Cephalerpeton ventriamatum (Moodie 1912) Mid-Pennsylvanian ~306 mya, snout-vent length ~10 cm, was the most primitive known reptile. Originally described as an amphibian, Cephalerpeton had been associated with microsaurs, like Tuditanus and romeriids, like Romeria texana, but only the latter is close to being valid.
Cephalerpeton was derived from a sister to Gephyrostegus. PhylogeneticallyCephalerpeton preceded Concordia at the base of the Lepidosauromorpha and Casineria at the base of the Archosauromorpha clade.
Distinct from Gephyrostegus, the skull of Cephalerpeton was relatively large with a large orbit. This pattern is similar to that of tiny Gephyrostegus watsoni. 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 premaxilla tipped down. The palate was relatively shorter. The transverse process of the pterygoid was more developed and had a single transverse row of teeth.
The cervicals were elongated and there were two more of them than in Gephyrostegus. 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 longer, more slender and hourglass-shaped. The radius and ulna were likewise more slender and relatively longer. Of the hand only the metacarpals were preserved. They appear even more assymmetrical with #4 still the longest.
A large skull and large orbit are traits typically associated with juveniles. Tiny Cephalerpeton nests at the end of a phylogenetic series of decreasing size beginning with Proterogyrinus and continuing with Silvanerpeton and Gephyrostegus among the basal Tetrapoda. Gephyrostegus watsoni (see below) was even smaller, half the size of Cephalerpeton. Smaller animals generally mature faster and thus reproduce more often, although their lifespans are also typically shorter. The fact that Cephalerpeton is so much smaller than Gephyrostegus is a key factor in the origin of the Reptilia (Carroll 1970). The first eggs provided with an amniotic membrane were probably small and laid by small adult females who lived and laid eggs in moist leaf litter, a transitional environment that stayed humid and protected both the adult and the egg from desiccation.
The first reptile invented the amniotic membrane, which surrounds and protects the developing embryo in all reptiles (including birds and mammals). The eggs of Cephalerpeton and Cassineria were more or less protected from desiccation and so could be laid away from water, but probably in damp leaf litter and moss rather than any hot and arid place. There is no fossil evidence that an eggshell was created at this node.
The term Amniota is commonly and scientifically used to encompass all reptiles, birds and mammals, but in the present tree, the inclusion set of taxa within the Amniota is redundant with the Reptilia. Hence the more recent term, Amniota, can now be dropped from usage.