The reptile family tree begins with Gephyrostegus because, by definition, the Reptilia includes turtles, lizards, snakes, crocodilians and birds, their last common ancestor and all of its descendants. That last common ancestor is Gephyrostegus, according to the present study. In the present study, distinct from all prior studies, all known reptiles nest in two major branches with turtles and lizards on one branch and crodilians, birds and mammals on the other.
Gephyrostegus bohemicus itself was probably too large to produce embryos enclosed within an amniotic membrane, the defining character of the Amniota (= Reptilia in this study). Gephyrostegus watsoni is a more likely candidates.
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.
Shortly after a very early sister to Cephalerpeton members of the Reptilia split into two clades represented at their bases by Westlothiana and Cephalerpeton. The new Lepidosauromorpha, are all those sharing a more recent common ancestor with living lizards, the Lepidosauria, than with Archosauria (Gauthier 1986). The new Archosauromorpha share more traits in common with living archosaurs (birds and crocs) than with Lepidosauria.
This primitive division has not been reported before because all prior studies were more limited in scope, without the benefit of an umbrella study like this one.
The more complete reptile family tree with archosauromorph and lepidosauromorph branches is found here. See a (slightly outdated) chronological phylogram of the same tree below. For those interested in obtaining the MacClade data file, click here and make your request.
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.