Koilops: a sister for Spathicephalus

Paleontologists have long recognized
that Spathicephalus (Fig. 1) was a close relative of baphetids, like Baphetes, ever since Watson (1929). Two main features link Spathicephalus with baphetids: antorbital fenestrae that have fused with the orbits, and a closed palate formed mostly from a pair of broad pterygoid bones.

Figure 1. Spathicephalus, a filter feeding temnospondyl with elongate orbits now nests with Koilops.

Figure 1. Spathicephalus, a filter feeding temnospondyl with elongate orbits now nests with Koilops.

Spathicephalus mirus (Watson 1926; Late Carboniferous, 320 mya) was described, “unlike that of any other early tetrapod, with a flattened, square-shaped skull and jaws lined with hundreds of very small chisel-like teeth.” The extended orbit shape traditionally allied Spathicephalus with Baphetes, but here in the large reptile tree (LRT, 978 taxa) it nests with two other flat-headed temnospondyls, Gerrerpeton and Koilops (Fig. 2), apart from Baphetes. The fossil does not show tooth replacement as every tooth is present without gaps. Distinct from derived temnospondyls, but like basal forms, the palate is closed on this bottom-feeder. Long before the publication of Koilops Milner et al. 2009 nested Spathicephalus close to Eucritta and Baphetes, but that relationship was not recovered in the LRT, despite sharing a closed palate. The elongate orbit without intrusions is convergent with that of Baphetes.

Figure 2. Koilops is a flat-headed sister to Spathicephalus, but with teeth, larger orbits and a shorter snout.

Figure 2. Koilops is a flat-headed sister to Spathicephalus, but with teeth, larger orbits and a shorter snout.

Koilops herma (Clack et al. 2016; NMS G. 2013.39/14) Tournasian, early Carboniferous ~375 mya) is a temnospondyl with a flat skull and large orbits nesting between Greererpeton and Spathicephalus. The nares were close to the rim of the short rorstrum. The pineal foramen was enormous. The teeth were small and sharp. The nasals were broad.

References
Clack et al. (14 other authors) 2016. Phylogenetic and environmental context of a Tournaisian tetrapod fauna. Nature ecology & evolution 1(0002):1-11.
Milner AC, Milner AR, Walsh SA 2009. A new specimen of Baphetes from Nýřany, Czech Republic and the intrinsic relationships of the Baphetidae. Acta Zoologica 90: 318.
Watson DMS 1929. Croonian Lecture. The evolution of the Amphibia. Philosophical Transactions of the Royal Society, London B 214:189-257.

wiki/Spathicephalus

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.

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

Distribution of ‘key’ traits in basal tetrapods

Before the advent of phylogenetic analysis,
paleontologists attempted to define clades with a short list of synapomorphies. In this way they were getting close to the dangers of pulling a Larry Martin. Many taxa, like pterosaurs and Vancleavea were (and are) considered enigmas because they seemed to appear suddenly in the fossil record with a short suite of traits that did not appear in other reptiles. That was only true back then because paleontologists were only considering short lists of traits.

After the advent of phylogenetic analysis
considering long lists of traits, the rule of maximum parsimony allowed clades to include members that do not have a short list of key traits. For instance some reptiles, like snakes, do not have limbs, but that’s okay based on the rule of maximum parsimony as demonstrated in the large reptile tree (LRT, 977 taxa, subsets shown in Figs. 1-5).

Before the advent of phylogenetic analysis
Carroll (1988) divided basal tetrapods into labyrinthodonts and lepospondyls and presented short lists of key traits.

Labyrinthodonts

  1. evolved directly from rhipidistian fish
  2. labyrinthine infolding of the dentine
  3. palate fangs and replacement pits
  4. vertebral centra composed of more than one element
  5. otic notch
  6. large in size

Lepospondyls

  1. a heterogeneous assemblage of groups with perhaps several origins from among various labyrinthodonts
  2. simple (non-labyrinthine) teeth
  3. no palate fangs
  4. vertebral centra composed of one element
  5. no otic notch
  6. small in size

By contrast,
the large reptile tree introduces a non-traditional topology in which lepospondyls have a single origin. Below (Figs. 1-5) the distribution of several traits are presented graphically.

Figure 1. Distribution of the solid and open palate architectures in basal tetrapods in the LRT topology.

Figure 1. Distribution of the solid and open palate architectures in basal tetrapods in the LRT topology.

Open palate distribution
Basal tetrapods have a solid palate (Fig. 1) in which the pterygoid is broad and leaves no space around the medial cultriform process. Other taxa have narrow pterygoids and large open spaces surrounding the cultriform process. Still others are midway between the two extremes. Traditional topologies attempt to put all open palate taxa into a single clade. Here the open palate evolved three times by convergence.

Figure 2. Size distribution among basal tetrapods in the LRT topology

Figure 2. Size distribution among basal tetrapods in the LRT topology

The length of basal tetrapods
falls below 60 cm in Eucritta and more derived taxa. It also falls below 60 cm in Ostelepis, at the origin of Tetrapoda and Paratetrapoda. Phlegethontia has a small skull, but is otherwise like an eel, and so does not fall below the 60 cm threshold.

Figure 3. Distribution of single vertebrae among basal tetrapods in the LRT.

Figure 3. Distribution of single vertebrae among basal tetrapods in the LRT.

Single piece centra
appear in frogs + salamanders, microsaurs and Phlegethontia, by convergence. Intercentra appear in all other taxa.

Figure 6. Distribution of palatal fangs among basal tetrapods in the LRT.

Figure 6. Distribution of palatal fangs among basal tetrapods in the LRT.

Palate fangs
appear in all basal paratetrapods and tetrapods except Phlegethontia, Spathicephalus and Gerrothorax. Exceptionally, Seymouria also had palate fangs.

Figure 7. Distribution of the otic notch among basal tetrapods in the LRT.

Figure 7. Distribution of the otic notch among basal tetrapods in the LRT.

The otic notch
is widespread among basal tetrapods. Those without an otic notch include

  1. One specimen of Phlegethontia that loses posterior skull bones
  2. Six flat-skulled temnospondyls in which the tabular contacts the squamosal. Some of these, like Greererpeton, have figure data that lack an otic notch, but photos that have one.
  3. Salamanders and frogs that greatly reduce posterior skull bones.
  4. All microsaurs more derived than Microbrachis

Let me know
if I overlooked or misrepresented any pertinent data. This weekend I should be able to look at and respond to the many dozen comments that have accumulated over the last few weeks.

 

Basal Tetrapods, slightly revised

Figure 1. Click to enlarge. With the addition of Panderichthys and Anthracosaurus the position of Koilops and Deltaherpeton have shifted to the base of the Temnospondyli.

Figure 1. Click to enlarge. With the addition of Panderichthys and Anthracosaurus the position of Koilops and Deltaherpeton have shifted to the base of the Temnospondyli. Some of that shifting is due to rescoring.

After earlier identifying
phylogenetic miniaturization at the bases of several major clades in the large reptile tree (LRT, 969 taxa), I wondered if similar size-related patterns appear in basal tetrapods.

  1. Osteolepis is smaller than Eusthenopteron. Has anyone removed the scales from the fore fins of Osteolepis to see what the bones inside look like?
  2. Pholidogaster is much larger than Osteolepis, but Colosteus and Phlegethontia are successively smaller with smaller limbs.
  3. Ventastaga and Pederpes are successively smaller than Ichthyostega.
  4. Koilops is much smaller than Ventastaga and Pederpes
  5. Eucritta is much smaller than Proterogyrinus, both in overall size and in relative torso length. Eucritta nests at the base of the Seymouriamorpha + Crown Tetrapoda.
Figure 2. Basal tetrapod skulls in dorsal view.

Figure 2. Basal tetrapod skulls in dorsal view. Tetrapoda arise with flattened skulls. Paratetrapoda retain skulls with a circular cross section. 

 

Anthracosaurus: beware the chimaera!

Figure 1. The complete skull of Anthracosaurus greatly resembles its relative, Neopteroplax.

Figure 1. The complete skull of Anthracosaurus greatly resembles its relative, Neopteroplax.

Anthracosaurus russelli (Huxley 1863, Panchen 1977, Clack 1987; Westphalian, Late Carboniferous, 310 mya, skull length 40cm; Figs. 1, 2) was originally considered a labyrinthodont. The wide, yet pointed, triangular skull and tall orbits recall traits found in labyrinthodonts, like Sclerocephalus, and in the basal tetrapod, Tiktaalik. Here, in the large reptile tree (LRT, 967 taxa),  Anthracosaurus nests with Neopteroplax (Fig. 3) as a derived embolomere, the clade that likely gave rise to Seymouriamorpha, Lepospondyli and Reptilia

At least one orbit
in Anthracosaurus has an inverted teardrop shape. The marginal and palatal fangs are quite large. Although flattened in dorsal view, comparisons suggest the jaw margin was convex, as in Neopteroplax.

Based on its size and nesting,
Anthracosaurus developed a labyrinthodont-like skull by convergence because Proterogyrinus is basal in the Embolomeri. Those giant marginal and palatal fangs indicate a predatory niche.

Figure 2. Left: Anthracosaurus chimaera from Clack 1987. Right: Older tracing in dorsal view of the complete skull and palatal view attributed to Anthracosaurus from an online photo.

Figure 2. Left: Anthracosaurus chimaera from Clack 1987. Right: Older tracing in dorsal view of the complete skull and palatal view attributed to Anthracosaurus from an online photo. The narrower skull is made of several different specimens (chimaera) and produces a loss of resolution in the LRT.

Clack 1987
illustrated a lateral and dorsal view of Anthracosaurus (Fig. 2) based on a chimaera of specimens. Unfortunately, plugging that data into the LRT produced loss of resolution over several nodes. Using the older single skull in dorsal view had no such problems.

We looked at the problems chimaera taxa produce
earlier here, and in six blogs that preceded that one.

Figure 3. Neopteroplax has a skull quite similar to the older single skull of Anthracosaurus and they nest together in the LRT.

Figure 3. Neopteroplax has a skull quite similar to the older single skull of Anthracosaurus and they nest together in the LRT.

The clade Anthracosauria has had problems
From Wikipedia: “Gauthier, Kluge and Rowe (1988) defined Anthracosauria as ‘Amniota plus all other tetrapods that are more closely related to amniotes than they are to amphibians” (Amphibia in turn was defined by these authors as a clade including Lissamphibia and those tetrapods that are more closely related to lissamphibians than they are to amniotes).”

In this definition non-amniote Anthracosauria does not include Anthracosaurus, but only Silvanerpeton and Gephyrostegus in the LRT because more basal taxa are also basal to amphibians.

“Similarly, Michel Laurin (1996) uses the term in a cladistic sense to refer to only the most advanced reptile-like amphibians. Thus his definition include the (Diadectomorpha and Solenodonsauridae) and the amniotes.”

In the LRT Diadectomorpha and Solenodonsauridae are amniotes.

“As Ruta, Coates and Quicke (2003) pointed out, this definition is problematic, because, depending on the exact phylogenetic position of Lissamphibia within Tetrapoda, using it might lead to the situation where some taxa traditionally classified as anthracosaurs, including even the genus Anthracosaurus itself, wouldn’t belong to Anthracosauria.

Indeed! And that happened in the LRT.

Laurin (2001) created a different phylogenetic definition of Anthracosauria, defining it as “the largest clade that includes Anthracosaurus russelli but not Ascaphus truei”.

In the LRT Ascaphus, the tailed frog, is derived from the large clade, the embolomeri, that includes Anthracosaurus. However the small clade that includes just Anthracosaurus and Neopteroplax does not include the tailed frog.

“However, Michael Benton (2000, 2004) makes the anthracosaurs a paraphyletic order within the superorder Reptiliomorpha, along with the orders Seymouriamorpha and Diadectomorpha, thus making the Anthracosaurians the “lower” reptile-like amphibians. In his definition, the group encompass the Embolomeri, Chroniosuchia and possibly the family Gephyrostegidae.”

In the LRT the Embolomeri are basal to Eucritta and the Seymouriamorpha, which are basal to the Reptilia (= Amniota) and Lepospondyli (including Amphibia). The Chroniosuchia and Gephyrostegus are both amphibian-like reptiles in the LRT.

The clade Reptilomorpha suffers the same definition problems.
As Wikipedia reported, “As the exact phylogenetic position of Lissamphibia within Tetrapoda remains uncertain, it also remains controversial which fossil tetrapods are more closely related to amniotes than to lissamphibians, and thus, which ones of them were reptiliomorphs in any meaning of the word.”

Wouldn’t it be great if someone could put together
a large gamut phylogenetic analysis that could settle all those controversial issues?

References
Clack JA 1987. Two new speciemens of Anthracosaurus (Amphibia: Anthracosauria) from the Northumberland coal measures. Palaeontology 30(1):15-26.
Huxley TH 1863. Description of Anthracosaurus russelli, a new labyrinthodont from the Lanarkshire coalfield. Quartery Journal of the Geological Society 19:56-58.
Panchen AL 1975. A new genus and species of anthracosaur amphibian from the Lower Carboniferous of Scotland and the status of Pholidogaster pisciformis Huxley. Philosophical Transactions of the Royal Society of London, B. 269: 581-640.
Panchen AL 1977. On Anthracosaurus russelli Huxley (Amphibia: Labyrinthodontia) and the family Anthracosauridae. Philosophical Transactions of the Royal Society B. 279 (968): 447–512.

 

The Aïstopods may be splitting apart

Those long, limbless amphibians,
the Aïstopoda, were once (in the 1920s) the oldest known tetrapods, known from Westphalian (310 mya) strata. Of course, since the publication of Ichthyostega (1932) and the rest of the Devonian tetrapods, that’s old news. Baird 1964 wrote: “The remarkable specialization [in aîstopods] already achieved by the early Mississippian implies an origin well back in Devonian time; a tetrapod ancestry rather than direct derivation from the crossopterygians fishes is indicated. Relationships of the order are obscure.” 

Interesting, that 1964 comment,
as some of the aïstopods continue to nest traditionally with lepospondyls, but others now nest with paratetrapods, closer to crossopterygian fishes.

Aistopods are traditionally considered lepospondyls
because the three parts of each vertebrae are fused to become one. Today three aîstopods were added to the large reptile tree (LRT, 967 taxa). Of those three, two did not nest within the Lepospondyli, or within the Tetrapoda.

Ophiderpeton and Oestocephalus
(Fig. 1) nests with Acherontiscus, within the Lepospondyli and within the Tetrapoda in the LRT. The orbits were far forward and the temples were fenestrated, narrowing the parietal. The supratemporal, tabular, jugal and squamosal and quadratojugal are all reduced, but still present.

Figure 1. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Figure 1. Ophiderpeton (dorsal view) and two specimens of Oestocephalus (tiny immature and larger mature).

Phlegethontia
Phlegethontia longissima (above; CGH 129)  Phlegethontia linnearis (below; Cope 1871, Anderson 2002, Fritsch 1875; Huxley and Wright 1867) was considered a aîstopod, but it does not nest with Ophiderpeton, despite the complete fusion of each vertebrae by convergence. Here Phlegethontia nests as a basal pro-tetrapod with Pholidogaster and Colosteus (Fig. 5). P. longissima CGH 129 (below) has not yet developed a temporal fenestra.

Unlike most other paratetrapods,
the premaxilla of P. longissima was drawn out to a very long tip and the premaxillary teeth were the largest. The large lateral naris became elongate over the maxilla and prefrontal, perhaps contacting the postorbital and indicating this was a full time air-breather. Not sure what is happening with the supratemporals, which appear to extend laterally. The naris is elongate atop the maxilla.

The vertebrae
of outgroup taxa, like Pholidogaster, are still tripartite. The vertebrae in the transitional taxon, Colosteus, are largely hidden by osteoderms. In Colosteus the forelimbs are vestiges compared to those in Pholidogaster. They disappear in Phlegethontia. Evidently snake-like taxa fuse the neural spine, intercentrum and pleurocentrum as they switch from limb locomotion to vertebral undulation.

Figure 2. Phlegethontia longissima skull (CGH 129) has relatively large temporal plates, a wide flat cranium and a long pointed rostrum.

Figure 2. Phlegethontia longissima skull (CGH 129) has relatively large temporal plates, a wide flat cranium and a long pointed rostrum.

Like Ophiderpeton
AMNH 6966
 (below) has a large temporal fenestra and the parietal is reduced to the portion anterior to the pineal foramen and fused to the fused frontals. The postorbital, suprateomporal and tabular are replaced by a larger occiput (braincase). The rostrum is also shorter.

Figure 2. Phlegethontia linearis and other congeneric taxa with bones identified. Here the traditional lacrimal and prefrontal identities are switched here.

Figure 3. Phlegethontia linearis and other congeneric taxa with bones identified. Here the traditional lacrimal and prefrontal identities are switched here. The squamosal and quadratojugal are fused. The parietal and other cranial bones are absent or vestiges.

Distinct from other paratetrapods,
the dorsal ostederms were absent. The ventral osteoderms had become elongate gastralia, convergent with tetrapods. Having just acquired limbs and girdles, this clade promptly got rid of them and emphasized cerebral undulation. The tiny’ gill bones’ illustrated by Fritsch 1875 (erased here, Fig. 4) are actually displaced gastralia, according to Baird 1964. The naris is larger in Phlegethontia compared to outgroup taxa with legs. So it was likely breathing air, rather than using gills.

Figure 4. Phlegethontia overall with neck and sacral bones colored red. The 'gill bones' are removed. They are gastralia.

Figure 4. Phlegethontia overall with neck and sacral bones colored red. The ‘gill bones’ are removed. They are gastralia.

Despite the many convergent traits
the LRT is able to separate Ophiderpeton from Phlegethontia and from all other long bodied, limbless tetrapods.

Figure 5. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are  transitional to the limbless condition in Phlegethontia.

Figure 5. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are transitional to the limbless condition in Phlegethontia.

The traditional basal taxon
for the Aïstopoda is Lethiscus (Viséan. 340 mya). Hopefully data will come in soon on that taxon so it can be added to the LRT. Wikipedia reports, “The skull is specialised and light, very like that of Ophiderpeton, with the orbits, far forward, and the cheek region unossified (lacking bone). There are approximately 30 closely spaced teeth on the maxilla and dentary, and a sutural pattern of the skull closely resembles that of the Late Carboniferous aïstopod Oestocephalus.”

References
Anderson JS 2002. Revision of the aïstopod genus Phlegethontia (Tetrapoda: Lepospondyli). Journal of Paleontology. 76 (6):1029–1046. Online here.
Baird D 1964. The aïstopod amphibians surveyed. Breviora 206:1-17.
Cope ED 1871. Stated Meeting, Nov. 3d, 1871. Proceedings of the American Philosophical Society 12:176-177
Fritsch A 1875. Über die Fauna der Gaskohle des Pilsner und Rakonitzer Beckens. Sitzungsberichtde er Böhemischen Gesellschaft der Wissenschaften. Prague: 70–79.
Huxley TH 1862. On new labyrinthodonts from the Edinburgh Coal-field. Quarterly Journal of the Geological Society London18:291-296.
Panchen AL 1975. A New Genus and Species of Anthracosaur Amphibian from the Lower Carboniferous of Scotland and the Status of Pholidogaster pisciformis Huxley. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 269(900):581-637.

wiki/Pholidogaster
/wiki/Phlegethontia
wiki/Oestocephalus
wiki/Ophiderpeton

Diplovertebron and amphibian finger loss patterns

Diplovertebron punctatum (Fritsch 1879, Waton 1926; Moscovian, Westphalian, Late Carboniferous, 300 mya, Fig. 1) was considered an anthracosaur or reptile-like amphibian. That is confirmed by the large reptile tree (LRT, subset Fig. 2), where it nests with  Utegenia transitional between basal seymouriamorpha, like Kotlassia, and basal amphibians, like Balanerpeton (Fig. 3), yet close to the origin of stem reptiles, like Silvanerpeton. Based on the nesting of Tulerpeton in the LRT, Diplovertebron had origins in the Late Devonian.

Figure 1. Diplovertebron nests at the base of the lineage of amphibians, close to the base of the reptiles, all derived from seymouriamorphs. Note the retention of five fingers. Wish I had better data than this.

Figure 1. Diplovertebron nests at the base of the lineage of amphibians, close to the base of the reptiles, all derived from seymouriamorphs. Note the retention of five fingers. Wish I had better data than this.

In Diplovertebron,
the vertebral structure is primitive. The notochord persisted in adults. The ribs were long and slender as in basal taxa, not shortened as in lepospondyl amphibians. Five manual digits were preserved with a 2-3-3-3-4 formula, a formula similar to amphibians, not like reptiles (2-3-4-5-5). The ilium is bifurcate with a long posterior process. The pubis did not ossify, as in several basal tetrapods including Crassigyrinus and derived Amphibia. Small scutes covered the entire torso ventrally, as in basalmost tetrapods and basal reptiles.

Figure 2. The gradual loss of basal tetrapod fingers. Unfortunately fingers are not known for every included taxon.

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.

The presence of five manual digits
in Diplovertebron and 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 2. Utegenia nests as a sister to Diplovertebron.

Figure 3. Utegenia nests as a sister to Diplovertebron.

Note the narrow frontals,
on Diplovertebron distinct from the wide frontals in Utegenia and Kotlassia, but more similar to those in Balanerpeton (Fig. 4), another basal amphibian, and Silvanerpeton, a stem reptile. Yet none have the hourglass shape found in Diplovertebron.

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

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.

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 2. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here.

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