Eusauropleura: now identified as a late-surviving basalmost reptile

The newest addition
to the large reptile tree (LRT, 1341 taxa) is Eusauropleura digitata (originally Sauropleura, Cope 1868; Romer 1930; Carroll 1970; Late Carboniferous, 310 mya; AMNH 6865; Figs. 1, 2) nests as a late-surviving basalmost reptile in the LRT.

The genesis for this genus
in the earliest Carboniferous is based on the more derived Silvanerpeton from the Viséan (335 mya). A dense layer of belly scales (not shown en masse), a larger manual digit 5, and a taller ilium, among other traits, distinguish this specimen from Gephyrostegus. A larger manus, ischium and giant caudal transverse processes (ribs) relative to the torso are unique traits among close relatives. Note the lack of ribs in the lumbar area, where large amniote eggs develop before they are laid. The eggs were relatively large based on the greater depth of the ischium.

Figure 1. Eusauropleura in situ and slightly reconstructed. Manus reconstruction with PILs enlarged.

Figure 1. Eusauropleura in situ and slightly reconstructed. Manus reconstruction with PILs enlarged.

Basal to Eusauropleura
are taxa close to the Reptilomorpha – Lepospondyli split. These include Eucritta and Utegenia (Fig. 2) all derived from the Late Devonian reptilomorph, Tulerpeton. This affirms the primitive state of basalmost reptiles, derived from Devonian tulerpetids. Further affirmation comes from the observation that the central vertebral elements of Eusauropleura “are very thin-walled, forming little more than a husk around the large notochord,” according to Carroll 1970.

Figure 2. Eusauropleura to scale with ancestral and descendant taxa including Eucritta, Utegenia, Silvanerpeton and Gephyrostegus, the last common ancestor of all reptiles.

Figure 2. Eusauropleura to scale with ancestral and descendant taxa including Eucritta, Utegenia, Silvanerpeton and Gephyrostegus, the last common ancestor of all reptiles. Note, none of these specimens preserves ossified carpals.

First considered a microsaur
(Cope 1868), then a gephyrostegid (Romer 1930, 1950; Carroll 1970), Eusauropleura was identified as more primitive than Gephyrostegus (Carroll 1970), but still terrestrial, not aquatic and close to the ancestry of reptiles, but not itself a reptile.

So what is a reptile?
As determined here in 2011, there is no list of traditional reptile skeletal traits that upholds the reptile status of Gephyrostegus. There is a new list. Irregardless of skeletal traits, only the nesting of Gephyrostegus as the last common ancestor of all reptiles in the LRT tells us it was laying eggs with an amnion, the ONLY trait needed to determine its reptile status. Silvanerpeton, from the earlier Viséan, was likely also a reptile because phylogenetic descendants of late-surviving Gephyrostegus are also found in coeval Viséan strata. Reptiles are that old. Given that the last common ancestor of Silvanerpeton and Gephyrostegus must also be a late-surviving member of that basalmost reptile radiation, whether the amnion was fully developed or not, something we may never know given the fragility of an amniotic membrane over 300 million years in stone. Earlier workers did not enter Eusauropleura, Silvanerpeton and Gephyrostegus into a wide gamut phylogenetic analysis and so did not recover a last common ancestor status for these amphibian-like reptiles.

Another specimen attributed to Eusauropleura
AMNH 6860 (Moodie 1909, Carroll 1970), is a bit more jumbled, more incomplete and more difficult to reconstruct. A complete ilium with an elongate posterior process is easy to see in this specimen. Such a process provides attachment points for more than one sacral rib, a traditional reptile trait, but this is difficult to determine in the scattered remains of the fossil. And is this really Eusauropleura?

Yet another specimen attributed to Eusauropleura
PU 16815 is an isolated pectoral girdle, bones lacking in the other specimens and therefore not readily comparable.

Scales
According to Carroll 1970, “Scales, both dorsal and ventral, are conspicuous in these specimens [Gephyrostegus and Eusauropleura]. The body was protected by heavy, oblong scales, overlapping to form a chevron pattern, between the pelvic and pectoral girdles. Were they not associated with the skeleton, they would be difficult to distinguish from those of [more primtive] embolomeres. Laterally the scales assume a more oval outline, become thinner, smaller and less extensively overlapping. The dorsal scales are small, thin and round. Where worn, all the scales exhibit a pattern of fine ridges, running parallel with the margins. These form a pattern of concentric ridges in the dorsal scales, similar to that of [more primitive] discosauriscids. Except for the heavier ossification of the dorsal scales, those of Eusauropleura are generally similar to those of Gephyrostegus.”

References
Carroll RL 1970. The ancestry of reptiles. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 257 (814):267–308. DOI: 10.1098/rstb.1970.0026
Cope ED 1868. Synopsis of the Extinct Batrachia of North America. Proceedings of the Academy of Natural Sciences of Philadelphia 1868:208-221.
Romer AS 1930. The Pennsylvanian tetrapods of Linton, Ohio. Bulletin of the American Museum of Natural History. 59 (2):144–147.
Romer AS 1950. The nature and relationships of the Paleozoic microsaurs: American Journal of Science 248:628-654.

wiki/Eusauropleura

Diplovertebron vs. Gephyrostegus

Updated June 13, 2017 with the realization that Watson’s 1926 Diplovertebron is the same specimen as Gephyrostegus watsoni (bohemicus). 

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.

Gleaning data from several papers, provided that 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.

So far, I have not found it. 60 cm may be an error.

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. That’s because Watson’s drawing (upper left) was poorly drafted. 

Revised backstory:
Diplovertebron punctatum (Fritsch 1879, Waton 1926; DMSW B.65, UMZC T.1222a; Moscovian, Westphalian, Late Carboniferous, 300 mya) aka:  Gephyrostegus watsoniBrough and Brough 1967) and  Gephyrostegus bohemicus (Carroll 1970; Klembara et al. 2014) after several name changes perhaps this specimen should revert back to its original name as it nests a few nodes away from Gephyrostegus.

Derived from a sister to EldeceeonDiplovertebron was basal to the larger Solenodonsaurusand the smaller BrouffiaCasineria and WestlothianaDiplovertebron was a contemporary ofGephyrostegus bohemicus, Upper Carboniferous (~310 mya), so it, too, was a late survivor.

Overall smaller and distinct from Eldeceeon, the skull of Diplovertebron had a shorter rostrum, larger orbit and greater quadrate lean. The dorsal vertebrae formed a hump and had elongate spines. The hind limbs were much longer than the forelimbs. The tail is incomplete, but appears to have been short and deep.

Seven sphere shapes were preserved alongside this specimen. They may be the most primitive amniote eggs known.

Watson 1926 attempted a freehand reconstruction (see below) that was so different from this specimen that for a time it nested as a separate taxon, now deleted.

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