The primitive cynodontProcynosuchus: functional anatomy of the skull and relationships

Abstract

An acetic acid prepared skull of the Upper Permian Karroo cynodont Procynosuchus delaharpeae Broom is described and an attempt is made to interpret its anatomy in functional terms. The dentition is adapted for an insectivorous habit with an incipient form of tooth occlusion between specific upper and lower postcanines. A specialized form of tooth replacement ensures that the posterior postcanine teeth remain functional for as long a period as possible. The adductor jaw musculature shows a masseter-like muscle between the zygomatic arch and the lateral surface of the angular bone, in addition to the large temporalis muscle which has invaded the lateral face of the coronoid process. The possibility of a monotreme-like detrahens muscle rather than a reptilian depressor mandibuli being present for jaw opening is suggested. The quadrate is moveable. The anatomy of the internal nares indicates that an arrangement of Jacobson's organ and associated nerves, blood vessels and glands comparable to that of monotremes was present. Interpretations of the various foramina of the snout are presented. Fleshy lips were probably present but the skin over the dorsal surface of the snout was tightly applied. The venous system of the head involves reduction of the vena capitis lateralis and the development of emissary veins comparable in general to those of mammals. The brain is comparable in size and relative development of its parts to that of living primitive mammals. There was incipient development of a monotreme-like membranous anterior lamina of the prootic. The stapes had a mechanical role associated with the moveable quadrate, in addition to its sound conducting role. It may have been capable of transmitting low frequency sound from the lower jaw as well as higher frequency sound from a tympanic membrane. The more advanced, Triassic cynodonts form a monophyletic group, of which Procynosuchus is the sister-group. The archaic but highly specialized Dvinia is the sister-group of all other known cynodonts.