DICYNODONT THERAPSIDS AND THE PERMO-TRIASSIC EXTINCTION

Dicynodont therapsids were the most diverse and abundant Late Permian terrestrial vertebrate herbivores, and they survived the End-Permian event to undergo a more limited radiation until their final extinction near the end of the Triassic. Our clearest picture of the effects of the extinction on dicynodonts comes from the intensively studied Karoo Basin of South Africa. The last Permian dicynodonts disappear in a step-wise fashion over a relatively short period of time in the Karoo, a pattern that may be consistent with a catastrophic event. There is also a short overlap in the stratigraphic ranges of Permian dicynodonts such as Dicynodon and the well known Triassic taxon Lystrosaurus. A similar pattern of overlap between Lystrosaurus and Dicynodon also has been reported from Zambia and China. However, smaller sample sizes and possible correlational problems make a detailed comparison to South Africa difficult. Important dicynodont faunas also are known from Russia and Tanzania, but the apparent absence of complete terrestrial boundary sections in these areas and their paucity of Early Triassic dicynodont fossils obscure the pattern of faunal change at the end of the Permian. Phylogenetic and taxonomic studies also have an important role to play in the study of dicynodonts at the Permo-Triassic boundary. For example, recent phylogenies emphasize that two important Permian lineages of dicynodonts survive the extinction, whereas two other major lineages do not. The two surviving lineages also differ markedly in their success during the Triassic. Ongoing taxonomic revisions allow more confidence in making faunal comparisons across basins, but the recent observation that the widely used index taxon Dicynodon may be paraphyletic emphasizes that this work is far from over. The long history of study surrounding dicynodonts and their rich fossil record offer great potential for studies of this group near the Permo-Triassic boundary, but many questions must be answered before a truly global picture can emerge.