QUESTIONING GRAY SHARK BIODIVERSITY IN THE ATLANTIC COASTAL PLAIN: MORPHOMETRIC ANALYSIS OF MIOCENE-PLIOCENE SHARK TEETH

The fossil teeth are often used for the taxonomic identification of ancient sharks due to their high preservation potential. Qualitative morphological analyses of Miocene-Pliocene shark teeth from the genus Carcharhinus from the Atlantic Coastal Plain have led experts to identify seven Carcharhinus species based on general tooth shape and size. However, recent discoveries from Miocene-Pliocene formations in the ACP suggest that various morphologies observed across Carcharhinus teeth may be gradational in nature rather than representing well-defined and discrete morphospecies. The goal of this study is to apply rigorous geometric morphometric techniques to teeth collected from the Calvert, Eastover, and Yorktown Formations of Virginia and Maryland to determine the nature of morphological variability of Carcharhinus teeth and the potential biological influence on tooth shape.

Preliminary analysis used five hundred carcharhinid upper teeth, which are the most commonly used in taxonomic studies. Each tooth was digitally photographed and x-y coordinates of geometrically and biologically identifying characteristics, ten lingually and nine labially, were obtained. Procrustes method was used to simultaneously fit the landmark points and derive shape coordinates that are invariant in respect to variations in size, rotation, and position of specimens. Principle component ordination of the Procrustes tangent coordinates was used to compare the overall, allometry–free shape differences between the teeth from different formations and geographic location. A separate size analysis was completed using tooth width and centroid size. Preliminary analysis reveals insignificant allometry-free morphological and size variation suggesting that tooth morphology remained constant throughout the Miocene and Pliocene. The gradational variation observed in the sampled teeth suggests that these specimens represent a morphological gradient rather than many discrete species. This implies that carcharhinid species diversity of the ACP cannot be determined based on isolated teeth. In addition, the temporal stability of shape morphospace suggests slow rates of morphological change of shark teeth during the Miocene and Pliocene.