The osteology and relationships of the Millerettidae (Reptilia: Cotylosauria)

Abstract

The morphology of the Millerettidae is described in detail, based on careful preparation using the acetic acid technique. This shows that the Millerettidae constitute a closely knit natural group and results in a more satisfactory classification. Common to the group is the possession of anthracosaur-like vomerine fangs. A second feature of major importance is that these animals do not have a solid lower temporal bar. It is suggested that the reason for this is to be sought in an understanding of cranial kinesis. The reason for the lack of a temporal bar can be traced back to Anthracosaurian amphibia and is due to expansion and contraction across the cheek region during jaw movements. (These movements were necessarily minimized during the transition from amphibian to reptile.) Following from this it is suggested that the Millerettidae may have given rise directly to the Squamata. The Archo-sauria and Rhynchocephalia probably had an entirely independent origin. Only three genera are considered valid, viz. Miileretta (Broom, 1948) in which the temporal opening first appears, Millerosaurus (Broom, 1948) in which contact between jugal and quadratojugal is lost, though the jugal retains a crenellated posterior edge, and a new genus Milleropsis based on Millerosaurus pricei (Watson, 1957) in which there is still greater separation between jugal and quadratojugal and in which the posterior border of the jugal is quite smooth.