Tooth replacement patterns in Eutheriodontia (Synapsida, Therapsida) from the South African Karoo Supergroup
Date
2020
Authors
Norton, Luke Allan
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Abstract
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
Description
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