We here report the results of a primarily histological study of jaw growth and tooth replacement in the Early Permian captorhinomorph reptile Captorhinus aguti. Preliminary histological examination shows that “drift” of teeth from the lingual to the labial side of the jaw apparently occurred through a combination of bone growth + remodelling. Simple graphical analysis of proportional changes in the C. aguti skull and jaws over a threefold span of size increase demonstrates that growth was essentially isometric. This fact and our preliminary histological observations are combined into a model of jaw growth plus tooth replacement, in which the positions and relative ages of teeth are described in terms of the Zahnreihen of Edmund (1960). Detailed histological observations are then used to test the model, which is confirmed in all essentials but shown to be incomplete as first formulated. With incorporation of the fact that during growth the older, smaller parts of the dentary and maxilla were progressively relocated anteriorly, our Zahnreihen-based model appears to be an accurate representation of tooth replacement in C. aguti. The C. aguti replacement system may have been derived with little change from that of single-rowed ancestors. It is uncertain whether this is true also of later multiple-rowed captorhinids. The exact mechanism of tooth loss in C. aguti and in modern reptiles, including control of timing, is uncertain.
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