The jaw function and adaptive radiation of the dicynodont mammal-like reptiles of the Karoo basin of South Africa

Abstract

The structure and functioning of the dicynodont jaw system are described. A pivoting action of the lower jaw around the palate posteromedian to the caniniform processes is as basic to the jaw movement as is the well-known double-convex jaw articulation. The sequence of origin of the characteristic features of the dicynodonts is analysed cladistically; this demonstrates the patterns of association of these characters into functional character-complexes. The structures of the palate and lower jaw, and their functional integration in feeding, are described and illustrated in standard format. Five different lineages of dicynodont can be identified:Eodicynodon;the robertoids (includingDiictodon); the dicynodontoids (includingDicynodonand the majority of the large dicynodont genera of the Permian and Triassic); the endothiodontoids (includingProdicynodon [=‘Chelydontops’]andPristerodon), and the emydopoids (includingCistecephalus, MyosaurusandKingoria).Eodicynodonor a similar form could have been ancestral to the other four lineages. The robertoids probably fed upon the stems and rhizomes of equisetaleans, while the varied dicynodontoids probably fed upon the varied glossopterid seed-ferns. The endothiodontoids, too, were probably herbivorous, but many, perhaps all, of the small emydopoids were burrowing and may have been omnivorous. The dicynodonts were probably ectothermal, and the dicynodontoids may have migrated to warmer latitudes in the winter. Onlyc. 20 genera of Karoo dicynodont are now recognized as valid, and it is suggested that this fauna is now almost completely known. Their distribution in the Karoo biozones is reviewed and correlated with environmental changes. The Permian ancestors of the Triassic dicynodonts, includingLystrosaurus, probably lived on higher, drier ground, and were therefore already adapted to the more fibrous food that spread into the basins as the climate became drier in the Triassic.