The Oldest vs. Most Primitive Reptile
The oldest known reptiles, Casineria and Westlothiana, were close to, but not THE most primitive known reptile. That title goes to Cephalerpeton (Moodie 1912, Figures 1, 2), the reptile phylogenetically closest to the non-reptile, Gephyrostegus. A single Cephalerpeton fossil was found in the Mazon Creek beds, dated to the Mid-Pennsylvanian about 300 mya. That’s 35-38 million years after Casineria and Westlothiana. So, the most primitive reptile was not the oldest reptile, testifying once again, to the longevity of many reptile taxa. Evidently Cephalerpeton had survival fitness, in the Darwinian sense.
A Tiny Gephyrostegid Able to Lay Eggs Protected with Membranes
Everyone knows that the one character that best unites all birds, mammals and reptiles of all sorts is the production of eggs protected by an amniotic membrane. By employing phylogenetic bracketing, we infer that the most primitive reptile also had this ability.
Cephalerpeton was originally considered to be an amphibian (Moodie 1912). Here it was derived from Gephyrostegus, a much larger “amphibian” that likewise survived for a long time on the planet. Fossils are known from 310 mya, at least 30 million years after the appearance of the oldest known fossil reptile, Westlothiana. Gephyrostegus bohemicus (Figure 2) had a snout-vent length of 20 cm, large enough to require two hands to hold it. Unlike most other “amphibians,” Gephyrostegus had a lizard-like build, well-suited to a terrestrial environment.
The Visionary Contribution of Robert Carroll (1970)
Dr. Robert Carroll (1970) got it right when he proposed that the first reptiles would be tiny, so their eggs would be tiny. Gephyrostegus watsoni (Brough and Brough 1967) was a tiny gephyrostegid (skull length ~1 cm, Figure 2) bearing an even closer resemblance to Cephalerpeton. The first eggs provided with an amniotic membrane were probably small and laid by small adult females who lived in and laid eggs in moist leaf litter, a transitional environment that stayed humid and protected both the adult and the egg from desiccation.
With a snout-vent length barely as long as just the skull of the holotype, tiny G. watsoni would have been a great candidate for “the most primitive reptile” based on its greatly reduced adult size. Unfortunately G. watsoni retained a discrete intertemporal bone, a trait no other reptile has. Cephalerpeton was twice its size. We’ll probably never know if G. watsoni laid eggs protected by an amnion, but I like the idea that it did.
More About Cephalerpeton, the Most Primitive Known Reptile
Distinct from Gephyrostegus, the skull of Cephalerpeton was relatively large with a large orbit. Such a pattern is similar to that of Gephyrostegus watsoni or what would be expected in a juvenile gephyrostegid. A discrete intertemporal bone was absent. The quadrate was aligned vertically. The otic notch was greatly reduced with a squamosal that had a near vertical posterior rim. The maxillary teeth were enormous. The mandible was concave dorsally in order to accommodate the upper teeth. The postorbital portion of the skull was shorter and no longer concave posteriorly. The postfrontal extended over the postorbital to mid orbit. The maxilla was slightly raised to just above the lower rim of the orbit. The premaxillary teeth were longest medially and the deeper premaxilla tipped down. The palate was relatively shorter. The transverse process of the pterygoid was more developed and had a transverse row of teeth.
The cervicals were elongated and there were two more of them. The pleurocentra were greatly enlarged, crowding out the intercentra.
The scapula and coracoid were unfused and as tall as the neural spines. The humerus was slender and hourglass-shaped. The radius and ulna were likewise more slender and relatively longer. Of the hand, only the metacarpals were preserved and they appear more assymmetrical with #4 still the longest.
The earliest known reptiles from both reptilian branches were similar in size to Cephalerpeton (Figure 2). One branch, the lepidosauromorpha, were largely herbivores. The other branch, the archosauromorpha, were larger insectivores grading toward carnivores.
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 B 257: 267–308.
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
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
Moodie RL 1912. The Pennsylvanic Amphibia of the Mazon Creek, Illinois, Shales. Kansas University Science Bulletin 6(2):232-259.