On the functional morphology of the gorgonopsid skull

Abstract

Cranial material of some gorgonopsids has been prepared by the acetic acid technique. A new species, Leontocephalus intactus , has been made for one of the skulls, and generic and specific diagnoses are presented. On the basis of composite information from all the skulls, some aspects of the functional morphology are discussed. From the form of the dentition and the pattern of tooth-wear, it is deduced that besides a direct closure of the jaws, the lower incisors must have been capable of interdigitating between the upper incisors to produce a shearing type of occlusion. This must necessarily have involved a propalinal shift of the lower jaw forwards, relative to the skull. However, there was no possibility of an anterior movement of the articular relative to the quadrate and the quadrate must therefore have been streptostylic. It is shown that the quadrate and squamosal together formed a basically ball-and-socket joint and that the quadrate rotated about a transverse axis, upon the epipterygoid. The epipterygoid itself was probably capable of a limited degree of movement relative to the pterygoid. A restoration of the jaw musculature is suggested, on the basis of new information about the lower jaw. A simple mathematical model is constructed to show the feasibility of the muscle restoration. The functional evolution of the gorgonopsid jaw mechanism and musculature is discussed. The organization of the nasal cavity is described and it is shown that the nasal capsule probably consisted of three parts—an anterior chamber restricted to the dorsal part of the skull, a large, purely olfactory posterior chamber, and a ventro-lateral diverticulum of the posterior chamber, the maxillary sinus. The probable presence of olfactory turbinal cartilages in the latter two parts is indicated. The respiratory air probably passed down a choanal tube, supported by extensive processes of the palatine. Evidence of the course of the naso-lachrymal duct is given. The position of Jacobson’s organ, and a possible function for the septomaxillary foramen, are discussed. On both functional and anatomical grounds, it is argued that the gorgonopsids had no trace of a secondary palate. The origin of the organization of the gorgonopsid snout is discussed. New details of the structure of the braincase are described and the homology of the elements are considered. It is shown that the braincase can be compared to the neurocranium of the mammals in several, but not all respects. On the basis of the internal form of the braincase, a reconstruction of the brain is given, suggesting that the telencephalon was relatively well developed. The phylogenetic position of the gorgonopsids as a whole is considered. They are compared with the Therocephalia and it is concluded that their functional organization, as indicated by an extensive suite of characters, differs radically from that group. The cynodonts are shown to be based on the therocephaliantype of organization and therefore to be relatively unrelated to the gorgonopsids. It is apparent that a common ancestor of the gorgonopsids and the therocephalians could have been but barely advanced over the pelycosaurian-grade of structure. Primitive therapsids from the Russian Kazanian deposits are briefly discussed and it is shown that there is some slight evidence for a dichotomy between them into ‘pregorgonopsid’ and ‘pretherocephalian’ stocks. It is formally proposed that the carnivorous therapsids should be classified into two equal ranks, the Gorgonopsia and the Theriodonta and that they were probably derived separately from the sphenacodont pelycosaurs.