Diplovertebron vs. Gephyrostegus

This blog had its genesis in a reader comment
that considered the taxon, Diplovertebron congeneric with the coeval Gephyrostegus bohemicus and G. watsoni (Fig. 1), echoing earlier authors. Although there may be some confusion here (see below), and several specimens have been attributed to Gephyrostegus by various authors, the specimen illustrated and labeled by Watson 1926 (Fig. 1) is not one of them, unless it was drawn very poorly. If anyone has in situ skeletal material, please send it along for an update.

Part of my confusion
lies in the Wikipedia article on Diplovertebron, which states it was 60 cm in length, at least 5x larger than the one illustrated by Watson and far larger than any of its sister taxa. There may be a paper I am unfamiliar with at present that clarifies the matter.

The Westphalian (310 mya) tetrapods
include some reptile-like amphibians and some amphibian-like reptiles. This strata is 30 million years younger than the Viséan, where members from the first great radiation of reptiles can be found. Several late-survivors of earlier radiations can still be found in Westphalian strata.

Earlier G. watsoni nested among basal archosauromorpha, apart from G. bohemicus at the base of the Reptilia and separated by Eldeceeon. So the three taxa in figure 1 are separated from each other by intervening genera and therefore cannot be congeneric.

With present data, flawed though it may be
Diplovertebron nests in the large reptile tree (LRT) with Utegenia, at the base of the Lepospondyli, the clade that ultimately gives us frogs, like Rana, salamanders, like Andrias, and caecilians, like Dermophis.

Figure 1. Diplovertebron, Gephyrostegus bohemicus and Gephyrostegus watsoni. None of these are congeneric.

Figure 1. Diplovertebron, Gephyrostegus bohemicus and Gephyrostegus watsoni. to scale  None of these are congeneric.

Some backstory:
Diplovertebron punctatum (Fritsch 1879, Waton 1926; Moscovian, Westphalian, Late Carboniferous, 300 mya) was considered an anthracosaur or reptile-like amphibian and that is confirmed by the large reptile tree, where it is transitional between basal seymouriamorpha, like Kotlassia, and Utegenia at the base of the amphibians, close to the origin of the Reptila (Silvanerpeton).

The vertebral structure is primitive, the notochord persisted in adults. The ribs were long and slender. Five digits were preserved with a 2-3-3-3-4 formula. The ilium is bifurcate with a long posterior process. The pubis did not ossify, as in derived Amhibia. Small scutes cover he entire torso ventrally. I’m particularly curious about the lumbar region, which includes ribs all the way back to the pelvis, a situation that does not occur in sister taxa.

References
Brough MC and Brough J 1967. The Genus Gephyrostegus. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 252 (776): 147–165. doi:10.1098/rstb.1967.0006
Carroll RL 1970. 
The Ancestry of Reptiles. Philosophical Transactions of the Royal Society London B 257:267–308. online pdf
Fritsch A 1879. Fauna der Gaskohle und der Kalksteine der Permformation “B¨ ohmens. Band 1, Heft 1. Selbstverlag, Prague: 1–92.
Jaeckel O 1902. Über Gephyrostegus bohemicus n.g. n.sp. Zeitschrift der Deutschen Geologischen Gesellschaft 54:127–132.
Klembara J, Clack J, Milner AR and Ruta M 2014. Cranial anatomy, ontogeny, and relationships of the Late Carboniferous tetrapod Gephyrostegus bohemicus Jaekel, 1902. Journal of Vertebrate Paleontology 34:774–792.
Ruta M, Jeffery JE and Coates MI 2003. A supertree of early tetrapods. Proceedings of teh Royal Society, London B (2003) 270, 2507–2516 DOI 10.1098/rspb.2003.2524 online pdf
Watson DMS 1926. VI. Croonian lecture. The evolution and origin of the Amphibia. Proceedings of the Zoological Society, London 214:189–257.

wiki/Gephyrostegus
wiki/Diplovertebron

 

Correcting mistakes on Brachydectes

Perhaps one of the most difficult skulls
in all of the Tetrapoda is Brachydectes newberryi ((Wellstead 1991; Latest Carboniferous, Fig. 1). Many bones are in their standard positions. However, the bones posterior to the orbit have moved around, fused or become lost. That’s where the trouble begins.

Figure 1. Brachydectes newberryi has some difficult to identify bones just aft of the orbit due to fusion and reduction. Brachydectes (Laysorophus) elongatus (Fig. 2) provides Rosetta Stone clues as to what is happening in this clade.

Figure 1. Brachydectes newberryi has some difficult to identify bones just aft of the orbit due to fusion and reduction. Brachydectes (Laysorophus tricarinatus) elongatus (Fig. 2) provides Rosetta Stone clues as to what is happening in this clade. Note the tabulars may be more of a square shape, as Pardo and Anderson drew, but did not identify as such. 

Finding data for
Brachydectes elongatus (formerly Lysorophus tricarinatus; Cope 1877, Carroll and Gaskill  1978, Wellstead 1991; Permian, 250 mya; AMNH 6172 ) provides many needed clues as to the identity of the mystery bones.  The data comes from Carroll and Gaskill 1978 and Wellstead 1991. Earlier hypotheses included errors that I want to correct now. Based on phylogenetic bracketing these taxa nest with the caecilians Eocaecilia and Dermophis all derived from elongate microsaurs close to Archerontiscus, Oestocephalus, Adelogyrinus, Adelospondylus and Microbrachis in the large reptile tree (LRT). Unfotunatey, the latter taxa do not reduce the cheek and temple elements. So they were of little help.

Figure 2. Brachydectes elongatus (Lysorophus tricarinatus) from Carroll and Gaskill 1978 and Wellstead 1991 with colors and new bone identities added.

Figure 2. Brachydectes elongatus (Lysorophus tricarinatus) from Carroll and Gaskill 1978 and Wellstead 1991 with colors and new bone identities added.

As you can see
in figure 2, most of the skull roofing bones and anterior skull bones of Brachydectes elongatus are in their standard spots and are therefore uncontroversial. So let’s nail down the rest of the bones with a parsimony check.

Figure 3. Brachydectes species compared to scale and not to scale. Size alone might warrant generic distinction.

Figure 3. Brachydectes species compared to scale and not to scale. Size alone might warrant generic distinction.

  1. No sister taxa have a large supraoccipital that contacts the parietals and extends over the skull roof. Here that light tan median bone is identified as a set of fused post parietals, as in sister taxa. A more typical supraoccipital may be peeking out as a sliver over the foramen magnum (spinal nerve opening, beneath the fused postparietals.
  2. No sister taxa separate the postparietals, so those in light red are identified here as tabulars, bones which typically form the posterior rim of sister taxa skulls and often provide corners to the skull.
  3. Typcially anterior to, but this time lateral to the new tabulars are the bright green supratemporals. As in sister taxa they maintain contact with the postorbitals (yellow/amber) and parietals (lavender/light purple). They form skull corners in B. elongulatus and rise above the plane of the cranium in B. newberryi – but still act as skull corners.
  4. The jugal is completely absent (unless a sliver of it is fused to the yellow-green quadratojugal lateral to the quadrate, The maxilla posterior to the eyeball is also absent.
  5. The postfrontal is fused to the parietal, with a slender strip maintaining contact with the postfrontal.
  6. The postorbital is in its standard position at the posterior orbit. Here it is roofed over by the supratemporal, as in Microbrachis.
  7. The squamosal is the tricky bone. It appears as a separate bright magenta element in B. elongulatus, but must be absent or fused to the postorbital in B. newberryi because it is otherwise not visible. I agree with previous workers on the identity of the squamosal in B. elongatus.

Bones may fuse, drift and change shape, but their connections to other bones often remain to help identify them using phylogenetic bracketing. Of course that requires a valid phylogenetic framework, one that minimizes taxon exclusion problems. The tabulars do not trade places with the postparietals in this hypothesis. The tabulars maintain their original places, lateral to the fused postparietals, bones which fuse by convergence in other taxa. Perhaps the concept of an autapomorphic oversized supraoccipittal was the source of earlier errors.

It’s interesting
that the opisthotics are posteriorly covered by the exoccipitals. That usually does not happen in most tetrapods, but is further emphasized in the caecilians, Eocaecilia and Dermophis. In competing candidate taxa Rhynchonkos, Batropetes and Microrator, a different pattern is present with the postparietals descending to cover large portions of the occiput and the tabulars are fused or absent.

Wellstead (1991) and perhaps others
made Brachydectes elongatus and Brachydectes newberryi congeneric, but I see enough differences here to warrant separate genera.

Pardo and Anderson 2016 reported, 
“Contra the proposals of some workers, we find no evidence of expected lissamphibian synapomorphies in the skull morphology in Brachydectes newberryi, and instead recognize a number of derived amniote characteristics within the braincase and suspensorium.

Our study reveals similarities between the braincase of Brachydectes and brachystelechid recumbirostrans, corroborating prior work suggesting a close relationship between these taxa.”

Pardo and Anderson freehand
a Brachydectes newberryi skull reconstruction to supplement their CT scans, but do not label the bones in the drawing. Present are paired bones posterior to the parietals and a single median bone posterior to those. Based on their text, the bones posterior to the parietals are identified as post parietals, “as in the majority of early tetrapods.’ Unfortunately, sister taxa among the microsaurs do not have a large supraoccipital. So this bone has to be reconsidered as a post parietal, which all related taxa have arching over the foramen magnum. Pardo and Anderson do not mention supratemporals, but all sister taxa in the LRT have them.

Recumbirostra
according to Wikipedia, are lepospondyl amphibians that include a large number of microsaurs. Of course, those are not derived amniotes. The LRT nests Brachydectes within the Microsauria (which is not a paraphyletic group here). The phylogenetic topology of Recumbirostrans recovered by Glienke (2012) do not create the same topology in the LRT, perhaps due to taxon exclusion. Glienke recovers Eocaecilia close to Rhynchonkos (in the absence of Adelospondyli). In both studies Microbrachis is basal.

The process of discovery
is often the process of correcting errors. And, as you can see, I’m glad to do so when errors are detected, whether out there or in here. Apologies for earlier errors. We’re all learning and helping each other to learn here.

 

References
Carroll RL and Gaskill P 1978. The order Microsauria. American Philosophical Society Memoires 126: 211 pp.
Cope ED 1877. Description of extinct Vertebrata from the Permian and Triassic formations of the United States. Proc. Am. Philos. Soc. 17: 182-193.
Pardo JD and Anderson JS 2016. Cranial Morphology of the Carboniferous-Permian Tetrapod Brachydectes newberryi (Lepospondyli, Lysorophia): New Data from μCT. PLoS ONE 11(8): e0161823. doi:10.1371/journal.pone.0161823
Wellstead C F 1991. Taxonomic revision of the Lysorophia, Permo-Carboniferous lepospondyl amphibians. Bulletin of the American Museum of Natural History 209: 1–90.

wiki/Lysorophus
wiki/Brachydectes

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

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

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

Dermophis, an extant caecilian gets the DGS treatment

Sometimes bones disappear.
Other times bones become fused to one another. The extant caecilian Dermophis (Fig. 1) might demonstrate one or the other or both. Coloring the bones helps to interpret and explain their presence despite the absence of sutures due to fusion or loss.

Figure 1. Dermophis, the extant Mexican caecilian, with bones, even if fused to one another, identified. The quadratojugal and squamosal are absent. Black and white image from Digimorph.org. Coloring the bones makes them so much easier to read and understand.

Figure 1. Dermophis, the extant Mexican caecilian, with bones, even if fused to one another, identified. The quadratojugal and squamosal are absent. Black and white image from Digimorph.org. Coloring the bones makes them so much easier to read and understand.

Dermophis mexicanus (Mexican caecilian, Peters 1880; extant) The nasal and premaxilla are fused. The maxilla, lacrimal, prefrontal and palatine are fused. The occipital elements and the paraspheniod are fused (= Os basale). The parietal and postparietal are fused. The jugal, squamosal, postfrontal and postorbital are fused. The dentary and surangular are fused. The splenial, articular and angular are fused. The pterygoid and quadrate are fused.

The cheek bones are traditionally labeled squamosals, but that may not be the whole story here. Different from nearly all other basal tetrapods (including other amphibians), caecilians shift the jaw joint forward, creating a large retroarticular process of the posterior mandible.

Dermophis lives in humid to dry soils beneath leaf-litter, logs, banana or coffee leaves and hulls or similar ground cover. It is viviparous.

Ontogeny should tell
The true identity of skull bones should be able to be determined by watching their growth from small disconnected bone buds in the embryo. Unfortunately, the references I’ve seen don’t make that growth clear in all cases. So, I’m stuck, for the present, with comparative anatomy within a phylogenetic framework that nests caecilians with Acherontiscus (Fig. 4) and kin, which have large and separate cheek bones.

FIgure 2. Eocaecilia has small limbs and a substantial tail.

FIgure 2. Eocaecilia has small limbs and a substantial tail. The tabular may be absent here unless it, too, is fused to the postorbital/squamosal. The tabular is tiny in Dermophis and probably useless.

Limbs and limb girdles
are absent in all extant caecilians and the majority of species also lack a tail. They have a terminal cloaca, like an earthworm. Limbs are vestigial in Eocaecilia (Fig. 2), and a substantial tail is present.

Figure 1. Eocaecilia skull with original and new bone identifications based on comparisons to sister taxa listed here. Like Brachydectes, the jaw joint has moved forward, beneath the jugal now fused to the quadratojugal creating a long retroarticular process, otherwise rare in amphibians. Also rare is the fusion of the squamosal with the postorbital.

Figure 3. Eocaecilia skull with original and new bone identifications based on comparisons to sister taxa listed here. Like Brachydectes, the jaw joint has moved forward, beneath the jugal now fused to the quadratojugal creating a long retroarticular process, otherwise rare in amphibians. Also rare is the fusion of the squamosal with the postorbital. Note the reduced supratomporal. here and in Dermophis.

The tentacle
Extant caecilians have a unique chemosensory organ located on the head called the tentacle. The tentacle exits the skull through the tentacular foramen (looks like an antorbital fenestra) located between the nares and orbit. Eocaecilia lacks this foramen (Fig. 3).

Figure 4. Acherotisicus has large cheek bones (squamosal, quadratojugal) that appear to fuse in Eocaecilia and Dermophis.

Figure 4. Acherotisicus has large cheek bones (squamosal, quadratojugal) that appear to fuse in Eocaecilia and Dermophis.

References
Peters WCH 1880 “1879”. Über die Eintheilung der Caecilien und insbesondere über die Gattungen Rhinatrema und Gymnopis. Monatsberichte der Königlichen Preussische Akademie des Wissenschaften zu Berlin 1879: 924–945.

Image above from Digimorph. org and used with permission.

wiki/Dermophis