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 (including Diictodon); the dicynodontoids (including Dicynodon and the majority of the large dicynodont genera of the Permian and Triassic); the endothiodontoids (including Prodicynodon [=‘Chelydontops’] and Pristerodon), and the emydopoids (including Cistecephalus, Myosaurus and Kingoria). Eodicynodon or 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. Only c. 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, including Lystrosaurus, 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.

Phylogenetic analysis of Russian Permian dicynodonts (Therapsida: Anomodontia): implications for Permian biostratigraphy and Pangaean biogeography

“…Cluver & King (1983) and King (1988, 1990) hypothesized that Emydops and Myosaurus were sister taxa, based on three synapomorphies (reduced upper teeth, long, straight anterior pterygoid rami, and a shortened basicranial region). A similar conclusion was reached by Haughton (1917) in his original description of Myosaurus as well as by Cox (1998). However, Angielczyk (2001) found that Myosaurus and Cistecephalus may be more closely related to each other than either is to Emydops.…”

“…Past phylogenetic analyses of Permian dicynodonts (e.g. Cluver & King, 1983; King, 1988, 1990; Cox, 1998; Angielczyk, 2001, 2002) did not include the Russian taxa examined here, making direct topological comparisons difficult. If only the taxa shared among these analyses (i.e.…”

“…Studies of dicynodont and anomodont phylogeny have been approached from a cladistic perspective for some time (e.g. Cluver & King, 1983; King, 1988, 1990; Cox, 1998), but only recently have these analyses begun to include character/taxon matrices and computerized searches for maximally parsimonious cladograms (Angielczyk, 1999, 2001, 2002; Modesto et al. , 1999; Hancox et al.…”

Do extraordinarily high growth rates in Permo-Triassic dicynodonts (Therapsida, Anomodontia) explain their success before and after the end-Permian extinction?

“…Numerous morphological studies have helped to shed light on the phylogeny, food acquisition and processing, locomotion, and biomechanical adaptations of these animals (e.g. Keyser, 1975; Cluver & Hotton, 1981; King & Rubidge, 1993; Cox, 1998; Angielczyk, 2004; Kemp, 2005; Sullivan & Reisz, 2005; Angielczyk & Walsh, 2008; Surkov & Benton, 2008). However, valuable information can also be gained from studying the microstructure, or histology, of dicynodont bones.…”

DOLICHURANUS PRIMAEVUS (THERAPSIDA: ANOMODONTIA) FROM THE MIDDLE TRIASSIC OF NAMIBIA AND ITS PHYLOGENETIC RELATIONSHIPS

“…The palatal pads of the palatines ( sensu Cox 1998; Text‐figs 1F, 2F, 4F) are larger and more strongly developed than those of Daptocephalus and Kannemeyeria . They have a rugose surface texture, and also a greater medial expansion anteriorly, giving them a teardrop shape.…”