessed high crested brow ridges. Carnotaurus also made T. rex look like it had Arnold Schwarzenegger arms as this 8.25m/ 27.1ft long dinosaur’s vestigial forelimbs were tiny.

Artist: teratophoneus  Genyodectes

In the superfamily Abelisauroidea (Eh-bel-ah-sawr-oi-dee-ah) are the two sister group families of noasaurids and abeilosaurids. Noasaurids appeared from the Late Jurassic until the Late Cretaceous 161-69 mya, while the temporal range of abelisaurids was from the Middle Jurassic until the end of the Late Cretaceous 170-66 mya.

Artist: cisiopurple  Abelosaurids

In the family, Abeilosauridae some unique characteristics were in the skull having ornamentation of brow ridges/horns with grooves and pits, very short but tall skulls due to the vertically lengthened premaxilla blunting the front of the snout, hind limbs comparable to ceratosaur legs and vestigial forelimbs. Abelisaurids formed a transitional intermediate relationship between Ceratosaurus and Carnotaurus and were closely related to both.

Artist: Wayne D. Barlow  Xenotarsosaurus

The abelisaurid, Xenotarsosaurus (Zee-no-tar-so-sawr-us) at 5.4m/18ft long occurred in the Late Cretaceous 95mya. Found in Argentina’s ‘Bajo Barreal Formation’, it had the unique ceratosaurian feature in showing a complete fusion between the astragalus and calcaneum ankle bones. With this complete fusion without any visible suturing and further fusion of the astragalus/calcaneum element to the widened distal end of the tibia, there is a prevailing thought to remove Xenotarsosaurus from the family, Abelisauridae and list it as a basal neoceratopsian with uncertain affinities. However, due to its’ remains showing numerous Carnotaurus features, as skull traits, it is still listed as an abelisaurid.

Artist: Damir G. Martin  Carnotaurus

One odd derived abelisaurid that occurred in the Late Cretaceous 72-69.9 mya ago was Carnotaurus. At 9m/29.5ft it had a very muscular neck with the deepened skull supporting thick bull-like horns just above the eyes. Also, the forelimbs were vestigial while the hind limbs were long and slender adapted for running. The horns, measuring 15cm/5.9in were formed from the frontal bones and most likely were sheathed in keratin. With the neck musculature, thick skull and horns, most likely the head was used as butting weapons against conspecifics (members of the same species). The eye orbits were small, so the eyes were better equipped to avoid injury during ramming bouts.

  

Artist: Philip Hood  Noasaurus attacking Saltasaurus 

Noasaurids are defined as all theropods closer to the ceratosaurian noasaurid, Noasaurus (No-ah-sawr-us) than to Carnotaurus. Unique features of noasaurids are a low rectangular arrangement of the mid caudal neural spines, a coracoid (a structure stationed anteriorly on the scapula) that is almost double the width than its length, a stout humeral head and the tibia possessing a flat anterior at the distal end.

Credit: Pixel Dust Studios  M. knopfleri

Masiakasaurus knopfleri (Phonetics: May-shuh-kuh-sawr-us = nop-fleur-eye) was a 1.9m/6.2ft noasaurid that lived during the Late Cretaceous 70 mya. It had heterodont teeth along the jawlines while the front teeth projected forwards. The species name, knopfleri was used due to the two paleontologists always jamming out to the rock band, ‘Dire Straits’ while working on the Madagascar fossil dig. Mark Knopfler was the band’s leader.

Tetanurae Cladogram

Tetanurae (Tet-nur-ee): The name refers to ‘stiff tails’ and is a clade including megalosauroids (temporal range: 170-93 mya), allosauroids (temporal range: 175.6-70 mya), tyrannosauroids (temporal range: 165-66 mya) and maniraptoriforms (temporal range: 167-0 mya) which includes maniraptors, ornithomimosaurs and birds. Tetanurans are those dinosaurs more related to birds than to their contemporaneous ceratosaurian cousins. In tetanuran evolution, there were multiple parallel diversifications of multiple lineages. Tetanurans are identified by two skull types. The first is more primitive being three times longer than tall and ends in a blunter snout than the second. The first skull also, along the lacrimal, nasal and frontal bones was ornamented with horns, crests and spikes. In the second skull type, it’s also longer than tall, but much lower and with less elaboration of ornamentation. Tetanurans and the ceratosaurian line diverged and split during the Late Cretaceous more than 200 mya. But the biggest transition from primitive theropods to the most derived occurs within the tetanuran clade.       

From tetanurans onwards, as big and diverse a clade as it was, only birds still exist just like modern humans are the only hominid that still exists, but with a much shorter temporal range than that of tetanurans. We’ve got a long way to go to match the success of tetanurans who roamed Earth for 100 million years. The oldest known hominids are Orrorin tugenensis and Sahelanthropus tchadensis in that both lived ~ 6 mya.

Artists: Lft. Durbed Crylophosaurus Mid. DinoEsculturas Dilophosaurus Rt. Alain Benetean Sinosaurus  

Two basal tetanurans were the previously discussed, Antarctic Elvis Presley crested, Cryolophosaurus and its sister taxon, Sinosaurus (Sye-no-sawr-us) that lived during the Early Jurassic 201-196 mya. As a medium sized dinosaur, at 5.6m/18.4ft, Sinosaurus was closely related to the North American Dilophosaurus, so much so that it was, until recently, once considered to be an Asian dilophosaurid. Just like the ceratosaur crest, the sinosaur crest was too fragile for defense so likely was used for display purposes and/or species recognition.

Orionedes simple Cladogram

Orionides (O-rye-o-nye-dees): This is a major tetanuran theropod clade that had a temporal range from the Middle Jurassic to the present 175.6-0.0 mya. The tetanuran clade includes most of the theropod groups, including birds. Large tetanurans are contained in the Orionides node, such as allosaurids, spinosaurids and carcharodontosaurids that reached lengths of 18m/59ft. The spinoraurid, Spinosaurus aegyptiacus (Phonetics: Spy-no-sawr-us = e-gyp-tuh-cus) is the largest terrestrial predator of all times. Orionides is comprised of the two clades, Megalosauroidea (Meg-uh-lo-sawr-oi-dee-uh) and Avetheropoda (Ay-vee-the-rop-o-duh), along with these two clades’ most recent common ancestors and descendants.

Megalosauroidea Cladogram

Megalosauroidea: Megalosauroideans temporal range was during the Middle Jurassic to the Late Cretaceous 170-93 mya. This clade is the sister group to Avetheropoda. Just to note, the term, ‘Spinosauroidea’ is synonymous to Megalosauroidea and is sometimes used in place of. The three genera members of the family, Piatnitzkysauridae (Pee-ut-nits-kee-sawr-ah-day) are basal megalosauroids composed of: Condorraptor (Con-door-rap-tor), Marshosaurus (Marsh-o-sawr-us) and Piatnitzkysaurus (Pee-ut-nits-kee-sawr-us), in which gave the name to the family. This gives the real definition of piantnitzkysaurids as those megalosauroids that are more closely related to Piatnitzkysaurus than to the other megalosauroids. Piatnitzkysaurid shared traits were in having, among others, vertically ridged paradental plates, anteriorly inclined posterior dorsal neural spines and a humerus with a canted (bent forward and away from body) distal condyle (rounded knob of humerus bone). Piatnitzkysaurus synapomorphies (distinguishing features) from the more derived megalosauroid are: a small or absent anterior process of the maxilla, moderately developed axis diapophyses (diapophysis ~ singular; transverse process of a vertebra) and an absence of axial pleurocoels (hollow depressions on lateral portions of vertebrae).

Credit: De Agostini Picture Library  Piatnitzkysaurus 

The sister group to Piatnitzkysaurids is, Megalosauria (Meg-uh-lo-sawr-e-uh) and it is composed of the basal megalosaurid, Streptospondylus (Strep-toe-spon-dill-us) that occurred in the Late Jurassic 161 mya and the two major sister families of, Megalosauridae (Meg-uh-lo-sawr-uh-day) and Spinosauridae (Spy-no-sawr-uh-day) that occurred respectively in the Late Jurassic 170-148 mya and in the Late Jurassic to the Late Cretaceous 148-93 mya.

Artist illustration: Sergey Krasovskiy  Torvosaurus

One megalosaurid was, Torvosaurus (Tor-voe-sawr-us) that existed in the Late Jurassic 153-148 mya and was the largest carnivore of the Jurassic Period with a length of 10m/33ft. Its narrow snout, with a kinked profile just above the large nostrils, had in its front most snout bone (premaxilla), three flat teeth oriented outwards with the front edge of the teeth crown overlapping the outer side of the rear edge of the preceding crown. Torvosaurus was an apex predator in Late Jurassic N. America and Portugal and most likely scavenged other predator kills and hunted either in packs or solitarily other larger dinosaur herbivores. There are two species in T. tanneri (tan-nur-ree) of USA, Colorado and T. gurneyi (grr-nee-ah) of Portugal. In the Late Jurassic N. America and Europe were connected where Torvosaurus evolved as one species. Once the continents split, it evolved through isolation and natural selection as two separate species.  

The oldest clutch of fossilized dinosaur eggs was found in what is now western Portugal and were laid down by a Torvosaurus mother between 152-145 mya. In addition to the eggs being shown to be single layered, the eggs also housed fossilized embryonic stages.

Artist: David Bonadonna  Spinosaurus aegypticus

Spinosaurids first appeared in the Late Jurassic 148 mya and went extinct 93 mya in the Late Cretaceous. Spinosaurids had elongated crocodile-like snouts with very little if any serrated teeth that were conical in shape. The conical teeth were excellent for grabbing onto slippery prey, such as fish, in which spinosaurids were the most aquatic of large dinosaurs in being semiaquatic. Also, the largest carnivore ever was in the spinosaurid, Spinosaurus aegypticus reaching a length of 18m/59ft. The major characteristic of spinosaurids was in the sail that the dorsal vertebral spines of the back supported. In fact, the term, Spinosaurus refers to ‘spine lizard’. These spines were nearly 2m/6.5ft tall. Along with the genus, Spinosaurus that occurred 112-92.5 mya, there were four other spinosaurid genera in, Baryonyx (Bar-ee-on-iks) 130-125 mya, Siamosaurus (Sye-am-o-sawr-us) 130 mya, Suchimimus (Su-ko-mie-mus) 125-112 mya, Irritator (Ear-re-tay-tor) 110 mya and occurring ~ 119 mya was Ichtyovenator (Ick-thee-o-ven-a-tor) that had two separate sails down its back. The sails of Ichtyovenator were separate with the first as the highest ending abruptly with the second sail from the separation rising down towards the tail. This is an indication that the sails of all spinosaurids were for display purposes for mating recognition and/or for appearing larger.

Through oxygen isotope studies of spinosaurid bones, as compared to other theropod fossils and extant turtles/crocodiles, the spinosaurid isotopic ratios were much closer to living turtles and crocodiles than to other theropod remains alluding to a semiaquatic lifestyle.  The main source of prey for spinosaurids was fish, but they were also opportunistic and if aquatic prey wasn’t available would hunt down terrestrial prey. Baryonyx and Spinosaurus both ate fish, but where Baryonyx was more of a carnivore than a piscivore, Spinosaurus was more of a piscivore than a carnivore.

There are four other genera listed under the family, Spinosauridae, but due to scant fossil material are only suspect spinosaurids as they might be synonymous to established spinosaurids or are simply nomen dubium.

Avetheropoda Cladogram

Avetheropoda clade with Allosauroidea ousting Carnosauria 

Avetheropoda: Avetheropods first occurred in the Late Jurassic 175.6 mya and still exist in the present as extant birds. As the sister taxon to Megalosauroidea, avetheropods refers to ‘bird footed dinosaurs’ and it is in a section of this clade that contains the major path to birds with its most common ancestors and all of its descendants. The famed Tyrannosaurus is also an avetheropod, although it was not in the direct line to birds. The simple relationship grouping below shows the avetheropod major groups.

The first sectioning of avetheropod relations is in the two clades, Coelurosauria (Phonetics: See-lure-o-sawr-ree-uh) and Allosauroidea (Phonetics: Al-lo-sawr-oi-dee-uh). You will find at times Carnosauria in place of Allosauroidea. I’m even using it in the tetanuran clade. That’s fine as both are still acceptable, but as time moves on Allosauroidea is going to be the permanent term.

Avetheropod shared characteristics are in possessing extremely complex vertebrate air sac chambers and in the loss or only a vestigial single metacarpal of the manus digit IV making the manus (hand), like the pes (foot), functionally tridactyl.

Allosauroidea Cladogram

Allosauroidea ~ occurred in the Middle Jurassic 175.6 mya to the Late Cretaceous 70 mya. It appears that there was a co-evolutionary race between herbivores and allosauroids, for as when herbivores started increasing in size and inheriting newer weaponry in spikes, clubs and bony plates, allosauroids began their increases in size, speed and agility. The two main derived sister taxa (plural for taxon) of allosauroids are Metriacanthosauridae (Phonetics: Met-rih-ah-kanth-o-sawr-uh-day) and Allosauria (Phonetics: Al-lo-sawr-e-ah).

Metriacanthosauridae Cladogram

The primitive allosauroid members all belong to the family, Metriacanthosauridae. Currently comprising six genera, metriacanthosaurids ranged in size from 4.5m/15ft to 10.8m/35.4ft long. The synapomorphies shared with the more derived allosauroids were: a fused distal end of the ischium, a manus shorter than the forearm, the laterosphenoid (lateral form of cranium’s sphenoid bone that is ossified cartilage) was articulated on the frontal and postorbital skull bones, sub-rectangular and sheet-like neural spines of the middle caudal vertebrae while the manus lacked digit V as well as the phalanges of digit IV.

Unique features of metriacanthosaurids were: a pronounced ventral keel on the anterior dorsal vertebrae, a straight posterior margin of the iliac post-acetabular process at the ventral inflection point, a ventrally curved ischial shaft and a bulbous fibular crest on the tibia.

The Allosauria order consists of two sister taxa in the family of, Allosauridae and the order of, Carcharodontosauria (Phonetics: Car-care-o-don-toe-sawr-e-uh). Occurring 155-146.8 mya during the Late Jurassic, Allosauridae consists of three genera in: Allosaurus, Epanterix and Saurophaganax (Sawr-o-fag-uh-nax). There are three species of Allosaurus in: A. fragilis, A. europaeus, and A. lucasi with A. fragilis being the more famous. Epanterias lived 146.8 mya at the end of the Late Jurassic and was large at 12m/40 ft. The largest allosaurid however, was Saurophaganax at 13m/43ft in length living 150 mya in the Late Jurassic.

Artist: atrox1  A. fragilis

Allosaurid anatomy was typical of large tetanuran theropods, but all teeth were serrated with five teeth seated in the premaxillary and sixteen in the maxilla. Also, digit I of the manus formed an opposable thumb. All allosaur fossils come from N. America’s ‘Morrison Formation’. It is mainly composed of shallow sea and alluvial sedimentary rock. Allosaurid fossil remains are found in the basin portion of the formation formed from the uplifting of the Rocky Mountains.

Artist: Manuel G. Jaramillo  Saurophaganax

One other suspect allosaurid found in the midsection of the Morrison Formation in what is now Oklahoma was, Saurophaganax. Being larger than the other allosaurids, it is also much rarer in fossil finds. The scenario in lack of fossils supports the idea that larger predators have smaller populations, due to the fact that larger animals are restricted to population growths as not to restrict food sources. However, as big as Saurophaganax was, it could easily steal a meal from a much smaller predator’s kill. Saurophaganax is Oklahoma’s state fossil.                

Carcharodontosauria encompasses the two families of, Neovenatoridae (Nee-o-vuh-nay-tor-uh-day) and Carcharodontosauridae. Neovenatorids had a temporal of 128-93 mya in the Early-Late Cretaceous. As compared to other allosauroids, their shoulder blades were much wider and shorter while the ilium was pocketed with cavities. Neovenatorids have had fossil discoveries in China, northern Africa, S. America and N. America. Neovenatorids consist of five genera plus the unranked file of, Megaraptora (Meg-uh-rap-tor-ah) that has another five genera. One with coelurosaurian characteristics has just been discovered in Australia, but with controversial relations to other allosauroids as they also have basal coelurosaurian anatomical traits. With the discovery of the as yet unnamed Australian fossil find dating back to 110 mya in the Early Cretaceous, its age makes it the oldest and most primitive megaraptoran. Therefore the evolution of megaraptorans originated in Australia and radiated out during the Gondwanan connection if it is finalized as a true maegaraptoran.

Artist: Jorge Gonzalez  Siats

Siats (See-atch) is a neovenatorid with remains discovered in the ‘Cedar Mountain Formation’ of Utah, U.S. from the Late Cretaceous 98.5 mya. The term, Siats comes from the Native American, Ute tribe meaning ‘man eating monster’ in their mythology. Siats autapomorphic unique traits were a subtriangular cross section of the distal caudal vertebrae, broad neural spines on the dorsal vertebrae and a transversely concaved acetabular rim of the iliac (relating to ilium) pubic peduncle. As one of the largest known N. American theropods, it reached a length of 11.9m/39ft and filled the large carnivore gap in N. America between the earlier carcharodontosaurs and the latter tyrannosaurs.

Artist: Julio Lacerda  Gualicho

Another neovenatorid was the Late Cretaceous 90 mya, Gualicho (Gwah-lee-cho). This 6.5m/21.3ft long neovenatorid had already possessed the reduction of forelimbs ending in only two fingers. The arms were about the size of a child toddler’s. This is an indication that allosaurians were evolving reduced forelimbs just as tyrannosauroids were, but independent of each other. Coming from what is now Argentina, Gualicho lived at a time when Argentina was a part of northern Patagonia after having split off from the supercontinent, Gondwana. Although Gualicho is a neovenatorid, it doesn’t fit nicely into any taxa grouping and is considered ‘mosaic’. Its anatomy suggests two things: either both megaraptorans and neovenatorids were carnosaurs, or that these two groups were a grade of theropods more closely related to coelurosaurs than to carnosaurs.

Artist: Damir G. Martin  Giganotosaurus 

Carcharodontosauridae ranges from the Late Jurassic to the Late Cretaceous 154-89 mya comprising nine genera, one subfamily (Carcharodontosaurinae) with one genus and within the subfamily one tribe (Giganotosaurini) with three genera. The defining term of Carcharodontosauridae is caecharodontosaurians that are closer in relations to, Carcharodontosaurus than to Allosaurus. Carcharodontosaurid features were foremost, no matter their size, a distinctly oversized head in comparison to their body, in which their fossil skulls are the largest of any dinosaur group. Acrocanthosaurus (Ak-row-can-tho-sawr-us), living in the Early Cretaceous 116-110 mya and measuring 11.5m/38ft in total length had a head measuring 1.3m/4.3ft. Giganotosaurus (Jig-uh-not-o-sawr-us) lived 98-97 mya during the Late Cretaceous was 13m/43ft long and had a head ~ 1.8m/5.9ft in total length. The skulls were not heavy though as they were filled with large fenestrae that decreased weight. Like most of the larger and later derived theropods, their forearms were much smaller than the hind limbs, but the carcharodontosaurid forelimb was robust with strong muscles as muscular attachments attest in the arms’ fossil bones.

Artist: Xing Lida  Acrocanthosaurus

Acrocanthosaurus had a row of tall neural spines at least 2.5 times that of the vertebra that ran the length of the back from the neck to down to the upper tail. These spines may have served as a fat reservoir and temperature control. There might only be a hypothesis of thermoregulation in Acrocanthosaurus, but in Giganotosaurus there is very good evidence that it was at least homeothermic. The fossil bones’ oxygen isotope patterns indicated an even heat distribution throughout the body which would have created a stable core temperature giving a metabolism between that of a mammal and a reptile. This in turn would have benefitted a rapid growth rate and an increase in cursorial (running) speed. In calculating the kinetic running speed limit in the time it would take for one of its sized legs to gain balance after the retraction of the other leg, it was found that the speed Giganotosaurus could safely obtain was 14m/45.9ft per second or an astounding 50km/hr-31mph. In running, the heavily stiffened tail acted as a counterbalance.        

One other carcharodontosaurid was, Concavenator (Con-kah-veh-nay-tuhr) that arose 130 mya in the Early Cretaceous. This medium sized 6m/20ft long theropod had some interesting features. Fossil remains of Concavenator’s posterolateral surface of the ulna (forelimb) possess homologous quill knobs, an anatomical trait found only in extant animals with large quilled forelimb feathers. Fossil impressions also reveal wide rectangular scales located underneath the tail and feet resembling extant birds’ covering of scutes and plantar pads on their undersides. The main trait though, is just in front and above the pelvis an extension of two elongated presacral vertebrae and along with a series of shorter, but more elongated than the rest of the neural spines, most likely supported a sail. The elongated spine row originated just in front of the hips and terminated at the base of the tail. From thermoregulation to fat storage, there is much speculation on why this hip sail evolved, so I’m going to throw in another.

Artist: Frank Lode  Concavenator

One thought of mine is in the body plan. Vertebrates have bilateral symmetry creating only one sagittal plane; basically creating a mirror image from both body halves when bisected. For a lying in wait ambush predator, the sail could’ve acted in a way as camouflage, distorting the potential prey’s viewpoint recognition of the predator’s general body outline. This would’ve given Concavenator an advantage of pouncing on and subduing prey at much closer range exerting less energy in the encounter as opposed to a chase and encounter.

Just like Convenator and Acrocanthosaurus, other orinidans, such as the spinosaurids, Metriacanthosaurus and Becklespinax all had sails, so they inherited it from a common ancestor ruling out convergent evolution.

Coelurosauria Cladogram

Coelurosauria ~ had a temporal range of 165-0.0 mya during the Late Jurassic to the present. This subgroup of Orionides comprises the compsognathids, tryrannosauroids, ornithomimosaurians and maniraptoriforms which include birds. The generic term, ‘coelurosaurus’ is a blanket moniker referring to the smaller coelurosaurians and does not represent any individual species. Although most were small, coelurosaurian sizes ranged from Parvicursor (Par-vee-cur-sor) at 39cm/29.5in long to Tyrannosaurus (Tie-ran-no-sawr-us) at 12.3m/40ft long with a potential of reaching 13m/42.7ft.

Virtually all of the smaller coelurosaurians from the most primitive to the most derived possessed feathering over parts or the entire body. Even some of the larger coelurosaurian fossil remains show evidence of feathering. Although there is no direct evidence that Tyarannosaurus rex possessed feathers, there is indirect evidence that it did. Dilong (Di-long), occurring 126 mya in the Early Cretaceous was a basal tyrannosauroid in the direct line to T. rex and it had feathers. The feathering was simple filaments and would’ve given the 2m/9.9ft long, Dilong a shaggy appearance.

Artist: Cheung Chun Tat  Yutyrannus

The recently discovered fossil remains of, Yutyrannus (U-ty-ran-nus) was even a more evolved ancestral tyrannosauroid in direct relations to T. rex and it had feathers all over its body. This 9m/29.5ft long tyrannosauroid lived 124.6 mya during the Early Cretaceous and is the largest known theropod with direct evidence in donning feathers. Even the name, Yutyrannus means ‘feathered tyrant’. The feathers of these two theropods would have served as insulation. While Dilong was cloaked in feathers and Yutyrannus was tucked in a tuft of plushy feathering, there are some T. rex scale impressions from fossil remains, but as yet no feathers.

Distinguishable traits found in all coelurosaurians are:

·        sacrum (vertebrae series attached to hips) longer than in other dinosaurs

·        tail stiffened towards the tip

·        bowed ulna (lower arm bone)

·        tibia (lower leg bone) that is longer than the femur (upper leg bone)

·        an intramandibular joint inherited from primitive to derived forms

Basal coelurosaurians were smaller than their allosauroid, megalosauroid and tyrannosauroid outgroups, but were also larger than their ornithomimosaur, oviraptorosaur and alvarezsaurid outgroups. So, coelurosaurs trended towards miniaturization as well as gigantism.

Artist: Michael B.H. Zuolong

Zuolong (Zu-o-long), occurring 161.2-155.2 mya in the Late Cretaceous ‘Shishugou Formation’ of China is the most basal coelurosaurian found thus far. It was 3.1m/10ft long and despite its coelurosaur primitiveness, it may turn out to be more derived than tyrannosauroids.  

There was some initial speculation that, Bicentenaria (By-sin-ten-ah-ree-ah) was a direct ancestor to all coelurosaurians. Living 90 mya in the Late Cretaceous, Bicentenaria most definitely is not the true ancestor to all coelurosaurs, as it is ~ 70 million years too late for that since coelurosaurs were already an established diverging group during the Late Jurassic 165 mya. But its fossil finds are unique as it is a transitional form in possessing both primitive and derived coelurosaurian anatomical traits.

Illustration Artist: Christian Masnaghetti  Bicentnaria

This 2.5-3m/8-10ft coelurosaur possessed primitive plesiomorphic traits such as having long slender tridactyl pes and manus (feet and hands) that were fashioned for speed running and grasping, while possessing ziphodont teeth, the laterally compressed, curved and serrated dentition allowed for slashing, grasping and seizing. A few of the more derived autapomorphies were in the humerus bearing a deep fossa on its surface, premaxillary teeth with mesial denticles only at the base of the crown and a spoon-shaped retroarticular process [ventral end of zygomatic arch (cheek bone)] was dorsoventrally depressed and transversely wide.     

The fossils, including subadult and adult remains were found as washed up into a pile in the Late Cretaceous Argentine sediment of the ‘Ezequiel Ramos Mexia Reservoir’ in Argentina’s Patagonia region. The name, Bicentenaria refers to Argentina’s bicentennial years as a nation. There had been speculation that it was a social animal due to the pile up of fossil bones, but once geological and taphonomic (taphonomy: the paleontology branch dealing with the processes of fossilization) considerations were employed, the fossils were individuals that had previously died in separate places when afterwards parts of their remains were washed away and heap deposited during separate time frames.

Artist: Tom Miller  Huaxiagnathus

Compsognathidae (Com-sog-nah-thuh-day) is a family of coelurosaurians that had a temporal range during the Late Jurassic to Early Cretaceous 151.5-108 mya. Huaxiagnathus (Wash-ag-nuh-thuse) and Sinocalliopteryx (Sye-no-cal-ee-op-ter-ix) were by far the largest at 1.8m/5.9ft and 2.4m/7.8ft long respectively, while most were just under or at 1m/3.3ft in total length. In being the largest, these two compsognathids were also two of the earliest compsognathids, showing that this group overall were trending towards miniaturization as compsognathid species were some of the smallest dinosaurs found so far.

Artist: Cheung Chun Tat  Sinocalliopteryx

In being small, compsognathids had the generic smallish coelurosaurian characteristics of large eyes, a three-fingered manus (hand) and an S-shaped neck and rigid long tail. Scales, filamentous feathering and skin impressions have been found among their fossils. Even though they had bird features their genetic line went extinct before the Late Cretaceous began.

The specific characteristic trait of compsognathid anatomical features was the shared feature in the distal end of the manus. As in all coelurosaurians, digit I is longer than all the other metacarpal digits, but in compsognathid manus morphology, the metacarpal in digit I is stockier and shorter than in other coelurosaurians. With compsognathids, there is also a projection from digit I.

Compsognathid diets most likely consisted of small vertebrates and large insects. They appear to have been especially fond of small lizards as the remains of lizards have been found in the digestive tracts of several compsognathid fossils.

Sinosauropteryx fossil

At 1.1m/3.6ft, Sinosauropteryx (Sye-no-sawr-op-tear-icks) and not to be confused with the aforementioned, Sinocalliopteryx, was a compsognathid that roamed open Early Cretaceous habitats 124.6-122 mya. Its fossil finds are very important due to the fact the remains still retained the feathering pigmentation, giving us evidence of its coloration. The pigments of Sinosauropteryx fossils come from the bristly filamentous feathering that weren’t for flight, but served as insulation and for camouflage. Melanosomes, the pigment that gives coloration to bird feathers, were found buried within the structure of the fossil feathers.

The two pigment storing melanosomes found were eumelanosome and phaeomelanosome embedded in the protein structuring of the feathers revealing darker striping down the tail. As melanosomes, these two pigments are organelles enclosed by their own membrane bound in the proteins of cells. Eumelanosome gives black pigmentation to the fossil while the phaeomelanosome gives a burnt orange with both colors alternating down the tail. Other colors, produced by pigments such as, carotenoids and porphyrins add more vivid colors, but due to the fact they’re not melanosomes, they degrade much easier over time and will not contribute to a fossil find. Eumelanosome pigment was also found along the orbitals of the eye sockets and nasal bridge masking the animal.

Fossilized Sinosauropteryx feathering

Along the body torso there was also evidence of countershading with a darker dorsal side (the back) over a lighter ventral side (the belly). This color scheme would aid Sinosauropteryx as camouflage from predators and prey with diffuse sunlight reigning down where there is little vegetation and shrubbery to hide. Feather coloration would have starkly shifted from dark to light, with paler plumes running across its chest distorting an observer’s view with Sinosauropteryx and its open environment backdrop. It is with this camouflage and its agility, quickness and speed that gave this small dinosaur an advantage in the open sunny habitat it lived in, in what is now northeastern China.

Sinosauropteryx fossils also reveal that females had two oviducts instead of just one as found in birds. This is evident in a well preserved fossil that had two developed eggs side-by-side where two oviducts would have been.

Artist: Bob Nicholls  Sinosauropteryx

Apparently, Sinosauropteryx was fond of lizards, for numerous fossils have the remains of lizards in the digestive tract, in particular, Dalinghosaurus, which was a small lizard contemporaneous to Sinosauropteryx as found in fossil sediment containing both animals. One fossil find had the remains in the gut region of, Sinobaatar, a small mammal. A further note on this little 35cm/13.8in multituberculate mammal is that it lived beyond the Cretaceous surviving the K-Pg extinction, living into the Eocene finally becoming extinct 34 mya. It was an arboreal herbivore that made it as a species nearly 100 million years. When you know that, Homo sapiens have only been around for ~ 200 thousand years, Sinobaatar had quite a long success story.

Tyrannoraptora Cladogram

Tyrannoraptora (Tuh-ran-o-rap-tor-uh) is the sister group to compsognathids. Occurring 170-0 mya in the Middle Jurassic to the present, Tyrannoraptora is further divided into the two clades of, Tyrannosauroidea and Maniraptoriformes (Man-ih-rap-tor-rih-formz). Under tyrannosauroids, tyrannoraptorans are an inclusive clade to all non-avian coelurosaurians that are more related to Tyrannosaurus rex and under maniraptoriforms; this clade represents closer relations to Passer domesticus (the modern pigeon).

Aorun skull fossil

Two tyrannoraptorans that were not in the direct, but as an indirect line (cousins) of tyrannosaur descendancy was, Aorun (Aw-roon) occurring 161.5mya and Ornitholestes that occurred 154 mya. Both of these Late Jurassic tyrannoraptorans were small with Aorun no more than 1m/3.3ft long and Ornitholestes (Or-nith-oh-less-tees) at best was 3m/9.9ft in length. Aorun is the oldest and most primitive of tyrannoraptorans. The fossil remains were of a sub adult that was no more than 1-yr-old, so the adult form would have been larger than demonstrated in the fossil find from the ‘Shishugou Formation’ in a remote part of Xinjiang, China. The fossil had very thin metacarpals in manus digits, III and IV approximating the hands of later more derived non-avian tyrannoraptorans. The thumb claw is large and curved while the other two finger claws are smaller exhibiting a straight underside. The tiny skull was just higher in height as a U.S. quarter.

Artist: Charles R. Knight  Ornitholestes

Built for speed and agility, Ornitholestes did not possess any strength for brute force as its latter day tyrannoraptorans would possess. Chasing after small vertebrate and larger insect prey, Ornitholestes most likely didn’t use its mouth and teeth to capture prey due to the small mouth and skull. It most likely was dependent on its longer forelimbs to seize small prey then, once captured, deliver a killing bite with its conical and slightly serrated frontal teeth. The back teeth were recurved making it easier to slice through meat, so may have also scavenged on larger carcasses.

The first major division of Tyrannoraptora is with the two sister groups, Tyrannosauroidea (Tuh-ran-o-sawr-oi-dee-uh) and Maniraptoriformes (Man-ih-rap-tor-rih-formz).

Tyrannosauroidea Cladogram

Tyrannosauroidea ~ The temporal range for tyrannosauroids was during the Late Jurassic to the Late Cretaceous 165-66 mya. Their special evolved features are in the skull and pelvis. Where earlier tyrannosauroids had a long, low and lightly built skull, later forms had taller and more massive skulls. Still there were similar distinct features, such as possessing a very tall premaxillary bone while the nasal bones were fused together. The tyrannosauroid pelvis had a concave notch at the upper front end of the ilium with a sharply defined vertical ridge on the outside surface of the ilium extending upwards from the acetabulum (hip socket). Also, a huge ‘boot’ on the end of the pubis was more than half as long as the shaft of the pubis itself. 

Beginning in the Jurassic Period, tyrannosauroids originated on the Laurasian supercontinent, but when Laurasia split from Gondwana in the Middle Jurassic it left tyrannosauroid species isolated to further evolve from the rest on the northern continents that later separated from Laurasia during the Cretaceous. By the end of the Cretaceous, tyrannosauroids radiated outwards becoming dominant predators in the Northern Hemisphere culminating in Tyrannosaurus rex.

Artist: Teratophoneus  Tyrannosauroids

In more derived forms, tyrannosauroids trended towards larger sizes and smaller forelimbs. In the more basal tyrannosauroids, long filamentous pennaceous structures, sometimes referred to as protofeathers and branching downy feathering were found in their fossil remains. Downy feathering covered most of the body in these fossils finds and was most likely used for insulation.

One of these basal tyrannosauroids with insulation feathering features was the afore mentioned, Dilong (Dye-long). Occurring 126 mya in the Early Cretaceous, this tyrannosauroid at 2m/6.6ft was just a bit larger than today’s average human and at 5kg/11lbs weighed no more than a regular sized turkey. As transitional, it is in anatomical appearance a derived coelurosaur, as well as a very primitive tyrannosaur. In body appearance, Dilong resembled juvenile tyrannosaurs, but had much longer forelimbs that could support weight if it chose to stand on all fours.

Credit: American Natural History Museum  Dilong

The anterior teeth of Dilong were D-shaped in cross-section giving it a cookie cutter bite. With sure balance and dexterity as its body, tail and hind limbs anatomically show, Dilong, could easily reach speeds of 32kph/20mph in chasing down its prey, which likely consisted of smaller dinosaurs and larger lizards.

Artist: Todd Marshall  Timurlingia

A recent 2016 Uzbekistan fossil find in the ‘Bissekty Formation’ of a tyrannosauroid bore out some interesting evidence that the brain size had increased before the body size of tyrannosauroids and later tyrannosaurs. Timurlingia (Tim-ur-len-gee-uh) was only 3.5m/11.5ft long, but already had the brain size of a horse according to its braincase. Occurring 90 mya in the Late Cretaceous, Timurlingia exemplifies that tyrannosaurs diversified upon a genus-to-genus basis with the lighter and longer limbed hunting down more swift smaller prey and the more derived shorter limbed genera with more brute force hunting down larger prey.

Artist: Durbed  Dryptosaurus

Dryptosaurus claw

Basal tyrannosauroids began increasing in size evolving finally into tyrannosaurids. At 7.5 m/24.6ft, one of the last most derived tyrannosauroids that wasn’t quite a tyrannosaurid anatomically was, Dryptosaurus. Living 67 mya in the Late Cretaceous in what is now the eastern USA, this tyrannosauroid’s only anatomical disposition to tyrannosaurs was its longer forelimbs. Its larger hands also had three fingers with 20.3cm/8in claws attached to each finger in conjunction with surangular (all jaw bones muscularly connected) lateral teeth. Equipped as such, this tyrannosauroid was merciless to its prey. It was also feathered at least on the anterior body portion.

Along with, Dryptosaurus, the tyrannosauroid, Appalachiosaurus (Ah-pah-lay-chee-oh-sawr-us) are the only two tyrannosauroids found in the eastern U.S.; the rest of tyrannosauroid fossil finds as tyrannosaurids were found in the western half of the USA. As a juvenile at 7m/23ft long, Appalachiosaurus’ 77 mya fossil remains were found in Alabama in the ‘Demopolis Chalk Formation’.

Artist: sbpaleoartist  Applalachiosaurus

The first beakdown of a tyrannosauroid family was in the more primitive, Proceratosauridae (Pro-seh-rat-o-sawr-ah-day) grouping. with a temporal range of of 165-120 mya during the Middle Jurassic to the Early Cretaceous, as a family, proceratosaurids were a difficult taxon as they had coelurosaur, ceratosaur (hence the nomenclature) and primitive tyrannosauroid affinities in anatomy. However, in having enough specific anatomy traits to list them as a separate taxon, they were, through maximum parsimony and Bayesian analyses, phylogenetically grouped together. Some main characteristics are: a cranial crest formed by the nasal bones starting at the junction of the premaxilla and nasals, a short and shallow concave step on the anterior margin of the maxilla, a short ventral margin of the premaxilla, a ventrally concave ischium and just ventral to the contact of the pubis and ilium the pubis retained a convex tubercule (knob).

Although proceratosaurids shared similar anatomical body traits with the more derived tyrannosaurs, they were generally smaller with distinct skull differences. First off the crests were distinct, the dentition was odd in having small premaxillary teeth, but large lateral teeth and the skull as a whole was highly pneumatized in the nasal, jugal, maxillary and cranial bones. Like the primitive tyrannosauroids, proceratosaurids also had longer forelimbs and fingers. Currently, five genera make up the family, Proceratosauridae.

Artist: Raul Martin  Guanlong

One proceratosaurid was, Guanlong (Gwon-long) which lived 160 mya in the Late Jurassic’s ‘Shishugou Formation’ of China. The above illustration portrays a Guanlong sub-adult and adult scene of how their fossil remains were interpreted. The juvenile became trapped in a muddy watery pit, perhaps a foot track made by a large sauropod. The adult apparently attempted a rescue and became mired in the mud as well, with both dying by suffocation as the mud sunk them under. The fossil remains were found with the juvenile underneath the adult.

At 3m/9.9ft, Guanlong lived 92 million years before its relative, Tyrannosaurus evolved. But unlike the tyrannosaurs, it had three forelimb fingers instead of just two with the forelimbs, although shorter than the hind legs were much longer than the tyrannosaurs. It was closer in relations to Dilong than to any tyrannosaurid. Having found two fossils remains of a six-yr-old juvenile and a 12-yr-old adult, paleontologists discovered that the crest was restricted to the forward portion of the snout in the juvenile where it extended to the back of the head in the adult. The cranial crest most likely, due to its fragility was merely for display. There is fossil evidence that Guanlong possessed integument feathering.

Tyrannosauridae Cladogram

Tyrannosauridae: This is the family that pinnacled into one of the largest and best known land predators of all time in, Tyrannosaurus rex. Tyrannosaurids appeared in the Late Cretaceous 80-66 mya. Although some tyrannosaurids technically support crests, they are not as elegant or elaborate as other crested dinosaurs as either being a row of bony bumps on the nasal bones as in Alioramus (Ah-lee-oh-ray-mus), or as a prominent horn in front of each eye as in Albertosaurus (Al-bear-toe-sawr-us). Also, Tarbosaurus and Tyrannosaurus had a small crescent-shaped crest behind each eye on the postorbital bone.

Tyrannosaurid species

Tyrannosaurids essentially had binocular vision veering away from stereoscopic vision as most actively hunting predators in any animal group do. There is still some ongoing argument that T. rex was more of a scavenger than a hunter due to its size hampering locomotion, but T. rex had one of the most sophisticated forms of binocular vision than any predator dinosaur, which would make it more of a hunter than a scavenger, in which scavengers rely more on stereoscopic vision for depth perception. However, Tarbosaurus did have orbits for the eyes that faced more sideways than they did in front, but fossil evidence proves that this tyrannosaurid relied more on the sense of smell rather than sight.

T. rex binocular forward vision

As in modern crocodiles, tyrannosaurids had flat snout scales that at the center were covered with small keratinized patches. In crocodiles, these scales cover bundles of sensory neurons that detect thermal, mechanical and chemical stimuli. It is thought that the tyrannosaurid snout scales also covered sensory neurons to identify objects and measure temperature ranges of their nests and to mechanically and gingerly move eggs and hatchlings.

Tyrannosaurids were at least homeothermic in regulating their own core body temperature putting them between ectothermic and endothermic animals. Histological growth rates in tyrannosaurids and in particular in Tyrannosaurus bear out that the young had pronounced growth rates whereas in adult forms growth halted. This is viewed in endothermic mammals and birds of today in limiting growth only in the immature stages. Indeterminate growth as seen in ectothermic vertebrates where growth continues throughout life is not evident in tyrannosaurid fossils, thus through studies it appears that tyrannosaurids maintained a constant temperature of the body’s core that was no more than 4-5 °C/7-9 °F higher than the extremities such as the tibia of the lower leg.

As in most cases though, there normally is an exception to the rule. Albertosaurines did have a rapid juvenile growth rate, but once reaching adulthood, although very slowly, still continued to grow throughout the adult stage.

Tyrannosauridae is further divided into three groups which comprise two subfamilies, one tribe, eleven genera and thirteen species. The two main divisions are in the subfamilies, Tyrannosaurinae, or Tyrannosaurine and Albertosaurinae which both are sister clades. Albertosaurinae consists of two genera in, Gorgosaurus (Gor-guh-sawr-us) and Albertosaurus with one holotype species in each genus. Albertosaurines had a temporal range of 76.6-68 mya during the Late Cretaceous and were lightly built with flatter skulls, shorter ilia and proportionally longer tibia when compared to tyrannosaurines. Albertosaurine adults measured ~ 9m/30ft.

Artist: Philip Whiteley  Gorgosaurus 

Both albertosaurines are from what is now N. America, with Gorgosaurus living in lush floodplains near the inland sea of the ‘Western Interior Seaway’, while Albertosaurus lived in areas where the seaway had receded comprising several environments due to the recession and transgression of the waters before the final recession. These coastal habitats included lagoonal, estuary and tidal flat environments.

Artist: Steve White  Albertosaurus

As the sister group to albertosaurines, Tyrannosaurinae includes the tribe, Alioramini (Ale-ee-ur-ray-muh-nee) consisting of two genera and three species in, Alioramus remotus (Ale-ee-ur-ray-mus = ruh-mo-tus), A. altai (al-tai) and Qianzhousaurus sinensis (Kin-zao-sawr-us = sigh-nen-sis). The two species of Alioramus occurred 70 mya in the Late Cretaceous and were from what is now the Gobi Desert of Mongolia. Q. sinensis’ almost complete remains were discovered in southern China within the city of Ganzhou at a construction site.

Artist: Zhao Chuang  Qianzhousaurus

Qianzhousaurus lived somewhere between 72-66 mya during the Late Cretaceous, which was contemporaneous with other tyrannosaurids that had the more typical tyrannosaurid bulky skull and heavily muscled and powerful deep set jaws. The smaller, Qianzhousaurus at no more than 6m/20ft had to compete against these much larger and stronger tyrannosaurids and the clue as to how it did lies in its nickname of ‘Pinocchio rex’. This tyrannosaurid had much weaker jaws and brute force than its other contemporary tyrannosaurids, but it had a much longer snout. This indicates that it left the much larger prey to the other tyrannosaurids while it filled a niche in pursuing smaller prey.

Credit: BBC  Daspletosaurus

The remaining seven genera are all direct tyrannosaurids with Daspletosaurus (Das-plet-o-sawr-us) containing two species giving a total of eight species. With a temporal range of 77-74 mya ago in the Late Cretaceous and up to 9m/30ft long, Daspletosaurus species were apex predators. They lived alongside another apex predator in, Gorgosaurus; a distinct difference in Qianzhousaurus’ situation where it filled a lesser niche in competition with other contemporaneous tyrannosaurids, like Tarbosaurus.

Artist: Lukas Panzarin  Lynthronax

The oldest tyrannosaurine fossil found thus far is, Lythronax (Lye-thru-nax), whose temporal range is in the Late Cretaceous 80 mya. This primitive tyrannosaur was 7.3m/24ft long and had T. rex skull traits with forward facing orbitals for binocular depth perception as well as the skull itself being short and wide. The serrated teeth were banana-shaped meat cleavers able to chop through the toughest of dinosaur hides and just like T. rex, would’ve been able to carve out large chunks of flesh and bone to swallow whole. In being a primitive tyrannosaurine, Lythronax had shortened forelimbs, but were still larger than the more derived tyrannosaurines; however, its postcranial morphology was very similar to that of the later tyrannosaurs giving a much earlier insight into tyrannosaur evolution.

Artist: paleo executor  Nanuqsaurus

Occurring 69.1 mya during the Late Cretaceous, Nanuqsaurus (Nah-nuk-sawr-us) was a 6m/20ft tyrannosaurine theropod that lived in what is now called the North Slope Borough of Alaska, USA. It is considered a highly derived tyrannosaur and is the sister taxon to the clade of Tarbosaurus and Tryrannosaurus. Although temperatures weren’t as cold in northern Alaska as they are today, Late Cretaceous Alaska, as part of the ancient subcontinent, Laramidia would have experienced cold snowy and long periods of dark winters. Food sources would’ve been scarce, so during the more bountiful summer months, with its smaller size and perhaps an ability to store fat, Nanuqsaurus managed to survive the harsher winter months supplemented by scavenging any carcasses it came across.     

The most derived tyrannosaurines were the two sister genera of, Tyrannosaurus and Tarbosaurus. In fact, when Tarbosaurus fossil remains were first found, scientists indeed thought that it was another T. rex fossil due to the vast similarities. The main differences between the two, is that the Tarbosaurus snout was thinner than the wider T. rex snout and Tarbosaurus forelimbs along with total body length were a bit shorter.

Artist: James Kuether  Tarbosaurus bare skin/feathered

Tarbosaurus lived 70 mya while Tyrannosaurus lived 68-66 mya. With both coming from Late Cretaceous, Tarbosaurus remains come from southern Mongolia, while Tyrannosaurus finds are from western North America. In Tarbosaurus being older, some paleontologists have hypothesized that T. rex evolved from Tarbosaurus, speculating that the Tyrannosaurus taxon line migrated from Asia into N. America from Siberia. To bear this out, more fossils will have to be discovered and evaluated in what occurred in between the two theropods’ timeframes.

T. rex skin fossil

As of yet, there is no direct evidence that either Tarbosaurus nor Tyrannosaurus possessed any type of feather covering. But due to close phylogenetic relations of tyrannosaurids that did possess feathers, most paleontologists consider it likely that there were at least on some parts of the T. rex body…feathering. Scales and scale impressions have been found in T. rex fossils including small mosaic scales. Even bare skin impressions have been found that would allude to feathering.

Artist, R.J. Palmer version of T. rex feathered

Back under ceratopsids we discussed a bit on the duels we perceive that might have occurred between T. rex and Triceratops and who would win the battle as we assume a fifty percent chance of either one winning. But we neglect the begging query as why would T. rex fight a Triceratops when it only had a 50-50 chance? The deal is...T. rex wouldn’t. He or she would only fight Triceratops if the odds were in its favor. It most definitely was and here is why...

 Artist: Vlad Konstantinov  T. rex vs. Triceratops

T. rex had a bite force of 562.5kg/sq. cm or 8,000 lbs/sq. in, while in the initial bite, the teeth’s tip exerted an astounding 30,302.3kg/sq. cm or 431,000 lbs/sq. in. That’s equivalent to the same pressure as having 33 full grown African elephants stacked atop one another atop your head. With this biting force supplemented by sixty razor sharp 30.5cm/12in serrated teeth, T. rex could slice through any tough dinosaur hide and chomp through bone to get at the marrow. So in getting back to the T. rex/Triceratops battles mentioned earlier under ceratopsids, in one quick bite hold, T. rex could have easily sliced through and severed Triceratops head and flung it; with a frill or no frill.

Simple Maniraptoriformes Cladogram

Maniraptoriformes ~ Quickly radiating out into new lands, basal coelurosaurs were predators of various Jurassic and Cretaceous environmental biomes. This created a rapid diversification and evolvement to fill in niches that never before were available to theropods. It also generated omnivorous and even herbivorous theropods filling in regions where small herbivores were absent. In regions where there were large herbivores but no large carnivores, the smaller theropods began enlarging their bodies. In ecology, this is known as ‘niche partitioning’ and is the process by which competing fauna utilize a biome differently than another species allowing a steady coexistence. This is the setting where the maniraptoriforms’ story unfolds.

As the sister taxon to Tyrannosauroidea, maniraptoriforms consist of two clades in Ornithomimosauria (Or-nith-uh-my-mo-sawr-ee-ah) and Maniraptora (Man-uh-rap-tor-ah). Maniraptoriforms had a temporal range from the Late Jurassic to the present from 167-0 mya. Maniraptoriforms consist of the clades that led directly to birds.

According to the cranial portion of the skulls, maniraptoriforms contained in the clade, Maniraptora (Man-uh-rap-tor-uh), all had brains that were at a minimum twice as large as the more basal coelurosaurs. Typical features are in having small phyllodont (leaf-shaped) teeth or no teeth at all in the more derived and, except for the most basal, often lacked grabbing claws, or no claws at all in the more derived. This would suggest that early in their evolvement, maniraptoriforms were trending on the whole toward an herbivorous diet. They all possessed feathering, even in some, broad pennaceous feathers branching off from a shaft. The tiny alvarezsaurid, Shuvuuia (Shu-vue-ee-ah) possessed a form of keratin consistent to that of modern day birds, while the ornithomimosaur, Pelecanimimus (Pel-ee-can-eye-mime-us) fossil finds bear impression evidence to the skin’s collagen fibers suggesting that under a feathered cloak, the body was without scales just as in birds.

Ornithomimosauria Cladogram

Ornithomimosauria ~ Called ornithomimosaurs, or ornithomimosaurians, but commonly known as ‘ostrich dinosaurs’ occurred 140-66 mya during the Early-Late Cretaceous. They are the most basal of maniraptoriforms with their taxa line coming to an end 66 mya. Compared to the large eyes the skulls were small. While they experienced a reduction in tooth size, the slender necks lengthened. The forelimbs were also long with the manus distal ends terminating into powerful hooked claws, used most likely for reaching and grasping. The hind limbs were also long and powerful with the pes (feet) ending in hoof-like claws making ornithomimosaurs one of the swiftest of dinosaur runners. Even though there are numerous fossils detailing ornithomimosaurs as prey, with their hind limbs, they weren’t totally defenseless. Just as ostriches can today in gutting a lion or even a human with their pes claws, the larger ornithomimosaurs could have done the same to their predators. Even a well-placed kick could do damage.

Deinocheirus forelimbs

Judging from ornithomimosaur fossil finds containing gastroliths, for the most part, it alludes to the fact they were herbivores, but some could also have been omnivores preying on small vertebrates and large invertebrates such as insects.

Ornithomimosaurs make up for a large group of species, with nine holotype genera and with two families in Deinocheiridae (Dee-in-no-care-ah-day) having three genera and Ornithomimidae (Or-nith-o-mi-muh-die) having eleven genera.

Found in South Africa’s ‘Kirkwood Formation’ that was laid down during the Early Cretaceous, Nqwebasaurus (Nuh-qway-ba-sawr-us) from 140 mya is the most basal ornithomimosaurian found thus far. As the earliest ornithomimosaur and coelurosaur from Gondwana, it was a 90cm/35.4in long three-year-old subadult. It had small reduced teeth and utilized gastroliths for digesting plant material. The hands were three-fingered including an opposable tipped with a claw. The well preserved fossil remains were found in the ‘Kirkwood Formation’ of South Africa making it the only African coelurosaur found.

Artist: Riley Chandler  Nqwebasaurus

The ‘mimo’ in Ornithomimosauria refers to the Latin word, ‘mimicus’ meaning mimic. So, Ornithomimosauria, means ‘bird mimic’, which in essence means they appeared to be birds but weren’t. Tooth reduction is an obvious evolutionary pattern in ornithomimosaurs, with Nqwebasaurus having reduced teeth as compared to other contemporaneous coelurosaur groups, while the latter ornithomimosaurian holotypes in the 130 mya Pelcanimimus (pelican mimic) had more reduced teeth than Nqwebasaurus and the 130 mya, Harpymimcus (harpy mimic) only had small cylindrical teeth in the lower jaw. By the time the ornithomimid, Gallimimus (chicken mimic) arrived 70 mya, its beak was totally toothless.

Deinocheridae-Ornithomimidae Clades

Deinocheiridae members occurred between 115-66 mya during the Late Cretaceous of Asia. Best known for their forelimbs, deinocherids had the longest arms of any other biped theropod. In general body anatomy, they were bulky but lightweight due to hollow bones. The three genera that make up the family are: Bishanlong (Bay-shan-long) from the ending of the Early Cretaceous 115-100 mya, Garudimimus (Gah-rue-die-mime-us) from the beginning of the Late Cretaceous 98-83 mya and Deinocheirus (Dee-in-no-care-us) from the ending of the Late Cretaceous 71-69 mya.

Artist: Damir Martin  Deinocheirus

The largest ornithomimosaurian was the deinocheirid, Deinocheirus at 11m/36ft long while weighing 6,350kg/14,000lb. The Gobi Desert fossil finds in Mongolia prove this ornithomimid to be one unique theropod. Most ornithomimosaurs were lithe built for speed, but this huge humpbacked ornithomimosaur was a slow moving gargantuan. It also had a duck-billed shape beak mounted on a horse-shaped skull. The last twelve back vertebrae supported bulky neural spines that became increasingly longer from front to back with the last one being 8.5 times longer than the centrum portion. This made the central to rear half of the back look hump-like. The spines supported interconnecting ligaments supporting the large abdomen. The forelimbs are the largest of any bipedal theropod.

Deinocheirus fossils have been found in sediment that were near rivers, small lakes and in mudflats. With at least 1,400 gizzard stones 8-87mm/0.32-3.3in in diameter found in fossil remains and also with the large abdomen, Deinocheirus ate soft plants and in particular with the duck-billed snout it foraged for water plants and browsed near the ground. It had no teeth but fish vertebrae and scales have been found mixed in with the gastroliths and whether the fish were ingested incidentally or not, makes it an omnivore. The musculature working the jaws were weak, so if it preyed on any land animals they would’ve had to been small, as the jaws were too weak to seize any prey with any larger size to it. Deinocheirus also had vertebrae spines in the middle of the back that supported a hump and like camels, it presumably stored fat.

Ornithomimidae is the last of the two families of ornithomimosaurians to radiate out into its own group. With a temporal range confined to the Late Cretaceous 92-66 mya, ornithomimids are known for their ostrich-like appearance in having large eyes, small heads and relatively long slender necks. With muscled long hind limbs, the legs were powerful terminating in a long foot with short but strong toes bearing hoof-like claws. As mentioned earlier, ornithomimosaurs were the fastest of dinosaurs, but ornithomimids were the fastest of ornithomimosaurs. The body’s skin was feathered rather than scaly.

Artist: Mohamad Haghani  Struthiomimus

Struthiomimus fossil

Another ornithomimid besides the aforementioned, Gallimimus was, Struthiomimus (Stru-fee-o-mime-us) which stands for ‘ostrich mimic’ and had a temporal range of 77-66 mya towards the end of the Late Cretaceous. At 4.3m/14ft long and 1.4m/4.6ft tall, Struthiomimus was the fastest runner of ornithomimids, therefore the swiftest of all dinosaurs with anatomical studies showing maximum speeds of up to 80kph/50mph. Struthiomimus was a gregarious herbivore. The anatomy supports an herbivorous diet in that the structure of the shoulder girdle was construed as to not allow the arms to reach in elevation, nor was the shoulders optimized for reaching to the ground. The II and III manus digits were equal in length and could not function independently suggesting that in life they were bound to work as a single unit by skin covering. The whole manus (hand) could not be fully flexed, so all this alludes to the fact that the manus was used as a hook and clamp for bringing plant material in reach of the mouth. This hand adaptation would also have served as the forelimb’s wing feather support.

Maniraptora Cladogram

Maniraptora ~ Genetically borrowed from their coelurosaur ancestry, maniraptorans and early birds inherited an increase in the size of the brain’s proportion devoted to the cerebrum. The cerebrum is that part of the brain responsible for the integration of complex sensory and neural functions and the initiation and coordination of voluntary activity in the body. However, there was much convergent evolution in maniraptoran groups as well, but have enough synapomorphies (common anatomical structures) to be phylogenetically united. With this understanding, keep in mind that maniraptorans as deeply nestled within coelurosaurs, are a branched clade closer in genetic relations to extinct and extant birds than they are to ornithomimosaurs.

Typical maniraptoran anatomical features were: a fused clavicle (shoulder blade) and sternum (breast bone), a bird pubis that points downwards rather than forwards, a shortened or absent tail ending in a stiffened pygostyle, a manus (hand) longer than the pes (foot) in the most derived and are feathered. With a unique modified wrist bone found only in maniraptorans called the, ‘semi-lunate carpal’ it originally made the wrist more flexible, but later on in the most derived maniraptorans, it made it possible for flight. Thus, the maniraptoran clade contains the only group of dinosaurs that evolved the capability of flight.

The temporal range for maniraptorans is from the Late Jurassic 167-0 mya. The major clades of maniraptorans are the: alvarezsauroids, therizinosaurs and the pennaraptorans which includes the avialan groups in which birds are a part of.

Alvarezsauroidea Cladogram

Artist: Matthew Martyniuk  Some alvarezsauroid sizes

The most basal of maniraptorans are from the clade, Alvarezsauroidea (Al-vuh-rez-sawr-oid-dee-ah) occurring in the Late Jurassic to the Late Cretaceous 160-66 mya. The oldest and most primitive alvarezsauroid is Haplocheirus (Hap-lo-chair-us) predating any other maniraptoran member species by 63 million years. It roamed through the Late Jurassic northwestern China semi-arid lands 260 mya. Haplocheirus was ~ 2m/6.6ft long with fossil exposing an almost complete individual. It had not yet developed the specific developed manus (hand) as the more evolved Cretaceous South American and Asian maniraptorans show with the singly enlarged digit as it still retained the use of two fingers with a well-developed thumb that became vestigial on most of the latter maniraptoran forms. It was covered in downy insulating feathers and possessed a deep keel (extension of the sternum). With long legs ending in three toes it was a swift runner and walked in digitigrade (walking on toes) fashion just as birds do.

Artist: Portia Sloan  Haplocheirus

Like most Jurassic theropods, Haplocheirus had relatively normal-sized curved and serrated front maxillary teeth. But the distinguishing dentition feature is that the back teeth rapidly decreased in size while the roots of the teeth became circular in cross section. There were at least 30 teeth in the maxilla and judging by this, Haplocheirus most likely preyed on small animals and larger insects.

Alvasauridae Cladogram

All other alvarezsauroids belong to the Alvarezsauridae family. This family is comprised of twelve genera with the two subfamilies, Patagonykinae (Pat-ah-go-nee-key-nee) and Parvicursorinae (Par-vye-kur-sor-uh-nay) in which has the two sister tribes, Ceratonykini (Suh-rat-toe-nee-key-nee) and Mononykini (Maw-no-nee-key-nee). Alvarezsaurids had a temporal range of 89-66 mya during the Late Cretaceous. Evolving from a primitive maniraptoran branch, alvarezsaurids were trending toward smaller body sizes and shorter forelimbs with Parvicursor (Par-vee-kur-sor) being no longer than 39cm/15.4in when fully grown. Alvarezsaurids only had one developed digit ending in a large claw. The other fingers, except in Linhenykus were vestigial nubs. This claw is presupposed to have been used in digging for insects, in particular to dig through termite mounds. Although the forearms were significantly short, the strong arm and breast muscles were adapted for tearing and digging.

Artist: Ezekiel Vera  Parvicusor

The Patagonykinae members are composed of the largest alvarezsaurids, with Patagonykus (Pat-ah-go-nee-cuss) at 2m/6.6ft in length and Bonapartenykus (Bo-nuh-par-tuh-nee-cuss) at 2.6m/8.5ft long. Both occurred in what is now, the Patagonia region of Argentina during the Late Cretaceous, with Patagonykus from ~ 89 mya and Bonapartenykus from 72 mya. They were primarily insectivores but also could handle small vertebrate prey. Both Patagonykinae and Parvicursorinae members came from a common coelurosaurian ancestor when their respective continents were connected and diverged when continental drift separated them.

Patagonykus skeletal anatomy

Patagonykus is a basal alvarezsaurid anchoring the subfamily, Patagonykinae. With primitive features for an alvarezsaurid, the last sacral centrum is ball-shaped caudally, a condition that is also found in the caudal vertebrae like the earlier coelurosaurs. But it possessed the derived alvarezsaurid, ‘pinched’ middle toe.

Artist: Gabriel Lio  Bonapartenykus

Bonapartenykus fossilized egg

An interesting feature of Bonapartenykus’ fossil remains is two eggs that were still retained in the oviducts of the female individual. Nearby of the specimen fossil site were found a nest of eggs that were contaminated with a fungus that killed the eggs; a clue that the mother had died post to just laying the eggs and unaided, the eggs were exposed to the fungal infection. The eggs are very unique dinosaur eggs and thus have been categorized under the oogenus (fossil eggs), Arriagadoolithus.

Artist: Karkemish  Alvarezsaurus

Alvarezsaurus is the sister member to the subfamily, Parvicursorinae. As a basal alvarezsaurid it had a longer tail, a curved scapula, an unfused astragalus and calcaneum unlike the more derived alvarezsaurids, but possessed typical alvarezsaurid features in its limb structures and its caudal vertebrae. It came from Late Cretaceous Argentina 86-83 mya.    

The Parvicursorinae (meaning: small runner) tribe members were all small swift sprinters. The name refers to an arctometatarsalian condition where the middle metatarsal (MT-III) is extremely pinched by the flanking two, MT-II and MT-IV. All were most likely primary insectivores, but with their minute pegged teeth they possibly could have snagged very small vertebrates and gulped them down as the jaw muscles were too weak to contend with any animal of size. They all occurred during the Late Cretaceous of Asia and N. America 84-66 mya.

Parvicursor is the only Parvicursorinae member that does not belong to a tribe but is the holotype sister to the tribe, Mononykini. Living 72 mya in the Late Cretaceous only weighed ~ 162g/5.7oz and as previously mentioned, was 39cm/15in in total length; pretty small considering our preconceived notion of dinosaur size.

Artist: Nobu Tamura  Ceratonykus

Ceratonykini is comprised of Ceratonykus (Suh-rat-toe-nee-cuss), Xixianykus (Zee-ee-awn-ee-cuss) and Albinykus (Al-bin-e-kus) where all members are most related to the last common ancestor of Ceratonykus.  Ceratonykus was 1.5m/4.9ft in length and with long legs was adapted to running in desert habitat. Xixianykus was 50cm/20in long and with legs that were 20cm/8in long, it had the longest leg to body ration of any dinosaur. Albinykus’ length is unknown due to incomplete but well preserved fossil remains. However, in life it could have stood comfortably fitting neatly in the palm of a human hand. The fossil remains were positioned in a crouching position with feet tucked underneath the body much like birds do today. Apparently a sand-slide doomed the individual burying it.

Artist: Matt van Rooijen  Albinykus

Mononykini is comprised of Mononykus (Maw-no-nee-cuss), Albertonykus (Al-bear-toe-nee-cuss), Linhenykus (Lin-he-nee-cuss) and Shuvuuia (Shu-vu-ee-ah). Mononykini members are more closely related to the common ancestor of Mononykus than they are to other alvarezsaurid species. Mononykini species all lived between 84-68.5 mya during the Late Cretaceous coming from what is now Asia and North America.

Artist: M. Haghani  Mononykus

Occurring 70 mya in the Late Cretaceous Mongolian floodplains, Mononykus was nimble and with a light skeleton and long legs was built for speed, which was useful on the flat mainly featureless terrain it lived in. With large eyes, it most likely sought out prey during cooler darkness when most of its predators were inactive. Prey consisted of insects and small vertebrates that it could snag with its small pointed teeth. It had a long thumb spike in digit I of the manus (hand) and a pubic bone that was triangular in cross section. In the pelvis, the ischium and ilium were fused together

Mononykus skeletal anatomy 

Artist: Karkemish  Albertonykus

Albertonykus occurred 68.5 mya during the Late Cretaceous with its fossil remains coming from the ‘Horseshoe Canyon Formation’ of Alberta, Canada. It measured only 70cm/2.5ft in total length. It also preyed on termites, but not the mound termites as the Asian alvarezsaurids did, as mound or earthen termites did not appear in N. America until the Eocene. As Albertonykus had arms too short for digging, they however would have been ideal for breaking the bark of trees to get at wood termites that were present in N. America as attested to termite burrows in petrified wood that lived during Albertonykus’ time. It surely would have accepted any other bark-living insects along with their grubs and larvae. Albertonykus is a pertinent fossil find for it supports the fact that alvarezsauroids originated in S. America then dispersed to Asia with the more derived species migrating to N. America when the continents were connected.

Artist: Julius T. Csotonyi  Linhenykus

Occurring, ~ 79.5 mya years ago in Late Cretaceous Mongolia, Linhenykus at 90cm/35.4in was a typical small alvarezsaurid with its single digit II long finger ending in a heavy claw. But rather than having any vestigial digit nubs it had no remnants of any other digits; all other digit metacarpals were absent. As a latter basal alvarezsaurid, Linhenykus, in retaining some archaic features, but having some distinct characteristics unique among its own from the more derived species represents a mosaic pattern in evolution. It most likely was an insectivore.

With a temporal range of 75 mya in Late Cretaceous Mongolia, Shuvuuia was no longer than 60cm/2ft long. Its forelimbs were unusually short but terminated with the manus supporting a massively enlarged digit I (thumb) and not quite vestigial, but reduced digits II and III. Living in localized fluvial plains of what is now the Gobi Desert, Shuvuuia scampered about chasing down insects or small vertebrates it could seize with its minute teeth that weren’t serrated or capable of biting into tough flesh. Once prey was captured it would be swallowed whole. It also, as other alvarezsaurids, most likely used the singular manus claws to dig into termite mounds.

Unique among the non-avian dinosaurs, Shuvuuia’s lightly built skull was capable of performing prokinesis where the upper jaw can flex independently of the skull’s braincase. One fossil find showed that the body was coated with small hollow tube-like structures resembling the rachis (central vane) of modern bird feathers.

Artist: Nattawut Wongta  Shuvuuia

Shuvuuia morphologically is very similar to Mononykus, but differs 1) in having cervical centra less compressed laterally bearing large pneumatic foramina, 2) deltopectoral crest of humerus continuous with humeral head, 3) femoral and tibiotarsal shafts bowed mediolaterally, 4) medial border of distal end of tibiotarsus with a sharp ridge, 5) medial margin of the ascending process of the astragalus less notched with lesser degree of co-ossification of proximal tarsals and metacarpals.             

Just to mention, there are two more alvarezsaurids that are not as yet fully classified. The first is, Alnashetri (Al-nah-shet-ree) that occurred during the Late Cretaceous 97 mya in what is now the northern edge of Patagonia. Its age makes for it to be a primitive alvarezsaurid and its location puts it nestled within the Patagonykinae tribe. However, the inclusion is based on a polyatomic phylogeny within a single anatomical region of the ankle and may create false results.  

The genus, Kol with the species name, K. ghuva (gah-vuh) is a recent Mongolian fossil find represented by a well preserved foot displaying the typical arctometatarsalian ‘pinched’ middle toe. It is an alvarezsaurid, but until more fossil discoveries, it isn’t clear to what tribe it belonged to.

Therizinosauria Cladogram

Therizinosauria (Phonetics: Ther-ih-zign-o-sawr-ee-ah) ~ The therizinosaurian clade order is composed of one superfamily, Therizinosauroidea and one family, Therizinosauridae. Within the whole clade there are 12 genera and 14 species. In the genus, Nanshiungosaurus (Nan-she-ung-sawr-us) there are currently listed two species in, N. brevispinus (bre-vis-pi-nus) and N. bohlini (boh-lee-nee), but with N. bohlini being much older and larger, it may later be designated its own genus. The therizinosaurian temporal range is from 130-70 mya during the Early-Late Cretaceous.

Artist: Raul Lunia  Therizinosaurus

For sure, among all the picks to choose from, therizinosaurs are the oddest of dinosaurs. First off, they came from flesh eating coelurosaurian theropods, even though a feast for them would be a buffet filled with dandelions, cycad leaves and conifer needles. They evolved the longest necks of any theropod and long forearms ending in the longest claws of any dinosaur, which were used for grasping vegetation and secondarily for ripping the guts open of an offending predator. The forelimbs also had a unique range of motion allowing therizinosaurs to have the farthest forward reach degree of any other known theropod. Another unique feature is that from shortened metatarsal foot bones, the more derived therizinosaurians began walking on four toes instead of the theropodal three middle toes.  

Even though the skulls were small, CT scan results of fossil skulls show that therizinosaurs had an acute sense of smell, hearing and balance. The torsos were wide and with unique hipbones pointing backwards, were one of the first theropods to develop the familiar ‘bird-like’ pubis, even though they are a non-avian maniraptoran. Therizinozaurians lived in lush forests of Eurasia North America with latter species moving into more harsh tundra regions. 

Falcarius skeletal anatomy

       

Artist: Apsarvis  Falcarius

The most basal therizinosaurian was, Falcarius (Fal-care-ee-us) that had a temporal of 126 mya during the Early Cretaceous. In Falcarius, the elongated neck and forelimbs were already an attribute. Its dentition alludes to an omnivorous diet, although the leaf-shaped teeth point to the transitioning of a more herbivorous diet. This therizinosaurian, at 3.7-4m/12.1-13.1ft long was much smaller than the more derived forms in which some latter species would top over 9.1m/30ft.

Jianchangosaurus fossil

As a sister to therizinosauroids, Jianchangosaurus (Jee-en-chan-go-sawr-us) occurred 126 mya in the Early Cretaceous and is considered more derived than Falcarius, but more primitive than the basal therizinosauroid, Beipiaosaurus. It was another smaller therizinosaurian measuring only 2m/6.6ft long, but had fully made the transition over to an herbivorous diet. The dental morphology in which the tooth surfaces facing the outside of the mouth is convex, teeth on the lower jaw possesses the reversed morphology, where the surface of the tooth facing the outside is concave. This maximized biting stresses during occlusion in cutting and masticating fibrous plant material.

Beipiaosaurus fossil

Artist: Masato Hattori  Suzhousaurus

Beipiaosaurus (Be-o-pee-uh-sawr-us) is the basal most primitive therizinosauroid and is from the Early Cretaceous 124.6 mya. From direct fossil evidence, Beipiaosaurus is one of the largest dinosaurs in possessing feathered covering and had the longest feathers of any dinosaur yet found. There were two kinds of feathering with a downy covering and the very large 12.5cm/5in quilled feathering that rose up through the down. Reaching a length of 2.2m/7ft, it still had reduced inner toes, but the fourth was enlarging, therefore transitioning to the four toes latter forms were walking on. The only teeth it possessed were in the cheek portion of the jaws. Its head and toothless beak were also larger than the more derived forms with the skull measuring about as large as the thighbone.

Credit: Anness Publishing Nothronychus 

The therizinosaurid family consisted of five genera with two Nothronychus species making for six species total in the family clade. The family temporal range was 94-66 mya during the Late Cretaceous. All therizinosaurid species possessed four weight bearing walking toes and the characteristic opisthopubic pelvis.  Suzhousaurus (Suz-how-sawr-us), living in the Early Cretaceous ~ 113 mya, was sister to and basal to the therizinosaurids. It lived in what is now China and is the closest relative to the two species in the N. American, Nothronychus (Noff-ron-ee-cuss) genus. This primarily verifies that therizinosaurs, along with other dinosaurs, regularly traversed a transitory land bridge connecting Asia to North America. Therizinosaurs were already enlarging body size in, Suzhousaurus, which had reached 6.5m/21ft in total body length. The distinguishing feature of, Suzhousaurus was its anteriorly straight and flattened ilium while the flattened pubis curved forward forming a shaft that had a hollow front edge. 

Artist: dinoraul  Therizinosaurus 

Therizinosauridae fossilized manus

Artist: Vlad Konstantinov  Therizinosaurus

Therizinosaurus, in being the namesake of Therizinosauridae and therizinosaurians overall, was the most derived and largest form of therizinosaurians. It had a temporal range of 70 mya in the Late Cretaceous. With a bulky body and perhaps lumbering gait walking on four digits, in which digit one was reduced to a dew claw, it reached a length of 10m/33ft, making them not only the largest therizinosaurian, but also the largest maniraptoran. Found only in the earlier and contemporaneous ornithischians and contemporaneous and later bird-line, Therizinosaurus had a fully developed opisthopubic (pubic bones pointing backwards) pelvis. At 1m/3.3ft, it also had the longest claws on each of the three digits of their forelimbs than any other dinosaur. In fact, in ignoring claw to body size ratios, the Therizinosaurus claw is the largest of any known animal. If we do look at body size ratio, Therizinosaurus with a 8.2-10% body/claw ratio, then today’s armadillo mops that up with a 22% body/claw ratio. The point is that Therizinosaurus had the biggest claws.

Artist: Paval Riha  Therizinosaurus fetus

Therizinosaurus’ were large herbivores competing for plant food with various sauropods in what is now, Mongolia. Seventeen clutches of Therizinosaurus eggs were found in near proximity of one another in the Gobi Desert and points to the fact that Therizinosaurus was indeed social and gregarious; at least during nesting.

Therizinosaurus fossilized eggs

In the maniraptoran clade, therizinosaurs were the sister group to pennaraptorans. As defined, the clade, Pennaraptora (Pen-nuh-rap-tor-ah) is the most recent common ancestor of Oviraptor philoceratops, Deinonychus antirrhopus and Passer domesticus (the house sparrow), along with all descendants thereof. The temporal range for pennaraptorans is from 167-0 mya during the Jurassic to the present. Anchiornis (An-chee-or-niss) occurring 160.89-160.25 mya during the Late Jurassic, so far is the earliest pennaraptoran discovered.

Simple Pennaraptora Cladogram

Although it represents a very small portion of the dinosaur form, the pennaraptoran order, Oviraptorosauria (O-vye-rup-tore-o-sawr-ee-ah) is comprised of at least 33 genera, 36 species and with 3 extra species listed as suspect oviraptorosaurian but not diagnostic, so are ‘nomen dubium’ due to scant fossil material. There is one primitive family in, Caudipteridae (Kaw-dee-tear-ah-day) one superfamily in, Caenagnathoidea (Suh-nawg-nah-thoi-dee-ah) that contains the two families, Caenagnathidae (Suh-nawg-nath-uh-day) and Oviraptoridae (O-vee-rap-tor-uh-day).

If someone today spotted an oviraptorosaurian they would swear they saw a bird, but they did not. The temporal range for oviraptorosaurians was 130-66 mya all in the Cretaceous Period. They are close relations to birds but did not form a direct line. They could not fly, but were fully feathered, except for the hind limbs, which were scaled like modern birds. Even though the shortened tails did not end in a pygostyle like birds, it was fully feathered with a bony structure at the distal end of the tail that acted much like a bird’s pygostyle in supporting long flight tail feathers. The ulna (long forearm bone) also was quilled with long feathering.  

They had evolved shortened rostrums (snouts), massive beak-like mandibles, extensive pneumatized skulls with the more primitive forms having only four pairs of premaxillary teeth while the latter forms had no dentition. Some supported a bony crest atop the skull. The forelimbs were ~ half the size of the hind limbs while the manus (hands) were long and tridactyl.

Artist: Matt Martniuk  Caudipteryx

Most oviraptorosaurians were no more than 2m/6.56ft long and ranged in size from, Caudipteryx (Caw-dip-teh-rix) at 1m/3.3ft long to Gigantoraptor (Gye-gan-toe-rap-tor) that was by far the largest oviraptorosaurian at 8m/26.3ft long. Even so that most were small, they all had a massive and large shoulder girdle when ratioed to body size.

Artist: M. Haghani Gigantoraptor

Protarchaeopteryx (Pro-tar-key-op-ter-ix), with a temporal range of 126.4 mya in the Early Cretaceous is either the precursor to oviraptorosaurians or is the most primitive of this order clade. In fact, it is closely related to Incisivosaurus and if more fossils are found, they may even be one and the same, or at least in the same genus. Coming from what is now China, its anatomy had hollow bones, a furcula (wishbone), long legs for running and although the manus (hands) supported three fingers ending in sharp curved claws similar to its coelurosaurian carnivorous predecessors, it most likely was an herbivore or at best an omnivore.

Protarchaeopteryx fossil

At 1m/3.3ft long, Protarchaeopteryx was the first dinosaur fossil find with solid proof of dinosaur feathering back in 1996, with the fossil exhibiting fine detail. They were not flight feathers, but were vaned symmetrically on the short tail and extended feathers were on its forearms. The body feathering most likely for insulation and the tail and forearm feathering was for display. All extant flightless birds have symmetrical tail feathers and Protarchaeopteryx’s skeletal structuring would not have supported flight. However it may have endeavored an arboreal lifestyle jumping from limb to limb and parachuting to the ground.      

As a bit of a conundrum, the name, Protarchaeopteryx literally means ‘before Archaeopteryx’. The only problem with that is Archaeopteryx lived ~ 25 million years before Protarchaeopteryx ever showed up in the fossil record. Don’t be too alarmed, for this isn’t a glitch. This is quite possible in the fact that both came from a common ancestor with Archaeopteryx evolving at a much quicker pace than Protarchaeopteryx’s more direct phylogeny. Look at horseshoe crabs, which really aren’t crabs in belonging to the subphylum, Chelicerata and in being so, are more related to spiders. But where horseshoe crabs, who’ve virtually remained unchanged physiologically and anatomically for the past 450 million years, the spider lineage has seen its line go from aquatic livelihoods to terrestrial ones and have diverged into nearly 40,000 species.

Artist: Pablo Lara  Incisisvosaurus

The earliest oviraptorosaurian so far discovered is, Incisivosaurus (In-sise-ee-vo-sawr-us). Occurring 126 mya in the Early Cretaceous, Incisivosaurus had a mouthful of variant teeth consisting of molars, premolars, incisors and canines for breaking into and grinding hard plant food material. The premaxillary bones anchored two large bucktoothed incisors used for gnawing much like today’s rodents. These prominent incisiform buckteeth is what gives this dinosaur its genus name.

Credit: Eudagraphics Insisisvosaurus head/skull

The dentition of Incisivosaurus would not have been adequate enough for a carnivore, so it can safely be said it took up an herbivorous diet of rough foods from nuts, twigs, saplings and tough cellulosic leaves. It used the buckteeth for gnawing at cellulosic material and probably to crack nut-like food to get at the seed contents. The molars were for masticating to initiate digestion. It also swallowed small stones to utilize as gastroliths.

Coming from what is now China, Incisivosaurus was 1m/3.3ft long and is included as the possible primitive ancestor to the more derived oviraptorosaurians. With a short cervical (neck) and slender lower jaw supporting a long fenestra this transitional maniraptoran shared in the traits of more derived oviraptorosaurians, while retaining more primitive ornithomimosaur features with reduced olfactory lobes and expanded optic lobes. This Incisivosaurus transitioning illustrates that many of the avian traits were not present in early members of the oviraptorosaurian group. This slices oviraptorosaurian away from the direct line of birds proving that the bird-like characteristics among later oviraptorosaurs were convergent evolving independently.

Artist: Emily Willoughby  A Caudipteryx family

Occurring 124.6-120 mya, the most basal family of oviraptorosaurians was, Caudipteridae (Caw-dee-tear-ah day) with fossils found in Early Cretaceous China from the Yixian and Jiufotang Formations. The distinguishing characteristic, besides full body downy feathering was a unique styled dagger-like pygostyle which anchored a fan of quilled feathers. There are three species in the family in being, Caudipteryx zoui, (Caw-dip-tuh-riks = zoo-we) C. dongi (dawn-gey) and Similicaudipteryx yixianensis (Suh-mil-uh-caw-dip-tuh-riks = see-icks-e-in-nin-sis). With minor differences between species such as body size, forearm primary feather lengths, breastbone size and minor variations in the forelimb and hip bones, caudipterids were very similar in anatomy, possessed long sharp and bulbous deep rooted dentition and along with gastrolith fossil finds suggest an omnivorous diet. C. zoui was the largest at 1m/3.3ft, C. dongi was .70m/2.3ft long and S. yixianensis was ~ 1m/3.3ft in length.

The 3 Caudipteryx species

With pennaceous and symmetrical tail feathers and forearm primary and secondary feathers, caudipterids could not fly, utilizing instead long hind limbs for moving the light body via swift running. However, no matter if they couldn’t fly, caudipterids were exhibiting a transitional progression towards feathered flight putting them into a more direct line to birds. Similicaudipteryx juvenile fossil specimens have been found and exhibit the characteristic caudipterid feather traits of downy body coverings and symmetrical vaned feathering on the tail and forelimbs. The vane feathers of the juvenile fossils though, showed a ribboning effect in the vane feathers.

Artists: Xing Lida/Song Qijin Similicaudipteryx adult/juvenile

As an expert in bird feather development, Richard Prum studied the fossils and explains that the apparently ribbon-like structure of the juveniles’ feathers were consistent with pennaceous feathers in the midst of moulting. In modern birds, new vaned feathers emerge from the feather follicle enclosed in a ‘pin feather’, which is a solid tube covered in keratin. Usually, the tip of this tube will fall away first, leaving a structure identical to that seen in the Similicaudipteryx fossil. Later, the rest of the sheath falls away when the entire feather has fully developed.

Prum also noted, as did Xu and his team who unearthed the fossils, that the structure of the Similicaudipteryx feathers is fundamentally different from other prehistoric birds with ribbon-like tail feathers. In those other species, the ribbon portion is formed from a flattened and expanded rachis (central quill) of the feather, with the feather barbs expanding out at the tip. In Similicaudipteryx, however, the ‘ribbon’ portion is the same width as the vaned tip. This is consistent with what is seen in bird feathers in the process of moulting. Prum concluded that rather than representing an instance of feathers changing in form as the animal aged, this specimen represents the first known fossil evidence of feather moulting.

Artist: Sydney Mohr Similicaudipteryx

Remiges (bird wing flight feathers) and retrices (Bird tail rudder flight feathers) had already evolved in caudipterids even though the feathering first served as display and insulation rather than for flight.

Caenagnathoidea Cladogram

The superfamily, Caenagnathoidea (Suh-nog-nah-thoi-dee-ah) has one basal caenagnathoid and two family groups that ranged from 125-66 mya during the Cretaceous and lived in what is now N. America and Asia. The main features were in the skull that supported short parrot-like beaks and often times a bony crest atop the cranium. They are defined as the descendants of their two most recent ancestors, Oviraptor (O-vur-rap-tur) and Caenagnathus (Suh-nog-nah-thuse).

This superfamily is characterized by members having the sacral centra and caudal centra possessing pleurocoels, which are a set of hollow depressions on the vertebrae’s lateral portions. Also, the skull displays a fused mandibular symphyses with the mandibular rami as bowed mid-length from a dorsal view and the rostrodorsal (region near back of the beak) margin of the dentary (anterior bone of lower jaw) is concave while the palatal shelf of the maxilla has two longitudinal ridges with a tooth-like ventral process.

Artist: Apsaravis  Avimimus

The most basal caenagnathoid was Avimimus (A-vee-my-mus) that occurred in the Late Cretaceous 70 mya in what is now Mongolia. At 1.5m/4.9ft long it was very bird-like in appearance. It did possess very small premaxillary teeth and was either an herbivore, or insectivore or both as an omnivore. The foramen magnum (the hole in the skull allowing the cervical column to connect with the brain was proportionately large compared to the rest of the skeletal anatomy. The hind limbs ended in bird-like three toes and the forelimbs were short with the manus (hand) bones fused together as it is in birds today. Due to the ilium’s position being oriented almost horizontally, the hips were exceptionally broad and anchored a long tail. This is the primary reason in the thinking that it couldn’t fly. Coming from the ‘Barun Goyot Formation’ shows that Avimimus lived in an arid environment full of sand dunes and intermittent streams.

In the family, Caenagnathidae there were eleven genera with twelve species and two specimens that are listed as ‘nomen dubium’. Caenagnathids had a temporal range during the Late Cretaceous 91-65 mya. Distinct features were in possessing long and shallow jaws (even the term: Caenagnathus, the genus the family is named after, means ‘recent jaws’). Inside the lower jaws bore a complex series of ridges and tooth-like processes, as well as a pair of horizontal, shelf-like structures. The jaws as well were hollow being air-filled, while supporting a fused dentary symphysis bearing distinct vascular grooves. All, but the most basal caenagnathid possessed bony crests above the skull. Caenagnathids were primarily herbivores, but most likely omnivores and depending on which species, dieted on vegetation, invertebrates, eggs, carcasses, small reptiles and young dinosaurs.

The presence of prominent notches (ulnar papillae) on the ulna (bone of the forelimb) indicates the presence of long quilled feathers on the arm. Ulnar papillae, or quill knobs are raised bumps or scars on the ulna bone exhibiting where muscle or tendons were attached to articulate feathering. In an Alberta, Canada fossil find of Apatoraptor (Ah-pat-o-rap-tur) the ulnar papillae are well preserved. It gives evidence that this 74 mya Late Cretaceous caenagnathid and its relatives could wave its short but strong feathered arms vigorously in showing off for display.

Artist: Zhao Chuang  Beibeilong nesting 

Caenagnathids made ringed nests in the earth and laid their eggs circular in fashion inside and on the edge of the nest. Beibeilong (Bay-bay-long) with a temporal range of ~ 93.9 mya in the Late Cretaceous is only known from a nest of eggs and a fully developed embryo. After a careful phylogenetic analysis, it was determined that the embryo was nested inside the family, Caenagnathidae belonging to a genus of its own. The embryo was dubbed, ‘Baby Louie’.

Artist: Vladimir Rimbala ‘Baby Louie’ fossil  

Caenagnathasia (Suh-nog-nath-a-sha), occurring 90 mya in the Late Cretaceous is so far the most primitive and smallest caenagnathid at 0.61m/2ft long and did not possess a crest. The relatively smooth beak bore a rounded and blunt tip, which is different than other caenagnathids, but all caenagnathid beaks varied, showing an evolvement to fulfill particular ecological niches. Although its fossils were found in Asia, Caenagnathasia is more closely related to the North American Campanian-Maastrichtian caenagnathids exhibiting a migration from Asia into N. America.

Artist: Luis Rey  Gigantoraptor fending

At 8m/26ft long and 5m/16ft tall, Gigantoraptor (Gee-gan-to-rap-tor) is not only the largest caenagnathid; it is also the world’s largest known feathered dinosaur. Living in the Late Cretaceous 85 mya, Gigantoraptor most likely had feathering as the other caenagnathids did, but not as much due to size inducing gigantothermy, such as ostriches in having bare skin on the hind limbs and neck. So feathering was decreased for insulation, but some still remained for displaying and the brooding of eggs and young. It hailed from what is now Mongolia.

Gigantoraptor skeleton

The thighbone was relatively short with the slender and elongated lower hind limb fibula and tibia bones supporting a pes (foot) ending in three large and strongly curved toe claws. This suggests that Gigantoraptor was an excellent runner. The forelimbs ended in elongated slender hands. Digit I (thumb) of the manus (hand) was short but strongly divergent. The frontal portion of the caudal (tail) vertebrae possessed long neural spines and was heavily pneumaticised with deep pleurocoels, while the middle section is somewhat stiffened by long prezygapophyses (superior articular vertebra processes). The caudal distal (end) was lightened with spongy bone. The jaws were toothless ending in a horny beak with the lower jaws fused into a broad shovel-like mandibula.

Oviraptoridae Cladogram

Artist: Esther van Hulsen  Oviraptor beak

Having close relations to caenagnathids, Oviraptoridae (O-vee-rap-tor-uh-day) had a temporal range in the Late Cretaceous 84-66 mya. They were small with most being between 1-2m/3.3-6.6ft long coming from Asia and in particular from what is now, Mongolia in the Gobi Desert. As caenagnathid relatives, the jaws were edentulous (no teeth), but instead, oviraptorids had two small bony conical projections protruding from the roof of the mouth. Most likely, all were herbivores as their beaks attest to an excellent tool for shearing vegetation, but may have been also omnivorous. The jaws ended in a short horny beak with the deep set mandibles possessing fenestrae (openings). The nostrils were set high and far back from the snout tip. In the four genera, Oviraptor, Citipati, Banji and Rinchenia and the recently found, Huanansaurus, there was a prominent midline cranial crest. All told there are 18 genera, 18 species and one ‘nomen dubium’ specimen.

Artist: Julius T. Cstonyi  Oviraptor nesting

Occurring 75 mya in the Late Cretaceous, Oviraptor (O-vee-rap-tur) is the oldest oviraptorid and defines the family members as those most closely related to it. At 2m/6.6ft long, due to fossilized nest sites of eggs, developed embryos, hatchlings and adults, we know that Oviraptor brooded nest contents and probably cared for the hatchlings and young. As in all oviraptorids, feathering covered the entire body and in addition, Oviraptor possessed a set of rib processes that kept the ribcage rigid, such as that found in extant birds.

Artist: Dennis Wilson  Oviraptor near full term hatchling

Oviraptor was primarily an herbivore, but supplemented its diet with animal protein as the remains of a lizard were found in the gut area of an Oviraptor fossil. Also, as its fossils have always been near at what were once bodies of water, it may have strolled along shorelines and banks to locate mussels to crack and pry open with its beak.

Artist: Nathane Rogers  Khaan

Khaan’s (Kahn) temporal range was during the Late Cretaceous ~ 75 mya with fossil remains coming from  the ‘Djadochta Formation’ of Mongolia. At 1.1m/3.6ft, it lacked the expansion of the third metacarpal as seen in other oviraptorids. Dieting on vegetation, it also supplemented its diet on small vertebrates. It appears that Khaan was sexually dimorphic. On some fossil specimens the caudal (tail) vertebrae have reduced chevrons and this trait is analogized as a female feature in increasing room for the laying of eggs. Chevrons are a series of bones on the underside (ventral) of reptile tails protecting nerves and blood vessels. Khaan was crestless.

Artist: Zhao Chuang   Huanansaurus

Huanansaurus skull fossil

Another oviraptorid is in the 2015 recently discovered Chinese fossil of, Huanansaurus (Hah-nan-sawr-us). Found during the construction of a railroad station in Jiangxi, Province, it had a temporal range in the Late Cretaceous 72 mya. At 24cm/9.5in long, Huanansaurus had some distinct features in the quadrate bone’s lower condyles being positioned behind the upper head. As crested, on the roof of the skull’s rear, the transverse nuchal crest wasn’t as prominent as in the front. The mandible (lower jawbones) is angular in form making up an outer side opening in the lower rim. In having a concave rim, the dentary (anterior bone of the lower jaw) covers the outer side opening of the lower jaw. The dentary bone is highly pneumatised.

Artist: Ville Sinkkonen  Banji

The last oviraptorid to be discussed here is the most derived in, Banji (Ban-gee) that lived 66 mya at the end of the Late Cretaceous in what is now Jiangxi Province, China from the ‘Nanxiong Formation’. Being a juvenile and not fully grown, the fossil is only 65cm/26in long. However, the adults would not have been much larger. Compared to other oviraptorids, the nasal cavity is unusually large that followed the crest’s curve almost to the eye orbit (socket). It was a crested oviraptorid which was formed by the nasal bones and premaxillae with the sides sporting a series of vertical striations along the sides. The crest ended in a stepwise caudal bearing two longitudinal grooves. Lightly built, it is thought as other oviraptorids that Banji was primarily an herbivore, but would also have chased down small invertebrates.

Paraves Cladogram

Paraves (Purr-aves) ~ With a temporal range from the Middle Jurassic to the present 165-0 mya, the ancestral paravian is the earliest common ancestor to birds and its stem lineage is more related to birds than to the oviraptorosaurs. We usually think of dinosaurs as gargantuan, but instead of enlarging, paravian ancestors started shrinking in size 200 mya in the Early Jurassic. In this evolving miniaturization, paravians also evolved new adaptations four times faster than other dinosaur groups. One case of this paravian paedomorphism is in progenesis where sexual development is accelerated. The exceptions though are in the dromaeosaurids and troodontids where the former went through three independent stages towards gigantism, while the latter, the troodontids went through one.

Dromaeosauridae Cladogram

This major clade is the crown group to modern birds, but as branch based there are no extant species or genetic material from their descendancy into their avian ascendants, even though all members of Paraves are a direct lineage to birds. Like the theropods before them they were bipeds with most walking on three toes. The basal or, earliest paravians, like the dromaeosaurids and troodontids developed a hyperextended ligament that held digit II (second toe) well above the ground, forcing them to bear weight on just digits III and IV. Digit II distally ended in a sickle shaped claw that became a formidable weapon and tool in hooking to gouge, but not necessarily in slicing. As weakly keeled on its underside, the claw was used more for climbing by puncturing and gripping. The climbing could have been performed on an object like a leaning tree trunk or on a large prey while they were hunting in packs. For smaller prey it would have been used for pinning it down as extant raptors do today.

Artist: Emily Willoughby  Balaur bondoc

The highly modified hallux (digit I or, first toe) became fully reversed in paravians. Of the paravian belonging to the Avialae clade that will be discussed later, Balaur bondoc (Bah-la-wur = bon-doc) took the hind limb’s digit I to a whole new level by having digit I run parallel to digit II. Its digits I and II were held as retracted bearing enlarged sickle-shaped claws on each digit.

All paravians were covered in feathers and in most cases the legs and feet were covered. They were the first to evolve true wings and in some forms...flight. In the most basal, the wings usually bore three large flexible fingers ending in claws, while in later more derived forms, the fingers became fused stiffening the forelimbs which is the case for all modern birds. Also, as borrowed from the earliest of coelurosaur ancestors, an increasingly asymmetric wrist joint evolved that modern birds would inherit as well. The importance to this wrist was that it allowed for a strong musculature flapping of the wings and along with the forelimb elongation, eventually led to flight.

Artist: delirio88  Microraptor in flight

Flight developed independently among the paravian groups with one of the earliest in being, Microraptor (My-crow-rap-tur). Analysis from extensive anatomical studies proved that Microraptor was capable of powered launching and flapping flight. But for most species, free flight was not obtained and were either flightless or gliders from heights to ground.

Paraves is divided up into two basal paravian families and one clade. The two families are Anchiornithidae (An-kee-or-nith-ah-day) and Scansoriopterygidae (San-soh-ree-op-tuh-rig-uh-day) while the clade is, Eumaniraptora (U-man-ee-rap-tor-uh). In the family, Anchiornithidae there are currently ten genera and in, Scansoriopterygidae there are at present three genera. Eumaniraptora includes the order, Deinonychosauria (Die-non-ih-ko-sawr-ree-ah) that consists of the two families, Dromaesauridae Dro-mee-o-sawr-ah-day and Troodontidae (Troh-o-don-tuh-day). As a eumaniraptoran sister clade to deinonychosaurians, Avialae (A-vee-l-ee) is in the direct line of dinosaurs to what we call birds.

Anchiornithidae Cladogram

Anchiornithids had a temporal range of 165-122 mya in the Late Jurassic-Early Cretaceous and due to their phylogenetic taxonomy might just very well be the most basal family of the bird line, even though none could fly. Fossil finds have mainly been found in China’s Late Jurassic deposits, although Yixianosaurus (Yi-chow-o- sawr-us) was found in China’s Early Cretaceous ‘Yixian Formation’ and Ostromia’s (Oss-trom-ee-uh) remains were discovered in Germany’s Late Jurassic Bavarian deposits attesting to a rapid Laurasian dispersal of eumaniraptorans.                   

Sharing features of other paravian groups, the more derived eumaniraptorans and basal avialans, anchiornithids were a transitional group in leading to birds. In fact, the term, Anchiornithidae refers to ‘near birds’. Anchiornithids are known for their long forearms, hind limbs and hands. They were lightly built, small and fully feathered. With much variety in feathering between genera, it is certain they possessed pennaceous feathers, although it is uncertain if the feathering possessed barbules on the forelimbs making them unspecialized for flight. Most had pennaceous feathers also running down the legs giving this eumaniraptoran family the dubbed moniker, “four-winged dinosaur.” Most had a feather vaned tail.

As in some avialans, the inside surface of the acromion margin (a bony process on the scapula) is bent hooking outwards. The small anterior dentary (mandible) teeth are more numerous and spaced closer together than the teeth in the middle of the tooth row, much like their eumaniraptoran cousins the troodontids. As in alvarezsaurids, therizinosaurids and avialan species, the inside surface of the fibula’s proximal part is flat.

A genus of Anchiornithidae is, Serikornis (Say-ree-kor-niss) that lived ~ 161-160 mya during the Middle Jurassic in what is now China in the ‘Tiaojishan Formation’. Serikornis had more than one feather type composed of short and symmetrical vaned forelimb feathers with both fuzz and long pennaceous feathers on the hind limbs. The tail is covered proximally by filaments and distally by fine rectrices (larger tail feathers). While the neck was covered in wispy feather bundles, the body possessed a pennaceous-like feather covering.

Serikornis fossil

Although Serikornis was ‘four-winged’, its skeletal anatomy and the complete absence of barbules on the feathers, the microstructure allowing feathers to resist air pressure during flight, attest it could not fly. However, it assuredly had the ability to flap its wings to increase hopping height and from a higher elevation had the capability to glide down.

A main distinguishing feature of Serikornis is in its dentition, which are small when thinking dinosaur teeth, but the anterior maxillary teeth were larger than any other anchiornithid. From luminography scans, the cervical vertebrae showed small pneumatic cavities. This 60cm/23.6in long basal anchiornithids was most likely an insectivore and carnivore in dieting on large insects and very small vertebrates.

Another anchiornithid was Eosinopteryx (E-oh-sigh-nop-tur-ix) which lived during the Middle Jurassic 160 mya. The virtually complete articulated fossil remains were also, as Serikornis’ remains, found in China’s ‘Tiaojishan Formation’. The holotype fossil is of a nearly fully grown subadult measuring 20cm/12in long. The adult would not be much larger, if at all, than that.

Artist: Emily Willoughby  Eosinopteryx

Fully feathered, if Eosinopteryx were alive today and if spotted by most anyone, they would say it was a bird, but they would be wrong, for it was very much a non-avian dinosaur. It however lacked the fully developed tail feathering that would have allowed for steering, so it would’ve been rudderless during flight. The wing’s primary feathering was twice as long as the humerus (upper arm bone) in which the arm structure was not designed for flapping. The hind limbs’ lower tarsals were featherless. The feet and toes were very slender and distally lacked any curved claws for defense, predation or climbing. However, the straight pedal claws would have allowed for swift cursorial terrestrial running.

The skeletal anatomy, which lacks a keel, is much like troodontids and so much so, it was originally classed as one. But numerous features such as a very short snout, a short tail and an unusual arrangement of wing bones placed it as an anchiornithid. With its dentition of unserrated, but sharp teeth, Eosinopteryx could have easily chased down and seized struggling insects and small vertebrates while crunching them in the mouth.

Artist Zhao Chuang  Caihong

There has been a new anchiornithid just described this year (2018) that was first discovered by a Chinese peasant in a quarry near the small village, Nanshimenzi and handed over to scientists in 2014. The new anchiornithids is, Caihong (Kay-hong) which literally translates in Chinese to, ‘rainbow’ and for good reason. A microscopic evaluation of the body, tail, neck and head feathering impressions revealed melanosome nanostructures that are similar to organelles that produce iridescence in extant birds. In addition, the feathers found on the head, chest, and the base of the tail preserve flattened sheets of platelet-like melanosomes very similar in shape to those which create brightly colored iridescent hues in the feathers of modern hummingbirds. These feather structures however, appear as solid lacking the air bubbles as seen in hummingbirds today, yet the feathering of Caihong represents the oldest known evidence of platelet-like melanosomes.

Caihung fossil feathering

Caihong at 40cm/16in long had a temporal range in the Late Jurassic 161 mya. Like other anchiornithids, it had a prominent lacrimal (small bone forming part of the eye socket) crest and possessed longer pennaceous feathering on the forelimbs and hind limbs and displayed large asymmetrical tail feathers. The recurved premaxilla teeth are more slender and tightly packed than the other teeth. The large hind limbs were 3.1 times longer than the torso. Ending in a hook-shaped foot, the pubic bone slightly directs toward the rear.

Caihong skull fossil

One more anchiornithid we’ll mention here, is the most basal paravian yet in, Yixianosaurus (Yi-chow-o-sawr-us). With an Early Cretaceous temporal range of 122 mya, the fossil remains come from the ‘Yixian Formation’ of northeastern China. With a feature known only among paravians, the mediolateral width of the humerus is much greater than the width of the scapular blade in Yixianosaurus. The lateral process of the ulna is sub-equal to the coronoid process in size, which is also a typical paravian trait. The manus (hands) and metacarpals (finger bones) are 140% longer than the length of the humerus (upper arm long bone). Digit II (second finger is the longest) but all three fingers supported bored large recurved claws. The long hands and short arms probably evolved independently within this genus. The large hands and curved claws most likely aided in capturing prey of small vertebrates and used in defense. At ~ 1m/3.3ft long, Yixianosaurus sported large pennaceous feathering on the forelimbs and arm anatomy, these features could have supported limited aerial locomotion.

Artist: Masato Hattori  Yixianosaurus

Scansoriopterygidae is a family consisting of three paravian genera that arose 165-156 mya in the Middle to Late Jurassic. All genera come from the ‘Tiaojishan Formation’ fossil beds of China. The family contains members that were the first non-avian dinosaurs that had adaptations to an arboreal (tree dwelling) lifestyle, including climbing and gliding. No more the size of a pigeon reaching maximum lengths of 16cm/6.3in long, scansoriopterygids were the smallest of all non-avian dinosaurs.

Artist:Román García Mora Scansoriopterygid fishing grubs

Scansoriopterygids had an extremely long third finger which was in contrast to other dinosaur groups that had a longer extended second finger, as in Yixianosaurus described just above. The elongated digit most likely aided in climbing as the term, scansoriopterygid refers to ‘climbing wings’ and fishing out grubs and adult insects from trees and bark, much like the extant aye-aye does today.

The scansoriopterygid skull is blunted making the snout short-faced while the anterior end of the lower jaw is slightly downturned and the teeth are procumbent. This skull shape superficially is similar to some insectivorous bats and is quite similar to that of early oviraptorosaurs, like Caudipteryx.

Scansoriopterygids’ paleoecology during the Middle-Late Jurassic consisted of a warm and humid climate supporting gymnosperm, ginkgopsid and coniferous trees, while other semitropical plants like lycopsids, horsetails and cycads abounded giving scansoriopterygids places to scamper, climb and hide and dine in as the ecology supported large insects, other invertebrates and small vertebrae populations.

Artist: Matt Martyniuk  Scansoriopteryx

With a temporal range in the Early to Late Jurassic 165-156 mya, Scansoriopteryx was ~ 35cm/13.8in long as an adult. The fossil remains of a hatchling at 13.5cm/5.3in long and a juvenile at 16.5cm/6.5in long holotype fossil were found in China from the fossil beds of the ‘Tiaojishan Formation’.

Scansoriopteryx (formerly known as, Epidendrosaurus) had a more opened non-perforated hip socket than most other dinosaurs where the pubis pointed backwards and downwards. Small pebbly scales were preserved in the fossil finds on the metatarsus (upper foot) where the foot ended in a large hallux (first toe). Also notable in the fossils is the wide rounded jaws with twelve teeth in the lower jaws that were larger in the front of the mouth than in the back. The long tail was 6.5 times the length of the femur.

Scansoriopteryx skeletal (Note long fingers)

The most notable features in Scansoriopteryx were the long fingers, in particular the third one. These forelimb distal digits give evidence of an arboreal lifestyle and they were fairly developed even at an early age as viewed in the hatchling fossil. This scansoriopterygid’s body possessed both downy and larger feathers.

Just to let ya know, with a slew of initial confusion in its naming, original provenance, taxon placement and whether it should be called a dinosaur versus a bird, the storm appears to have abated as it is now comfortably nestled in the family, Scansoriopterygidae which was named after this tiny dinosaur we call, Scansoriopteryx.

Artist: Pablo Lara  Epidexipteryx

Epidexipteryx (Ep-ee-dex-teh-ricks) was a small scansoriopterygid that lived in the Late Jurassic 160 mya in the ‘Dauhugou Beds’ of what is now Inner Mongolia. It was around the size of a pigeon at 44.5cm/17.5in long including the tail. The retrices’ (tail feathers) four long vanes weren’t branched into individual filaments as extant birds are, but were made up of a single ribbon-like sheath. The body was covered in more primitive dinosaur feathering, but also some feathers arose from a membranous base like modern birds do and may be the beginnings in the evolution of the bird feather and its anchoring.

Resembling oviraptorosaurs skulls, the skull of Epidexipteryx had some unique features. It only had teeth in front of the jaw that angled outwards instead of downwards and the lower jaw curved down and away from the upper jaw. This arrangement made the lower teeth point outwards instead up into the upper jaw. Also, Epidexipteryx lacked remiges (wing feathers) but may have had a membrane supported by the forelimbs and fingers.

Epidexipteryx skeletal anatomy

Although the skeletal anatomy of Epidexipteryx proves a close relation to Scansoriopteryx, there is contention that it’s ancestral to oviraptorosaurs.

The third member of scansoriopterygids I’m not going to use the genus name only in describing it, but am going to also use its specific epithet in giving the binomial species name. The reason for doing so is while we are used to long winded species names, this scansoriopterygid has only four letters and is tied with a bat species for the shortest species nomenclature. The genus is merely, Yi, while the specific name is simply, qi. So, there ya have it, we’ll be describing this little guy by it species name of, Yi qi (Yee-chee).

In speaking of the bat species name of, Ia io, Yi qi gave the impressionable appearance of a bat straight out from the bowels Hades. This scansoriopterygid with a temporal range during the Late Jurassic 160 mya had a bony rod called a ‘styliform element’ jutting backward from each wrist that, along with the elongated fingers supported a patagium (skin membrane) reminiscent of a bat wing.

Artist: Emily Willoughby  Yi qi 

Unique to Yi qi from all other dinosaurs was in possessing a plane of skin membrane between each finger as evidenced from wrinkled skin in between each finger of the fossil. Extant birds of flight do have a short membrane called a propatagium, but Ti qi’s membrane expanded more like a bat’s patagium. Whether it flew or glided, the mechanics of the forelimb are still under study, but most likely it used its wings for gliding as extant gliders, like flying squirrels and the greater Petauroides Volans both have styliform elements for membrane support.

In addition, the head was short with a blunt snout and a downturned lower jaw that held teeth angled forward. With four on each side, the larger upper front teeth pointed slightly forward. There were no other teeth except for those in the front tip of the mouth much like its scansoriopterygid relative, Epidexipteryx.  

Approximately 32cm/12.6in long, Yi qi’s body, from the base of the snout to the caudal (tail) was covered in paintbrush-like quilled feathers. Only the tip of the snout, the patagium, ankles and feet had no feathering, although the bottom leg covering was long enough to layer over the top half of the foot’s metatarsus. Behind the shins and long slender forelimbs sported Yi qi’s longest feathers although the patagium lacked any feathering cover.          

Below is a short video clip put out by Nature about Yi qi:

Eumaniraptora:

As the sister clade to Oviraptorsauria, Eumaniraptora is composed of members having a temporal range from the Middle Jurassic to the present 165-0 mya. The two eumaniraptoran subgroups are Deinonychosauria (Die-non-ih-ko-sawr-e-uh) and Avialae (A-vee-ale-e). There was a global distribution of eumaniraptorans with fossil finds showing up on all continents within diverse environments. Under the avialan line eumaniraptorans are the only dinosaur group to evolve flight eventually leading to dinosaurs called birds. Depending on species, the diets are highly varied in the forms of carnivores, insectivores, piscivores, omnivores, frugivores and herbivores.  Eumaniraptorans are the last remaining theropod group and all other dinosaur groups to survive the K-T Extinction.

Eumaniraptora Cladogram

All eumaniraptorans strictly have an obligate bipedal gait with a distally placed metatarsal forcing pedal (foot) digit I to touch ground or wrap around objects like branches. The claws were sickle-shaped, while pedal digit II was retractable on the more primitive deinonychosaurians. These theropods were small overall in body size with long slender limbs and proportionately long hands. They all were bird hipped with a backward facing pubis and also had long leg, arm and tail feathers. The tail itself was mobile at the base and stiffened the rest of the length. The limbs had long feathering and the more derived had conquered limited free flight.

Deinonychosauria  Cladogram

Deinonychosauria: The more basal group of eumaniraptorans is from the order, Deinonychosauria that had a temporal range from the Early to Late Cretaceous 143-66 mya. The name is derived from Greek inferring ‘terrible claw’ in reference to the retractable sickle-shaped claw found distally on pedal digit II. Most fossil finds have been discovered in Asia and N. America, although there have been fragmentary remains discovered in northern Africa. These are all, based on Gondwana diversity, but with two finds of dromaeosaurid fossils in what is now Argentina’s Patagonia region during the Late Cretaceous of, Neuquenraptor (90 mya) and Unenlagia (~89 mya), bears out that worldwide distribution of deinonychosaurians was well underway at least by the beginning of the Cretaceous and before the breakup and drifting apart of the northern (Gondwana) and southern (Laurasia) hemispheres. During the Jurassic, there was greater connection between the two hemispheres allowing for dromaeosaurid migrations further evidenced by scant European fossil finds including the dromaeosaurid trademark pedal digit II sickle-shaped retractable claw. Again, Deinonychosauria is divided into Dromaeosauridae (Dro-me-o-sawr-us) and Troodontidae (Tru-o-don-tee-day.)

Dromaeosauridae Cladogram

Dromaeosauridae: Dromaeosaurids are the most basal of the deinonychosaurians that have an Early to Late Cretaceous temporal range of 130-66 mya. Most were small, but a few were over 5m/16.4ft. The smallest is the microraptorian, Hesperonychus (Hes-purr-uh-nie-kus) at 50cm/19.7in, while Achillobator (Ah-kill-o-bay-tur) was the longest at 6m/19.7ft long. The miniaturization in size is a feathered dinosaur’s hallmark trait towards the path to the origins of bird flight.

Dromaeosaurids are quite extensive with the Dromaeosauridae clade’s taxa containing three subfamilies and one subgroup node order with three subfamilies of its own. Thus far, altogether, there are at least 43 genera, which don’t include suspect dromaeosaurid scant or extensively disarticulated fossils.           

Dromaeosaurid caudal (tail) vertebrae were tightly locked, much more so than in other theropod groups. In fact with the microraptorians and eudromaeosaurs, chevron and neural arch extensions grew extremely long turning the tail into a unique and effective stabilizer. As the more basal dromaeosaurids had elongate metatarsi displaying a primitive form of the arctometatarsalian condition in which the proximal part of the middle metatarsal is pinched between the surrounding metatarsals, the more derived had developed short and stout metatarsi. The arctometatarsus aided in speed but not so much in agility. With the dromaeosaurid metatarsal and stiffened long tail transition, it looks as if they opted out speed for agility. Still able to run, just not as fast as their predecessors, the latter dromaeosaurids could now commit quick agile turns in pursuing prey or evading predators.

Source: Encyclopedia Britannica Dromaeosaurid sickle claw

The sickle-shaped and retractable large claw was held off the ground and used as a killing implement. When muscles of the toe (digit II) were contracted, the claw swept downwards slashing and impaling whatever animal was the dromaeosaurid’s intended victim. The claw mechanics were backed by a claw base to tip angled momentum maximizing the transmission of forces from the leg to the tip of the claw. With the second toe held up, dromaeosaurids walked only with two weight bearing toes. All dromaeosaurids were carnivorous and aided by excellent eyesight, as witnessed by the size of their skull’s orbits and scleral rings, were cathemeral in being active both night and day.

There are nine dromaeosaurid genera taxa that are holotype dromaeosaurids and do not belong to any other subgroupings. Primarily from Europe, out of these nine, there are at least five fossil finds that are definitively dromaeosaurids, but further classification is difficult due to current scant remains. They are: Nuthetes (New-thet-eez) from England 143 mya, Ornithodesmus (Or-nith-o-dez-mus) from the Isle of Wight 125 mya, Variraptor (Var-e-rap-tor) from France 70mya, Pyroraptor (Pi-row-rap-tor) from southern France 70.6 mya and Adasaurus (Aye-dah-sawr-us) from Mongolia 70 mya. The remaining four genera are basal dromaeosaurids and are: Shanag (Shan-ag) from Mongolia 130 mya, Zhenyuanglong (Zen-yu-an-long) from China 125 mya, Pamparaptor (Pam-pa-rap-tor) from Patagonia, Argentina 89 mya and Luanchuanraptor (Loo-an-choo-an-rap-tor) from China ~77.5 mya.

Jonathan Kuo Pyroraptor

On a couple of light notes, Pyroraptor wasn’t named so because it breathed fire like a dragon, no, it’s not that colorful; it was named due to the fact that when a fire swept through a French forest, it uncovered the fossil remains. Also, the term ‘raptor’ for theropods refers to dromaeosaurids due to their shared characteristics in the Digit II (second toe) sickle claw, grasping three-fingered manus (hands) and relatively large brain sizes. However, not too many scientists like referencing the term ‘raptor’, preferring instead the term dromaeosaurids as today’s birds of prey are considered raptors.    

While speaking of dragons, Zhenyuanglong literally means, ‘Zhenyuan’s dragon’ in reference to the man, Zhenyuang Sun who secured the fossil for study from a local farmer who found it in the ‘Yixian Formation’ of Liaoning, China. At 1.7m/5.6ft, it was a medium sized dromaeosaurid. For a dromaeosaurid it had short forelimbs that supported long well developed pennaceous feathers while the body was coated with simple ‘hairy’ feathered filaments. In fact, the short limbs were the shortest dromaeosaurid forelimbs once an arm to leg ratio was conducted. Along the length of the tail, long vaned feathering was evident while the fossil gives no evidence of long leg feathering as the forelimbs possessed; something unusual as all other dromaeosaurids with long arm feathers also had long leg feathering.

Artist: Chuang Zhao  Zhenyuangalong

With all this feathering, Zhenyuangalong still couldn’t fly as there were no strong muscle attachment scars that could’ve afforded flapping wings. Also, its estimated weight of 20kg/44lb, for its size, it would’ve even been too heavy to even glide. But, its weight and length makes it the largest non-avian dinosaur with full blown bird-like feathered wings. The feathered forearms were most likely used for display in threatening rivals and impressing potential mates.

Zhenyuangalong Fossil

Halszkaraptorinae (Houz-kuh-rap-tor-uh-nay) ~ is the most basal subfamily of dromaeosaurids and through phylogenetic analysis contains three Late Cretaceous genera, which are: Mahakala (Ma-ha-kah-la) from ~ 80 mya, Halszkaraptor (Houz-kuh-rap-tor) from 75-71 mya and Hulsanpes (Hul-san-pees) from 70 mya. All halszkaraptorines came from Mongolia with a paleoecology consisting of a semiarid climate, extensive sand dunes, alluvial settings and steppes interspersed with intermittent streams. Where the more permanent bodies of water were is also where halszkaraptorine fossils have been found. They were attracted to the waters in a harsh dry climate and in doing so, Halszkaraptor evolved into a semiaquatic dinosaur.

Artist: Frank Ipollito  Mahakala

Mahakala was the most primitive of the primitive halszkaraptorines. At 70cm/28in long, Mahakala was not in the direct line of avian dinosaurs, but shows that basal feathered dinosaurs were downsizing to quickly find food and shelter. With its wing-like forelimbs and biped hind limbs, it gives a consensus portrayal of the dinosaur’s ancestral lineage in leading up to their final descendants…birds.

Although the ilium, ulna femur and tail vertebrae were primitive in Mahakala and unlike most other dromaeosaurids its middle metatarsal was not pinched, as well as possessing short forelimbs, it still possessed the dromaeosaurid expanded claw on digit II. It also had a combination of shared characteristics among other dromaeosaurids, troodontid members and birds.

Credit: St. Thomas Pro.  Halzkaraptor   

With a long neck, a goose-like beak and postural adaptations similar to short-tailed aquatic fowl, Halszkaraptor looked a lot like a swan. In addition, Halszkaraptor possessed eleven sharp needle-like teeth in a flattened premaxillae, did not possess the typical dinosaurian epiophyses (bony projections of the cervical vertebrae) in which the cervical (neck) vertebrae had reduced neural spines, external nostrils were situated behind the premaxillae but in front of the maxillae and digit III of the forelimb was longer than digit II.

Halzkaraptor Fossil

At 60cm/23.6in long, Halszkaraptor, as mentioned earlier due to the arid conditions it and the other halszkaraptorines lived in, it not only lived near bodies of water, it lived in them evolving into a semiaquatic lifestyle. Through synchrotron radiation scans, the intricate inner ear was visible and evidence of snout nerve channels, like in crocodiles, were found that gave heightened sensitivity to touch. Along with the elongated neck, this afforded Halszkaraptor to snag small fish prey as it swam with the head underwater seeking out fish with its sensitive snout. With the forelimbs used as paddles and slightly webbed hands while the long tail shifted its center of gravity forward, it swam through water with ease.  

Halszkaraptor, as a biped could easily run, maneuver with ease using the long tail for stability and dwell on land as well for nesting and hunting small vertebrates and large insects in utilizing its sharp claws.

Unenlagiinae (Un-en-lay-gye-nay) ~ is a subgroup clade of ancestral Gondwanan dromaeosaurid theropods with remains being found in South America and Antarctica. Unenlagiines have a distinct anatomy from the Laurasian dromaeosaurids which was consequential from Pangaea’s breakup into Gondwana and Laurasia resulting in isolating unenlagiines from the rest of the dromaeosaurid family members. This initiated separate evolutionary paths. The temporal range is 94 -70 mya during the Late Cretaceous when the tip of S. America and Antarctica were connected.

Unenlagiines possessed the typical dromaeosaurid extended and sickle-shaped claws, but distinguished by even lengthier chevrons and superior processes in caudal (tail) stiffening and a posteriorly oriented pubis. Although they were showing dental (teeth) reduction as other dromaeosaurid groups were, they still retained a higher dental count. The small size of the teeth was pronounced when compared to skull height, while in addition, unenlagiines possessed longitudinal grooves on the tooth crowns.

There are a total of six unenlagiine genera, although it has been pretty much in agreement that Rahonavis (Ray-ho-nay-viss), due to its morphology is going to be reclassified under Avialae as being closer to birds. Rahonavis displays both dromaeosaurid and avian dinosaurian features, in which these transitional features make for much difficulty in where it should be classified. The temporal range for this subgroup is from the Late Cretaceous 94-70 mya.

The other five unenlagiines are: Buitreraptor (Bwee-tree-rap-tor) from 94 mya, Neuguenraptor (Nu-kin-rap-tor) from 90 mya, Unenlagia (U-nen-lag-ee-ah) from ~89 mya, Unquillosaurus (Un-key-o-sawr-us) from 75 mya and Austroraptor (Aw-stro-rap-tor) from 70 mya. Unenlagiines are known for their low hanging and long heads and smaller conical teeth when compared to the northern hemisphere dromaeosaurids’.

Artist: Joshua N. Edder  Buitreraptor

Buitreraptor, at only1.5m/4.9ft in total length (excluding Rahonavis at 70cm/27.6in) was one of the smaller dromaeosaurids and smallest of the unenlagiines. With its fossil remains coming from what is now, Patagonia, Argentina, Buitreraptor is the most basal unenlagiine.   

It had a rather long slender snout most likely to poke into crevices and holes to either snag or scare out small reptiles and mammals from their hiding places. It most likely also waded along shorelines to snatch small fish and aquatic vertebrates. The snout was filled with tiny unserrated teeth that were grooved, flattened and strongly recurved in structure, perfect for seizing struggling and slippery small prey.

Buitreraptor skeletal anatomy

Buitreraptor’s remains show a mosaic of anatomical features when compared to other dromaeosaurids, troodontids and even the more advanced avialans. But, it was more primitively distinctive with shorter fingers that were essentially the same length. It did possess the sickle claw on the second toe and was most likely, due to its smaller size was used to pin down prey rather than utilizing it to kill prey as the other larger dromaeosaurids did.

Artist: Fred Wierum  Austroraptor

At 5-6m/16.4-19.7ft, Austroraptor (Aw-stro-rap-tor) was one of the, if not largest dromaeosaurid, but for sure is the largest dromaeosaurid found in the southern hemisphere; Austroraptor may even have reached a length of 6.5m/21ft. Its fossil remains were found in rocks of the ‘Allen Formation’ from the far southern region of Argentina in the Río Negro Province. It must have subsisted on larger prey as the juvenile sauropods, Rocasaurus and Saltasaurus that shared the same paleoenvironment, would have been accessible for this predator.  

Like all unenlagiines, it had small non-serrated conical teeth. The descending process of the lacrimal (bone situated on front part of eye orbit’s medial wall) curves anteriorly. Several skull bones resemble features of Deinonychus, a troodontid, but the forelimbs were distinctive to other dromaeosaurids in being shorter proportionately to the rest of the body with the humerus being half the length of the leg’s femur. The small humerus might be due to Austroraptor’s size increase, as most theropods did lose forearm length while increasing body size.

Microraptorinae Clade

Microraptorinae (My-crow-rap-tor-uh-nay) ~ is a subfamily clade of basal dromaeosaurids with an Early to Late Cretaceous temporal range of 125-76.5 mya. Microraptoria, as an order is frequently found in literature as synonymous to Microraptorinae. Microraptorines, or as ya like, microraptorians first appeared in what is now China and the specific region of the ‘Yixian and Jifuotang Formations’ of Liaoning County, China is where they all came from. The only exception to this is, Hesperonychus, in which its fossil remains were found in the ‘Dinosaur Park Formation’ of Alberta, Canada.

Microraptorines were some of the smallest dinosaurs with the three species in the genus Microraptor reaching lengths of only .77-.9m/2.53-2.95ft. The limbs were slim and long with all four limbs and the tail supporting advanced feathering on some species positing them in the ‘four-winged’ dinosaurs. The largest ‘four-winged’ dinosaur since found is the microraptorine, Changyuraptor that reached 1.2m/3.9ft in length.

Artist: Qilong Genus Microraptor

Microraptorine distinctive dromaeosaurid characteristics are: a maxilla laterally sculpted by small pits, very short manual phalanx III-2, shortened first digit of the hand, splatulate (rounded) pubic symphysis, metatarsal III with a pinched proximal end and a slender metatarsal II.

It is not known if microraptorines were capable of free flight, but due to anatomical features and morphology as evidenced from extremely well preserved fossils, there certainly is good argument they could glide. One thing that we do know though is that Microraptor, from 120 mya during the Early Cretaceous, not only had feathering, it also had iridescent plumage.

Artist: Jason Brougham  Microraptor gui

Comparing the feather melanosome (melanin containing organelles) remains of Microraptor to those of modern birds with iridescent feathers shows that M. gui’s feathers were black and iridescent in color and structure. From camouflage, thermoregulation, signaling (mating, competition) and to flight, feather colors have played main roles that birds picked up from their dinosaur ancestries.

M. gui Fossil with iridescent feathering 

Below is a short video of Microraptor’s iridescent plumage:

Tianyuraptor was the most primitive microraptorine and did not possess these typical microraptorine traits: a large oval fenestra in the coracoid, significantly shortened penultimate manual phalanges, the posterior end of the ilium extending ventral to the ischial peduncle, lateral projections halfway down the pubis and a strongly anteriorly curved pubic shaft.

Tianyuraptor Fossil

Within the microraptorine subgroup are seven genera with three species in Microraptor while the rest only have one species in each. The genera are: Graciliraptor (Gray-sil-e-rap-tor) of 126 mya, Changyuraptor (Chang-you-rap-tor) of 125 mya, Zhongjianosaurus between 125-120 mya, Sinornithosaurus (Sign-or-nith-o-sawr-us) between 124.6-122 mya, Tianyuraptor (Tee-an-you-rap-tor) of 122 mya, Microraptor (My-crow-rap-tor) of 120 mya and Hesperonychus (Hess-puh-ruh-nye-cuss of 76.5 mya.

Artist: Midiaou Diallo  Zhongjianosaurus

Zhongjianosaurus, at 70cm/28in is the smallest of the dromaeosaurids and thus far the smallest of the non-avian dinosaurs. In sharing dromaeosaurid traits, it is distinguishable with proportionally long ossified uncinated processes fused to the dorsal ribs. It also had a widely arched furcula with slender and posteriorly curved clavicular rami. The humeral proximal end was strongly offset medially from the humeral shaft and the internal tuberosity of the humeral proximal end was short. A large fenestra perforates the humeral deltopectoral crest, while the humeral ulnar condyle was hypertrophied.

Zhongjianosaurus’ miniaturized size suggests that there was during its time, a sufficient amount of niche partitioning. Possibly being omnivorous and arboreal, niche partitioning may have aided in preferential habitat differences.

As a dromaeosaurid, Sinornithosaurus had the typical sickle-shaped toe claw and was a close relative to the microraptors. It possessed two types of feathers with one being formed as tufts from several filaments joining together, while the second type as exhibited on the forelimbs, legs and tail were composed of rows of filament joined together along the main shaft (rachis) much like modern birds. Although it couldn’t perform free flight, it could glide short distances after jumping from trees. Microscopic cell structure studies of the feather fossil remains show a variance in body color of reddish-brown, yellow, black and grays.

Artist: AMHR R. Mickens  Sinornithosaurus  

However, what makes Sinornithosaurus so distinct is that it may have been able to deliver a venomous bite. Just as rear-fanged snakes and gila monsters, Sinornithosaurus had upper rear-fangs that were grooved and just like these other reptiles, it appears that Sinornithosaurus had a venom gland just above the back of the jaw connected to the grooved teeth via a canal. Rear-fanged snakes and gila monsters must bite then chew to allow the venom to flow into the wound. It is thought by the initial paleontologists who studied and evaluated the Sinornithosaurus fossils that it also did the same thing.

Artist: christopher252  Sinornithosaurus venom delivery 

Sinornithosaurus

There is a group of paleontologist that are now disputing this in saying there were other dinosaur teeth with grooves, so it doesn’t prove that Sinornithosaurus had the capability to deliver venom. But with the assemblage of the grooved teeth, the cavity where the venom gland could’ve been placed and the tube-like canal connecting the two, it certainly seems reasonable that Sinornithosaurus through convergent evolution could have developed a venomous delivery system. Below is a BBC video of Sinornithosaurus and in it, the video explains the venom assembly. So you be the judge in whether there was a dinosaur that had a venomous bite.           

       

Hesperonychus is the only N. American microraptorine thus far discovered and as mentioned earlier, was found in Alberta, Canada. Its name means, ‘western claw’. It was small measuring ~ 60cm/23.6in in total length. Even though it was a miniature dinosaur, it was well equipped with dagger-like teeth and razor-sharp claws with the distinctive dromaeosaurid sickle claw on the second toes of each foot. Most likely it was chiefly an insectivore supplementing the diet on terrestrial invertebrates, small mammals and quite possibly on occasion a carcass whenever it came across one.

Credit: Alchetron.com  Hesperonychus

The pubic bones of Hesperonychus were fused with the rest of the skeletal anatomy made up of gracile bones. The pubic symphysis as a cartilaginous joint was spatulate or rounded. In being small, it played an important role in the ecosystem it lived in by infusing the predator/prey relationship within smaller animals.

Eudromaeosauria Cladogram

Eudromaeosauria: The next main subgroup of dromaeosaurids is in the class node of, Eudromaeosauria (U-dro-me-o-sawr-e-uh) with the meaning of ‘true dromaeosaur’. With a temporal range of 143-66 mya ago, eudromaeosaurines are the most derived of dromaeosaurids, were large bodied as compared to other dromaeosaurid groups and were feathered hypercarnivores with a diet consisting almost entirely of vertebrate flesh. Eudromaeosaurines filled ecological niches as small carnivores and piscivores.

All eudromaeosaurines possessed an expanded and large heel jutting from the second phalange (toe) that also bore the sickle-shaped claw. However unlike their dromaeosaurid cousins with a broader based claw, eudromaeosaurines’ sickle claw was thinner, sharper and more blade-like.

Eudromaeosaurines are further divided into three subfamilies in being: Saurornitholestinae (Sawr-or-nih-tho-lest-uh-nay), Dromaeosaurinae (Dro-me-o-sawr-uh-nay) and Velociraptorinae (Vuh-loh-suh-rap-tor-uh-nay).

Saurornitholestinae ~ Saurornitholestine fossils have a temporal range of 77-70 mya during the Late Cretaceous and include three medium-sized monotypic genera with all from western N. America. The three genera of Saurornitholestinae are: Saurornitholestes (Sawr-or-nih-tho-les-teez), the one the subfamily is named after and is from 75 mya in the Late Cretaceous; Bambiraptor (Bam-bee-rap-tor) from 72 mya in the Late Cretaceous and Atrociraptor (Ah-tross-rap-tor) from 68 mya.

Artist: Emily Willoughby  Saurornitholestes

Saurornitholestes fossils come from what is now Alberta, Canada and in the U.S. states of Alabama, Montana, New Mexico, North Carolina and South Carolina. There are two species in, S. langstoni (lang-sto-nee) and S. sullivani (sull-la-vah-nee) with both reaching lengths of 1.8m/5.9ft. Besides the typical dromaeosaurid features like the sickle-shaped second toed claw, both species were distinct. With fang-like teeth in front of the mouth as other dromaeosaurids had, additionally the skull had an unusually large olfactory bulb, suggesting it had the keenest sense of hearing over all other dromaeosaurids along with an excellent hearing sense.

Artist: Jonathan Kuo S. sullivani

Found on both sides of the ‘Western Interior Seaway’ Saurornitholestes fossil discoveries have been numerous with dozens more of indeterminate scant finds that are suggestive of being Saurornitholestes remains.

Artist: Peter Schouten  S. langstoni

As a small biped carnivore and scavenger, Saurornitholestes competed with other small theropods, like Troodon and Dromaeosaurus, but with their agility ran from the contemporaneous tyrannosaurids. In one S. langstoni fossil, there were teeth serration marks on the dentary (lower jaw bone) that match the teeth of a juvenile tyrannosaurid, such as Daspletosaurus or Gorgosaurus. On the other hand, a fossil of a juvenile pterosaur, Quetzalcoatlus had a Saurornitholestes front tooth embedded in a wing bone. Even a juvenile Quetzalcoatlus would’ve been way too large for Saurornitholestes to hunt down and kill as prey, for predators throughout the eons normally seek out prey half their size to lessen injury. What is surmised is that both were feasting as scavengers on some carrion when a tiff between the two occurred resulting in a good bite by the saurornitholestine inflicted onto the pterosaur. In other words, Saurornitholestes was stating, “Bug-off, I got dibs.”

Bambiraptor fully grown would’ve been ~1.3m/4.3ft. Although there have been adult specimens, the first Bambiraptor fossil found was of a younger juvenile, hence the name, ‘Bambi’. This juvenile find was also discovered by a juvenile, a 16-yr-old boy combing his ranchland for fossils near ‘Glacier National Park’ in Montana, USA. The find was significant with its anatomical features representing bird-like dinosaurs evolving into birds. As birds, it possessed the characteristic V-shaped ‘wishbone’ or furcula, which allows birds to flap wings. It also possessed very long forelimbs and manus phalanges (fingers) that approached the length required for flight, the forearms folded as well.

Artist: Taena Doman  Bambiraptor

What is unique to Bambiraptor is that it quite possibly had arm and manus (hand) maneuverability features for the manus to reach its mouth, therefore enabling it to hold its food while it ate. There were no feathers or feathering impressions of the fossil finds, but since Bambiraptor’s anatomy shows classic troodontid anatomy, such as the sickle-shaped second toed claw, raptorial manus (hands) and a cladistic confirmed analysis of its phylogenetic bracketing within feathered Paraves.

Artist: Michael B.H.  Atrociraptor

Atrociraptor is a saurornitholestine also discovered in N. America in Alberta, Canada’s ‘Horseshoe Canyon Formation’. At 2m/6.6ft long it was the largest of the three saurornitholestines and was a bit hefty at 15kg/33lbs. Atrociraptor’s massive skull was short, but tall with a slender lower jaw filled with large closely packed recurved and serrated teeth. The outer teeth were paced closer to the fulcrum of the jaw’s articulation. Although identical in form to one another, the teeth varied in size. All this dentition arrangement just might point out that Atrociraptor was a specialist predator preying primarily on one particular prey item.

Dromaeosaurinae Clade

Dromaeosaurinae ~ Dromaeosaurines are another subgroup of eudromaeosaurine dromaeosaurids that had a temporal range of ~ 130-66 mya in the Early-Late Cretaceous. Dromaeosaurines all have distinctive sub-equal denticles (teeth), although they are very closely related to velociraptorines in other ways and it has created much difficulty in distinguishing the slight varying taxonomies between the two groups. Fossils have been found in N. America, Mongolia and Denmark.

Currently there are nine dromeaosaurine genera and eight of them are: Yurgovuchia (Yur-go-vu-chee-ah) from the Early Cretaceous 130-125 mya; Utahraptor (U-tah-rap-tor) from the Early Cretaceous 126 mya; Deinonychus (Die-non-e-cuss) from the Early Cretaceous 115-108 mya; Itemirus (I-tem-e-rus) from the Late Cretaceous 91 mya; Achillobator (Ah-kill-o-bait-or) from the Late Cretaceous 89 mya; and Dakotaraptor (Duh-ko-tah-rap-tor) from the ending of the Late Cretaceous 66 mya.

The ninth one is Dromaeosauroides (Dro-may-o-sawr-roy-deez) which is known as a ‘tooth taxon’ since teeth are the only remains found thus far, unless ya count the coprolite (fossilized do-do) flecked with fishbone attributed as being dumped by Dromaeosauroides. The teeth are definitively dromeaosaurine and are much like Dromaeosaurus with small distinctions, hence the name, Dromaeosauroides meaning, ‘Dromaeosaurus-like’. Dromaeosauroides hails from 140 mya Denmark and is the only European confirmed dromeaosaurine. Most of the 140 million year ago Danish Mesozoic deposits are now under seawater. If the definitive evidence holds up, Dromaeosauroides pushes eudromaeosaurines’ existence back millions of years.

Artist: Francesco Delrio  Deinonychus

Deinonychus was once listed as a velociraptorine, but is now considered a basal dromeaosaurine transitional to velociraptorines. At 3.4m/11ft, it is the fourth largest dromeaosaurine behind Dromaeosauroides (3.5m/11.5ft), Utahraptor (5.7m/19ft) and Achillobator (6m/19.7ft). Deinonychus’ multiple fossil remains of the aged and young have been found in the U.S. states of Montana, Oklahoma, Utah and Wyoming.

Deinonychus claws

The manus (hands) of Deinonychus were very large with the distal second as the longest and the first digit as the shortest. The pes (foot) possessed the typical dromaeosaurid sickle claw on digit II (second toe) that was large and formidable while used in predation.

Artist: Masato Hattori Deinonychus

With juvenile and adult Deinonychus teeth and bone bite marks commonly found among ornithopod fossils, in particular Tetnotosaurus, it appears that Deinonychus hunted in packs and fed on the prey in a shared communalism. However, Deinonychus did not have that strong of jaw muscles to bite through bone; only to scratch it, which indeed knocked out a lot of teeth. With a bite force of only 15.7% that of an extant American alligator, Deinonychus’ bite is more comparable to that of a wolf’s. Still strong, but not as strong as some articles that compared it to an alligator’s biting force of 1450 N (newtons), which is one of the strongest bites known. It’s comparable to the jolt one would feel when a pickup truck reached the end of a rope once pushed off a cliff taller than the rope is long with that one holding onto the other end of the rope.     

Coming from the 98-83 million-year-old ‘Bayan Shireh Formation’ of Dornogovi Province, Mongolia; Achillobator lived roughly 89 mya in the Late Cretaceous. Carrying a body weight of ~ 350kg/771.6lbs and growing to 6m/19.7ft, unlike a host of its arboreal dromaeosaurid relatives, Achillobator definitely was a ground dwelling predator. In addition to remaining on the ground, the name Achillobator refers to ‘Achilles Hero’ and for good reason; its Achilles tendons were well developed to carry that weight and length around on terra firma.

Source: imgur.com  Achillobator

When compared to other dromaeosaurines, Achillobator possesses all the related features in the pes (feet), manus (hands) dentition (teeth) and feathering. But the one distinctive trait that sets it apart from all dromaeosaurines and dromaeosaurids is its pro-pubic pelvis. The most forward pelvic bone is the pubis. While all other dromaeosaurine pubic bones point backwards as in birds, Achillobator’s points vertically downwards. This simply is a plesiomorphic (primitive) genetic holdover from its very distant saurischian cousin’s pelvic trait.

Artist: Nobu Tamura  Yurgovuchia

At 2.5m/8.2ft long, although most of that length is the tail, Yurgovuchia fossil remains were found in the ‘Cedar Mountain’ and ‘Yellow Cat Member’ formations of Utah, USA. Yurgovuchia is closely related to Dromaeosaurus, Achillobator and is even in closer phylogenetic association to Utharaptor as an advanced dromaeosaurine displaying a rapidly evolving species descended from Utahraptor.

Velociraptorinae ~ Velociraptorines are the last subfamily of eudromaeosaurine dromaeosaurids, with its members occurring at the end of the Late Jurassic and Late Cretaceous Periods 151-66 mya. Before we go any further with this, let me just state that at the moment, Velociraptorinae is a mess. Very recently, it has been decided that all members of this subfamily are to be moved out except for Velociraptor. Of those that have been removed, I will list here as they are still monotypic dromaeosaurids and do show relations to velociraptor in skeletal anatomy and morphology. Do not be too concerned by this for science is not static, but changing constantly. When newer revelations have come to light and are vigorously analyzed and tested, if that new data is closer to the truth, then that material will be fitted into the subject of questionable description either by complimentary addition, or outright replacement of the older data.         

It appears that these former Velociraptorinae members are now going to be relegated as monotypic dromaeosaurids; they are: Nuthetes (New-thet-eez) from the Early Cretaceous 143 mya; Linheraptor (Lynn-huh-rap-tor) from the Late Cretaceous ~78.5 mya; Tsaagan (Sar-gan) from the Late Cretaceous 75 mya; Boreonykus (Boh-ree-o-nye-cuss) from the Late Cretaceous 73 mya and Acheroraptor (Ah-cher-o-rap-tor) from the ending of the Late Cretaceous 66 mya.

Currently, in the genus, Velociraptor (Vuh-loh-suh-rap-tor) there are two species in, V. mongoliensis (Mon-gol-e-in-sis) and the lesser known, V. osmolskae (Aus-mol-ski). V. mongoliensis fossils, which are numerous, come from the sedimentary redbeds of the ‘Djadochta Formation’ of what is now the Mongolian province of Ömnögovi. V. osmolskae fossil remains are from the same redbeds of what is now Inner Mongolia, China in the ‘Bayan Mandahu Formation’. The redbeds are the result of an arid environment filled with sand dunes.

Artist: Jonathan Kuo Lft: V. mongoliensis Rt: V. osmolskae

Both were small averaging 2.1m/6.9ft and the fossils reveal quil knobs on the forearms that once held pennaceous feathers. As in most all dromaeosaurids the tail was stiffened with long bony projections (prezygapophyses) located on the upper surfaces of the vertebrae beginning with the tenth vertebra extending forward in bracing four to ten more vertebrae. However, the prezygophyses allowed horizontal tail movement.  But unlike other dromaeosaurids, on the anterior end, the skull curved upwards and was concave-shaped on the upper surface, while convex on the lower.

The pes (foot) digit II (second toe) had the typical dromaeosaurid sickle claw, but in the velociraptors, the claw was shaped straighter used more for stabbing rather than for slashing. If you read Et Tunc Nulla Erat VII and recall protoceratopsids, I mentioned a fossil find with Velociraptor and Protoceratops preserved in a mortal combat pose after being buried alive in a sand dune slide. Velociraptor was stabbing at the protoceratopsid’s neck in which was an intelligent move, for that is where the carotid artery and jugular vein are most exposed.

Artist: Raul Martin  Velociraptor & Protoceratops’ fatal duel

The slight differences between the two velociraptors was that in addition to both having a single row of holes called neurovascular foramina that in life housed blood vessels and nerves in life, V. osmolskae had a bone ridge near the neurovascular foramina whereas, V. mongoliensis didn’t. Also, although both had jaws lined with 26-28 widely spaced teeth more strongly serrated on the back edge than the front, there were slight differences in tooth structure between the two. Both were scavengers and carnivores most likely hunting in packs at times.         

I will mention here two former velociraptorines. Nuthetes (Nuth-e-tease) was a small predator with a maximum length of 2m/6.6ft with its fossil remains coming from the ‘Lulworth Formation’, a subunit of the limestone ‘Purbeck Group’, living 143 mya in what is now England. The significance of Nuthetes is that if its scant remains hold up as a dromaeosaurid, along with Dromaeosauroides, it will push the family lineage back tens of millions of years. The fossil remains attributed to a sub adult consist of thecodont (teeth set in sockets) teeth and a partial jaw. Nuthetes lived along the shorelines of a ‘Purbeck Group’ lagoon.

Artist: Mark Witton  Nutthetes

Acheroraptor was also 2m/6.6ft long with its fossil remains coming from the Maastrichtian ‘Hell Creek Formation’ of Montana, USA. Acheroraptor, living 66 mya at the ending of the Cretaceous, is the most derived dromaeosaurid. Although its total length was smaller than T. rex’s head, with its advanced sickle second toed claws it was a deadly predator to smaller animals.

Artist: arvalis  Acheroraptor

The skull of Acheroraptor was long-snouted along  jaws lined with dagger-like ridged teeth. Acheroraptor differed from other dromaeosaurids in possessing a hypertrophied post-antrum (cavity) wall that projects posteriorly into the antorbital fenestra, a maxillary fenestra positioned low in the antorbital fossa and directly posterior to the promaxillary fenestra, and distinctive dentition with marked apicobasal (apical membrane facing the outside of the body or lumen of internal cavities) ridges.   

Acheroraptor is a significant N. American find, for it gives N. American dromaeosaurids a phylogenetic framework. This phylogenetic analysis specifies that Acheroraptor is closer in relations to Asian dromaeosaurids than it is to any N. American dromaeosaurid. As paleontologist, Dr. David Evans points out, “The close evolutionary relationship of Acheroraptor to a small group of late-occurring Asian species that includes Velociraptor suggests migration from Asia continued to shape North American dinosaur communities right up until the end of the Cretaceous Period.”

Troodontidae Cladogram

Troodontidae: Troodontids are the other deinonychosaurian subgroup besides the dromaeosaurids which are in the family clade, Troodontidae (Tru-don-tuh-day). Existing 130-66 mya during the Early and Late Cretaceous, troodontids are the most basal of the deinonychosaurian clade. Remains have been found in North America, China and Mongolia. Except for, Borogovia, which had a straight and flat claw on the second toe that could not be held above ground, the rest of the troodontids possessed the typical deinonychosaurian sickle claw on the second toe that was held off the ground. 

Borogovia pes; no sickle toed claw

Basically, the size of troodontids was small ranging from Latenivenatrix in being the largest at 3.5m/11.5ft, down to Mei, which was about the size of a peregrine falcon at 60cm/23.6in. The distinctive features of troodontids were in the long skull filled with closely spaced recurved saw-edged teeth. The hind limbs were slender and long used for rapid locomotion. The manus (hands) were large and flexible making it capable enough to maneuver precise grasping.  

The main feature of troodontids was that the skull as the braincase, relative to body size is the largest of all known dinosaur groups. With very large orbits (eye sockets) and a well-developed brain cavity where sight and hearing nerve brain centers were located, troodontids had keen diurnal and nocturnal sight as well as hearing. Many conclude that troodontids with brain sizes proportionate to extant non-flying birds were intelligent, but it is hard to really measure intelligence. They might have been the smartest dinosaur, but for sure their cranial anatomy gives evidence that they were very agile, speedy carnivores with a keen sense of sight and sound.

Troodontid fossils and fossils of their nest sites containing embryos show that, like birds, at least some troodontids brooded shedding light on troodontid reproduction, again, as similar to birds, but ground nesting birds. The troodontid body was covered in feathers and was carnivorous, but along with some troodontid remains found in other dinosaur groups’ nesting sites, like an oviraptorid nest with eggs, they may have also been egg eaters. But, it could also suggest that they were prey themselves brought to the nest site by the oviraptorid parents, or that troodontids were nest parasites like the extant cowbird that lays its eggs in another birds nest to be raised by the other birds. Overall, troodontids were fast and agile dinosaurs with acute senses.

Troodontidae is divided into three subfamilies and they are: Jinfingopterginae (Gin-feng-op-tuh-ridge-uh-nay), Sinovenatorinae (Sy-no-vuh-tor-uh-nay), and Troodontinae (Tru-o-dont-uh-nay).

Jinfingopterginae ~ is a separate branch of troodontids that are more closely related to Jinfingopteryx than to Troodon formosus. They are basal troodontids with a temporal range of 122-71 mya during the Early-Late Cretaceous.

The two genera of Jinfengopteryginae are: Jinfengopteryx (Gin-feng-op-tuh-rics) from the Early Cretaceous 122 mya and Almas (Al-mass) from the Late Cretaceous 75-71 mya. This subfamily also contains a nomen dubium fossil that is a suspect jinfengopterygine numbered as WDC DML 001 that as yet, has not been evaluated and named.

Artist: Emily Willoughby  Jinfengopteryx

At 60cm/23.6in, Jinfengopteryx was small with its remains discovered in the ‘Huajiying Formation’ of Hebei, Province in China. It had extensive pennaceous feathering on its forelimbs and tail, but lacked any feathers on the hind limbs. However, it could not fly. When first discovered in 2005, it was thought to be one of the most primitive of birds, but quick studies and a final review, found it to be a troodontid.

J. elegans skeletal anatomy

Jinfengopteryx, due to its small size most likely was an omnivore chasing after large invertebrates and small invertebrates. Tiny reddish-yellow looking ‘pearls’ found in its remains’ digestive cavity are identified as grains proving that all troodontids weren’t strict carnivores.      

Sinovenatorinae ~ includes all troodontids most closely related to Sinovenator and are exclusively from Early Cretaceous China. The four genera of Sinovenatorinae are: Daliansaurus (Dah-lye-uhn-sawr-us) from the Early Cretaceous 126 mya; Mei (May) from the Early Cretaceous 125 mya; Sinovenator (Sy-no-vah-nay-tur) from the Early Cretaceous 125 mya; Mei (My) from the Early Cretaceous 125 mya and Sinusonasus (Sih-no-so-nah-sus) from the Early Cretaceous 125 mya.

Artist: Zhao Chuang Dalianosaurus

Described just in 2017, Daliansaurus comes from the ‘Yixian Formation’ of Liaoning Province, China. Its remains were well preserved showing new details of Chinese troodontids. In using osteohistological thin-sectioning and high-resolution synchrotron-based imaging due to the excellent condition of the fossil, scientists were able to analyze the histology of its forelimbs. In the collective analysis, it showed that the fossil specimen’s growth characteristics within the forelimb layered bone depositions that this Daliansaurus had passed the exponential growth phase within its first year perishing three years later. But while even four-years old, it still had not quite attained maximum somatic size.

Dalianosaurus fossil

Skeletal features of the forelimbs had the humerus of the upper arm as rather long with the lower arm ulna distally ending in three fingers on each manus (hands). Digit I’s (thumb) metacarpal being is strongly curved in a vertical plane while the digit II (second finger) is shorter than digit III. Digit I is the most robust with a claw bearing a distinctive deep groove on the side.

The hind limbs’ strongly bowed femur’s process (lateral crest) is lower than the posterior trochanter (bony protuberances where muscles are attached to the upper part of thigh) process. The tibia is 1.4 times longer than the femur. With its long and well-built metatarsals and skeletal leg anatomy, at 1m/3.3ft in total length, Daliansaurus’ running capabilities were far more developed than other troodontids.

Generally it was a typical troodontid with the neural arches fused to the vertebral centra in the dorsal vertebrae. But, in addition to the dromaeosaurid second toe sickle claw, Daliansaurus had an extra-large claw on its fourth toe as well, exacting the same length of the claw on the second. The projection of the premaxilla above the nostril is flattened, while another projection below the nostril is straight. This is a feature of the premaxilla where other troodontids had only one of the projections, but not both as Dalinsaurus had. It had 21 teeth in the upper jaw and 34 in the lower with the back teeth being having serrations on the rear side.

Artist: Todd Marshall  Sinovenator

Sinovenator, as mentioned again is the namesake to sinovenatorines. At 1m/3.3ft, Sinovenator had the typical troodontid skull, closely spaced large numbers of teeth, sickle-like second toed claws and raptorial manus (hands). Some distinct features were that the antorbital fenestra (opening in skull in front of orbit or eye socket) was straight and vertically oriented. Also, the upper branch of the premaxillae excludes the maxillae from the nostrils and the maxillary tooth row is positioned some distance from the jaw rim. The ilium is small in comparison to the rest of the pelvis, the ischium is bird-like, the pubic bone was oriented to the rear and the shin bone was wide at the top having a rectangular-like lower joint surface.

Sinovenator was a primitive troodontid bearing similar features with the most basal dromaeosaurids, suggesting the time of a transitional split between the two groups. The pubic bone orientation to the rear is one such feature shared. This orientation also points out that the forward pointing pubic bone in later more derived troodontids was not inherited from older theropods, but evolved within the troodontid clade.

Artist: Craig Chesek  M. long

Mei, or with the genus and specific name, Mei long was a small troodontid at 70cm/27.6in total in length found in the lower ‘Yixian Formation’ of Liaoning Province, China. As a basal troodontid like the previously mentioned Sinovenator, but unlike the advanced troodontids, it had bird-like hips. Mei had unique small and low maxillae with tiny fenestrae, an extremely large naris extending caudally over one-half of the maxillary tooth row and closely packed middle maxillary teeth with the maxillary tooth row extending caudally to the level of the preorbital bar.

Artist: Esther van Hulsen Mei slept just like birds do today

Artist: Mick Ellison Mei 

In Mei, paleontologists have found their Sleeping Beauty; Mei in Chinese even means ‘sleep soundly’. With the two Mei fossil remains, both were detailed in a 3-D sleeping pose. The pose is much like birds roosting with the head tucked in between a wing and the body, while the body rested over the bent legs. The surrounding matrix of the fossil sites is volcanic ash and it appears the two asphyxiated from volcanic gaseous then quickly buried by the ash; much like what occurred during the Pompeii eruption.

With the 3-D fossilization imaging of the two Mei remains, the evidence points to the fact they died in a life sleeping pose rather than coincidental rigor mortis. This form of sleeping suggests that Mei was warm-blooded.        

Troodontinae Clade

Troodontinae ~ are a group of closely related troodontids descended from their common ancestors of Gobivenator and Zanabazar. The ten genera from Troodontinae are: Sinornithoides (Sy-no-nif-oi-deez) from the Early Cretaceous 113mya; Urbacodon (Ur-bah-koh-don) from the Late Cretaceous ~ 95 mya; Troodon (Tru-o-don) from the Late Creataceous 77.5-76.5 mya; Stenonychosaurus (Sten-o-nick-o-sawr-us) from the Late Cretaceous 76 mya; Latenivenatrix (La-ten-eye-vuh-nay-trics) from the Late Cretaceous 75.5 mya; Saurornithoides (Sor-orn-nih-thoy-deez) from the Late Cretaceous 75 mya; Linhevenator (Lin-he-ven-ah-tor) from the Late Cretaceous 75 mya; Philovenator (Fie-lo-ven-ah-tor) from the Late Cretaceous 75 mya; Gobivenator (Go-be-ven-ah-tor) from the Late Cretaceous 72 mya and Zanabazar (Zah-nah-bah-zar) from the Late Cretaceous 70 mya.

Stenonychosaurus comes from the Late Cretaceous ‘Dinosaur Park Formation’ of Alberta, Canada and nesting sites in ‘Two Medicine Formation of Montana’. It was 2.4m/7.9ft in length and stood up to 0.9m/3ft in height. As mentioned earlier, where troodontids had the largest brain size of any other dinosaur group when compared to body size, Stenonychosaurus had the larger brain when compared to all other troodontids. When compared to other species with the same body weight, Stenonychosaurus shows a steady increase in the encephalization quotient (EQ) of brain weight. Its eyes, as testified by the fossils’ orbits (eye sockets) were also the largest troodontid eyes that probably afforded excellent diurnal sight, but superb nocturnal sight. The position of the orbits slightly faced forward, which also enhanced vision with depth perception.

Credit: Thomas Henry Huxley Paleozoology Gardens  Stenonychosaurus 

The jaws of Stenonychosaurus met in a broad U-shaped in an adhesion of fused fibrocartilaginous tissue known as ‘symphysis’.  The denticles (teeth) bore large serrations with pits between each denticle (tooth). According to the type of teeth, Stenonychosaurus may have been an omnivore. The grasping manus (hands) ended in three flexible fingers. Analyzing Stenonychosaurus’ fossils’ ring bones, it reached full maturity by 3-5 years.

Stenonychosaurus fossilized eggs

Studying nesting fossil sites and their embryos in Montana’s ‘Two Medicine Formation’, it has been determined that they are Stenonychosaurus embryos. The nests were dish-shaped fashioned, rimmed with hardened mud and filled with sediment. Up to sixteen elongate and teardrop-shaped eggs were laid being positioned vertically with the tapered end buried in the pebble sediment.            

Saurornithoides (Sawr-or-nih-thoi-deez) at 1.8m/5.9ft appears to be an Asian counterpart to the North American, Troodon as they both share many morphological and physiological features. Both shared similar size large orbits (eye sockets) and due to the large braincase that housed a large brain compared to body size, both shared similar intelligence.

Artist: Linda Krattiger  Saurornithoides

Saurornithoides was discovered in the ‘Djadochta Formation’ of what is now Mongolia and had the typical troodontid large number of closely spaced serrated teeth on the rear side. However, its evolvement is transitional as Saurornithoides still retained a basal presence in possessing a recessus tympanicus dorsalis (the upper one of three small openings on the side of the braincase within the inner ear region), but exemplifies a more derived form in lacking a small opening at the front of the snout called a fenestra promaxillaris. In possessing more derived traits, it also had large denticles on the rear tooth edges as well as the high number of six sacral vertebrae.

With a body length of 3.5m/11.5ft, Latenivenatrix was the largest of troodontids so far discovered. Also coming from the upper ‘Dinosaur Park Formation’, it was discovered in conjunction with the Stenonychosaurus remains. The distinction between the two is due to the varying frontals, Digit II (metatarsal two) structures and of course size. Both of these troodontines were originally assigned as Troodon. As well, both species phylogenetically are closer to the Mongolian troodontid taxa indicating an earlier Asian troodontid form migrating into North America.

Artist: Julius T. Csotonyi  Latenivenatrix

Latenivenatrix differed from other troodontines in having a slightly retroverted (tipped backwards) pubis with a shaft that curved anteroventrally, while slightly proximal to the pubic boot is a large muscle scar on the lateral surface of the pubic shaft. It maintained troodontine traits in possessing an elongated ambiens process and has a horizontal ventral margin of the postacetabular process and of course had the dromaeosaurid and troodontid pes digit II (second toe) sickle-shaped claw. As far as paleopathology goes, in one of its fossils, a catalogued parietal bone possessed an aperture that was caused by a cyst resulting in a bite wound.     

Gobivenator’s remains were found in, you guessed it, what is now the Gobi Desert of Mongolia from the ‘Djadokhta Formation’ that is part of the 75-71 million-year-old ‘Campanian’ stratigraphic range. The Gobivenator fossil remains are the most complete and articulated yet of any troodontid.

Gobivenator fossil

The typical characteristics of troodontids were also displayed in Gobivenator’s remains such as the digit II (second toe) sickle claw, raptorial manus (hands), closely spaced teeth in the lower jaw and a graceful frame geared for swift running. What makes it unique is in its autapomorphic (derived trait) features having the parietal bones fused ending anteriorly in a pointed tip, the lower jaws’ surangular (jaw bone found in all land vertebrates, except mammals that connects all jaw bones) has a fossa (a bone depression) and the skull had a large fenestra (opening).

Artist: Eloy Manzanero  Gobivenator

Gobivenator possessed an akinetic skull, however it had lost the epipterygoid (a slender skull bone found in reptiles) bone and there was a reduction in contact between palate bones. These were the preconditioned initial features evolving towards cranial kinesis found in birds.        

There are nine troodontid genera which are monotypic outside the subfamilies; they are: Albertavenator (Al-burr-tuh-vah-nay-tur) from 130 mya; Geminiraptor (Gem-in-i-rap-tur) from 127 mya; Liaoningvenator (Lee-ow-ning-o-sawr-us) from 126 mya; Jianianhualong (Chon-e-an-wuh-lon) from 124.4 mya; Xixiasaurus (Zee-zee-ah-sawr-us) from the Late Cretaceous 86 mya; Byronosaurus (By-ruh-no-sawr-us) from the Late Cretaceous 80-75 mya; Talos (Tay-luhs) from the Late Cretaceous 75.95 mya; Borogovia (Bow-roe-go-vee-ah) from the Late Cretaceous 70-65 mya and Tochisaurus (To-chee-sawr-us) from 69 mya of the Late Cretaceous.

Artist: Weldon Owen   Byronosaurus

Discovered in Mongolia’s Gobi Desert on separate occasions, Byronosaurus adult remains were found in two sites, while in a third location nearby at a nesting site, two near hatchling embryo skulls were found still with egg pieces clung to them. This poses a bit of a conundrum, for the nesting site was not a Byronosaurus nest, but an oviraptorid nest with eggs in it while one egg having a preserved oviraptorid embryo. How did they get there? Were the Byronosaurus hatchlings preying on the eggs for their first meal, or did the oviraptorid parents seize the Byronosaurus hatchlings or near full term embryos for their meals, or, was Byronosaurus a nest parasite?

Byronosaurus embryonic skulls

At 1.5m/4.9ft long, Byronosaurus shares troodontid traits, but the teeth were not serrated. The teeth were needle-like most suited for grasping and seizing hold of small vertebrates. Living in a desert, the fossils were found near streams, so with its teeth built for capturing and holding on to struggling or slippery prey, it may have been a piscivore as well. The metatarsals (foot bones) were proportionally long compared to the body and along with the gracile long hind limbs, it was built for speed.

Artist: Julius T. Csotonyi  Jianianhualong

Jianianhualong fossil remains were found in what was Early Cretaceous China. At 1m/3.3ft long, the fossil is exceptional in giving vivid detail on skeletal anatomy and feather impressions. Jianianhualong possessed both features of basal and derived troodontids. Unique among troodontids, it also had the neural spine of the axis convex shaped on the top margin with the portion strongly expanded. The dentary bones supported 25 teeth on each side while the maxillaries supported 21 on each side. The teeth were closely packed up front and not serrated differing from the back teeth that were more spaced and had serrations on the back side.

Jianianhualong fossil

The tail and forelimb feathering are pennaceous and asymmetrical, the feathers that birds actually take flight with. Although Jianianhualong could not fly, with its long hind limbs, it was a probable and agile sprinter and the tail wings would have afforded drag in maneuvering. But, with asymmetrical feathering already in place as so explicitly detailed in Jianianhualong’s 124.4 mya fossil remains, through a bit more flight evolvement, birds were able to take wing and fly.       

Talos is a troodontid representative from the N. American ‘Kaiparowits Formation’ in what is now Utah. At 2m/6.6ft, it was one of the larger troodontids and coming from the same region and time, in what was then the southern portion of the Laramida Island, as T. rex and other tyrannosaurs like the genus, Teratophoneus or Lythronax may have been one of the larger dinosaur’s conceivable snacks.

Artist: Jorge Gonzalez  Talos & its sharp claws

It had the troodontid characteristics of a feathered body, raptorial manus (hands), a retractable second toed sickle claw, bore weight on the other two toes of each leg and had a large brain. In addition it had its own distinct features in unlike most troodontids, the Talos neural arch of the mid dorsal was pneumatic. Where most troodontid mid dorsal neural arch spines were short and fan-shaped, the Talus mid dorsal neural arch spines were long. As shared with Mei, projection of the dorsal periphery of the parapophyses (ventral transverse process of a vertebra) may reflect incipient development of the stalk-like condition in dromaeosaurids. The most distinct feature was that the neural spine tapers to a dorsocaudal (top of tail) point in lateral profile.

Artist: Julio Lacerda  Troodon in Alaskan winter

Talos’ paleoenvironment showed that by living in forested wetlands and floodplains, which was in contrast to the desert paleoenvironment of its Mongolian cousins, troodontids were adaptive. Even in Cretaceous Alaska where the average winter temperature was 2° C/36 F° and snowed during winter months of long cold darkness, troodontids were able to successfully inhabit these climatic conditions. In fact, from the number of fossil finds and amount of loose teeth, Troodon was the most common dinosaur found in Alaska’s ‘Prince Creek Formation’ making upwards to two-thirds of all species. This feathered Troodon and the other two troodontids gives us insights into how animals physically and physiologically adapted to environmental contrasts.

Artist: Oliver Demuth  Albertavenator

Found in the ‘Horsehoe Canyon Formation’ of Alberta, Canada, Albertavenator is another N. America troodontid reaching ~ 1.8m/6ft in length. The supraciliary foramen (bone ridge located above eye-sockets) is truncated anteriorly by the lacrimal (small bone forming the eye-socket) contact. Dentition is very similar to the synonym, Troodon formosus from the ‘Dinosaur Park Formation’, but the skull overall is much more robust and the ectocranial (outside cranial surface) of the frontal bone is much shorter than all other troodontids.

In the next Et Tunc Nulla Erat series we’ll dive into dinosaurs that begin to not only appear as birds, but evolve as birds.

Apologies for the delay in getting this article published. I had already written it by May, but to edit it and upload the photos had to take a back seat to other concerns. Anyway, here it is…

Here’s to hoping that ya all had a Happy Turkey Day and, from the Grand ol’ White Bearded One (Santa Claus), are hand delivered a most joyous occasion this Christmas season!

Life’s Been Swell!

BJA

05/14-26/2018