Et tunc nulla
erat IV
(And Then There Was)
AMPHIBIA
In ‘Et Tunc Nulla Erat III’, we left off
during the late Devonian (382.7-372.2 mya) with the appearance of the truly
tetrapodal labyrinthodonts arising from lobe-finned fish. These creatures
radiated out in speciation on land with some species members becoming ancestral
to reptiles and lissamphibians. Tetrapodal essentially means possessing four
limbs.
To addend a bit where
we finished up last time, during the Late Devonian 370 mya there was massive
effort in vertebrate speciation to radiate out onto land due primarily from
heavy aquatic predation, new food sources and poorly oxygenated shallow waters.
During this late
Devonian period, the seas and even rivers and lakes were teeming with life of
both invertebrates and vertebrate. The land however was relegated to plants and
insect arthropods until our ancestors first crawled onto land.
Before we dig into the
amphibian line it would be worth mentioning the animal, Bothriolepis (Bo-three-o-lep-is). The creature lived from the mid
to the end of the Devonian 387-360 mya and was from the placoderm line of fish
although it didn’t look much like a fish. At 20.3-30.4cm/8-12in (although one
species B. maxima had a carapace measuring 100cm/39.4in), Bothriolepis was heavily armor plated and had long dagger-like spines
for fins that went the length from just behind the head to the trunk of the
body.
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| Bothriolepis during the Devonian |
One reason I’m dwelling
on Bothriolepis is that it was one of
the first shallow marine animals to populate freshwater environs. The gills
were relatively short but broad allowing more surface area to make transitions
and adaptation to freshwater more reliable. The dermal skeleton was made of
cellular bone making it one of the first animals in utilizing bone for support and
protection to transition onto land environments.
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| Pandericthys chomping on Bothriolepis |
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| Eusthenopteron |
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| Pandericthys |
In resultant
descendants of the sarcopterygian lobe-finned fishes, natural selection
transitions produced Eusthenopteron
(Phonetics: You-sten-op-teh-ron) and although it was still relegated to an
aquatic life, its adaptations began the progress to life on land with fin bones
verging to limbs. Pandericththys
(Phonetics: Pan-der-ik-thees) was well suited for muddy shallow bottoms with
lobed limbs supported by digits. Acanthostega,
which was still dependent to watery environs, had developed webbed feet with
eight digits that could pull the body onto shore. Ichthyostega (Phonetics: Ik-thee-o-stega) possessed larger limbs
with adults relying more on lungs rather than gills for obtaining oxygen.
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| Acanthtostega/Ichtyostega comparison Click to Enlarge |
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| Acanthostega |
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| Ichthyostega |
Finally, around 372-370
mya, there was Tiktaalik (Phonetics:
Tik-ta-lik) that exhibited the link between fish and amphibians. Its webbed
feet were weight bearing, possessed wrists, crocodilian-like shoulders and
elbows, simple rays reminiscent of fingers and had gills plus primitive lungs.
It had no fish bony plates in the gill area to restrict up or down and sideways
head movement. This trait gave rise to one of the first necks ever to be held
by an animal. In addition, Tiktaalik’s
ribcage was robust enough to support body weight once outside of aquatic
environs.
Tiktaalik’s physiological
endowments were to lead the way to all future terrain tetrapodal lifestyles in
which labyrinthodonts were later to begin making their appearance to continue
carrying the tetrapod torch.
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| Tiktaalik |
The genus, Tulerepton (Phonetics: Tu-lur-ep-tahn)
of the Late Devonian was from the Ichthyostegalia
order, just as Acanthostega and Ichthyostega, but in contrast, had a
much more strengthened limb structure used however more for paddling than
walking. This creature though, in adult form had lost its gills and was totally
dependent on lungs for acquiring oxygen and with a disconnection of
the head from the pectoral girdle, allowed for much more head movement.
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| Tulerepeton |
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| Pederpes |
The tetrapod genus Pederpes (Phonetics: Ped-er-pees) was
the first tetrapod to have five digits on each limb, although there was a sixth
vestigial nub on the forelimbs. The feet were not paddle-like and were
fashioned to accommodate walking on land. Pederpes
thus far, is the first of the fossil record in exhibiting true terrestrial
locomotion.
Labyrinthodont skeletal structures differed from true reptiles in having teeth that exhibited complexly in-folded enamel surfaces. These teeth were grooved strongly reinforcing the whole dentition structure enhancing the seizing/holding of prey. The dental arrangement was borrowed from Late Paleozoic Era trending amphibian-like labyrinthodonts, who in turn borrowed it from lobed-finned fish. The term Labyrinthodontia (Phonetics: Lay-be-rinth-o-don-chee-ah) in itself is Greek meaning ‘maze tooth’.
Also borrowed from the
early amphibian type ancestry were eyes placed at the top of the skull; a
holdover from watery ancestral origins for better field sight on a flat head
when floating on the surface.
All labyrinthodonts
possessed two otic notches, one behind each eye orbit that invaginated as the
posterior margin of the skull roof. The otic notch was an auditory structure
supporting a tympanum much like as found in modern day frogs.
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| Labyrinthodont skull with otic notches |
Varying from the size
of a small salamander to a modern day crocodile, labyrinthodonts possessed
stocky lizard-like bodies with short limbs. The subclass, Labyrinthodontia is not monophyletic in that the group or clade
does not include all ancestral and descendent species, but is rather
paraphyletic consisting of the common ancestor, but not all of the descendants.
In this regard, a lot of researchers have abandoned the Labyrinthodontia
subclass, but due to their direct relationship to modern amphibians and even to
the first reptiles, I still include the subclass.
Supporting a massive
skull possessing dentine armor, numerous labyrinthodonts were one of the first
known land animals to utilize scutes or scales for protective covering. They
also maintained topside eye openings, a pair of nostrils and a parietal eye
that could sense photoreception. Today, the primitive tuatara, some anoles,
iguanas and juvenile bullfrogs also exhibit a distinct parietal eye located
between the normal eyes.
In the more primitive
water-bound labyrinthodont species along with the more advanced land species,
their fossil records show an otic notch behind each eye that served as an open
spiracle or water breathing tube in the earliest specimens, which later evolved
into a tympanic (ear) membrane in more evolved land forms. The otic notch
evolved away allowing for stronger bites by jaw muscle rearrangement reducing
stresses and deformation during jaw movement.
During the last 20
million years of the Devonian, ~72% of all animal life became extinct in
particular marine organisms were hardest hit. But through this, the earliest of
tetrapods survived to be the first vertebrate to walk on firma terra.
Labyrinthodonts
survived and radiated outwards onto land from the early water restricted Late
Devonian forms to the more advanced land forms in the Early Mesozoic Era with
all becoming extinct around 210 mya during the Triassic.
One thing to take away
from all this before we dig deeper is that saltwater fish, in order to regulate
proper blood salinity drink saltwater retaining the water while getting rid of
the excess salt as waste. The first freshwater lobed-fin fish, in
order to adapt reversed the process in maintaining blood salinity by drinking
water continuously and getting rid of the copious amounts of water through
urination while retaining the lower percentage of salts.
This retaining of salt
in blood amounts to roughly 9 g/l or 0.9% salt solution in the blood. Further,
this is about one-third the salt of seawater. All vertebrates, even today, whether on land or in water, maintains this exact same percentage of essential
salts. This also includes man in bearing vestiges of his very earliest fish
origins in carrying a little bit of the seas within us.
The Bridge:
What is now called
‘early amphibians’ or ‘primitive amphibians’ or ‘basal amphibians’ were in all
actuality aquatic or semiaquatic labyrinthodonts. Composed of the superorders, Osteolepiformes (Phonetics:
Os-te-o-lep-iss-for-mees), Elpistosteglia
(Phonetics: L-piss-toe-steg-lee-ah), and Ichthyostegalia,
the class, Tetrapoda (Phonetics:
Tet-rah-po-dah) is the crown jewel of all land animals with four limbs
including snakes, cetaceans, amphisbaenians and others that have since lost
limbs through Hox gene evolution. Out of the three, all lines of evolutionary
species became extinct, except for Ichthyostegalia
which is the root base for all tetrapod animals past and present.
Ichthyostega and the more primitive
Acanthostega were still dependent
upon an amphibious lifestyle, but both had a defined osteology formed for
tetrapodal locomotion on dry land just beyond shorelines. Limbs with digits had
evolved in the waters.
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| Another Devonian Ichthyostega scene |
From Ichthyostega serving as the basal
ancestor, three main branches of labyrinthodont orders arose with side orders
becoming extinct leaving no present day survivors. The three main orders were Diadectomorpha (Phonetics:
Die-uh-dec-toe-mor-pha), a reptiliomorph labyrinthodont, Lepospondyli (Phonetics: Le-pos-pon-duh-lie) and Temnospondyli (Phonetics:
Tem-nos-pon-duh-lie).
Stereospondyli (Phonetics:
Steer-e-os-pon-duh-lie), with a simpler backbone composed of a single
intercentrum, had branched off from temnospondyl labyrinthodonts around the
Permian/Triassic border 255-251 mya leading to a dominant but now extinct
lineage. The capitosaurs as viewed from the otic notch sketch were
stereospondyls that produced the first crocodilian morphology in losing the otic notch, although they
were not an ancestral line to crocodiles.
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| Capitosaur trend in losing the otic notch |
The order, Nectridea (Phonetics: Nec-tree-day-a)
branched off early around 300-299 mya from the newly evolved Lepospondyli labyrinthodont producing
the fairly-well known genus, Diplocaulus
(Phonetics: Dip-low-cawl-us) only for this genus to die off by the time the
Triassic arrived. But lepospondyls weren’t finished in experimenting with new
clades and families that were going to succeed up to the present.
Early amphibian
labyrinthodonts were the first anchored side of the bridge. The bridge span
itself was lepospondyls, temnospondyls and reptiliomorphs. The bridge spanned
to the other end of the anchored bridge that produced the first true lissamphibians
(modern day amphibians) and reptiles. Let’s journey over that bridge.
Routes Taken:
A side note before we
begin is an oddity that I feel unique in the sequencing of evolution. It
concerns tetrapodomorph fish (aquatic vertebrates possessing four lobed limbs
but no feet). Osteolepis (Phonetics:
Os-te-o-lep-is), a lobe-finned fish basal to all tetrapods (extinct and extant
land vertebrates with four limbs ending in feet) are more closely related to four limbed land vertebrates than they
are to present-day lobe-finned lungfish. Even though lungfish retain numerous
ancestral characteristics of lobe-finned fish going all the way through the
Devonian 425 mya, through verified DNA analysis, the ancient/present day
lungfish is far more distantly related to osteolepids than say camels are. This
is due to lungfish early side branching.
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| Osteolepis fossil |
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| Osteolepis ~ closer to a camel than to lungfish |
Lepospondyls are
characterized in having spool-shaped vertebra that did not ossify from
cartilage as lungfish currently possess. The upper portion of the vertebra
known as the neural arch was fused to the centrum which was the main body of
the vertebrae. Most had smooth skin and were clawless. Lepospondyls ranged from
the end of the Devonian 350 mya to Late Permian 255 mya.
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| Amphibia Tree of Life |
In the photo below is a rare fossil of the temnospondyl Scelerocephalus which choked to death over too big a meal.
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| Scelerocephalus choked to death (Note the eaten animal's head within the body cavity) |
Both groups had aquatic, semi-aquatic and terrestrial representatives where some temnospondyls were totally terrestrial and could run. Again, the fairly known lepospondyl was Diplocaulus while the genus Eryops (Phonetics: Eh-ry-ops) were temnospondyl specimens.
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| Diplocaulus |
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| Eryops |
There are physiological
and morphological lending evidence that supports this. Yes, all amphibians today
derived from labyrinthodonts, but were the result of divergence within the
labyrinthodont class. Frogs and toads came from temnospondyl labyrinthodonts
that were losing their tails and replacing socketed or rooted teeth for
pedicellate teeth or none at all. Salamanders and caecilians came from those
lepospondyl labyrinthodonts that retained tails and teeth. Most caecilians have
very few tail vertebrae which support very small tails or hardly any tail at
all. The reason for this will be explained shortly, but first we’re going to
discuss the order Anura (frogs and
toads).
The origins of present
day lissamphibians are a route of divergence. No lissamphibian group came
solely from one branch or clade, but rather, while all shared an ancient
labyrinthodont basal ancestor, there were further numerous branching that each
modern day lissamphibian group evolved from as separate from one another.
In other words, where
salamanders and caecilians are kissing cousins, frogs are their third
cousin.
Leading to Anura:
With frogs and toads
coming from the labyrinthodont temnospondyl line, they are only distantly related
to the more closely related salamanders and caecilians that arose from two
divergent lepospondyl groups. This has been validated from an intensive
molecular phylogeny study on a 2005 rDNA analysis.
For lissamphibians, the
more distant relationships of frogs to the more related salamanders and
caecilians has a lot to do with geological events that took place during the
very late Paleozoic and very early Mesozoic 250 mya. The event occurred just
before the breakup of the continent Pangaea, but after temnospondyl and
lepospondyl divergences from true lobe-finned species. So, although the groups
originally shared a more basal ancestor like Gerobatrachus (Phonetics:
Geh-row-bah-track-us) that appeared 290 mya in the Permian, the breakup of
Pangaea isolated amphibian phylogeny where divergent evolution proceeded in
coming up with the three modern day amphibians.
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| Gerobatrachus |
In the fossil record,
frogs first appeared on Pangaea land in what is now India and Africa when
western India was conjoined to central eastern Africa. Salamanders have eastern
Asian origins when China and Mongolia were conjoined to eastern Russia and
Kazakhstan, while the later origins of caecilians appeared in the tropical
jungles of the Pangaea Triassic. All of this is corroborated with eight
mitochondrial genomes of current lissamphibians to the phylogenies of amphibian
sequencing.
Slightly more evolved
away from its contemporary temnospondyl cousins, a creature known as Amphibamus (Phonetics: Am-phi-bay-mus)
showed up in the Late Carboniferous during the Pennsylvanian 300 mya. This
animal had begun the process of a frog’s anatomy with much larger hind limbs
than forelimbs along with a larger pelvis, while the ribcage and tail became
shortened. This animal no longer possessed scales.
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| Amphibamus |
Utegenia, (Phonetics: U-tuh-gin-e-a) a
basal Seymouriamorpha of Late Carboniferous
to Early Permian, is a probable sister taxon to Amphibamus, thus a predecessor to frogs and is the lineage point
where lissamphibian frogs split from reptiles as Utegenia is also a basal
reptiliomorph. Utegenia lived in the
latest of the Carboniferous 300 mya down into the Permian 290 mya.
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| Utegenia various morphal stages |
From the Early Permian
during a time of very humid biomes, Utegenia
along with Doleserpeton (Phonetics:
Doe-le-sir-pe-tawn), although were aquatic dependent in larval stages, ambient
humidty allowed the adult to roam land. Phylogenetically, these two preceded Gerobatrachus (Phonetics:
Ger-o-bah-trak-us) and were transitional from the usual Seymouriamorpha
morphologies.
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| Utegenia fossil |
Again, Utegenia is the species that gives the
true split of lissamphibian frog lineage from living reptiles. Doleserpeton had four digit forelimbs
and five digit hind limbs; the formula virtually all modern day frogs
follow.
Utegenia and Doleserpeton had narrower snouts than modern day lissamphibian
frogs, but were more broadened than other contemporary Seymouriamorpha. Today’s frog leap lengths are due to this
evolutionary dropping in weight and broadening of the overall skull with no
tail to lessen counterweight and drag.
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| Doleserpeton |
Gerobatrachus, swimming the Early
Permian swamps and frequenting the humid jungles 290 mya had a frog head and
salamander tail. This creature was without doubt a temnospondyl and had only
borrowed the salamander-like features from ancestral labyrinthodonts. With a
shorter vertebral column than lepospondyls and even other temnospondyls
existing at the time, along with a shortening vomer facial bone and as in most
frog mouths exhibiting the palatine bone as a narrow strip along the side of
the palate, Gerobatrachus known as
the ‘frogamander’, was well on its way to being the closest basal ancestor to
modern frogs and toads.
By 250 mya in the
Triassic, Triadobatrachus (Phonetics:
Tri-ad-o-bah-trak-us) had made its appearance in what is now known as
Madagascar when the current island was sandwiched between India and Africa. It
appears that this animal lived partly in water and on land as both aquatic and
land plants have been found with the fossils.
Triadobatrachus, with fourteen
vertebrae had six of them supporting a small retained tail. Of course, modern
day frogs have only four to nine vertebrae with no tail. Triadobatrachus had large hind legs but without the ability to hop or
jump, they were used for kicking in swimming.
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| Triadobatrachus fossil |
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| Triadobatrachus |
Triadobatrachus is a basal ancestor to
true frogs giving way to the first known frog, Prosalirus (Phonetics: Pro-say-le-rus) that lived 190 mya during
the Jurassic. This primitive frog had long hip and hind limb bones that indeed
were made for jumping and possessed a skeletal structure to absorb forces
resulting from landings. Prosalirus
is aptly named coming from the Latin word, ‘prosalire’ meaning ‘to leap
forward’, for it truly was the leap into modern day frogs.
During the early
Jurassic 190 mya, the tiny (1in/2.54cm) Vieraella
(Phonetics: Vee-eh-rye-ell-a) had all the characteristics of an extant frog
with a typical frog head possessing large eyes. Its hind limbs may have been
tiny as well, but were well equipped to conduct long jumps.
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| Vieraella |
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| Prosalirus |
Although it is now an
extinct species, Callobatrachus
(Phonetics: Call-lo-bah-trak-us) is one of the first known lissamphibian
showing up 125 mya in the Early Cretaceous. In every way it is like all extant
frogs with pedicellate teeth where the crown is separated from the root by
fibrous tissue.
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| Callobatrachus |
The term toad has no
taxonomic value, as a toad is simply a special type frog that hops more than jumps
and is usually encased in warty skin with a pair of parotoid glands located on
the sides of the head that manufacture the steroid lactone toxin bufotoxin.
Toads independently lost their teeth from extant frogs that also exhibit the
absence of teeth. Toads are rather late arrivals first appearing in the Upper
Paleocene 62 mya.
As easily witnessed
today, frogs have enormous skulls and hind limbs as opposed to the rest of its
skeletal structure. While the hind limbs are powerfully built, the skull had to
be lightweight while still being relatively massive. In order to lose weight,
frogs forego the fenestrae while reducing all the other skull bones to a bare
minimum in broadening the overall skull. This skull trending, along with
shortening of the tail can be traced back all the way through the
aforementioned extinct species.
Leading to Caecilian:
Lepospondyls came in
all physiological body forms that have been categorically put into five main
branches. The five groups recognized are: 1) Adelospondyli ~ (aquatic elongate bodies with short but well
developed limbs) arriving and dying out during the Early Carboniferous’
Mississippian period; 2) Aistopoda ~
(aquatic limbless snake-like bodies) first appearing in the Early
Carboniferous’ Mississippian then ending in the early Permian; 3) Lysorophia ~ (elongate aquatic bodies
with very small limbs) arising during the Late Carboniferous’ Pennsylvanian
becoming extinct by the Early Permian; 4) Nectridea
~ (aquatic urodele-like in appearance) arising during the Late Carboniferous’
Pennsylvanian while becoming extinct by the mid Permian; 5) Microsauria ~ (aquatic and fully
terrestrial forms possessing short tails, four small limbs and feet) first
appearing in the Late Carboniferous’ Pennsylvanian while becoming extinct by
the Early Permian.
As one can see,
lepospondyl species spans are only from the Carboniferous to the Early Permian,
but these amphibian-like animals are the ancestors that gave a direct rise to
caecilians and urodeles (living salamanders/newts). Perhaps, even caecilians
are a survivor in the direct line of lepospondyls that lost their legs due to a
fossorial lifestyle.
A firm conclusion is
still out on whether caecilians evolved from lepospondyls or temnospondyls with
the debate scooting more towards temnospondyl lineage. However, here they will
be treated as from the lepospondyl line and attempts will be added to reinforce
this path. But, also included will be the possible temnospondyl line.
The reason for the
continuing caecilian argument is that until caecilians had first made their
appearance, there is no clear cut ancestry in the fossil records just yet
leading to caecilians. Indeed, large morphological and topological gaps in the
caecilian fossil record owe to the ongoing debate.
To live underground,
adaptations had to be met such as a more cylindrical body plan, numerous
vertebrae, a reinforced skull or forearms for efficient tunneling and
hemoglobin uptake to extract more oxygen from poor oxygenated environments. In
a dark and narrow environment, other physiological adaptations to conserve
energy in dropping once important surface features would be the degenerative
loss of eyes and the drastic shortening of long tails and limbs.
No tail or a highly
shortened tail facilitates caecilian locomotion. In leading a strict fossorial
lifestyle, caecilians have developed a body hydrostatic mechanism for
burrowing. This kinematic mobile force is dependent on skin to vertebral
independence, where a longer tail would interfere with this type of mobility,
for performing this whole body and internal form of concertina locomotion, a
long tail would be a hindrance and add drag.
Lepospondyl microsaurs
and lysorophians had species that were already trending to a fossorial
lifestyle in osteological (skeletal)/physiological caecilian traits. It is
within one of these two orders that direct caecilian lineage is derived. This
may become a mute issue as Microsauria
is now considered paraphyletic which includes lysorophians. Since argument is convincing for either one, we will discuss both.
So first is the microsaurs proper followed afterwards by lysorophians.
Microsauria is paraphyletic in
being a crown base and along with all its suborders and their families, Aistopoda, Lysorophia and Nectridea are
nestled within the order. These three lepospondyls in clade format appear to
have arisen from the microsaur genus, Rhynchonkos
(Phonetics: Rin-chon-kos) which was very salamander-like.
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| Rhynchonkos |
All microsaurs had
short limbs and short tails and Rhynchonkos
was no exception, but it also possessed an elongate body with degenerate limbs
that was uncommon for other microsaurs.
In leaning as well
toward reptile physical characteristics, Rhynchonkos,
for about the only difference between the early true reptiles and numerous
microsaurs skeletal structure is that microsaurs have two side-by-side condyles
where reptiles have one. However Rhynchonkos
is only a very distant relative to reptiles. It can be argued though as
appearing to be the basal ancestor to the order, Gymnophiona (caecilians).
Rhynchonkos carried with it
reptilian features found in other groups such as Eocaecilia (E-o-say-see-le-ha) that both groups had borrowed from
an earlier linked lepospondyl ancestry. In other words, it was not convergent
evolution where both carried the similar traits due to independently evolving
them in adapting to analogous environments. We will get back to Eocaecilia shortly.
Whether in paraphyletic
(consisting of an ancestor, but not all of its ancestors) groupings, or
phylogenetic analyses, the microsaur clade, Recumbirostra
(Phonetics: Re-cum-bir-os-tra) appears to be the ancestral base to all
caecilians. Altenglanerpeton
(Phonetics: Alt-en-glan-er-pe-tawn) belonged to this clade 299 mya with a
temporal range at the very end of the Carboniferous and very beginning of the
Permian.
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| A Permian scene |
Thus far, there is only
one species to date of Alteglanerpeton
which is A. schroederi. If for sure
other fossil finds uncover another direct link to caecilians, it will be a very
close relative to Altenglanerpeton.
Altenglanerpeton fossil remains come
from the Carboniferous/Permian timeline 299 mya. It had all the ear markings of
transitioning to a caecilian body structure. In life, this animal sported a
very long slender body though with a shortened tail. The limbs were greatly reduced.
The triangular skull was robust with an upper labial snout overbite. The skull
also supported widely spaced eye sockets with the jugal bones extending well in
front of the socket orbits, while the body support consisted of ~ 30 spool
shaped vertebrae.
Altenglanerpeton possessed lungs, but
was not totally terrestrial or fossorial. It used its elongated body to
undulate while swimming and its triangular shaped skull to wedge into aquatic
bed sediment and debris; a precursor to tunneling. The degenerative limbs aided
in gaining access to tight squeezes. Its fossil in fact was found in lake
sediment, but this does not mean the animal did not venture onto land.
Both Batropetes and Pelodosotis belonged to families as sister taxons within the Recumbirostra clade, so therefore were
earlier relatives of Altenglanerpeton, Rynchonkos and thus eventually producing the family Eocaecilia originated in.
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| Pelodosotis |
There is an approximate
100 million year gap between Altenglanerpeton
and Eocaecilia, the first known
caecilian that shows up in the Jurassic and perhaps future fossil finds will
link the two due to their common morphologies.
Eocaecilia is essentially a
caecilian with vestigial tiny limbs. Fossil finds dating in the Jurassic from
199.6-175.6 mya shows characteristics that it also shared a few similar traits
with extinct microsaurs and extant salamanders. Although Eocaecilia was not totally fossorial it lived an insectivorous
lifestyle foraging under forest floor litter and debris.
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| Eocaecilia |
The Eocaecilia braincase analyzed from
computed tomography gave a more reliable phylogenetic indicator than simply
studying peripheral skull regions. In the tomography analysis, E. micropodia, the only species
representative of Eocaecilia, showed
that the animal possessed long anterolateral processes on the sphenethmoid (an
unpaired skull bone on the neurocranium), paired olfactory nerve foramina and
ossified nasal septum along with an ossified anterior wall of the sphenethmoid.
All these traits are now known to only exist in extant caecilians.
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| Eocaecilia's ventrum |
Caecilians, unique
among other animal groups have evolved a dual jaw closing system where the
upper maxillary bone pulls up on the lower jaw mandible much like a third order
lever system. This mouthing process is served by more developed posterior interhyoideus jaw muscles in
closing the jaw by pulling back then down on a process located just behind the
lower jaw hinge. These muscles are strong giving the animal a greatly
strengthened biting force.
This is a novel
function found only in caecilians, where before in caecilian ancestry jaw
components served more as a ventral constrictor. Only in the most primitive of
caecilians, the rhinatrematids is this lever system mouthing process poorly developed.
What led to this more
evolved caecilian component is the fully solid roofed skull in modern
caecilians to facilitate a fossorial life of burrowing and tunneling. The
skulls of caecilian ancestry all had a temporal fossa which became fully closed
in more recently derived caecilians.
Caecilian independent
evolvement of stegokrotaphic skull features, according to the most revered and
respected herpetologist specializing in caecilians, R.A. Nussbaum, directs
evidence to open temporal region lysorophid microsaurs as the more likely
direct ancestors to lissamphibian caecilians.
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| Caecilian stegokrotaphic skull features |
Lysorophians are
closely related to microsaurs; in fact just might be a microsaurid and if not,
most certainly an extension. However, they were highly specialized creatures
for their time existing from the Middle Pennsylvanian to the very Early
Permian. With very early specialization features such as reverse evolutionary
engineering in reducing limbs rather than extending them, fenestrate skulls
bearing short mandibles and sutured neural arch halves at the body’s midline,
their early appearance and exit makes this problematic in comparison to the
microsaur line in general.
 |
| Lysorophians Left: Brachydectes Right: Lysorophus |
Those who argue that caecilians arose from temnospondyls claim that the temnospondyl family Amphibamidae shared similar skull structure, dental arrangement and auditory structure. Phylogenetically, this can be pointed out, but the order Temnospondyli literally means cut vertebra because each vertebra is divided into several parts; there is no morphological evidence of caecilian backbones evolving from such a vertebral column.
Leading to Urodele:
Morphology would
suggest that microsaurs could have been directly ancestral to urodeles, but
with backbone structure exhibiting the transverse process as located on the
anterior end of the dorsal vertebrae such as in the microsaur, Cardiocephalus (Phonetics:
Car-di-o-ceph-ah-lus); they are not.
However, with both
nectridians and aistopods arising from microsaur lineage, microsaurs share an
indirect relationship to urodeles. Having the transverse process located near
the middle of the vertebrae, nectridians and aistopods share a unique common
feature with urodeles in which the bony projection is also located near the
middle of the backbone. Phylogenetically, it appears that urodeles (all modern
day salamanders) are the off-shoot branch of nectridians serving as the crown
ancestor with aistopods serving as the stem branch.
As in caecilians, there
is debate that salamanders derived directly from temnospondyls through the Batrachia group along with frogs. Recent
discoveries of earliest Late Jurassic-Middle Cretaceous urodele well preserved
fossils in China volcanic deposits provides evidence leaning towards the
divergence of the lissamphibian Cryptobranchus
from the lissamphibian Hynobiidae.
This diverging line can be argued as being within the temnospondyl clade. Hynobiids, found primarily in Asia have a
biphasic life cycle with aquatic gilled larvae and aquatic or terrestrial adult
forms.
 |
| Which lissamphibian route taken in the cladogram? |
Also, hynobiids
fertilize externally, have a greater degree of ossification, possess an angular
bone in the jaw and carry a rather large number of micro-chromosomes. These traits give rise to consideration that
hynobiids are the basal common ancestor to all other urodeles. This arrangement
would make the order Nectridea a
polyphyletic taxon rendering the order as not a true clade in evolutionary
grouping. Most researchers agree with this route.
But not bending under that pressure, I’m presenting my thoughts on
urodeles as evolving through lepospondyl nectrideans. Amphibian evolution is
very complex; it is not cut and dry and it may be later proven that modern day
salamanders had many
basal evolutionary marks coming from numerous ordered species lines. This is
what makes science shine as it is flexible enough to correct a wrong hypothesis
no matter how long it was supported.
In going the nectridean
path, it is with full intention in my following paragraphs, an attempt in
shedding light on this debate.
 |
| Nectridean trunk and tail vertebrae |
Nectridea was a diverse small in size group
of animals very similar to today’s terrestrial salamanders and aquatic newt
forms. Most were 2cm/2.36in-15cm/5.91in in length, although Diplocaulidae (Keraterpetontidae) family members did reach 100cm/39.37in in total
length.
 |
| Pennsylavanian scene Cacops |
There was an abundance
of nectrideans during the swampy forests of the Middle Carboniferous to Middle
Permian periods 318-270 mya. It was a diverse group coming up with the bizarre
boomerang shaped head in the genus
Diplocaulus, but all shared long compressed tails (that is flattened from
side-side), well developed interdigitate spinal hind limbs with a set of five hind
limb toes and four toes on each forelimb. Nectridean fossils also exhibited
symmetrical neural and hemal spines, complex trunk vertebrae articulations and
arches constructing the vertebra.
The lepospondyl clade
leading to microsaur nectridians had lost their characteristic labyrinthodont
teeth infolding patterns of dentin and enamel early opting for paired palatal
small fangs. These small fangs could have been precursors in easily evolving
into present day urodele vomerine teeth.
 |
| The Middle Pennsylvanian |
During the Middle
Pennsylvanian, around 308 mya in the Carboniferous Period lived Utaherpeton, (Phonetics: U-tah-erp-uh-tawn)
who showed characteristic microsaur features such as small cervical ribs and
hind limbs larger than forelimbs but with the hind feet being unusually larger
than the rest of the hind limbs. It was also salamander-like in appearance. Utaherpeton’s body length was no more
than 4cm/1.6in.
 |
| Utahrepeton |
As far as lepospondyls
go, completions of phylogenetic and morphological analyses show that Utaherpeton is the most basal member of a
separate clade including all lepospondyl members. In addition, with the
prefrontal extending to the premaxilla more at the front of the skull than found
in other microsaurs, this skull bone configuration leads to hints of indirect
nectridean evolvement.
A bit later after Utaherpeton’s appearance, during the Late Carboniferous around 304 mya, Hyloplesion (Phonetics:
Hy-lo-pleas-e-un) first made its presence. This microsaurid nectridean was also
salamander-like in body form with a total length of 7.7cm/3.03in.
 |
| Utaherpeton |
The scapulocoracoid is
the pectoral girdle where the scapula links the humerus to the body and the
clavicle to the sternum with the beak-shaped coracoid as a paired bone sharing
in the overall assemblage. Results from geometric morphometrics show that
overall cranial and postcranial growth was isometric primarily and in comparing
allometric data to all other Paleozoic tetrapodal taxa where isometric growth
instead of metamorphical is an ancestral feature. This shows that Hyloplesion, in morphological change did
not go through larval to adult metamorphosis as most modern day salamanders do
which includes skeletal reorganization. But as metamorphosis is not evident in
any early day amphibian group, it only bears out that metamorphosis is a
derived mode of development found only in extant salamanders and their closest
caudate relatives.
 |
| Hyloplesion |
However, where all temnospondyls
never possessed this type pectoral girdle arrangement and while other
lepospondyl groups were losing their scapulocoracoid, Hyloplesion was enlarging it, which is present in most all modern
day vertebrates except for therians; [marsupial (metatheria) and placental
(eutheria) mammals.] Certainly the scapulocoracoid is found in all extant
urodeles.
 |
| Hyloplesion skeletal |
Crossotelos (Phonetics:
Cros-so-tel-os) is a true nectridean and most likely derived in the early
Permian around 295-292 mya. This animal, along with fine abdominal ribs and
laterally compressed body, also possessed intervertebral nerve openings that
are the foundational layout of current urodele spinal nerve systems throughout
the vertebral columns. Various intervertebral nerves of extant salamanders have
evolved from within the urodele group from the fossil associated primitive
intervertebral nerve passageways of cryptobranchs and hynobiids to the more
defined three spinal nerve exits through the posterior foramina in more modern
salamanders such as Ambystomatoidea
(Phonetics: Am-be-sto-ma-toi-de-ah).
 |
| Chunerpeton |
Skull fossils of Cricotillus brachydens and Trimerorhachis leptorhynchus were first
incorrectly labelled as temnospondyls which had urodele characteristics. Now
though, these two species have been confirmed and properly assigned to the nectridean
group as Crossotelos juveniles.
 |
| Chunerpeton fossil |
Crossotelos, a urocordylid species
were pelagic piscivores being almost wholly aquatic. The creature still
retained an abundance of abdomen scales covering its belly. Whether directly or
indirectly through close relatives, Crossotelos
most likely gave rise to Chunerpeton
(Phonetics: Chu-ner-pe-tawn) 172 mya in the Middle Jurassic. It is the earliest
known crown caudate and would be listed as a urodele if it had not gone
extinct. Chunerpeton in all aspects
considered was a member of the suborder, Cryptobranchoidea
(Phonetics: Cryp-toe-brank-coid-dee-ah) and being neotenic retained several
juvenile features into adulthood such as external gills.
In the photo below, an
unidentified larval cryptobranchoid fossil defines soft tissue and a belly full
of ingested conchostracans, an extant crustracean first appearing during the Devonian.
 |
| Fine detail of a cryptobranchoid larva's soft tissue |
During the Late
Jurassic the salamander family, Karauridae
appeared 150-147 mya with two genera, Karaurus
(Phonetics: Kah-row-rus) and Kokartus
(Phonetics: Ko-kar-tus). Karaurus retained
a lacrimal bone found in most extant salamandrids and an angular bone in the
mandible which all hynobiids and cryptobranchiods still possess. Although an
aquatic neotenic salamander with retained external gills, anatomically Karaurus fossil remains resemble urodele
terrestrial salamanders.
 |
| Karaurus |
 |
| Karaurus fossil |
Please recall that
caudates are basically salamanders that have become extinct while urodeles are
salamanders that are extant. The first caudate that branched from crypotbranchs
arrived during the Mesozoic Era in the Middle-Late Jurassic 164-146 mya. This
species called, L. daohugouensis
(Phonetics: Dowel-hu-gal-en-sis) is from the newly formed Liaoxitriton (Phonetics: Le-ow-ip-te-rus) genus. Although not quite a member of the Hynobiidae (Phonetics: Hy-no-be-ah-day)
family, in full anatomy it was trending that way and most likely is the
hynobiid crown group from which most all other modern day salamanders are tied
to.
Daohugouensis still retained some of
the cryptobranch vestiges such as anterolaterally extended VTR’s (vomerine
tooth rows), rostral morphology and a wide/round rostrum. But it also possessed
a few trending hynobiid traits.
Although orientation of
VTR’s differs between daohugouensis and hynobiids, teeth are of the same
arrangement, while the unicapitate ribs expanded proximally in daohugoensis and
early hynobiids. Also, vertebrae transverse processes are around half the
length of the centra (the vertebra component that supports the arches ~ centrum
as singular).
 |
| L. daohugouensis |
Daohugouensis may not
be considered a hynobiid familial base unit, but its sister taxon, L. zhongjiani certainly is one of the
first true hynobiids. Arising in the Early Cretaceous just after Daohugoensis 145-124 mya, Zhongjiani (Phonetics: Zun-gee-un-ee),
aside from extant cryptobranchs is the crown base for all modern day urodele
families.
Through hynobiid
speciation, other hynobiid groups evolved that also speciated into the current
family groups of salamanders. Zhongjiani
possessed gill rakers into adulthood. Neoteny does not show up in all modern
day salamanders, but for the ones that independently developed it like the
axolotl, are actually reverting back to expressing the Zhongjiani gene they’ve inherited, as markers for gene expression
had become passive or inactivated in air breathing salamanders.
 |
| L. zhongjiani |
Views expressed here
under amphibian evolution is mostly my own and certainly should not be followed
as gospel. When dealing with 300 million year old animals that could be
pedomorphic (retention of aquatic larval form into adult life) or peramorphic
(juvenile condition modified from ancestral traits where further adult form
substantially modifies juvenile condition), cladistics may virtually become too
impossible a task. So there you are, working with a pedomorphic lepospondyl
that in all fossilized appearances looks much the same as a peramorphic
temnospondyl.
The next excerpt will
be diapsids with the path leading to crocodilians and turtles.
 |
| Frog evolutionary trends |
 |
| A creationist view of frog evolution |
In
Evolving Form,
BJA
06/19/2015
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