ONTOGENY OF THE EARLY TRIASSIC THRINAXODON LIORHINUS (THERAPSIDA, CYNODONTIA). DENTAL MORPHOLOGY AND REPLACEMENT

ABDALA, F.; JASINOSKI, S. C.; FERNANDEZ, V.

RALEIGH

Congreso; SEVENTY-SECOND ANNUAL MEETING SOCIETY OF VERTEBRATE PALEONTOLOGY; 2012

SOCIETY OF VERTEBRATE PALEONTOLOGY

The non-mammaliaform cynodont Thrinaxodon liorhinus is one of the most common representatives of the post Permian-extinction fauna of the Lystrosaurus Assemblage Zone of the South African Karoo Basin. Thrinaxodon is also one of the best known Triassic cynodonts with a well-known cranio-dental and postcranial anatomy. Here we present a detailed study on variation of the dental morphology and replacement in Thrinaxodon liorhinus. For this study we analysed five specimens ranging from 37 to 87 mm in skull length using micro computed tomography (μCT) scanning techniques, which were supplemented by detailed anatomical analysis of 41 specimens with a basal skull length of approximately 30 mm to 96 mm. Our results confirm the alternate replacement of the postcanines and the posterior migration of the postcanine series (including the loss without replacement of the anteriormost postcanines). Even when most of the observations point to a posterior-to-anterior replacement wave, the evidence is not clear-cut. A new classification was designed to describe accurately the morphology of postcanines, taking into consideration the number of sectorial cusps, presence and pattern of the lingual cingulum, and presence of labial cingular cusps. The lower postcanines are clearly more complex (and more numerous) than the upper postcanines; only the lower postcanines have more than three sectorial cusps and a cingular collar on the lingual margin. Complexity of the postcanines increases from the smallest individual to specimens of 75 mm of skull length, but complexity decreases in larger specimens. On several specimens, erupting canines can be observed through the replacement pit while the complexity of the replacement pattern was assessed using X-ray micro-computed tomography. The virtual extraction of functional and replacement teeth permitted us to conclude that in most of the cases, the upper canines were replaced anteriorly while lower canines were replaced posteriorly. The presence of two simultaneous replacements of the upper canine tooth was observed in two small juveniles, suggesting a higher rate of canine replacement at younger age. Incisors also had a sequential replacement pattern, and more replacement teeth were present in medium-sized individuals.