egg differentiates the reptiles, birds, and mammals from their ancestral group, the amphibians. Before the end of the Mississippian Period, three great stocks of reptiles had diverged from one another to become independent groups: one that gave rise to mammals, a second to turtles, and a third to the other reptilian groups and to the birds. The fossil record shows that there are many individual species making up these three. A relatively rich fossil record has delineated the evolutionary pathway of these groups. It has also required a reevaluation of just what a “reptile” is. As customarily defined, the class Reptilia includes the living turtles, squamates, and crocodiles. Technically, reptiles can now be defined by what they are not: they are amniotes that lack the specialized characters of birds and mammals. The fossil record suggests that the “amniotes” are “monophyletic”—that they have but one common ancestor—rather than suggesting the possibility that amniotic eggs arose independently more than once.
Reptiles are considered to be a monophyletic stock that diverged from amphibian ancestors perhaps sometime in the Mississippian Period of more than 320 million years ago. But while genetic evidence of this divergence, obtained by the “molecular clock” analysis technique, can be dated back to as long ago as 340 million years, fossils that are ascribed to the first reptiles (instead of terrestrial amphibians) have been recovered from several localities globally. Fossils of small reptiles named Hylonomus and Paleothyris have been found interred in fossilized tree stumps of early Pennsylvanian age, and it may be that the fossil record of this later appearance is more valid than the assumption of a Mississippian evolution of the group. In either case, these first reptiles were very small—only about 4 to 6 inches long.
That these small reptiles laid the first amniotic eggs is still speculation. There are no fossil eggs in the stratigraphic record until the lower Permian, and this single find remains controversial. But the pathway to the amniotic condition probably passed through an amphibian-like egg without a membrane that would reduce desiccation, which laid in moist places on land. It would have been the evolution of a series of membranes surrounding the embryo (the chorion and amnion), covered by either a leathery or a calcareous but porous egg that was required for fully terrestrial reproduction. One possibility seemingly