Tulerpeton pes options

Earlier the pedal elements of the amphibian-like reptile Tulerpeton were moved around to produce a reasonable reconstruction. Today I offer a few more options (Fig. 1) including one with six toes. All appear to be reasonable.

Figure 1. Tulerpeton pes reconstruction options using published images of the in situ fossil.

Figure 1. Tulerpeton pes reconstruction options using published images of the in situ fossil.

None of these reconstructions
changes the nesting of Tulerpeton as the basalmost Reptile (=Amniote). Such long toes with so many phalanges in these patterns of relative length are not found in basal tetrapods.

Figure 1. Tulerpeton parts from Lebedev and Coates 1995 here colorized and newly reconstructed. Manus and pes enlarged in figure 2.

Figure 2 Tulerpeton parts from Lebedev and Coates 1995 here colorized and newly reconstructed. Manus and pes enlarged in figure 2.

A little backstory
Tulerpeton curtum (Lebedev 1984, Fammenian, Latest Devonian, 365 mya) was described as, “one of the first true tetrapods to have arisen.” Here it nests as the basalmost reptile, pushing Gephyostegus bohemicus back to the pre-amniotes. Very little other than the limbs are known. In life it would have been similar to and the size of Gephyrostegus, Urumqia and EldeceeonTulerpeton lived in shallow marine waters.

References
Coates MI and Ruta M 2001 2002. Fins to limbs: What the fossils say. Evolution & Development 4(5): 390–401.
Lebedev OA 1984. The first find of a Devonian tetrapod in USSR. Doklady Akad. Navk. SSSR. 278: 1407–1413.
Lebedev OA and Clack JA 1993. Upper Devonian tetrapods from Andreyeva, Tula Region, Russia. Paleontology36: 721-734.
Lebedev OA and Coates MI 1995. postcranial skeleton of the Devonian tetrapod Tulerpeton curtum Lebedev. Zoological Journal of the Linnean Society. 114 (3): 307–348.

wiki/Tulerpeton

 

 

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

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

 

Basal tetrapods revised with more taxa

Full resolution during a Heuristic search was not enough.
Full resolution with high Bootstrap scores was the goal. Reexamination of the data would hopefully get to that goal, as it did so many times before. Sometimes it takes awhile. It’s a learning process, and I learned a lot over the last several weeks, sometimes from difficult and scrappy data. Here’s the result:

Figure 1. Subset of the large reptile tree (LRT) focusing on basal tetrapods.

Figure 1. Subset of the large reptile tree (LRT) focusing on basal tetrapods.

Some interesting results here. 

  1. Large temnospondyls are now split in two  (with, as before, many former small temnospondyls joining the equally small lepospondyls).
  2. Ichthyostega, now not so primitive, nests closer to Reptilomorpha.
  3. New reconstructions are offered for some taxa, like Tuditanus and Utaherpeton.
  4. Basal diplocaulids, like Keraterapeton, were added.
  5. Two taxa known as Trematosaurus, one with a shorter rostrum, one with a longer one, are split apart on the tree. Gavial-like snouts are not monophyletic at present, but long-nouted forms do not have long snouts as juveniles. This is a well-known quagmire I may get into later.

Look for more basal tetrapods with legs, not fins in the Late Devonian.
Not sure where they are, but they are out there. Apparently there were several ventures onto land, not just one fin-to-finger transition.

In a few days
I’ll start with some of the interesting details as time allows, but basically this completes the task, the tree, and the broad strokes that hypothetically echo the origin of reptiles and the variation that followed thereafter.

 

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

Trimerorhachis: a late survivor of the fin/finger transition?

Figure 1. Trimerorhachis was considered a dvinosaurian temnospondyl. Here both Trimerorhachis and Dvinosaurus nest low on the basal tetrapod tree, close to the fin/finger transition.

Figure 1. Flattened Trimerorhachis was considered a dvinosaurian temnospondyl. Here both Trimerorhachis and Dvinosaurus nest low on the basal tetrapod tree, close to the fin/finger transition, not within the Temnospondyli. Both are late survivors of a Devonian radiation.

Wikipedia reports:
Trimerorhachis (Early Permian, (Cope 1878, Case 1935, Schoch 2013; up to 1m in length) is an extinct genus of dvinosaurian temnospondyl within the family Trimerorhachidae. The trunk is long and the limbs are relatively short. Many bones are poorly ossified, indicating that Trimerorhachis was poorly suited for movement on land. The presence of a branchial apparatus indicates that Trimerorhachis had external gills in life. The body of Trimerorhachis is also completely covered by small and very thin osteoderms, which overlap and can be up to 20 layers thick. The scales were more similar to fish scales than they were to reptile scales, according to Colbert 1955. However, Olson 1979 disputed that interpretation. Specimens are often preserved as masses of bones that are mixed together and densely packed in slabs of rock”

Figure 2. Trimerorhachis forelimb and hind limb in situ and reconstructed.

Figure 2. Trimerorhachis forelimb and hind limb in situ and reconstructed. Pawley 1979 did not report metacarpals or a pubis. It is possible and perhaps likely that only 4 metacarpals were present along with two phalanges, but its worth exploring all possibilities. 

As a late (Early Permian) survivor of a Late Devonian radiation
Trimerorhachis evolved by convergence certain traits found in other more derived tetrapods, like a longer femur and open palate (narrow, bowed pterygoids). Testing all possibilities while minimizing assumptions is the most valuable benefit of a large gamut phylogenetic analysis conducted by unbiased software. Workers used to eyeball specimens in the pre-computer days.

Figure 2. Trimerorhachis pelvis. The pubis is not ossified.

Figure 3. Trimerorhachis pelvis. The pubis is not ossified here, according to Pawley 1979, but see Fig. 1.

Like other workers,
Pawley 1979 considered Trimerorhachis close to Dvinosaurus (Fig. 7) and both thought to be derived from the basal temnospondyl Balanerpeton and Dendrerpeton. The large reptile tree (LRT) nests both taxa at the base of the Lepodpondyli, not closely related to Trimerorhachis and distinct from Temnospondyli. Pawley supports the hypothesis that aquatic ‘temnospondyls,’ like Trimerorhachis, had terrestrial ancestors. By contrast, the LRT nests Trimerorhachis with weak-limbed taxa more primitive than any temnospondyl.

Additionally
the LRT nests Batrachosaurus and Gerrothorax in the Dvinosaurus / Trimerorhachis clade. This clade features horizontally opposed dorsal ribs and an equally flattened skull. Another flattened taxon, Ossinodus, is closely related. I have not seen limb material for any of these taxa. Acanthostega is the closest taxon that preserves limbs.

Figure 3. Trimerorhachis hind limb and pes from Pawley 1979.

Figure 4. Trimerorhachis hind limb and pes from Pawley 1979 and reconstructed here.

Pawley 1979 noted,
“The vast majority of the [Trimerorhachis] specimens consists of ornamental cranial and pectoral girdle bones, intercentra, and larger elements of the appendicular skeleton. Neural arches, pleurocentra, ribs and distal limb elements are rare.” No sacrals were found by Pawley. No dorsal ribs had uncinate processes (like those in Ichthyostega and Eryops). The chevrons were long and tapered distally (creating a fin?). The interclavicle was diamond-shaped with a longer anterior portion.

Figure 4. Trimerorhachis humerus changes during ontogeny

Figure 5. Trimerorhachis humerus changes during ontogeny

The humerus
(Fig. 5) was  L-shaped and the degree of torsion varied between specimens from 45º to 90º. The distal end always exhibited a low degree of ossification.

Figure 6. Trimerorhachis cladogram. Gray area is the Temnospondyli clade.

Figure 6. Trimerorhachis cladogram. Gray area is the Temnospondyli clade.

Pawley considered
Trimerorhachis a secondarily adapted aquatic temnospondyl. All workers have noted the wide open palate vacuities that characterize most, but not all members of the Temnospondyli. By contrast, the LRT nests Trimerorhachis with taxa that had not yet left the water completely and shared a flat morphology with Tiktaalik and Panderichthys.

This is the second time
elongate limbs and digits have appeared by convergence in basal tetrapods. Earlier Pholidogaster and kin provided the first exceptions to the rule. Note that all known specimens of Trimerorhachis are Early Permian, some tens of millions of years later than the Late Devonian radiation of that clade. The Ichthyostega line is the one that ultimately produced crown Tetrapoda via a sister to Eucritta.

FIgure 8. Dvinosaurus nests with Trimerorhachis and also has ceratobranchial (gill) bones.

FIgure 7. Dvinosaurus nests with Trimerorhachis and also has ceratobranchial (gill) bones. The loss of the intertemoral is shown here in light green merging to the postorbital in orange. 

If these nestings are not correct
and Trimerorhachis ultimately nests higher on the basal tetrapod tree, then we’re witnessing massive convergence of another sort, convergence that allies Trimerorhachis with tetrapods at the fin/finger transition. I’d like to see limbs for Gerrothorax or any other plagiosaur, if available.

Figure 9. Ossinodus is a close relative of Trimerorhachis in the LRT.

Figure 8. Ossinodus is a close relative of Trimerorhachis in the LRT. 

By the way, I find this fascinating…
week after week, far and away the most popular page(s) on this blog continue to be on the origin of bats.

References
Berman DS and Reisz RR 1980. A new species of Trimerorhachis (Amphibia, Temnospondyli) from the Lower Permian Abo Formation of New Mexico, with discussion of Permian faunal distributions in that state. Annals of the Carnegie Museum. 49: 455–485.
Case EC 1935. Description of a collection of associated skeletons of Trimerorhachis. University of Michigan Contributions from the Museum of Paleontology. 4 (13): 227–274.
Colbert EH 1955. Scales in the Permian amphibian Trimerorhachis. American Museum Novitates. 1740: 1–17.
Olson EC 1979. Aspects of the biology of Trimerorhachis (Amphibia: Temnospondyli). Journal of Paleontology. 53 (1): 1–17.
Pawley K 2007. The postcranial skeleton of Trimerorhachis insignis Cope, 1878 (Temnospondyli: Trimerorhachidae): a plesiomorphic temnospondyl from the Lower Permian of North America. Journal of Paleontology. 81 (5):
Williston SW 1915. Trimerorhachis, a Permian temnospondyl amphibian. The Journal of Geology. 23 (3): 246–255.
Williston SW 1916. The skeleton of Trimerorhachis. The Journal of Geology. 24 (3): 291–297.

wiki/Trimerorhachis

Trimerorhachis and kin to scale

Updated April 23 with a revision to the tabulars of Panderichthys. Thanks DM! My bad. 

Yesterday we took a revisionary look at Trimerorhachis insignis (Cope 1878, Case 1935, Schoch 2013; Early Permian; 1m in length; Fig. 1). Today we take a quick peek at the taxa that surround it in the large reptile tree (LRT, 980 taxa, Fig. 1) all presented to scale. Several of these interrelationships have gone previously unrecognized. Hopefully seeing related taxa together will help one focus on their similarities and differences.

Figure 1. Trimerorhachis and kin to scale. Here are Panderichthys, Tiktaalik, Ossinodus, Dvinosaurus, Acanthostega, Batrachosuchus and Gerrothorax. Maybe those tabular horns on Acanthostega are really supratemporal horns, based on comparisons to related taxa.

Figure 1. Trimerorhachis and kin to scale. Here are Panderichthys, Tiktaalik, Ossinodus, Dvinosaurus, Acanthostega, Batrachosuchus and Gerrothorax. Maybe those tabular horns on Acanthostega are really supratemporal horns, based on comparisons to related taxa.

And once again
phylogenetic miniaturization appears at the base of a tetrapod clade. Note: the small size of Trimerorhachis (Fig. 1) may be due to the tens of millions of years that separate it in the Early Permian from its initial radiation in the Late Devonian, at which time similar specimens might have been larger. Provisionallly, we have to go with available evidence.

We start with…

Panderichthys rhombolepis (Gross 1941; Frasnian, Late Devonian, 380 mya; 90-130cm long; Fig. 1). Distinct from basal taxa, like Osteolepis, Pandericthys had a wide low skull, a wide low torso, a short tail and five digits (or metacarpals). No interfrontal was present. The orbits were further back and higher on the skull. Dorsal ribs, a pelvis and large bones within the four limbs were present.

Tiktaalik roseae (Daeschler, Shubin and Jenkins 2006; Late Devonian, 375mya: Fig. 1) nests between Pandericthys and Trimerorhachis in the LRT. Distinct from Panderichthys the opercular bones were absent and the orbits were even further back on the skull.

Ossinodus pueri (Warren and Turner 2004; Viséan, Lower Carboniferous; Fig. 1) was orignally considered close to Whatcheeria. Here it nests between Trimerorhachis and Acanthostega. The presence of an intertemporal appears likely. Distinct from Acanthostega, the skull is flatter, the naris is larger. Distinct from sister taxa, the maxilla is deep and houses twin canine fangs. A third fang arises from the palatine.

Acanthostega gunnari (Jarvik 1952; Clack 2006; Famennian, Late Devonian, 365mya; 60cm in length; Fig. 1) was an early tetrapod documenting the transition from fins to fingers and toes. Based on its size and placement, the nearly circular bone surrounding the otic notch is here identified as a supratemporal, not a tabular, which appears to be lost or a vestige fused to the supratemporal. This taxon is derived from a sister to Ossinodus and appears to have been an evolutionary dead end.

Trimerorhachis insignis (Cope 1878, Case 1935, Schoch 2013; Early Permian; 1m in length; Fig. 1) was considered a temnospondyl close to Dvinosaurus, but here nests as a late surviving basal tetrapod from the Late Devonian fin to finger transition. It is close to Ossinodus and still basal to Dvinosaurus (Fig. 1) and the plagiosaurs. As a late survivor, Trimerorhachis evolved certain traits found in other more derived tetrapods by convergence, like a longer femur and open palate. The presence of a branchial apparatus indicates that Trimerorhachis had gills in life. Dorsally Trimerorhachis was covered with elongated scales, similar to fish scales.

Dvinosaurus primus (Amalitzky 1921; Late Permian; PIN2005/35; Fig. 1) Dvinosauria traditionally include Neldasaurus among tested taxa. Here Dvinosaurus nests basal to plagiosaurs like Batrachosuchus and Gerrothorax and was derived from a sister to Trimerorhachis.

Batrachosuchus browni (Broom 1903; Early Triassic, 250 mya; Fig. 1) nests with Gerrothorax, but does not have quite so wide a skull.

Gerrothorax pulcherrimus (Nilsson 1934, Jenkins et al. 2008; Late Triassic; Fig. 1) was originally considered a plagiosaurine temnospondyl. Here it nests with the Trimerorhachis clade some of which  share a lack of a supratemporal-tabular rim, straight lateral ribs and other traits.

This clade of flathead basal tetrapods
is convergent with the flat-headed Spathicephalus and Metoposaurus clades and several others.

References
Berman DS and Reisz RR 1980. A new species of Trimerorhachis (Amphibia, Temnospondyli) from the Lower Permian Abo Formation of New Mexico, with discussion of Permian faunal distributions in that state. Annals of the Carnegie Museum. 49: 455–485.
Broom R 1903. On a new Stegocephalian (Batrachosuchus browni) from the Karroo Beds of Aliwal North, South Africa. Geological Magazine, New Series, Decade IV 10(11):499-501
Case EC 1935. 
Description of a collection of associated skeletons of Trimerorhachis. University of Michigan Contributions from the Museum of Paleontology. 4 (13): 227–274.
Clack JA 2006. The emergence of early tetrapods. Palaeogeography Palaeoclimatology Palaeoecology. 232: 167–189.
Clack JA 2009. The fin to limb transition: new data, interpretations, and hypotheses from paleontology and developmental biology. Annual Review of Earth and Planetary Sciences. 37: 163–179.
Coates MI 2014. The Devonian tetrapod Acanthostega gunnari Jarvik: Postcranial anatomy, basal tetrapod interrelationships and patterns of skeletal evolution. Earth and Environmental Science Transactions of the Royal Society of Edinburgh.
Coates MI and Clack JA 1990. Polydactly in the earliest known tetrapod limbs. Nature 347: 66-69.
Colbert EH 1955. Scales in the Permian amphibian. American Museum Novitates. 1740: 1–17.
Daeschler EB, Shubin NH and Jenkins FA, Jr 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature. 440 (7085): 757–763.
Gross W 1941. Über den Unterkiefer einiger devonischer Crossopterygier (About the lower jaw of some Devonian crossopterygians), Abhandlungen der preußischen Akademie der Wissenschaften Jahrgang.
Jarvik E 1952. On the fish-like tail in the ichtyhyostegid stegocephalians. Meddelelser om Grønland 114: 1–90.
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wiki/Ossinodus
wiki/Acanthostega
wiki/Tiktaalik
wiki/Panderichthys
wiki/Trimerorhachis
wiki/Gerrothorax
wiki/Batrachosuchus