QUANTIFYING HETERODONTY IN THE LATE DEVONIAN (UPPER FAMENNIAN) SHARKS CLADOSELACHE AND CTENACANTHUS FROM THE OHIO SHALE, USA

Differentiation of tooth size and shape (i.e. heterodonty) within the Neoselachian jaw is an expected pattern. Various forms of heterodonty may be observed, including differentiation within an individual set of jaws (monognathy), between upper and lower jaws (dignathy), within an individual tooth file or developmental position (ontogeny), or between males and females (gynandry). These patterns primarily result from natural selection for dietary diversification. However, the presence, types, and the degree of heterodonty present in Devonian sharks, Cladoselache and Ctenacanthus, have not previously been described in the literature. In order to test for, and quantify, various types of heterodonty in representatives of these two genera, we analyzed a number of associated dentitions collected from the Cleveland Shale Member of the Ohio Shale (upper Famennian; Upper Devonian). Geometric morphometrics and linear measurements were used to describe tooth shape. The resulting measurements were regressed with jaw position, tooth file position, and upper versus lower jaw to test for heterodonty. Teeth shape from Cladoselache and Ctenacanthus dentitions that were examined did not show any form of heterodonty. However, tooth size did vary slightly with jaw position, indicating a degree of ontogenic variation and symphyseal teeth at the lower jaw symphysis indicated a difference between upper and lower jaws. The long period of tooth retention as indicated by the tooth files creates a unique record of ontogenetic heterodonty within a tooth file that can be seen as an increase in tooth size lingually. Although heterodonty was not found in either taxa, significant morphometric differences between the two genera were evident. These findings strengthen the validity of the recognized genera and species and thus provide insights into the evolution of these Devonian sharks.