The postcranial skeleton of the Early Triassic non-mammalian cynodont Galesaurus planiceps: implications for biology and lifestyle

Abstract

Newly discovered skeletons of the Early Triassic basal cynodont, Galesaurus planiceps, has enabled a detailed morphological redescription of the postcrania of this genus. The examination of Galesaurus reveals two distinct morphs, namely a gracile and a robust morph. The primary differences between each morph lie in the pectoral and pelvic girdles with further subtle differences in the fore- and hind limbs. The morphological differences between the two morphs may be attributed to ontogeny, sexual dimorphism or the presence of two subspecies. The morphology and high cortical thickness in the limb bones of Galesaurus indicates that it was a more robust animal compared to its closely related sister taxon Thrinaxodon liorhinus. Galesaurus was thus, capable of being an active burrower and may have used burrows to escape the harsh environmental conditions of the Early Triassic. The bone microstructure of Galesaurus reveals uninterrupted fibro-lamellar bone, indicating fast continuous initial grow th, with a change to lamellar-zonal bone, indicating a decrease in growth rate. T he presence of annuli and LAGs in the peripheral lamellar-zonal bone indicates interrupted slow growth and suggests that Galesaurus may have been susceptible to environmental fluctuations. The growth patterns of Galesaurus and Thrinaxodon are similar, but can be distinguished from one another by the presence of lamellar-zonal bone in the former and parallel-fibred bone in the la tter genus. Annuli and LAGs are absent in Thrinaxodon , implying that Thrinaxodon was less susceptible to environmental fluctuations than Galesaurus, as growth did not decrease or cease periodically. Abstract iv Galesaurus, with a short biostratigraphic range from the Palingkloof Member, Balfour Formation and lowermost Katberg Formation of the Lystrosaurus Assemblage Zone, was previously known only from cranial and poorly preserved, isolated postcranial fragments. In contrast, extensive research has been conducted on the more abundant better-known Thrinaxodon, which has a biostratigraphic range that extends the entire Lystrosaurus Assemblage Zone. It was previously assumed that the postcr anial skeletons of basal cynodonts were indistinguishable. However, this study has revealed morphological differences between Galesaurus and Thrinaxodon , allowing the taxa to be distinguished in the absence of cranial material. Examining postcranial material previously identified as Thrinaxodon and ensuring that collection material has been correctly identified can now test the short stratigraphic range of Galesaurus .