Ontogeny of the Early Triassic cynodont Thrinaxodon liorhinus (Therapsida): dental morphology and replacement

ABSTRACT

Here for the first time we use micro-computed tomography (μCT) scanning techniques to study dental replacement in the Early Triassic cynodont Thrinaxodon liorhinus. We analyzed five specimens ranging 37–87 mm in skull length using μCT scanning, which were supplemented by detailed anatomical analysis of 48 specimens with a basal skull length of 30–96 mm. Our results indicate that lower postcanines are more numerous and present a more complex morphology than the upper postcanines, even in the same individual; 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 with a skull length of 75 mm, but complexity decreases in larger specimens. Our results confirm the alternate replacement of the postcanines and the posterior migration of the postcanine series (including the loss without replacement of the anterior-most postcanines). Observations point to a posterior-to-anterior replacement wave in lower postcanines, but the evidence is not clear-cut for the upper series. The virtual extraction of functional and replacement teeth permitted us to conclude that in most of the cases the upper canines were replaced anteriorly, whereas 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 a younger age. Incisors also had a sequential replacement pattern, and more replacement teeth were present in medium-sized individuals.

ACKNOWLEDGMENTS

We thank C. Churms (DebTech, DeBeers, South Africa) and P. Keanly (X-Sight, South Africa) for scanning specimens of Thrinaxodon from the SAM and TM. We are also grateful to all the members of the staff of the ID17 and ID19 beamlines and especially to P. Tafforeau at the European Synchrotron Radiation Facility for their help during the different microtomographic experiments on the BPI specimens. The scanning of the specimen BP/1/5558 was performed during experiment EC-847, allocated to our team. The curators and collection managers of the following institutions are acknowledged for allowing us access to their specimens: AMMM, BP, BSP, CAMZM, FMNH, NHMUK, NM, SAM, TM, UCMP, and USNM. Comments from W. Clemens, P. Ungar, and Z.-X. Luo on an early draft, and from C. Kammerer, Z.-X. Luo, and the editor S. Modesto on the final version of the manuscript are especially appreciated. S.C.J. thanks B. D. Reddy (Cerecam, University of Cape Town) for allowing access to 3D-imaging software. E. Abdala helped to achieve the final version of Figures 5, 7, 8, 10, and 11. Funding for this project was provided by the Claude Leon Foundation (to S.C.J.), National Research Foundation of South Africa, and ANPCyT PICT-2008-1798, Argentina (to F.A.).

Handling editor: Sean Modesto