Proteus, the blind cave salamander, enters the LRT

Losing its maxilla
(Fig. 1) did not stop this taxon from sporting a lot of other teeth in the palatine, ectopterygoid and maybe the vomer, even though only the premaxillary teeth line up with dentary teeth.

Figure 1. Skull of Proteus the white olm. Colors added. Note the lack of a lacrimal and maxilla.

 Proteus the white olm
(Figs. 1, 2), is a blind cave salamander with a long torso, tiny limbs and external gills (Fig. 2).

Figure 2. Skeleton of Proteus, the white olm.

Proteus anguinus
(Laurenti 1768) nests with Necturus, the mudpuppy. The lacrimal and maxilla are absent. The postorbital and postfrontal are stretched out. External gills enable Proteus to remain underwater. Apparently the dorsal portion of the vertebral column is very short (about 5 vertebrae), with the majority comprised of lumbar vertebrae (without dorsal ribs).

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References
Laurenti JN 1768. Synopsin Reptilium. J.T. de Trattnern, Viennae, pp. 35–36.

wiki/Olm

Triassurus: a tiny Triassic salamander?

Summary for those just skimming:
Including more taxa and using DGS to gather more data from fossils upsets academic results again.

Shoch et al. 2020 discuss the origin of salamanders
by directing their attention to a tiny juvenile Triassic tetrapod, Triassurus (PIN-2584/10, skull length 3.8 mm; Ivakhnenko 1978; Figs. 1–3). The discovery of a larger referred specimen (FG 596/V/20), provided Schoch et al. a reason to reexamine the type.

Science first learned about tiny Triassurus
from Ivakhnenko 1978 (here reproduced in a book, Fig. 1). Not much detail back then.

Figure 1. Illustration and description Triassurus (Ivakhnenko 1978). Little more than the outline of the skull was illustrated back then.

The Schoch et al. tracing of the tiny holotype
(Fig. 2) likewise offers few details. DGS colors (Fig. 2) provide more and different details.

Figure 2. Triassurus in situ with tracing from Schoch et al. 2020. DGS color tracing added here. See figure 3 for a reconstruction.

Pushing those DGS details around
to create a reconstruction (Fig. 3) helps one understand the anatomy of Triassurus, enough to score it.

Figure 3. The type specimen of Triassurus in situ and reconstructed.

Scoring the Middle Triassic Triassurus type
nests it with Early Permian Gerrobatrachus (Fig. 4), one node away from extant salamanders in the large reptile tree (LRT, 1690+ taxa; subset Fig. 5). So, if Triassic Triassurus is the earliest known salamander (as Schoch et al report), then Early Permian Gerobatrachus is one, too. And it is tens of millions of year older. If Gerobatrachus is not a salamander, then both are not salamanders.

Figure 4. Gerrobatrachus adult.

The LRT tree topology
is distinctly different than the cladogram published by Shoch et al. 2020 (Fig. 6).

Figure 5. Subset of the LRT focusing on basal tetrapods including Triassurus, type and referred specimen FG 596 V 20.

Unfortunately,
the cladogram employed by Schoch et al. 2020 (Fig. 6) needs more taxa. Currently it nests Proterogyrinus as the outgroup taxon. That creates problems. Relative to the LRT (Fig. 5) one branch of the Schoch et al. cladogram is inverted such that the basalmost tetrapods, Siderops and Gerrothorax, nest as highly derived terminal taxa. The other branch with caecilians, frogs and salamanders is not inverted relative to the LRT, but in the LRT caecilians do not nest with frogs and salamanders. Caecilians nest with microsaurs in the LRT.

Figure 6. Cladogram from Schoch et al. 2020 nests both specimens of Triassurus close to salamanders. Colors match colors in figure 5.

Schoch et al. 2020 reported on Triassicus,
“to reconstruct crucial steps in the evolution of the salamander body plan, sharing numerous features with ancient amphibians, the temnospondyls. These finds push back the rock record of salamanders by 60 to 74 Ma and at the same time bridge the wide anatomic gap among salamanders, frogs, and temnospondyls.”

In the LRT the salamander body plan goes back to the Early Permian, at least.

From the abstract:
“The origin of extant amphibians remains largely obscure, with only a few early Mesozoic stem taxa known, as opposed to a much better fossil record from the mid-Jurassic on.”

In the LRT the origin of extant amphibians can be traced in detail over several dozen taxa (Fig. 5) back to Cambrian chordates.

Figure 7. Cladogram from Schoch et al. 2020. They insert Eocaecilia here derived from Doleserpeton. The LRT nests Eocaecilia with microsaurs. Note how the morphology does not fit here. Where is Apteon in this cladogram?

From the abstract:
“Yet the most ancient stem-salamanders, known from mid-Jurassic rocks, shed little light on the origin of the clade.Here we report a new specimen of Triassurus sixtelae, a hitherto enigmatic tetrapod from the Middle/Late Triassic of Kyrgyzstan, which we identify as the geologically oldest stem-group salamander.”

“The new, second specimen is derived from the same beds as the holotype, the Madygen Formation of southwestern Kyrgyzstan. It reveals a range of salamander characters in this taxon, pushing back the rock record of urodeles by at least 60 to 74 Ma (Carnian–Bathonian). In addition, this stem-salamander shares plesiomorphic characters with temnospondyls, especially branchiosaurids and amphibamiforms.”

FIgure 8. The FG 596 V20 specimen that Schoch et al. referred to Triassurus does not nest with Triassurus in the LRT. See Figure 9 for reconstruction.

Speaking of that second specimen…
a reconstruction of the narrow-snouted FG 596 V20 specimen (Fig. 8) does not look like the wide-mouth type of Triassurus. In the LRT the second larger FG 596 V20 specimen nests with the CGH129 specimen of the legless microsaur, Phlegethontia (Fig. 9) far from Triassurus and other salamanders.

Figure 9. Reconstruction of the specimen referred to Triassurus, which does not nest with Triassurus in the LRT.

What Schoch et al. identified as a large horizontal quadrate (q)
in the FG 596 V20 specimenn(Fig. 8) is re-identified here as a large humerus, larger than the ones they identified. The forelimb goes into the left mouth. The left maxilla is displaced back to the occiput.

Figure 10. Phlegethontia longissima skull (CGH 129) has relatively large temporal plates, a wide flat cranium and a long pointed rostrum.

In conclusion, the tiny Triassurus type
nests close to salamanders, but closer to Early Permian Gerobatrachus (Fig. 4). The larger referred FG 596 V20 specimen with legs (Fig. 9) nests with legless Late Carboniferous Phelgethontia (Fig. 10), far from salamanders. Adding taxa and getting deeper into the details using the DGS method upsets the simpler and inaccurate Schoch et al. tracings, tree topology and conclusions.

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References
Ivakhnenko M 1978. Tailed amphibians from the Triassic and Jurassic of Middle Asia. Paleontological Journal 1978(3):84-89.
Schoch RR et al. 2020. A Triassic stem-salamander from Kyrgyzstan and the origin of salamanders, Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.2001424117

https://phys.org/news/2020-05-triassurus-sixtelae-fossil-kyrgyzstan-oldest.html

Newt pre-pollux phase shift?

Wagner and Chiu 2001
printed an X-ray image of a newt (genus: Triturus) foot with six toes (Fig. 1). They call the new one a pre-pollux.

Figure 1. Left to right: Original illustration from Wagner and Chiu 2001. Cleaned up and labeled. Placed on top of an in vivo pes with a phase shift movement of the metatarsals. Some toes are curled, so the graphic bones extend beyond the curl.I have been calling a similar medial manus digit
‘digit zero‘ for the last three years.

The problem I had with the Wagner and Chiu newt
was matching toes 1–5 to the toes in a photo of another specimen. It was impossible to do so while retaining the original connections to the distal tarsals (Fig. 1). Instead the toes had to be shifted medially with regard to the Wagner and Chiu tarsal images.

In other words, when digit zero is present
digits 4 and 5 arise from the lateral distal carpal and digit 1 arises from distal carpal 2.

When digit zero is absent
digits 4 and 5 each have their own distal carpal and digit 1 arises from distal carpal 1.

Still thinking about this one.
Not sure what the ramifications are with regard to newts and other tetrapods. Where is the error? Perhaps there is no error. It is what it is. There was a phase shift.

Earlier we looked at digit zero on the manus
of the theropod Limusaurus (which opened up the whole phase shift question in theropods), and the screamer, Chauna (which could have done the same with its ‘spike’, but never did).

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References
Wagner GP and Chiu C-H 2001. The tetrapod limb: a hypothesis on its origin. Journal of Experimental Zoology (Mol Dev Evol) 291:226–240.

The largest amphibians of all time

Yesterday we looked at Siderops, a big plagiosaur and peeked at Koolasuchus, a giant plagiosaur. That makes today a good today to review the largest amphibians of all time (Fig. 1, click to enlarge).

Figure 1. Click to enlarge. The largest amphibians of all time include Mastodonsaurus, Prionosuchus, Koolasuchus, Siderops, Crassigyrinus and the extant Andrias, the giant Chinese salamander.

Figure 2. The cover of Giants, the book that launched my adult interest in dinosaurs, pterosaurs and everything inbetween.

Back in 1989
and eager to locate the largest amphibian of all time, I added Prionosuchus (Fig. 1) to the popular natural history book, “Giants of Land, Sea and Air – Past and Present” (Fig. 2). Since the largest Prionosuchus is only known from fragments and slivers, much had to be restored using phylogenetic bracketing.

Now
Mastodonsaurus is probably just as long, but much bulkier, making it the largest amphibian of all time. It was the size of a Hippopotamus (Fig. 3).

The largest living amphibian is Andrias, the Chinese salamander.

Mastodonsaurus jaegeri (Jaeger 1828; Schoch 1999; Middle Triassic; skull length 1.2m; overall length 6m) is the largest lepospondyl in the LRT. Traditionally it was considered a temnospondyl, but if so that would make all amniotes temnospondyls, too, which was not the intention of the definition. Anterior dentary tusks fit through new skull openings (in red above) anterior to the nares. Intercostal plates overlapped succeeding ribs. Mastodonsaurusinhabited swampy ponds.

Andrias davidianus (Blanchard 1871; 1.8m in length; extant) the Chinese giant salamander, is a sister to Rana, the bullfrog and derived from a sister to Gerobatrachus.

Figure 3. Living hippopotamus, an amphibious mammal related to Mesonyx.

References
Blanchard É 1871.Note sur une nouvelle Salamandre gigantesque (Sieboldia Davidiana Blanch.) de la Chine occidentale. Comptes Rendus Hebdomadaires des Séances de l’Académie des Sciences. Paris 73: 79.
Jaeger GF 1828. Über die fossile Reptilien, welche in Württemberg aufgefunden worden sind. 48 pp., 6 pls.; Stuttgart (Metzler).
Schoch RR 1999. Comparative osteology of Mastodonsaurus giganteus (Jaeger, 1828) from the Middle Triassic (Lettenkeuper: Longobardian) of Germany (Baden-Württemberg, Bayern, Thüringen). Stuttgarter Beiträge zur Naturkunde Serie B. 278: 1–175. PDF

wiki/Mastodonsaurus
wiki/Andrias

SVP 2018: Seeking the origin of living amphibians

Danto et al. 2018 reports,
“Despite increasing knowledge about the fossil record of lissamphibians (frogs,salamanders, and caecilians), their origin is still unresolved and different origins within Paleozoic early tetrapods are proposed.”

By contrast
the large reptile tree (LRT, 1313 taxa; subset Fig. 1) confidently recovers a monophyletic and completely resolved lissamphibia that also includes microsaurs (Fig. 1) and other non-traditional members. Utegina is close to the ancestry of frogs + salamanders + the more distantly related caecilians.

Figure 1. Subset of the LRT focusing on basal tetrapods, colorized according to chronology. Note the wide dispersal of Early Carboniferous taxa, suggesting a Late Devonian radiation as yet largely undiscovered. Utegenia is close to the ancestry of all living amphibians (in white).

The Danto team sought the answer
to their enigma not in a phylogenetic analysis, but in a study of developing salamanders. They report, “In this study, we sought to investigate if the ossification sequence of neural arches and centra and the mode of centrum formation in extinct and extant forms could be indicative of phylogenetic relationships.  Here we demonstrate that the mode of centrum formation is highly variable in early tetrapods and lissamphibians and cannot be used to determine the origin of lissamphibians within early tetrapods.”

References
Danto M et al. 2018. The implication of the vertebral development on the origin of lissamphibians. SVP abstract

 

 

 

The ‘armadillo’ ‘frog’: Dissorophus

Figure 1. Images from Cope 1896 of the armored dissorophid, Dissorophus (= Otocoelus). At first I did not see the limbs preserved with the armor and skull. Did you?

Dissorophus multicinctus (Cope 1895; Late Carboniferous, 280 mya; 13 cm skull length) had a large head and short trunk, but more extensive dermal and sub dermal ossifications than the related Cacops, a basal lepospondyl in the large reptile tree (LRT, 1166 taxa). This terrestrial basal tetrapod was originally considered a “bratrachian armadillo” with its double-layer armor. Distinct from most basal tetrapods, (but like members of the sister clade Reptilia!) the limbs were quite large. Together with the armor, and with comparisons to sister taxa, Dissorophus was fully terrestrial

What were its tadpoles/juveniles like?
I don’t think we’ve found any. Let me know if any are known.

Wikipedia reports:
Dissorophus was a temnospondyl. The online cladogram of Dissorophus relatives from Schoch 2010 lists all lepospondyls in the LRT. Temnospondyls, like Metaposaurus, split off earlier in the LRT.

Figure 1. Cacops and its sisters.

References
Cope ED 1895. A batrachian armadillo. The American Naturalist 29:998
Cope ED 1896. The Ancestry of the Testudinata. The American Naturalist 30(353):398-400
Cope ED 1896. Second contribution to the history of the Cotylosauria. Proceedings of the American Philosophical Society 35(151):122-139
DeMar RE 1966. Longiscitula houghae, a new genus of dissorophid amphibian from the Permian of Texas. Fieldiana: Geology 16:45-53
Schoch RR 2013. The evolution of major temnospondyl clades: an inclusive phylogenetic analysis. Journal of Systematic Palaeontology [R. Butler/R. Butler]
Schoch RR and Milner AR. 2014. Handbook of Paleoherpetology Part 3A2 Temnospondyli I.

https://en.wikipedia.org/wiki/Dissorophus

Synonyms:
Dissorophus articulatus Cope 1896 (no. 345457)
Longiscitula houghae DeMar 1966 (no. 345456)
Otocoelus mimeticus Cope 1896 (no. 138240)
Otocoelus testudineus Cope 1896 (no. 138239)

Ichthyostega and Acanthostega: secondarily more aquatic

More heresy here
as the large reptile tree (LRT, 1036 taxa) flips the traditional order of fins-to-feet upside down. Traditionally the late Devonian Ichthyostega and Acanthostega, bridge the gap between lobe-fin sarcopterygians, like Osteolepis.

In the LRT
Acanthostega, ‘the fish with limbs’, nests at a more derived node than its precursor, the more fully limbed, Ossinodus (Fig. 1). Evidently neotony, the retention of juvenile traits into adulthood, was the driving force behind the derived appearance of Acanthostega, with its smaller size, stunted limbs, smaller skull, longer more flexible torso and longer fin tail.

Figure 1. Ossinodus is the more primitive taxon in the LRT compared to the smaller Acanthostega, essentially the neotenous ‘tadpole’ of the two.

Likewise
Ichthyostega is more derived than both fully-limbed Ossinodus and Pederpes, which had five toes. As in Acanthostega, the return to water added digits to the pes of Ichthyostega. In both taxa the interosseus space between the tibia and fibula filled in to produce a less flexible crus.

Figure 2. Long-limbed Ossinodus and Pederpes were more primitive than the more aquatic Icthyostega.

So, Acanthostega and Ichthyostega were not STEM tetrapods.
Instead, they were both firmly nested within the clade Tetrapoda. Ossinodus lies at the base of the Tetrapoda. The proximal outgroups are similarly flattened Panderichthys and Tiktaalik. The extra digits displayed by Acanthostega and Ichthyostega may or may not tell us what happened in the transition from fins to feet. We need to find a derived Tiktaalik with fingers and toes.

Figure 3. Tiktaalik specimens compared to Ossinodus.

In cases like these
it’s good to remember that ontogeny recapitulates phylogeny. Today and generally young amphibians are more fish-like (with gills and fins) than older amphibians.

It’s also good to remember
that the return to the water happened many times in the evolution of tetrapods. There’s nothing that strange about it. Also the first Devonian footprints precede the Late Devonian by tens of millions of years.

Figure 4. From the NY Times, the traditional view of tetrapod origins. 

Phylogenetic analysis teaches us things
you can’t see just by looking at the bones of an individual specimen. A cladogram is a powerful tool. The LRT is the basis for many of the heretical claims made here. You don’t have to trust these results. Anyone can duplicate this experiment to find out for themselves. Taxon exclusion is still the number one problem that is largely solved by the LRT.

You might remember
earlier the cylindrical and very fish-like Colosteus and Pholidogaster convergently produced limbs independently of flattened Ossinodus, here the most primitive taxon with limbs that are retained by every living tetrapod. By contrast, the Colosteus/Pholidogaster experiment did not survive into the Permian.

References
Ahlberg PE, Clack JA and Blom H 2005. The axial skeleton of the Devonian trtrapod Ichthyostega. Nature 437(1): 137-140.
Clack JA 2002. Gaining Ground: The origin and evolution of tetrapods. Indiana University Press.
Clack JA 2002. An early tetrapod from ‘Romer’s Gap’. Nature. 418 (6893): 72–76. doi:10.1038/nature00824
Clack JA 2006. The emergence of early tetrapods. Palaeogeography Palaeoclimatology Palaeoecology. 232: 167–189.
Jarvik E 1952. On the fish-like tail in the ichtyhyostegid stegocephalians. Meddelelser om Grønland 114: 1–90.
Jarvik E 1996. The Devonian tetrapod Ichthyostega. Fossils and Strata. 40:1-213.
Säve-Söderbergh G 1932. Preliminary notes on Devonian stegocephalians from East Greenland. Meddelelser øm Grönland 94: 1-211.
Warren A and Turner S 2004. The first stem tetrapod from the Lower Carboniferous of Gondwana. Palaeontology 47(1):151-184.
Warren A 2007. New data on Ossinodus pueri, a stem tetrapod from the Early Carboniferous of Australia. Journal of Vertebrate Paleontology 27(4):850-862.

wiki/Ichthyostega
wiki/Acanthostega
wiki/Ossinodus
wiki/Pederpes

Lethiscus: oldest of the tetrapod crown group?

Figure 1. Lethiscus stocki skull, drawing from Pardo et al. 2017 and colorized here. Note the loss of the postfrontal and the large orbit. Pardo et al. nest this taxon between Acanthostega and Pederpes in figure 3. There is very little that is plesiomorphic about this long-bodied legless or virtually legless taxon. Thus it should nest as a derived taxon, not a basal plesiomorphic one.

Pardo et al. 2017
bring us new CT scan data on Lethiscus stocki (Wellstead 1982; Viséan, Early Carboniferous, 340 mya) a snake-like basal tetrapod related to Ophiderpeton (Fig. 2) in the large reptile tree (LRT, 1018 taxa), but with larger orbits.

Figure 2. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Lethiscus is indeed very old (Middle Viséan)
but several reptiles are almost as old and Tulerpeton, a basal amniote, comes from the even older Late Devonian. So the radiation of small burrowing and walking tetrapods from shallow water waders must have occurred even earlier and Tulerpeton is actually the oldest crown tetrapod.

Figure 3. Pardo et al. cladogram nesting Lethiscus between vertebrates with fins and vertebrates with fingers. They also nest microsaurs as amniotes (reptiles), resurrecting an old idea not supported in the LRT. Actually not much of this topology is supported by the LRT.

Pardo et al. nested Lethicus
between Acanthostega (Fig. 4) and Pederpes (Fig. 3) using a matrix that was heavily weighted toward brain case traits. Ophiderpeton and Oestocephalus (Fig. 2) were not included in their taxon list, though the clade is mentioned in the text: “Overall, the skull morphology demonstrates underlying similarities with the morphologies of both phlegethontiid and oestocephalid aïstopods of the Carboniferous and Permian periods.” So I’m concerned here about taxon exclusion. No other basal tetrapods share a lateral temporal fenestra or share more cranial traits than do Lethiscus, Ophiderpeton, Oestocephalus and Rileymillerus. All bones are identified here as they are in Pardo et al. so bone ID is not at issue. I can’t comment on the Pardo team’s braincase traits because so few are examined in the LRT. Dr. Pardo said they chose taxa in which the brain case traits were well known and excluded others.

Figure 4. Acanthostega is basal to Lethiscus in the Partdo et al. tree.

Pardo et al. considered
the barely perceptible notch between the tabular and squamosal in Lethiscus (Fig. 1) to be a “spiracular notch” despite its tiny size. I think they were reaching beyond reason in that regard. They also note: “The supratemporal bone is an elongate structure that forms most of the dorsal margin of the temporal fenestra, and is prevented from contacting the posterior process of the postorbital bone by a lateral flange of the parietal bone.” The only other taxon in the LRT that shares this morphology is Oestocephalus, Together they nest within the Lepospondyli (Fig. 3) in the LRT. I think it is inexcusable that Pardo et al. excluded  Ophiderpeton and Oestocephalus. 

Figure 4. Subset of the LRT with the addition of Lethiscus as a sister to Oestocephalus, far from the transition between fins and feet. Here the microsaurs are not derived from basal reptiles

Summarizing,
Pardo et al. report, “The braincase and its dermal investing bones [of Lethiscus] are strongly indicative of a very basal position among stem tetrapods.”  and “The aïstopod braincase was organized in a manner distinct from those of other lepospondyls but consistent with that seen in Devonian stem tetrapods.” It should also be noted that the skull, body and limbs were likewise distinct from those of other lepospondyls, yet they still nest with them in the LRT because no other included taxa (1018) share more traits. ‘Distinct’ doesn’t really cut it, in scientific terms. As I mentioned in an email to Dr. Pardo, it would have been valuable to show whatever bone in Lethiscus compared to its counterpart in Acanthostega and Oestocephalus if they really wanted to drive home a point. As it is, we casual to semi-professional readers are left guessing.

Pardo et al. references the clade Recumbirostra.
Wikipedia lists a number of microsaurs in this clade with Microbrachis at its base, all within the order Microsauria within the subclass Leposondyli. Pardo et al. report, “Recumbirostrans and lysorophians are found to be amniotes, sister taxa to captorhinids and diapsids.” The LRT does not support this nesting. Pardo et al. also report, “This result is consistent with early understandings of microsaur relationships and also reflects historical difficulties in differentiating between recumbirostrans and early eureptiles.” Yes, but the later studies do not support that relationship. Those early understandings were shown to be misunderstandings that have been invalidated in the LRT and elsewhere, but now resurrected by Pardo et al.

Ophiderpeton granulosum (Wright and Huxley 1871; Early Carboniferous–Early Permian, 345-295mya; 70cm+ length; Fig. 2, dorsal view)

Oestocephalus amphiuminus (Cope 1868; Fig. 2,  lateral views) is known from tiny immature and larger mature specimens.

Figure 5. A series of Phlegethontia skulls showing progressive lengthening of the premaxilla and other changes.

A side note:
The recent addition of several basal tetrapod taxa has shifted the two Phlegethontia taxa (Fig.5) away from Colosteus to nest with Lethiscus and Oestocephalus, their traditional aistopod relatives. That also removes an odd-bedfellow, tiny, slender taxon from a list of large robust stem tetrapods.

References
Pardo JD,Szostakiwskyj M, Ahlberg PE and Anderson JS 2017. Hidden morphological diversity among early tetrapods. Nature (advance online publication) doi:10.1038/nature22966
Wellstead CF 1982. A Lower Carboniferous aïstopod amphibian from Scotland. Palaeontology. 25: 193–208.
Wright EPand Huxley TH 1871. On a Collection of Fossil Vertebrata, from the Jarrow Colliery, County of Kilkenny, Ireland. Transactions of the Royal Irish Academy 24:351-370

wiki/Acherontiscus
wiki/Adelospondylus
wiki/Adelogyrinus
wiki/Dolichopareias
wiki/Ophiderpeton
wiki/Oestocephalus
wiki/Rileymillerus
wiki/Acherontiscus

Apateon and the origin of salamanders + frogs

Figure 1. Apateon overall and the skull in palatal and dorsal views. This taxon nests between Doleserpeton and Gerobatrachus in the LRT.

Apateon pedestris (von Meyer 1844, Early Permian, 295mya; 12 cm in length) was long considered a temnospondyl in the family Branchiosauridae. Here Apateon nests between Doleserpeton and Gerobatrachus in the lepospondyl lineage of frogs, like Rana and salamanders like Andrias.

Resembling a small salamander with a long, laterally flattened tail, Apateon had a shorter rostrum and large orbits than Doleserpeton. The pineal opening was larger. The ilium was more erect. The pubis was missing. The ectopterygoid did not contact the maxilla and the palatine did so only with a narrow process. At present, no other taxa in the LRT (978 taxa) do this.

Small scales covered the body. Three pairs of external gills were present for underwater respiration. Many species are known, as well as a good ontogenetic series.

Anderson 2008 reported, 
“Branchiosaurs [including Apateon] are closely related to amphibamids, if not included in the latter group, and have been suggested to be closely related to salamanders because of shared similarities in the sequence of cranial ossification.”

“New transitional fossils like the stem batrachian Gerobatrachus have filled in the morphological gap between amphibamid temnospondyls and the earliest frogs and salamanders, and this portion of the lissamphibian origins question appears very well supported.”

The LRT recovers
Amphibamus much closer to the base of the lepospondyls, about 5 nodes distant from Apateon. Of course, neither are closely associated with temnospondyls in the LRT, despite the open palate, otic notch and other convergent traits.

Neotony
The apparent lack of gill-less adults among all of the apparent larval gilled specimens of Apateon was a cause of consternation for awhile. The new largest specimen (Frobisch and Schoch 2009) appears to indicate an adult specimen. It had partially interdigitating and tight sutures of the skull roof, a high degree of ossification and differentiation of the postcranium as compared to smaller larval specimens. Uncinate processes indicate that this specimen represents an adult. However, it lacks ossifications of the exoccipitals and quadrates, intercentra, and the coracoid as seen in metamorphosed specimens. Frobisch and Schoch conclude, “The anatomical evidence at hand clearly indicates that both life history strategies, metamorphosis and neoteny, were established in Paleozoic branchiosaurids.”

References
Anderson JS 2008. Focal Reviews: The Origin(s) of Modern Amphibians. Eovlutionary Biology 35:231-247.
Anderson JS et al. 2008.  A stem batrachian from the Early Permian of Texas
and the origin of frogs and salamanders. Nature 453 (7194): 515–518.
Frobisch N and Schoch RR 2009. The largest specimen of Apateon and the life history pathway of neotony in the Paleozoic temnospondyl family Branchiosauridae. Fossil Record 12(1):83-90.
von Meyer H 1844. Briefliche Mittheilung an Prof. Bronn gerichtet. Neues Jahrbuch für Geognosie, Geologie und Petrefakten-Kunde 1844: 329-340.

wiki/Gerobatrachus
wiki/Apateon

Diplovertebron and amphibian finger loss patterns

Updated June 13, 2017 with the fact that Diplovertebron is the same specimen I earlier illustrated as Gephyrostegus watsoni. And the Watson 1926 version of Diplovertebron (Fig. 1) was so inaccurately drawn (by freehand) that the data nested is apart from the DGS tracing. Hence this post had deadly errors now deleted.

Figure 2. The gradual loss of basal tetrapod fingers. Unfortunately fingers are not known for every included taxon. Odd Tulerpeton with 6 fingers may result from taphonomic layering of the other manus peeking out below the top one. See figure 6. Mentally delete Diplovertebron from this chart. 

The presence of five manual digits
in Balanerpeton (Figs. 4, 5) sheds light on their retention in Acheloma + Cacops. There is a direct phylogenetic path between them (Fig. 2). Note that all other related clades lose a finger or more. Basal and stem reptiles also retain five fingers.

Figure 3. Utegenia nests as a sister to Diplovertebron.

Distinct from the wide frontals
in Utegenia and Kotlassia,  Balanerpeton (Fig. 4) had narrower frontals like those of Silvanerpeton, a stem reptile.

Figure 4. The basal amphibian, Balanerpeton apparently has five fingers (see figure 5).

As reported
earlier, finger five was lost in amphibians,while finger one was lost in temonospondyls. Now, based on the longest metacarpal in Caerorhachis and Amphibamus (second from medial), apparently manual digit one was lost in that clade also, distinct from the separate frog and microsaur clades. In summary, loss from five digits down to four was several times convergent in basal tetrapods.

Figure 5. DGS recovers five fingers in Balanerpeton with a Diplovertebron-like phalangeal pattern. Two 5-second frames are shown here.

Finally, we have to talk about
Tulerpeton (Fig. 6). The evidence shows that the sixth manual digit is either a new structure – OR – all post-Devonian taxa lose the sixth digit by convergence, since they all had five fingers. Finger 6 has distinct phalangeal proportions, so it is NOT an exposed finger coincident rom the other otherwise unexposed hand in the fossil matrix.

Figure 6. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here. Digit 6 is either a new structure, or a vestige that disappears in all post-Devonian taxa.

References
Fritsch A 1879. Fauna der Gaskohle und der Kalksteine der Permformation “B¨ ohmens. Band 1, Heft 1. Selbstverlag, Prague: 1–92.
Kuznetzov VV and Ivakhnenko MF 1981. Discosauriscids from the Upper Paleozoic in Southern Kazakhstan. Paleontological Journal 1981:101-108.
Watson DMS 1926. VI. Croonian lecture. The evolution and origin of the Amphibia. Proceedings of the Zoological Society, London 214:189–257.

wiki/Diplovertebron