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

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. Note the twin supratemporals on Panderichthys determined by comparison to related taxa. And 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. Note the twin supratemporals on Panderichthys determined by comparison to related taxa. And 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. he presence of a branchial apparatus indicates that Trimerorhachis had external 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.
Jenkins FA Jr, Shubin NH, Gates SM and Warren A 2008. Gerrothorax pulcherrimus from the Upper Triassic Fleming Fjord Formation of East Greenland and a reassessment of head lifting in temnospondyl feeding. Journal of Vertebrate Paleontology. 28 (4): 935–950.
Nilsson T 1934. Vorläufige mitteilung über einen Stegocephalenfund aus dem Rhät Schonens. Geologiska Föreningens I Stockholm Förehandlingar 56:428-442.
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):
Warren A and Turner S 2004. The first stem tetrapod from the Lower Carboniferous of Gondwana. Palaeontology 47(1):151-184.
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/Ossinodus
wiki/Acanthostega
wiki/Tiktaalik
wiki/Panderichthys
wiki/Trimerorhachis
wiki/Gerrothorax
wiki/Batrachosuchus

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

Distribution of ‘key’ traits in basal tetrapods

Before the advent of phylogenetic analysis,
paleontologists attempted to define clades with a short list of synapomorphies. In this way they were getting close to the dangers of pulling a Larry Martin. Many taxa, like pterosaurs and Vancleavea were (and are) considered enigmas because they seemed to appear suddenly in the fossil record with a short suite of traits that did not appear in other reptiles. That was only true back then because paleontologists were only considering short lists of traits.

After the advent of phylogenetic analysis
considering long lists of traits, the rule of maximum parsimony allowed clades to include members that do not have a short list of key traits. For instance some reptiles, like snakes, do not have limbs, but that’s okay based on the rule of maximum parsimony as demonstrated in the large reptile tree (LRT, 977 taxa, subsets shown in Figs. 1-5).

Before the advent of phylogenetic analysis
Carroll (1988) divided basal tetrapods into labyrinthodonts and lepospondyls and presented short lists of key traits.

Labyrinthodonts

  1. evolved directly from rhipidistian fish
  2. labyrinthine infolding of the dentine
  3. palate fangs and replacement pits
  4. vertebral centra composed of more than one element
  5. otic notch
  6. large in size

Lepospondyls

  1. a heterogeneous assemblage of groups with perhaps several origins from among various labyrinthodonts
  2. simple (non-labyrinthine) teeth
  3. no palate fangs
  4. vertebral centra composed of one element
  5. no otic notch
  6. small in size

By contrast,
the large reptile tree introduces a non-traditional topology in which lepospondyls have a single origin. Below (Figs. 1-5) the distribution of several traits are presented graphically.

Figure 1. Distribution of the solid and open palate architectures in basal tetrapods in the LRT topology.

Figure 1. Distribution of the solid and open palate architectures in basal tetrapods in the LRT topology.

Open palate distribution
Basal tetrapods have a solid palate (Fig. 1) in which the pterygoid is broad and leaves no space around the medial cultriform process. Other taxa have narrow pterygoids and large open spaces surrounding the cultriform process. Still others are midway between the two extremes. Traditional topologies attempt to put all open palate taxa into a single clade. Here the open palate evolved three times by convergence.

Figure 2. Size distribution among basal tetrapods in the LRT topology

Figure 2. Size distribution among basal tetrapods in the LRT topology

The length of basal tetrapods
falls below 60 cm in Eucritta and more derived taxa. It also falls below 60 cm in Ostelepis, at the origin of Tetrapoda and Paratetrapoda. Phlegethontia has a small skull, but is otherwise like an eel, and so does not fall below the 60 cm threshold.

Figure 3. Distribution of single vertebrae among basal tetrapods in the LRT.

Figure 3. Distribution of single vertebrae among basal tetrapods in the LRT.

Single piece centra
appear in frogs + salamanders, microsaurs and Phlegethontia, by convergence. Intercentra appear in all other taxa.

Figure 6. Distribution of palatal fangs among basal tetrapods in the LRT.

Figure 6. Distribution of palatal fangs among basal tetrapods in the LRT.

Palate fangs
appear in all basal paratetrapods and tetrapods except Phlegethontia, Spathicephalus and Gerrothorax. Exceptionally, Seymouria also had palate fangs.

Figure 7. Distribution of the otic notch among basal tetrapods in the LRT.

Figure 7. Distribution of the otic notch among basal tetrapods in the LRT.

The otic notch
is widespread among basal tetrapods. Those without an otic notch include

  1. One specimen of Phlegethontia that loses posterior skull bones
  2. Six flat-skulled temnospondyls in which the tabular contacts the squamosal. Some of these, like Greererpeton, have figure data that lack an otic notch, but photos that have one.
  3. Salamanders and frogs that greatly reduce posterior skull bones.
  4. All microsaurs more derived than Microbrachis

Let me know
if I overlooked or misrepresented any pertinent data. This weekend I should be able to look at and respond to the many dozen comments that have accumulated over the last few weeks.

 

Basal Tetrapods, slightly revised

Figure 1. Click to enlarge. With the addition of Panderichthys and Anthracosaurus the position of Koilops and Deltaherpeton have shifted to the base of the Temnospondyli.

Figure 1. Click to enlarge. With the addition of Panderichthys and Anthracosaurus the position of Koilops and Deltaherpeton have shifted to the base of the Temnospondyli. Some of that shifting is due to rescoring.

After earlier identifying
phylogenetic miniaturization at the bases of several major clades in the large reptile tree (LRT, 969 taxa), I wondered if similar size-related patterns appear in basal tetrapods.

  1. Osteolepis is smaller than Eusthenopteron. Has anyone removed the scales from the fore fins of Osteolepis to see what the bones inside look like?
  2. Pholidogaster is much larger than Osteolepis, but Colosteus and Phlegethontia are successively smaller with smaller limbs.
  3. Ventastaga and Pederpes are successively smaller than Ichthyostega.
  4. Koilops is much smaller than Ventastaga and Pederpes
  5. Eucritta is much smaller than Proterogyrinus, both in overall size and in relative torso length. Eucritta nests at the base of the Seymouriamorpha + Crown Tetrapoda.
Figure 2. Basal tetrapod skulls in dorsal view.

Figure 2. Basal tetrapod skulls in dorsal view. Tetrapoda arise with flattened skulls. Paratetrapoda retain skulls with a circular cross section. 

 

Anthracosaurus: beware the chimaera!

Figure 1. The complete skull of Anthracosaurus greatly resembles its relative, Neopteroplax.

Figure 1. The complete skull of Anthracosaurus greatly resembles its relative, Neopteroplax.

Anthracosaurus russelli (Huxley 1863, Panchen 1977, Clack 1987; Westphalian, Late Carboniferous, 310 mya, skull length 40cm; Figs. 1, 2) was originally considered a labyrinthodont. The wide, yet pointed, triangular skull and tall orbits recall traits found in labyrinthodonts, like Sclerocephalus, and in the basal tetrapod, Tiktaalik. Here, in the large reptile tree (LRT, 967 taxa),  Anthracosaurus nests with Neopteroplax (Fig. 3) as a derived embolomere, the clade that likely gave rise to Seymouriamorpha, Lepospondyli and Reptilia

At least one orbit
in Anthracosaurus has an inverted teardrop shape. The marginal and palatal fangs are quite large. Although flattened in dorsal view, comparisons suggest the jaw margin was convex, as in Neopteroplax.

Based on its size and nesting,
Anthracosaurus developed a labyrinthodont-like skull by convergence because Proterogyrinus is basal in the Embolomeri. Those giant marginal and palatal fangs indicate a predatory niche.

Figure 2. Left: Anthracosaurus chimaera from Clack 1987. Right: Older tracing in dorsal view of the complete skull and palatal view attributed to Anthracosaurus from an online photo.

Figure 2. Left: Anthracosaurus chimaera from Clack 1987. Right: Older tracing in dorsal view of the complete skull and palatal view attributed to Anthracosaurus from an online photo. The narrower skull is made of several different specimens (chimaera) and produces a loss of resolution in the LRT.

Clack 1987
illustrated a lateral and dorsal view of Anthracosaurus (Fig. 2) based on a chimaera of specimens. Unfortunately, plugging that data into the LRT produced loss of resolution over several nodes. Using the older single skull in dorsal view had no such problems.

We looked at the problems chimaera taxa produce
earlier here, and in six blogs that preceded that one.

Figure 3. Neopteroplax has a skull quite similar to the older single skull of Anthracosaurus and they nest together in the LRT.

Figure 3. Neopteroplax has a skull quite similar to the older single skull of Anthracosaurus and they nest together in the LRT.

The clade Anthracosauria has had problems
From Wikipedia: “Gauthier, Kluge and Rowe (1988) defined Anthracosauria as ‘Amniota plus all other tetrapods that are more closely related to amniotes than they are to amphibians” (Amphibia in turn was defined by these authors as a clade including Lissamphibia and those tetrapods that are more closely related to lissamphibians than they are to amniotes).”

In this definition non-amniote Anthracosauria does not include Anthracosaurus, but only Silvanerpeton and Gephyrostegus in the LRT because more basal taxa are also basal to amphibians.

“Similarly, Michel Laurin (1996) uses the term in a cladistic sense to refer to only the most advanced reptile-like amphibians. Thus his definition include the (Diadectomorpha and Solenodonsauridae) and the amniotes.”

In the LRT Diadectomorpha and Solenodonsauridae are amniotes.

“As Ruta, Coates and Quicke (2003) pointed out, this definition is problematic, because, depending on the exact phylogenetic position of Lissamphibia within Tetrapoda, using it might lead to the situation where some taxa traditionally classified as anthracosaurs, including even the genus Anthracosaurus itself, wouldn’t belong to Anthracosauria.

Indeed! And that happened in the LRT.

Laurin (2001) created a different phylogenetic definition of Anthracosauria, defining it as “the largest clade that includes Anthracosaurus russelli but not Ascaphus truei”.

In the LRT Ascaphus, the tailed frog, is derived from the large clade, the embolomeri, that includes Anthracosaurus. However the small clade that includes just Anthracosaurus and Neopteroplax does not include the tailed frog.

“However, Michael Benton (2000, 2004) makes the anthracosaurs a paraphyletic order within the superorder Reptiliomorpha, along with the orders Seymouriamorpha and Diadectomorpha, thus making the Anthracosaurians the “lower” reptile-like amphibians. In his definition, the group encompass the Embolomeri, Chroniosuchia and possibly the family Gephyrostegidae.”

In the LRT the Embolomeri are basal to Eucritta and the Seymouriamorpha, which are basal to the Reptilia (= Amniota) and Lepospondyli (including Amphibia). The Chroniosuchia and Gephyrostegus are both amphibian-like reptiles in the LRT.

The clade Reptilomorpha suffers the same definition problems.
As Wikipedia reported, “As the exact phylogenetic position of Lissamphibia within Tetrapoda remains uncertain, it also remains controversial which fossil tetrapods are more closely related to amniotes than to lissamphibians, and thus, which ones of them were reptiliomorphs in any meaning of the word.”

Wouldn’t it be great if someone could put together
a large gamut phylogenetic analysis that could settle all those controversial issues?

References
Clack JA 1987. Two new speciemens of Anthracosaurus (Amphibia: Anthracosauria) from the Northumberland coal measures. Palaeontology 30(1):15-26.
Huxley TH 1863. Description of Anthracosaurus russelli, a new labyrinthodont from the Lanarkshire coalfield. Quartery Journal of the Geological Society 19:56-58.
Panchen AL 1975. A new genus and species of anthracosaur amphibian from the Lower Carboniferous of Scotland and the status of Pholidogaster pisciformis Huxley. Philosophical Transactions of the Royal Society of London, B. 269: 581-640.
Panchen AL 1977. On Anthracosaurus russelli Huxley (Amphibia: Labyrinthodontia) and the family Anthracosauridae. Philosophical Transactions of the Royal Society B. 279 (968): 447–512.