Partial skull of late cretaceous durophagous shark, Ptychodus Occidentals (Elasmobranchii: Ptychodontidae), from Nebraska, U.S.A

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.