The ‘armadillo’ ‘frog’: Dissorophus

Figure 1. Images from Cope 1896 of the armored dissorophid, Dissorophus (=Otocoelus)

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.

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)

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The origin of Lissamphibia (frogs, salamanders, caecilians)

The origin of modern amphibians has been controversial.
A new paper by Pérez-Ben et al. 2018 seeks to clarify the issue. According to Wikpedia, “Currently, the three prevailing theories of lissamphibian origin are:

  1. Monophyletic within the temnospondyli
  2. Monophyletic within lepospondyli
  3. Diphyletic (two separate ancestries) with apodans within the lepospondyls and salamanders and frogs within the temnospondyli.”

From the Pérez-Ben et al. abstract:
Current hypotheses propose that the living amphibians (lissamphibians) originated within a clade of Paleozoic dwarfed dissorophoid temnospondyls. Morphological traits shared by these small dissorophoids have been interpreted as resulting from constraints imposed by the extreme size reduction, but these statements were based only on qualitative observations. Herein, we assess quantitatively morphological changes in the skull previously associated with miniaturization in the lissamphibian stem lineage by comparing evolutionary and ontogenetic allometries in dissorophoids. Our results show that these features are not comparable to the morphological consequences of extreme size reduction as documented in extant miniature amphibians, but instead they resemble immature conditions of larger temnospondyls. We conclude that the truncation of the ancestral ontogeny, and not constraints related to miniaturization, might have been the factor that played a major role in the morphological evolution of small dissorophoids.

The authors appear to be dividing
tiny (miniaturized) frogs from frogs in general (= immature temnospondyls). And that’s a good start.

The second hypothesis (above)
is supported and recovered in the large reptile tree (LRT (1154 taxa, subset in figure 1) in which Lissamphibians are indeed derived from dissorophids, but dissorophids are lepospondyls (yellow-green clade below), which are derived from reptilomorphs and seymouriamorphs (orange clade below), while temnospondyls are much more primitive and diphyletic (pink and blue clades below). The phylogenetic miniaturization occurred much earlier than Lissamphibia, which is a much larger clade if it is still defined by the inclusion of the more distantly related caecilians, deep within the Microsauria.

FIgure 2. Subset of the LRT has a larger gamut of taxa. Here lepospondyls nest together when more basal tetrapods are added to the taxon list than are present in figure 1.

FIgure 2. Subset of the LRT has a larger gamut of taxa. Here lepospondyls nest together when more basal tetrapods are added to the taxon list than are present in figure 1.

Quantitive approaches
have never trumped phylogenetic approaches.

First: Recover the cladogram.
Let it tell you what happened, and when and how. The dissorophids are indeed derived from more primitive temnospondyls, but several intervening transitional clades must be accounted for. 

References
Pérez-Ben CM, Schoch RR and Báez  AM 2018. Miniaturization and morphological evolution in Paleozoic relatives of living amphibians: a quantitative approach
https://doi.org/10.1017/pab.2017.22Published online: 23 January 2018

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, the tadpole of the two.

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. Ossinodus, Pederpes were more primitive than the more aquatic Icthyostega.

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.

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.  Red comment was added by me.

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 stock skull, drawing from Pardo et al. 2017 and colorized here.

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

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 2. Pardo et al. cladogram nesting Lethiscus between vertebrates with fins and vertebrates with fingers. They also nest microsaurs as amniotes (reptiles). None of this is supported by the LRT.

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, OphiderpetonOestocephalus and RileymillerusAll 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 does not have much of a neck.

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

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

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.

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.

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

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