Dentition of Late Cretaceous shark, Ptychodus mortoni (Elasmobranchii, Ptychodontidae)

ABSTRACT

Ptychodus (Elasmobranchii, Ptychodontidae) is an enigmatic durophagous shark that lived in Cretaceous seas. Based on multiple articulated tooth plates of Ptychodus mortoni from the Niobrara Chalk in Kansas, the dental pattern of P. mortoni and its paleobiological implications were examined. Each tooth plate consists of one medial tooth row and about nine lateral tooth rows on each side. One individual shark possesses a total of slightly over 550 teeth, with approximately 220 functional upper teeth and 260 functional lower teeth. The largest tooth plate of the species likely measured about 55 cm in length and 45 cm in width. Although this study does not resolve the ordinal placement of Ptychodontidae, it demonstrates that there are two different patterns of tooth plate organization in Ptychodus: a plesiomorphic condition characterized by juxtaposed, non-overlapping tooth rows (e.g., P. decurrens, P. marginalis, and P. occidentalis) and an apomorphic condition characterized by imbricated tooth rows (e.g., P. mortoni). It is hypothesized here that the imbrications of tooth rows in P. mortoni likely helped distribute the bite-induced load on its dentition more widely, and its sharp-tipped crowns were effective in shattering animals with comparably thin, brittle shells (e.g., bivalves and crustaceans). The recognized difference in tooth plate organization adds another level of complexity to the dental evolution of Ptychodus as well as to the already complex evolutionary history of predatory behaviors in cartilaginous fishes.

ACKNOWLEDGMENTS

I thank the following present and past museum associates for allowing me access to specimens in respective institutions in their care: R. J. Zakrzewski (FHSM), J. Chorn, C. Fielitz, D. Miao, and H.-P. Schultze (KUVP), K. Seymour (ROM), and G. R. Corner (UNSM). I also thank M. J. Everhart and M. Mortimer for useful discussions. I also thank G. Cuny and M. Siverson as well as the editors for making useful comments and suggestions on the manuscript that improved the quality of this paper.

Handling editor: Charlie Underwood