Currently Cephalerpeton is the basalmost reptile we know of, the most primitive one most likely to have laid that first amniotic egg. It nests at the base of all other reptiles and descended from a tiny Gephyrostegus watsoni (Figs. 2 and 3).
The problem is Cephalerpeton was twice the size of G. watsoni. Cephalerpeton had those giant teeth, both in the premaxilla and maxilla. These traits had morphologically skewed Cephalerpeton toward the plant-eating side of the basal Reptilia beginning with the captorhinids and millerettids.
Plus, Cephalerpeton has a few autapomorphies, like a long robust neck and the deeply concave mandible. Then again, Cephalerpeton arrives rather late on the scene in the fossil record.
The tiny Gephyrostegus watsoni (Fig. 3) is probably closer to the size expected following Carroll’s (1970) hypothesis on egg-laying. All it needs is to fuse the intermedium to the supratemporal or postorbital. Or, the intermedium may have become reduced to nothing, as it never reappears in derived taxa.
Okay, so to find the basalmost reptile…
We’re looking for a twin to G. watsoni with the following changes: 1) loss of the intermedium; 2) pelvis without two dorsal processes; 3) a smaller, narrower interclavicle; 4) perhaps a taller, more ossified scapula; 5) better ossified vertebrae.
If something like this is languishing unloved in some museum drawer, well, let’s bring it out!
Speaking of something old, something new on Romeriscus coming soon.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
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
Carroll RL and Baird D 1972. Carboniferous Stem-Reptiles of the Family Romeriidae. Bulletin of the Museum of Comparative Zoology 143(5):321-363. online pdf
Gauthier JA 1986. Saurischian monophyly and the origin of birds, p. 1-55. In K. Padian (ed.) The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Science 8, Berkeley, California.
Gregory JT 1948. The structure of Cephalerpeton and affinities of the Microsauria. American Journal of Science, 246:550-568 doi:10.2475/ajs.246.9.550
Jaeckel O 1902. Über Gephyrostegus bohemicus n.g. n.sp. Zeitschrift der Deutschen Geologischen Gesellschaft 54:127–132.
Moodie RL 1912. The Pennsylvanic Amphibia of the Mazon Creek, Illinois, Shales. Kansas University Science Bulletin 6(2):232-259.