Basal Tetrapods, slightly revised

Updated June 23, 2017 with the replacement of the cladogram with one containing more recently added taxa.

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

Figure 1. Subset of the LRT showing basal tetrapods (amphibians).

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. Ventastaga and Pederpes are successively smaller than Ichthyostega.
  3. Koilops is much smaller than Panderichthys.. 
  4. 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.

 

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.

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 2. Utegenia nests as a sister to Diplovertebron.

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).

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

Origin of bipedalism in dinosaurs: Overlooking Carrier’s Constraint

Persons and Currie 2017 debunk on old theory
on bipedalism in dinosaurs and introduce a new one that suffers from taxon exclusion while overlooking a very popular theory from the last thirty years: Carrier’s Constraint (Carrier 1987).

From the abstract:
“Bipedalism is a trait basal to, and widespread among, dinosaurs. It has been previously argued that bipedalism arose in the ancestors of dinosaurs for the function of freeing the forelimbs to serve as predatory weapons.”

I never heard of this reason before. Predatory weapons only happen as a result and much later phylogenetically and only sometimes.

“However, this argument does not explain why bipedalism was retained among numerous herbivorous groups of dinosaurs. We argue that bipedalism arose in the dinosaur line for the purpose of enhanced cursoriality.”

The term ‘enhanced’ is pretty vague. Does it mean ‘better’? But can that be proven? The fastest animals on land now are quadrupedal cheetahs. Bipedal Chlamydosaurus does not have greater speed or endurance. Persons and Currie bring up the “tripping on one’s own forefeet” hypothesis and that, IMHO, has some validity.

“Modern facultatively bipedal lizards offer an analog for the first stages in the evolution of dinosaurian bipedalism. Many extant lizards assume a bipedal stance while attempting to flee predators at maximum speed.”

But quadrupedal lizards are just as fast as bipedal ones. Lizards gain no speed when switching to bipedal locomotion as Persons and Currie also note.

Bipedal lizard video marker

Figure 1. Click to play video. Just how fast can quadrupedal/bipedal lizards run? This video documents 11 meters/second in a Callisaurus at the Bruce Jayne lab.

“Bipedalism, when combined with a caudofemoralis musculature, has cursorial advantages because the caudofemoralis provides a greater source of propulsion to the hindlimbs than is generally available to the forelimbs.”

Yes, at first, especially when the forelimbs are lifted from the ground! Persons and Currie stay clear of the bipedal ability of fenestrasaurs including pterosaurs. There, in taxa like Cosesaurus, the driving force switches to the hips.

“That cursorial advantage explains the relative abundance of cursorial facultative bipeds and obligate bipeds among fossil diapsids and the relative scarcity of either among mammals.”

Actually there is no abundance of bipeds anywhere among diapsids, except in the Fenestrasauria (not related to archosaur-line diapsids) and Archosauria + Poposauria. Persons and Currie also stay clear of the inverted bipeds among mammals, the bats, and they are numerous.

None of the so-called ‘reasons’ why are pertinent
without the random evolution of longer hind limbs than forelimbs and the ability to balance over the hind limbs, whether running or standing still. It also helps to have even a small anterior addition to the ilium, according to Shine and Lambeck 1989. The pubic foot of theropods and the prepubis of pterosaurs also provide femoral muscle anchors.

Unfortunately

  1. Persons and Currie do not indicate the node at which bipedalism arose in the last common ancestor of bipedal crocs and dinosaurs: Gracilisuchus and Turfanosuchus at the base of the Poposauria. In the large reptile tree (LRT)  Gracilisuchus (Fig. is the last common ancestor of bipedal crocs, like Scleromochlus, and bipedal pro-dinosaurs, like Lewisuchus.
  2. Persons and Currie subscribe to the outdated hypothesis of “Avemetatarsalia” in which former members, like pterosaurs now nest with lepidosaurs and Lagerpeton now nests with chanaresuchids.
  3. Persons and Currie also avoid the likely bipeds, Arizonasaurus and Postosuchus.
  4. Persons and Currie discuss the the likely biped, Eudibamus, but incorrectly ascribe it to the bolosaurs.
  5. Persons and Currie overlooked Carrier’s Constraint, which holds that,“air-breathing vertebrates which have two lungs and flex their bodies sideways during locomotion find it very difficult to move and breathe at the same time, because the sideways flexing expands one lung and compresses the other, shunting stale air from lung to lung instead of expelling it completely to make room for fresh air.” — but is that the reason to go bipedal? or just the first and biggest advantage narrow-gauge bipedal reptiles enjoy?
Click to enlarge. Squamates, tritosaurs and fenestrasaurs in the phylogenetic lineage preceding the origin of the Pterosauria.

Figure 2. Tritosaurs and fenestrasaurs in the phylogenetic lineage preceding the origin of the Pterosauria.

What fenestrasaurs gain by a bipedal configuration

  1. height dominance over conspecific rivals for mating privileges. This is emphasized in Langobardisaurus with its long neck. This is emphasized by Cosesaurus by flapping and leaping, both working to increase height.
  2. Ability to breathe while running for added endurance
Chlamydosaurus, the Austrlian frill-neck lizard

Fig. 3. Chlamydosaurus, the Austrlian frill-neck lizard with an erect spine and elevated tail. At one time some paleontologists did not believe what you can see here, that this lizard can stand bipedally. Such was their bias.

What the lizard, Chlamydosaurus, gains by bipedal configuration

  1. combined with their frightfully opening frill neck, dominance over rivals and interlopers, which they charge bipedally.
  2. better ability to survey the local area for rivals (principally) and predators while on the ground, — but Chlamydosaurus is primarily (90%) arboreal for the same reason and 90% bipedal while on the ground, not just while running, which some paleontologists are not aware of or did not believe (Hone and Benton 2007, 2009).
Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

What Gracilisuchus gained by a bipedal configuration

  1. Gracilisuchus is not much taller bipedally. Remember, archosaurs had no scales at this point. Feather quills would appear on dino backs. Osteoderms appeared along croc backs to support their longer spinal columns. So, standing erect might have just been sexy at first.
  2. Overcoming Carrier’s Constraint: greater endurance by not having to undulate while breathing and so continue breathing while running.

What do bipedal reptiles have in common?

  1. Other than sauropods and other reptiles that adopt a tripodal pose bipedal reptiles are generally small, having experienced phylogenetic miniaturization.
  2. Other than Tanystropheus, bipeds are terrestrial and/or arboreal
  3. Longer hind limbs than forelimbs
  4. Anterior process of the illiim, no matter how small
  5. Typically stronger or more sacral connections to the ilium
  6. Typically a long neck and short torso (but Longisquama (Fig. 2), as a lemur analog, and lemurs themselves break that rule).
Figure 1. The ancestry of Scleromochlus going back to Lewisuchus, Saltoposuchus, Terrestrisuchus, SMNS 12591 and Gracilisuchus.

Figure 1. The ancestry of Scleromochlus going back to Lewisuchus, Saltoposuchus, Terrestrisuchus, SMNS 12591 and Gracilisuchus.

It’s easy to overlook the most obvious.
I have a feeling that this will not be the first time Persons and Currie are going to be reminded of Carrier 1987.

References
Carrier DR 1987. The evolution of locomotor stamina in tetrapods: circumventing a mechanical constraint. Paleobiology (13): 326–341.
Clemente CJ, Withers PC, Thompson G, Lloyd D 2008. Why Go Bipedal? Locomotion and Morphology in Australian Agamid Lizards.J. Exp. Bio. 211: 2058-2065
Peters D 2000b. A reexamination of four prolacertiforms with implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106: 293–336.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Persons WS and Currie PJ 2017. The functional origin of dinosaur bipedalism: Cumulative evidence from bipedally inclined reptiles and disinclined mammals. Journal of Theoretical Biology, 2017; 420: 1 DOI: 10.1016/j.jtbi.2017.02.032
Shine R and Lambeck R 1989. Ecology of Frillneck Lizards, Chlamydosaurus kingii (Agamidae), in Tropical Australia. Aust. Wildl. res. Vol. 16: 491-500.
Snyder RC 1954. The anatomy and function of the pelvic girdle and hind limb in lizard locomotion. American Journal of Anatomy 95:1-46

Little red flags for a Saharastega reconstruction

Updated April 17, 2017 with a removal of the nonexistent dorsal tusk holes in Nigerpeton. Thanks DM!

Saharastega moradiensis (Sidor et al., 2005; Late Permian; Fig. 1) is a large, flat-headed, temnospondyl basal tetrapod. According to the original reconstruction (Fig. 1) it is the only temnospondyl in the large reptile tree (LRT, now 962 taxa) in which the jugal has no posterior process and the quadratojugal contacts the postorbital. Those autapomorphies raised red flags that started the present investigation.

Figure 1. Saharastega fossil skull, tracing of fossil skull, freehand reconstruction, all by Sidor et al., followed by color tracing that finds nares at the dorsal rostrum, concave dorsal rostrum and posterior jugal separating the quadratojugal from the postorbital overlooked by Sidor et al.

Figure 1. Saharastega fossil skull, tracing of fossil skull, freehand reconstruction, all by Sidor et al., followed by color tracing that finds nares at the concave dorsal rostrum and posterior jugal separating the quadratojugal from the postorbital

Taking the Saharastega freehand reconstruction at face value
Saharastega was scored and it nested with the coeval Nigerpeton (Fig. 2) which has dorsal nares and anterior fang holes along with a concave rostral profile. These are traits not shared by Saharastega according to the freehand reconstruction (Fig. 1).

Going back to the fossil
and colorizing the bones of Saharastega reveals a skull more like that of Nigerpeton than the freehand reconstruction indicates. Fang holes are not presesent, according to those who have seen the fossil (see below), so they are removed here. Both share dorsal nares and a concave rostral profile, together with a jugal that separates the quadratojugal from the postorbital. Note the placement of the internal nares relative to the external nares in Nigerpeton (Fig. 2). That pattern is more or less shared by Saharastega (Fig. 1).

Figure 2. Nigerpeton nests with its contemporary, Saharastega (figure 1) and has dorsal nares and a concave rostrum.

Figure 2. Nigerpeton nests with its contemporary, Saharastega (figure 1) and has dorsal nares and a concave rostrum.

The two taxa, Nigerpeton and Saharastega,
are not congeneric, but they do appear to share more traits than the authors originally indicated. The crack across the rostrum in Saharastega somewhat obliterated the nares. Otherwise they would have not been overlooked.

References
Sidor CA, O’Keefe FR, Damiani R, Steyer JS, Smith RMH, Larsson HCE, Sereno PC, Ide O and Maga A 2005. Permian tetrapods from the Sahara show climate-controlled endemism in Pangaea. Nature. 434 (7035): 886–889. doi:10.1038/nature03393. PMID 15829962.
Damiani R, Sidor CA, Steyer JS. Smith RMH, Larsson HCE, Maga A and Ide O 2006. The vertebrate fauna of the Upper Permian of Niger. V. The primitive temnospondyl Saharastega moradiensis. Journal of Vertebrate Paleontology. 26 (3): 559–572. doi:
wiki/Saharastega

Rough chronology of basal tetrapods and basal reptiles

Today we’ll look at WHEN
we find fossils of basal tetrapods and basal reptiles. According to the large reptile tree (959 taxa, LRT, subset shown in Fig. 1), oftentimes we find late survivors of earlier radiations in higher strata. The origin of Reptilia (amphibian-like amniotes) extends back to the Devonian and Early Carboniferous now, not the Late Carboniferous as Wikipedia reports and as the Tree of Life project reports.

Figure 1. Color coded chronology of basal tetrapods and reptiles.We're lucky to know these few taxa out of a time span of several tens of millions of years.

Figure 1. Color coded chronology of basal tetrapods and reptiles.We’re lucky to know these few taxa out of a time span of several tens of millions of years. Click to enlarge.

The Late Devonian 390–360 mya
Here we find late survivors of an earlier radiation: Cheirolepis, a basal member of the Actinopterygii (ray-fin fish) together with Eusthenopteron and other members of the Sarcopterygii (lobe-fin fish). Coeval are basal tetrapods, like Acanthostega and basal reptiles, like Tulerpeton. These last two launch the radiations we find in the next period. The presence of Tulerpeton in the Late Devonian tells us that basal Seymouriamorpha and Reptilomorpha are waiting to be found in Devonian strata. We’ve already found basal Whatcheeriidae in the Late Devonian taxa Ichthyostega and Ventastega.

Early Carboniferous 360–322 mya
Here we find the first radiations of basal reptilomorphs, basal reptiles, basal temnospondyls,  basal lepospondyls and microsaurs, lacking only basal seymouriamorphs unless Eucritta is counted among them. It nests outside that clade in the LRT.

Late Carboniferous 322–300 mya
Here we find more temnospondyls, lepospondyls and phylogenetically miniaturized archosauromorphs, likely avoiding the larger predators and/or finding new niches. Note the first prodiapsids, like Erpetonyx and Archaeovenator, appear in this period, indicating that predecessor taxa like Protorothyris and Vaughnictis had an older, Late Carboniferous, origin. Not shown are the large basal lepidosauromorphs, Limnoscelis and Eocasea and the small archosauromorphs, Petrolacosaurus and Spinoaequalis.

Early Permian 300–280 mya
Here we find the first fossil Seymouriamorpha and the last of the lepospondyls other than those that give rise to extant amphibians, like Rana, the frog. Here are further radiations of basal Lepidosauromorpha, basal Archosauromorpha (including small prodiapsids), along with the first radiations of large synapsids.

Late Permian 280–252 mya
Here we find the next radiation of large and small synapsids, the last seymouriamorphs, and derived taxa not shown in the present LRT subset.

Early/Mid Triassic 252 mya–235 mya
Among the remaining basal taxa few have their origins here other than therapsids close to mammals. Afterwards, the last few basal taxa  listed here, principally among the Synapsida, occur later in the Late Triassic, the Jurassic and into the Recent. Other taxa are listed at the LRT.

What you should glean from this graphic
Taxa are found in only the few strata where fossilization occurred. So fossils are incredibly rare and somewhat randomly discovered. The origin of a taxa must often be inferred from phylogenetic bracketing. And that’s okay. This chart acts like a BINGO card, nesting known taxa while leaving spaces for taxa we all hope will someday fill out our card.

 

 

Platyhystrix: closer to Acheloma than to Cacops?

Platyhystrix was a dissorophid lepospondyl anamniote that had a dorsal sail (Figs. 1, 2 ), not quite like those  of its more famous reptilian/synapsid contemporaries, Dimetrodon and Edaphosaurus.

There must have been something in the air back then,
and those dorsal sails were there to catch it!

Figure 1. Platyhistrix skull reconstructed from slightly disassociated parts.

Figure 1. Platyhistrix skull reconstructed from slightly disassociated parts. And the Lewis and Vaughn 1965 dorsal sail, distinct from the others in figure 2. The skull here appears to have a confluent naris and antorbital finestra, as in Acheloma, but there are other bones missing there, too, like most of the maxilla.

Dissorophids are traditionally nested with
temnospondyls, but here, at the large reptile tree (LRT, now 959 taxa), they arise from a sister to the basal seymouriamorph, Utegenia and continue to be generally smaller taxa (< 60cm).

Figure 2. Other Platyhystrix specimens known chiefly from dorsal spines.

Figure 2. Other Platyhystrix specimens known chiefly from dorsal spines. That old skull from Williston 1911 is missing the central area, here imagined from the more complete specimen in figure 1.

Distinct from Acheloma
the skull of Platyhysterix does not appear to have giant palatal fangs, or such large marginal teeth. The jugal nearly separates the postorbital from the supratemporal. The postorbital is larger and much knobbier.

Like Acheloma
The rostrum may include a confluent nairs/antorbital fenestra, a constricted rostrum (in dorsal view), a naris of similar laterally wavy shape, robust premaxillary ascending processes, large tabulars and other traits relatively exclusive to these two.

A fair amount of reassembly
is required of the Platyhystrix skull. The random neural spine below the lower right jaw line allies the skull with specimens that also have long neural spines.

Figure 1. Acheloma dunni skull with a confluent antorbital fenestra and naris.

Figure 3. Acheloma dunni skull with a confluent antorbital fenestra and naris.

Wouldn’t it be interesting 
to see hatchlings and juveniles of Platyhystrix? It is widely considered, along with its double-armored kin, Dissorophus, to have been fully terrestrial. So, did these two have a swimming tadpole stage? And then develop spines and armor in adulthood? Or did they converge with reptiles, laying protected eggs on land, skipping the tadpole stage? Let’s keep an eye out for little finbacks.

References
Berman DS, Reisz RR and Fracasso MA 1981. Skull of the Lower Permian dissorophid amphibian Platyhystrix rugosus. Annals of the Carnegie Museum 50 (17):391-416.
Case EC 1911. Revision of the Amphibia and Pisces of the Permian of North America. Publ. Carnegie Inst. Washington 146:1-179.
Dilkes DW and Reisz R 1987. Trematops milleri identified as a junior synonym ofAcheloma cumminsi with a revision of the genus. American Museum Novitates 2902.
Lewis GE and Vaughn PP 1965. Early Permian vertebrates from the Cutler Formation of the Placerville area, Colorado, with a section on Footprints from the Cutler Formation by Donald Baird: U.S. Geol. Survey Prof. Paper 503-C, p. 1-50.
Williston SW 1911a. A new family of reptiles from the Permian of New Mexico. American Journal of Science 31:378-398.
Williston SW 1911b. American Permian vertebrates. University of Chicago Press: 145 pp.

wiki/Acheloma
wiki/Platyhystrix