Thrinaxodon

Abstract: The growth dynamics of the Early Triassic non-mammalian cynodont Thrinaxodon liorhinus were assessed through bone histology. Several limb bones of various sizes were examined, revealing a rapidly deposited, uninterrupted, fibro-lamellar bone tissue. A region of slowly deposited parallel-fibred bone occurs peripherally in most skeletal elements studied, becoming more extensively developed in the larger limb bones. On the basis of the bone histology, it is proposed that Thrinaxodon liorhinus grew rapidly during early ontogeny, and at a slower rate with increasing age, possibly once sexual maturity was reached. Variation in bone tissue patterns at different stages of ontogeny is noted and discussed. Given that growth rings are generally absent from the skeletal elements studied, and that the environment was seasonal, it appears that Thrinaxodon liorhinus growth was unaffected by environmental fluctuations.

Abstract: Two new tetrapod trace fossil records from the Triassic of Antarctica are described. The first is a giant terminal chamber collected from the lower Fremouw Formation (Lower Triassic) at Wahl Glacier (Beardmore Glacier region, central Transantarctic Mountains). Comparison to South African burrows known to contain the cynodont Thrinaxodon liorhinus, suggests that the Antarctic fossil was created by a tetrapod of similar size. The second set of tetrapod burrow casts was collected from Member ‘A’ of the Lashly Formation (lower Middle Triassic) and, if correctly interpreted, represent the first evidence of tetrapods from the Middle Triassic of Victoria Land. Our findings demonstrate that tetrapods were present in Victoria Land during the Middle Triassic, despite not being recovered as body fossils until the Upper Triassic in that region. Comparison to morphologically similar South African trace fossils suggests that procolophonids might have produced the Antarctic burrows, but this attribution is nece...

Abstract: Remains of a peculiar traversodontid cynodont, Protuberum cabralensis gen. et sp. nov., are described herein. The material was collected from two outcrops representing the Therapsid Cenozone (Middle Triassic) of the Santa Maria Formation, and consists of a cranium with most of its dentition preserved and an associated postcranial skeleton. The upper postcanines have two sharp cusps that are connected by a medial crest on unworn postcanines. The specimens possess several autapomorphies, including: (1) presence of thickened bone on the dorsal surface of the skull; (2) thick dorsal ribs, with remarkable processes situated on their dorsal borders that decrease in size distally; and (3) an iliac blade with a series of rugosities along its dorsal border. The lumbar ribs bear overlapping costal plates and have distally projecting rib shafts that differ from the pattern observed in Thrinaxodon, Pascualgnathus and Cynognathus.

Abstract: A new galesaurid cynodont, Progalesaurus lootsbergensis gen. et sp. nov., is described on the basis of a well-preserved skull, lower jaw, right scapula and left atlantal neural arch. Autapomorphies of Progalesaurus include postcanine teeth bearing numerous mesial and distal accessory cusps that flank a recurved main cusp, a post-temporal fenestra bordered by the squamosal ventrally and a large external naris. Progalesaurus is similar to Galesaurus in possessing a poorly defined masseteric fossa on the dentary, a strongly recurved main cusp of the postcanine dentition, an incomplete secondary palate and a similar basisphenoid-parasphenoid morphology. A cladistic analysis of ten early cynodont genera resolves a monophyletic Galesauridae encompassing Cynosaurus, Progalesaurus and Galesaurus, although support for this clade is weak. Procynosuchus and Dvinia are placed at the base of Cynodontia whereas Thrinaxodon and Platycraniellus are positioned higher, but outside of Eucynodontia. The holotype and only known specimen of Progalesaurus was collected during systematic prospecting of Permo/Triassic boundary strata at New Lootsberg Pass, Graaff-Reinet District, South Africa. The discovery of Progalesaurus increases the number of valid Early Triassic cynodonts to four and sheds light on the tempo of early cynodont diversification after the end-Permian mass extinction.