Entelognathus: revisions

Yesterday we looked at Entelognathus (Figs. 1-3; Zhu et al. 2013), a Silurian placoderm transitional to bony fish. That was my first placoderm and I made some errors that have since been corrected. Those errors were corrected when I realized the frontal (pineal in placoderms and Cheirolepis) originated as a tiny median (purple) triangle that included the pineal opening. I was also confused by the splitting of the parietal in Osteolepis, which I thought gave rise to the parietal/postparietal split, but instead that is an autapomorphy arising only in certain Osteolepis specimens. Further confusion comes from the fusion of bones, the splitting of bones and the different names given to the same bone in Silurian to Devonian taxa. Because of this, today and today only I will call the bones by the colors provided by Zhu et al. A key to their various names is provided (Fig. 1).

I was also surprised
to see that Zhu et al. 2013 found no trace of a purple/orange division in Entelognathus (Fig. 1f). This is odd for a transitional taxon, but still possible. Worth looking into. Equally odd, Zhu et al. did not color the purple bone consistently (Fig. 1).

The pineal opening drift
from the purple to the orange bones attends the lengthening of the rostrum and perhaps the brain and olfactory regions. The purple bone invades the paired orange bones and at the posterior tip of the parietal is the pineal opening. So the purple bone more or less delivers the pineal opening more or less in the middle of the orange bones.

Figure 1. From Zhu et al. 2013 SuppData showing placoderm and other basal vertebrate skull roofs. Note: Entelognathus is the only taxon without frontals, which I found in the photos of the fossil, figure 2.

Figure 1. From Zhu et al. 2013 SuppData showing placoderm and other basal vertebrate skull roofs. Note: Entelognathus is the only taxon without a frontal/parietal split, which I found in the photos of the fossil, figure 2 and corrected at the tip of the long arrow.

I traced bone sutures on photos of the specimen
and found that purple/orange division. So now Entelognathus has a complete set of skull roofing bones from the nasal to the frontal to the parietal and post parietal. I may have even seen where the yellow green intertemporal splits from the orange parietal.

Figure 2. Entelognathus fossil. Scale bar = 1 cm. Here the frontal/parietal division is shown.

Figure 2. Entelognathus fossil. Scale bar = 1 cm. Here the frontal/parietal division is shown. Rather than a median uture, one finds a medial ridge.

I hope to never do another fish.
But happy that I was able to resolve some earlier questions and move on. Feelings aside, mistakes that go on unnoticed are worse than mistakes you, or others, find and correct.

Figure 1. Entelognathus drawings from Zhu et al. 2013, with colors and homologous tetrapod bone. abbreviations added.

Figure e. Entelognathus drawings from Zhu et al. 2013, with colors and homologous tetrapod bone. abbreviations added. Corrected from an earlier version.

References
Zhu M, Yu X-B, Ahlberg PE, Choo B and 8 others 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 502:188–193.

Cheirolepis fossil images
wiki/Cheirolepis
wiki/Eusthenopteron
wiki/Entelognathus

 

Placoderm Entelognathus skull bones re-identified with tetrapod homologies

Images repaired May 18, 2017 after studying photos of the specimen, comparing related taxa and dispensing with false paradigms. Click here for more details. 

Barford 2013 wrote: 
“It may be hard to see, but you seem to share a family resemblance with Entelognathus primordialis. The fish, which lived 419 million years ago in an area that is now part of China, is the earliest known species with a modern jaw.” Here (Fig. 1) one can identify a complete set of homologous tetrapod skull bones understood by the original authors, who identified the bones with traditional placoderm names. (Ala, placoderms, bony fish and sacropterygians, including tetrapods, have different names for the same bone). And they made a mistake or two along the way, none of which negate their conclusions, but cement them.

I never thought I’d be featuring any placoderm fish in this blog
or in ReptileEvolution.com, but Entelognathus, as everyone already knows — and I just learned, is something very special. A major discovery. And this was my first day studying placoderms.

Barford 2013 reported, “Palaeontologists have traditionally believed that the fishes’ features bore no relation to ours. They assumed that the placoderm face was lost to evolutionary history, and most thought that the last common ancestor of living jawed vertebrates had no distinct jawbones — that it was similar to a shark, with a skeleton made mostly of cartilage and at most a covering of little bony plates. The theory went that the bony fishes evolved later, independently developing large facial bones and inventing the ‘modern’ jaw. Such fishes went on to dominate the seas and ultimately gave rise to land vertebrates. [Entelognathus] has what looks like a bony fish’s jaw, even though it is older than the earliest known sharks and bony fishes.”

According to Wikipedia
Entelognathus
 primordialis
 (Zhu et al. 2013; Late Ludlow, Silurian, 419 mya; IVPP V18620) “is a genus of placoderm fish with dermal marginal jaw bones (premaxilla,
maxilla and dentary), features previously restricted to Osteichthyes (bony fish).”

More than that,
all of the skull bones find homologies in tetrapods and bony fish (Figs. 1, 2) when certain bones are correctly identified or homologized. It just takes a few colors here and there to make it all clear.

Figure 1. Entelognathus drawings from Zhu et al. 2013, with colors and homologous tetrapod bone. abbreviations added.

Figure 1. Entelognathus drawings from Zhu et al. 2013, with colors and homologous tetrapod bone. abbreviations added. This revised image adds a small triangular frontal between the anterior processes of the parietal and the rest of the bones follow suit. 

All of the bones in the skull of Entelognathus
find homologies with those in Cheirolepis (Whiteaves 1881; Fig. 2) and also with tetrapods. Entelognathus lived 59 million years before the appearance of tetrapods like Ichthyostega. and is someday going to be a part of the story behind those Middle Devonian footprints.

Here new labels and colors
repair original errors and indicate tetrapod homologies in Entelognathus (Zhu et al. 2013).

  1. Three purported sclerotic bones are circumorbital bones (prefrontal, postfrontal, jugal)
  2. The purported jugal is the dorsal half of the maxilla before these bones fused.
  3. The purported quadratojugal is the posterior of the maxilla
  4. The rostral is the nasal
  5. The triangular frontal was overlooked
  6. The pineal plate is a pair of parietals
  7. The central plate is a pair of postparietals
  8. The marginal plate is the supratemporal
  9. The anterior paranuchal plate is the tabular
  10. The opercular is the quadratojugal
Figure 2. Cheirolepis skull (left) with skull bones colorized as in Osteolepis (right) and Enteognathus, figure 1. Colors make bone identification much easier. Note the post opercular bone differences between Osteolepis and Cheirolepis indicating separate and convergent derivation, based on present data.

Figure 2. Cheirolepis skull (left) with skull bones colorized as in Osteolepis (right) and Enteognathus, figure 1. Colors make bone identification much easier. Note the post opercular bone differences between Osteolepis and Cheirolepis indicating separate and convergent derivation, based on present data.

On the subject of nomenclature
Zhu et al. 2013 (SuppData) list the various names given to fish skull bones and their homologies in other fish clades. Some of the more confusing include:

  1. The parietal in sarcopterygians is the frontal in actinopterygians and the preorbital in placoderms.
  2. The postparietal in sarcopterygians is the parietal in actinopterygians and the central in placoderms.
  3. The supratemporal in sarcopterygians is the intertemporal in actinopterygians and the marginal in placoderms.
  4. The tabular in sarcopterygians is the supratemporal in actinopterygians and the anterior paranuchal in placoderms.
  5. And there are others…

Where is the authority that can fix this problem?
But if we fix it, then what? Then all prior literature will have to be translated. Either way, we’re hosed. Maybe we should just colorize homologous bones and leave it at that, as Zhu et al. did in their SuppData.

Entelognathus precedes Cheirolepis by 29 million years.
Preopercular and opercular bones do not appear in Entelognathus, but are present in Cheirolepis. So they are new bones in osteichythys.

The ‘al’ bone in Entelognathus (Fig. 1) is the cleithrum, supporting the pectoral fin.

The split (spiracle) between the skull roofing bones (intertemporal. supratemporal, tabular) and cheek bone (squamosal) do not appear in Entelognathus, but do so in Cheirolepis.

Sclerotic rings are not necessary in such small and well-protected eyes as in Entelognathus and if present, would have been very tiny and fragile.

Comparisons of the circumorbital bones in Entelognathus and Cheirolepis are strikingly similar down to the small post-orbit depression in the jugal in Entelognathus that becomes a notch in Cheirolepis.

Comparisons of the postopercular bones
of Cheirolepis and Osteolepis (Fig. 2) show little to no homology, suggesting a possible separate but convergent derivation.

Note some skull bones
later split apart at the median, while others fuse together. It’s their shapes and locations that identify them. “The large hexagonal central plate seems to have a single ossification centre, whereas most placoderms have paired centrals,” reports Zhu et al, making a case in point. A pineal opening is not present in the pineal plate (fused parietals) of Enteleognathus. This is further evidence that the pineal opening migrated from the frontals to the parietals over tens of millions of years. More on that tomorrow.

Barford 2013 concludes
“There remains a chance that E. primordialis evolved its jaw independently from the bony fish, so that we did not inherit it, and the resemblance is an illusion.” I don’t agree with this conclusion. The evidence for homology elsewhere overwhelms any competing hypotheses.

Friedman and Brazeau (2013) also comment on this discovery.
First, Entelognathus alwaybranches outside the radiation of living jawed vertebrates, meaning that key components othe osteichthyan face are no longer unique innovations of that group. Second, acanthodians — that pivotal assortment of extinct shark-like fishes — are shifted, en masse, tthe branch containing the cartilaginous fishes. This triggers a cascade of implications. If all acanthodians are early cartilaginous fishes, then their shark-like features are not generalities of jawed vertebrates, but specializations of the cartilaginous-fish branch. The most recent common ancestor of jawed vertebrates was thus probably clad in bonarmor othe sort common to both placoderms anbony fishes. This inversion of a classic scenario in vertebrate evolution raises an obvious question: how did we get it so wrong?”

In summary
Even when someone gets it right, some of the details may still be correctable – and the present corrections do not overturn the conclusion, but support it. As usual, I have not seen the fossil firsthand. I have not added Entelognathus to the LRT. I simply make comparisons to published figures of Cheirolepis, which was one source of the earlier problems I had, no hopefully settled.

Thanks to David M.
for directing me to the Entelognathus paper. : – )

Please let me know
if someone else has drawn the same insight in the last 4 years since the publication of Zhu et al. 2013. If so, I am unaware of it.

References
Barford E 2013. Ancient fish face shows roots of modern jaw. Nature News. online here.
Friedman M and Brazeau 2013. A jaw-dropping fossil fish. Nature 502:175-177. online here.
Whiteaves JF 1881. On some remarkable fossil fishes from the Devonian rocks of Scaumenac Bay, in the Province of Quebec. Annals and Magazine of Natural History. 8: 159–162.
Zhu M, Yu X-B, Ahlberg PE, Choo B and 8 others 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 502:188–193.

wiki/Cheirolepis
wiki/Entelognathus

Lambdotherium: not a basal brontothere — it’s another pig relative!

Earlier a putative stem brontothere, Danjiangia, was re-nested with basal artiodactyls in the large reptile tree (LRT, 1005 taxa).

Here another putative stem brontothere,
Lambdotherium (Cope 1880, Mader 1998; Eocene, 50mya; Fig. 1) likewise moves away from the basal brontothere, Eotitanops. In the LRT  Lambdotherium nests with Ancodus (Fig. 2), another basal artiodactyl close to extant pigs.

Figure 1. Lambdotherium traditionally nests with the basal brontothere, Eotitanops, but here nests with Ancodus, a basal artiodactyl.

Figure 1. Lambdotherium traditionally nests with the basal brontothere, Eotitanops (ghosted here), but here nests with Ancodus, a basal artiodactyl. Brontotheres have a very tall naris. Pigs do not. 

I don’t know of any post-crania
for Lambdotherium. Note that Ancodus (Fig. 2), like Eotitanops, has a pentadatyl manus. Lambdotherium was traditionally considered a brontothere based on its teeth. The LRT employs relatively few dental traits. And maybe some specimens need to be reexamined. The very high arch of the Lambdotherium squamosal, among many other traits, is more similar to pig-like taxa, than to basal brontotheres, which here nest closer to rhinos, than to horses, contra the Wikipedia report on brontotheres.

Distinct from both rhinos and horses,
brontotheres have four toes on the forefeet. All are derived from a sister to Hyrachyus, which likewise has four toes.

Figure 1. Ancodus nests as a more derived sister to Sus and it retains digit 1 on the manus and pes.

Figure 2. Ancodus nests as a more derived sister to Sus and it retains digit 1 on the manus and pes.

References
Cope ED 1880. The bad lands of the Wind River and their fauna. The American Naturalist 14(10):745-748.
Mader BJ 1998. Brontotheriidae. In Janis CM, Scott KM, and Jacobs LL (eds.), Evolution of Tertiary Mammals of North America 1:525-536.
Mihlbachler MC 2004. Phylogenetic Systematics of the Brontotheriidae (Mammalia, Perissodactyla). PhD dissertation. Columbia University. p. 757.
Mihlbachler MC 2008. Species taxonomy, phylogeny and biogeography of teh Brontotheriidae (Mammalia: Perissodactyla). Bulletin of the American Museum of Natural History 311:475pp.

wiki/Eotitanops
wiki/Lambdotherium

Chongmingia: no longer an enigma bird

Wang et al. 2016
reported on a head-less, ‘tail-less’ basal bird fossil, which they named Chongmingia (Fig. 1).

Figure 1. Chowmingia from Wang et al. 2016. Little red spots are added. Hand reconstructed differently from the original. Foot reconstructed.

Figure 1. Chongmingia from Wang et al. 2016. Little red spots are added. Hand reconstructed differently from the original. Foot reconstructed.

Unfortunately the team had some difficulty nesting Chongmingia.
They reported: “For the first analysis using the coelurosaurian matrix, the analysis produced 630 most parsimonious trees of 4523 steps (Consistency index = 0.266, Retention index = 0.578). The strict consensus tree placed Chongmingia within basal Avialae, and Chongmingia is the sister taxon of Ornithothoraces. For the second analysis focusing on phylogeny of Mesozoic birds, the analysis produced four most parsimonious trees of 1009 steps. The strict consensus tree places Chongmingia as the sister to all avialans except for Archaeopteryx, and thus Chongmingia represents the most primitive bird from the Jehol Biota uncovered to date and one of the most primitive Cretaceous birds known. However, this phylogenetic hypothesis was weakly supported by both Bremer and Bootstrap values.”

Unfortunately the team did not use several Solnhofen birds 
in their phylogenetic analysis. Perhaps if they did so, like the large reptile tree (LRT, 998 taxa) does, then they might have recovered a single tree in which Chongmingia nests within basal Enantiornithes in the LRT.

I was able to see in the published photo of Chongmingia

  1. a small string of diminishing caudal vertebrae
  2. the dorsal portion of the scapula
  3. the distinction between the scapula and coracoid (they thought it was fused)
  4. Manual digit 1 (beneath the metacarpus)

Such a small tail
like the similarly short-changed Protopteryxwould not have accommodated many rectrices (tail feathers). So Chongmingia might not have been as great a flyer as some of its relatives with pygostyles. Several other enantiornithes likewise do not preserve a tail of any sort.

References
Wang M, Wang X, Wang Y and  Zhou Z 2016. A new basal bird from China with implications for morphological diversity in early birds. Nature Scientific Reports 6, art. 19700, 2016.

wiki/Chongmingia

Pectodens: basal to tanystropheids and pterosaurs

It’s always good
to see another tritosaur. That’s the lineage that gave rise to a menagerie of taxa, including pterosaurs. That’s a heretical hypothesis of relationships recovered by the large reptile tree (LRT, 997 taxa).

Figure 1. Pectodens reconstructed using the original tracings of the in situ fossil in Li et al. 2017.

Figure 1. Pectodens reconstructed using the original tracings of the in situ fossil in Li et al. 2017.

Li et al. 2017 conclude:
“A new, small terrestrial tetrapod is described from the Middle Triassic of Yunnan, China. Pectodens zhenyuensis n. gen. n. sp. bears very characteristic elongate teeth forming a comb-like marginal dentition. The elongate cervicals of Pectodens zhenyuensis n. gen. n. sp. with low neural spines together with the morphology of the cervical ribs are features consistent with protorosaurs, such as Macrocnemus. However, the imperforate puboischiadic plate, simple rounded proximal tarsals, and a straight 5th metatarsal are primitive characteristics. Unlike tanystropheids, but in common with Protorosaurus (personal observation, N.C. Fraser, 2013), both lack a thyroid fenestra in the pelvis.”

Figure 2. Pectodens skull traced using DGS techniques and reassembled below.

Figure 2. Pectodens skull traced using DGS techniques and reassembled below. Here a quadratojugal process of the jugal is identified and other parts are assembled with greater accuracy than a freehand sketch (Fig. 1).

Pectodens zhenyuensis (Li et al. 2017; IVPP V18578; Anisian, Middle Triassic; 38cm in length) was originally considered a diapsid and a possible protorosaur. Here Pectodens nests between Macrocnemus and Langobardisaurus (Fig. 3). Originally the interclavicle, sternum and quadratojugal were overlooked.

Note the large orbit, the long metarsal 5 and the perforated pubis. The elongate caudal transverse processes anchor powerful leg muscles.

Occasionally within the Tritosauria
metatarsal 5 is not short, but elongate. It is always axially twisted. The pubis and ischium typically angle away from one another, but sometimes produce a thyroid fenestra. Tritosaurs have a sternum, like many other lepidosaurs do. Protorosaurs do not have a sternum.

Li et al. did not attempt a phylogenetic analysis.
Instead they made educated guesses as to the affinities of Pectodens, overlooking the variation present in related taxa revealed in a cladogram. Pulling a Larry Martin (highlighting or letting yourself get confused by one or two traits) is never a good idea. Better to let hundreds of traits determine the exact nesting of a taxon without bias. Let the taxa nest themselves. Let the convergent traits simply be convergent traits.

Earlier we looked at the pectoral girdle and sternum of Langobardisaurus, Huehuecuetzpalli and other tritosaurs. Pectodens fits right in.

The posterior maxillary teeth in Pectodens
are wider at their base presaging the grinding teeth found in Cosesaurus, basal pterosaurs and Langobardisaurus.

Note the way the fingers and toes
bend anteriorly during use. That’s a lepidosaur trait. Pectodens would have had sprawingling hind limbs given its simple femoral head. Tracks matching such curved toes are known from the Middle Triassic.

Li et al. considered Pectodens to be the first terrestrial taxon
from the its locality. And that’s definitely a probability. However, given that Tanystropheus and others may have been underwater bipedal predators (squid parts were found in their torso), let’s leave open the possibility that Pectodens was maybe dipping its toe in the water.

Figure 1. Subset of the LRT focusing on Tritosauria. Pectodens nests here basal to the Characiopoda (Tanystropheids + Fenestrasauria including pterosaurs).

Figure 1. Subset of the LRT focusing on Tritosauria. Pectodens nests here basal to the Characiopoda (Tanystropheids + Fenestrasauria including pterosaurs).

Let’s not continue to nest tanystropheids
with protorosaurs. Sure they share several traits by convergence, but they are not related to one another as determined by a large gamut analysis, the LRT.

References
Li C, Fraser NC, Rieppel O, Zhao L-J and Wang L-T 2017. A new diapsid from the Middle Triassic of southern China. Journal of Paleontology.7 pp. doi: 10.1017/jpa.2017.12

 

Ichthyostega’s toes – evidence of regeneration?

Figue 1. The pes (foot) of Ichthyostega has 7 digits. Those five that most parsimoniously match related taxa are  listed. The vestigial digit between 2 and 3 may be the result of injury and rejuvenation.

Figue 1. The pes (foot) of Ichthyostega has 7 digits. Those five that most parsimoniously match related taxa are listed. The vestigial digit between 2 and 3 may be the result of injury and imperfect or unfinished regeneration.

You might remember
earlier the basal tetrapod Ichthyostega (Fig. 1) shifted its nesting closer to Proterogyrinus (Figs. 2, 3) and Eucritta (Fig. 4) at the base of the Reptilomorpha. One of the reasons for that shift was a reexamination of the pes of Ichthyostega, which has seven digits. Which digits are homologous with the five that are found in many other higher tetrapods?

Figure 2. Proterogyrinus pes according to Holmes.

Figure 2. Proterogyrinus pes according to Holmes.

Metatarsal and phalangeal proportions 
provide clues. If the above digit identities ares used, there is a pretty close match to related taxa. Acanthostega, for instance, has eight pedal digits with metatarsal 3 about twice as long as the more medial metatarsals. Distinct from Ichthyostega, Acanthostega has only one phalanx on digit 1 and only 2 phalanges on digit 2, but in keeping with the ‘one less’ phalangeal formula, digits 3–7 stop at 3 phalanges. In Ichthyostega digits 4 and 5 each add a phalanx, approaching the pattern seen in Proterogyrinus.

Figure 3. Proterogyrinus pedes in situ (black) and restored (blue).

Figure 3. Proterogyrinus pedes in situ (black) and restored (blue).

Holmes 1984
reconstructed the pes of Proterogyrinus (Fig. 2). If one takes the data from in situ drawings provided by Holmes (Fig. 3), reconstructions of both pedes can be created to check the accuracy of the Holmes reconstruction while removing any freehand bias.

Figure 4. Eucritta in situ and reconstructed. Note the large pes in green.

Figure 4. Eucritta in situ and reconstructed. Note the large pes in green.

The pes of the related Eucrtta also bears another look.
It is more difficult to reconstruct based on the taphonomic scattering of the elements. If you’ll notice the medial three digits of Eucritta each appear to have one less phalanx, as in Acanthostega.

 Which makes one wonder about Ichthyostega.
The vestigial digit between 2 and 3 in particular gives one pause. We know that salamanders can regrow their extremities. Based on the unusual apparent binding of pedal digits 1 and 2 in Ichthyostega, along with the vestige of a digit between 2 and 3, One may wonder if that unusual morphology is the result of an accident or injury with subsequent imperfect or unfinished regeneration. Another identical Ichthyostega pes would falsify this hypothesis.

References
Ahlberg PE, Clack JA and Blom H 2005. The axial skeleton of the Devonian trtrapod Ichthyostega. Nature 437(1): 137-140.
Clack JA 1998. A new Early Carboniferous tetrapod with a mélange of crown group characters. Nature 394: 66-69.
Clack JA 2007. Eucritta melanolimnetes from the Early Carboniferous of Scotland, a stem tetrapod showing a mosaic of characteristics. Transactions of The Royal Society of Edinburgh 92:75-95.
Holmes R 1984. The Carboniferous Amphibian Proterogyrinus scheelei Romer, and the Early Evolution of Tetrapods. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 306: 431-524.
Jarvik E 1952. On the fish-like tail in the ichthyostegid stegocephalians. Meddelelser om Grønland 114: 1-90.
Jarvik E 1996. The Devonian tetrapod Ichthyostega. Fossils and Strata. 40:1-213.
Romer AS 1970. A new anthracosaurian labyrinthodont, Proterogyrinus scheelei, from the Lower Carboniferous. Kirtlandia 10:1-16.
Ruta M, Jeffery JE and Coates MI 2003. A supertree of early tetrapods. Proc. R. Soc. Lond. B (2003) 270, 2507–2516 DOI 10.1098/rspb.2003.2524 online pdf
Säve-Söderbergh G 1932. Preliminary notes on Devonian stegocephalians from East Greenland. Meddelelser øm Grönland 94: 1-211.

wiki/Ichthyostega
wiki/Eucritta
wiki/Proterogyrinus

Stegops spikes?

Stegops divaricata (Cope 1885; AMNH 2559; 5.6 cm skull length; Westphalian, Late Carboniferous, 310 mya) is a basal tetrapod that has bounced around the family tree without settling down.

Moodie 1916 reported 
the skull of Stegops was small, oval and “the quadrate angles project into sharp horns.” One can presume Moodie meant the squamosal had horns, because that’s how he drew them (Fig. 1). The quadrates in this and related taxa are hidden beneath the cheek bones. He considered Stegops a microsaur.

Figure 1. Stegops does not have the squamosal spikes shown by Moodie 1916, but does have a deep squamosal roofed over by an extended cranium with long tabulars. And little spikes appear to be present on several temporal bones.

Figure 1. Stegops does not have the squamosal spikes shown by Moodie 1916, but does have a deep squamosal roofed over by an extended cranium with long tabulars. And little spikes appear to be present on several temporal bones. You’ll have to look hard to see them.

According to Wikipedia:
“Stegops is an extinct genus of euskelian temnospondyl from the Late Carboniferous of the eastern United States. Fossils are known from the Pennsylvanian coal deposits of Linton, Ohio. It was once classified in the eryopoid family Zatrachydidae because it and other zatrachydids have spikes extending from the margins of its skull, but it is now classified as a dissorophoid that independently evolved spikes.”

After Moodie 1916,
this taxon was largely ignored for decades until about ten years ago.

Then Milner and Schoch 2005 reported:
“The spiky-headed temnospondyl amphibian Stegops divaricata from the Middle Pennsylvanian coal of Linton, Ohio has remained neglected and enigmatic for several decades. It has been argued to be the ancestor of the Permian Zatrachydidae, also spiky-headed temnospondyls, although there are few resemblances other than the spikes. An examination of previously undescribed material of Stegops, along with a re-evaluation of the original specimens, permits a redescription and partial systematic assignment of it. All specimens have bony spikes on the tabular, quadratojugal and angular, but in apparent dimorphism, only some have squamosal and supratemporal spikes. A phylogenetic analysis of 52 characters in 15 temnospondyl taxa places Stegops within the dissorophoid clade but leaves its position uncertain within that clade. The Zatrachydidae, represented by Acanthostomatops, fall outside the Dissorophoidea, and the zatrachydid affinities of Stegops asserted by previous workers are based on homoplasious similarities in ornamentation. Internal relationships of the Dissorophoidea remain unresolved and Stegops shares conflicting similarities with Amphibamidae in some resolutions and with an Ecolsonia + Dissorophidae + Trematopidae clade in others.”

Figure 2. Dissorophus nests with Stegops among basal lepospondyls in the LRT.

Figure 2. Dissorophus nests with Stegops among basal lepospondyls in the LRT.

After phylogenetic analysis
Stegops nested with Dissorophus (Fig. 2) agreeing with Milner and Schoch. The new reconstruction bears little resemblance to the Moodie illustration (Fig. 1). The open palate with palatine exposure on the cheek, together with a deeply emarginated squamosal roofed over by large supratemporals and tabulars are traits uniting thiese taxa. In the large reptile tree (LRT) dissorphids nest with basal lepospondyls.

References
Milner AR and Schoch RR 2005. Stegops. A problematic spiky-headed temnospondyl
SVPCA Platform Presentation, (London)
Moodie RL 1909. Journal of Geology 17(1):79
Moodie RL 1916. The microsaurian family stegpidae. The coal measures amphibia of North America. Carnegie Institution of Washintion 238: 222pp.

wiki/Stegops