Gephyrostegus moves one node to the base of the Archosauromorpha

Gephyrostegus bohemicus (Figs. 1, 2) was one of the first taxa added to the large reptile tree (LRT, 2003+ taxa) almost eleven years ago when this online experiment had its genesis at 200 or so taxa. Distinct from traditional studies, Gephyrostegus nested at or near the base of the Reptilia (= Amniota), occasionally trading places with Silvanerpeton (Figs. 1, 2) from the Viséan (Early Carboniferous). Thereafter reptiles split into two clades in the LRT: Lepidosauromorpha and Archosauromorpha (Fig. 1). This also breaks from textbooks and university traditions based on taxon exclusion. These vertebrate paleontology institutions are now ten years out of date.

Academic workers do not nest amphibian-like Gephyrostegus
within the Reptilia. Similarly, academic workers do not nest amphibian-like Limnoscelis and Diadectes within the Reptilia. This is unfortunate because Gephyrostegus is in the lineage of synapsids, mammals and humans in the LRT. It’s morphology teaches something about how one branch of reptiles laid larger eggs and crawled further from the water.

We’ve known about this basal dichotomy
for the last ten years. Add taxa to your own cladogram to confirm or refute this novel hypothesis because current workers are reticent to do so.

The robust torso and robust long legs of Gephyrostegus
(Fig. 1) seemed to link it to similar basal archosauromorphs, like Eldeceeon (Fig. 1) and Diplovertebron (= Gephyrostegus watsoni, Fig. 1). This is distinct from basal lepidosauromorphs with shorter legs, and more like their last common ancestor, Silvanerpeton (Fig. 2).

A review of ten-year-old scores for taxa
now surrounded by ten years of taxon inclusion revealed several dozen scoring errors that were corrected based on a photo of the skull published by Klembara et al. 2014 (Fig. 2) rather than tracings and freehand reconstructions by Carroll 1970. The many subtle changes found in the skull shifted Gephyrostegus to the base of the Archosauromorpha joining other taxa sharing a robust torso and larger limbs. Notably, there is no score for ‘large hind limbs’ or ‘humpback torso’ in the LRT MacClade matrix, which has enough characters, so long as they are correctly scored.

The number of corrections in the LRT
continues to rise, erasing former mistakes. Even so, the early LRT did a pretty good job, errors and all. Taxon inclusion continues to trump character inclusion. That’s because the LRT lumps and separates taxa, not characters.

Gephyrostegus bohemicus
(Jaeckel 1902) Upper Carboniferous (~310 mya)~22 cm snout-vent length, is the basalmost archosauromorph, derived from Viséan Silvanerpeton, the basalmost reptile. Gephyrostegus is 30 million years younger. Gephyrostregus phylogenetically preceded the basal archosauromorph, Eldeceeon, also from the Viséan.

Distinct from Silvanerpeton, the presacral vertebral count was reduced to 24. All four limbs were larger and robust. Manual digits IV and V were longer. The girdles were more robust. The intermedium is fused to create the astragalus.

Gephyrostegus bohemicus has no traditional amniote characters, but nests within the Reptilia. Gephyrostegus had more terrestrial, longer legs, fewer dorsal ribs, a fused astragalus, and a deeper pelvis. Phylogenetic bracketing indicates Gephyrostegus laid amniotic eggs, the key trait of the Amniota = Reptilia.

While most early tetrapods lived their lives in water, Gephyrostegus was among the few that preferred land (= moss covered swampy coal forest logs). Tiny circular scales covered the body except ventrally where large V-shaped scales were present.

Whenever the LRT seems to stumble or stall,
better data revealed by DGS tracings (Fig. 2) correct errant scores making possible the lumping and splitting that the LRT is built to do and generally does well based on minimizing taxon exclusion.

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
Gauthier JA 1986. Saurischian monophyly and the origin of birds. Memoirs of the California Academy of Science. 8: 1–55.
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.
Moodie RL 1916. Journal of The coal measures Amphibia of North America. Carnegie Institution of Washington #238. 222 pp.
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

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