Partial skull of Late Cretaceous durophagous shark,Ptychodus occidentalis(Elasmobranchii: Ptychodontidae), from Nebraska, U.S.A.

ABSTRACT ABSTRACT Ptychodus (Elasmobranchii: Ptychodontidae) is an enigmatic durophagous shark known from Cretaceous marine deposits nearly worldwide based primarily on isolated teeth. Here, we describe a specimen of P. occidentalis Leidy from the Greenhorn Limestone (middle Cenomanian–early Turonian) in Nebraska, U.S.A. The specimen does not offer any new taxonomic information concerning the unresolved ordinal placement of Ptychodus. However, it is significant because it preserves a partial skull including teeth, incomplete jaws, and neurocranial fragments as well as placoid scales and vertebrae, all of which provide a wealth of new anatomical information. The specimen contains at least 267 teeth and shows that the two dental plates are anteroposteriorly elongate. The partial paired Meckel's cartilages show a long, fused jaw symphysis and elongate jaw rami. The distribution of presumed neurocranial fragments and placoid scales with respect to the position of the dental plates suggests that the shark had a broad head with a narrow, subterminal mouth. The morphology of placoid scales suggests that the shark was a rather sluggish swimmer and had a stout, streamlined body similar to that in the extant orectolobiform sharks (e.g., Ginglymostoma). Based on the jaw length, the individual of P. occidentalis was approximately 2 m in total length (TL), while some of the largest Ptychodus known from younger Cretaceous horizons in the North American Western Interior likely attained at least 7 m TL. Ptychodus occidentalis is inferred to be an opportunistic generalist (rather than a hard prey specialist) that fed on a variety of readily available prey using inertial suction feeding.