Origin of the mammal-like reptiles

Abstract

The first major adaptive radiation of reptiles consisted of the Synapsida, or mammal-like reptiles, which are now totally extinct, but which dominated the terrestrial fauna from the late Pennsylvania, throughout the Permian and for much of the Triassic periods. The mammals arose from advanced synapsids in the Upper Triassic, at which time the dinosaur reptiles were rapidly replacing the mammal-like reptiles as the principal terrestrial forms. The orthodox view1,2 of the ancestry of this important stage in vertebrate history is that the synapsids arose from a hypothetical stem-reptile of the family Romeriidae, the group closest to the ancestry of all the other reptile lineages. The principal difference between romeriids and the primitive synapsids is the development of a space, or temporal fenestra in the cheek region of the skull of the latter. However, the structure of this part of the skull of the romeriids cannot satisfactorily be considered ancestral to the synapsid temporal region on either comparative anatomical or functional grounds. As I propose here, the kind of skull which would be expected to evolve into that of a synapsid is the type possessed by the archaic ‘quasireptile’ Limnoscelis and its possible relative Romeriscus.