The manus of Tetracynodon (Therapsida: Therocephalia) provides evidence for survival strategies following the Permo-Triassic extinction

Abstract

We present a comprehensive qualitative and quantitative study of the manus of a new therocephalian specimen referable to Tetracynodon from the Early Triassic of South Africa. We examined 18 specimens, representing at least 12 genera, including basal therocephalians (Lycosuchidae and Scylacosauridae) and eutherocephalians (Akidnognathidae, Whaitsioidea and Baurioidea). A temporal range of 23 million years through the Permo-Triassic (Wordian to Anisian) was surveyed. A principal component analysis of the therocephalian manus indicates that (1) metacarpals II, III, and IV acted as a module of which the medial and lateral elements (metacarpals I and V) were independent from each other; and (2) the proximal carpals, ulnare, and radiale lengths show contrasting variation in their measurements (e.g., groups with longer ulnares tend to have shorter radiales). The manus of Permian and Triassic taxa occupy separate regions in morphospace. This segregation pattern suggests selection for a manus with slender, elongated second to fourth metacarpals during and after the Permo-Triassic mass extinction. We show a heterogeneous condition of the fifth distal carpal bone. Although usually interpreted as fused to the fourth distal carpal or absent, the fifth distal carpal was present as a cartilaginous element in Tetracynodon. Strong and proportionally long unguals in relation to length of digits III and IV, wide and stocky basal phalanges, and short non-ungual phalanges strongly suggest that Tetracynodon was a scratch-digger. This supports the proposition that burrowing was an important behavioral strategy of terrestrial taxa during and after the Permo-Triassic mass extinction.