Tooth replacement patterns in Eutheriodontia (Synapsida, Therapsida) from the South African Karoo Supergroup

Norton, Luke Allan
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This is the first comprehensive study, using micro-computed tomography, of eutheriodont tooth replacement patterns through ontogeny in therocephalians (Lycosuchus and Bauria) and cynodonts (Cynosaurus and Galesaurus). Comparison of tooth replacement patterns of the incisors, canines and postcanines revealed that this varied the most in the postcanines, followed by the canines. The incisor replacement pattern is conservative, with all four taxa exhibiting alternating replacement. Lycosuchidae retain the basal synapsid condition of two maxillary canine loci, whereas Bauria and the epicynodonts Cynosaurus and Galesaurus have only a single maxillary canine. Maxillary canine replacement occurred several times through ontogeny in the two epicynodonts with cessation of canine replacement coinciding with attainment of skeletal maturity. This differs from the condition previously reported for the epicynodont Thrinaxodon, in which canine replacement continued well into adulthood. In contrast, there is no evidence of canine replacement in Bauria. Alternating postcanine replacement occurs in Lycosuchus, Cynosaurus, and Galesaurus, with the pattern of Cynosaurus more closely resembling that previously described in Thrinaxodon. In Cynosaurus, replacement waves move along the jaw from front-to-back in multiples of two in the mandible, and three in the maxilla. It is hypothesised that the first maxillary postcanine locus became dormant after two replacements, causing a distal shift in the postcanine series. Conversely, Galesaurus, does not exhibit cessation of replacement and the first maxillary postcanine is replaced even in the largest specimens. Additional teeth are added distally to the postcanine series in Galesaurus, such that larger specimens have more postcanine teeth. Only one Bauria specimen manifests postcanine replacement, suggesting that reduction in replacement activity is an adaptation to maintain precise occlusion. As each of the study taxa exhibit different replacement patterns, especially with regard to the postcanines, this study highlights a previously unrecognised diversity in tooth replacement patterns amongst the Eutheriodontia
A thesis submitted in fulfilment of the requirements for the degree Doctor of Philosophy in Palaeontology to the Faculty of Science, University of the Witwatersrand, Johannesburg, 2020