THE FIRST UNEQUIVOCAL NORTH AMERICAN OCCURRENCE OF THE MOSASAUR HAINOSAURUS (REPTILIA) FROM THE CRETACEOUS PIERRE SHALE OF THE MISSOURI RIVER TRENCH, SOUTHERN SOUTH DAKOTA

One of the largest reptiles to occur in the seas during the end of the Cretaceous was the mosasaur, Hainosaurus. The mosasaur attained lengths of over 10 meters with a meter-long skull and is a tylosaurine mosasaur, a top predator. Hainosaurus bernardi was originally described in 1885 from Maastrichtian deposits in Belgium based upon a partial skeleton and a second skull. Other reports of Hainosaurus from the Old World have occurred, but its occurrence in North America has been questioned. The first report of the genus in North America, Hainosaurus pembinensis was from the early Campanian Pembina Member of the Pierre Shale in Canada. Later workers have doubted the assignment of the Canadian taxon to Hainosaurus, and many authors have considered the taxon as a species of the closely allied genus, Tylosaurus. A number of characters have been utilized to define Hainosaurus, but many have been found in other taxa. Perhaps the most definitive character of Hainosaurus is that the pineal opening is shared by the frontal and parietal bones, rather than being confined to the parietal. Until now, no other North American mosasaur, including the Canadian taxon, exhibits a clearly shared pineal opening. Now, a new specimen found by Mr. Paul Neumiller from the late Campanian DeGrey Member of the Pierre Shale along the shore of the Missouri River in Gregory County exhibits the shared pineal opening. In addition, other characters suggested as characteristic of Hainosaurus including relatively smooth surfaces on the teeth, interlocking premaxillary-maxillary suture, and a prominent internal process of the suprastapedial process of the quadrate occur on the newly discovered specimen. Other apomorphies suggest that the specimen represents a new species. Evidently, tylosaurine specimens from the early Campanian exhibit the initial divergence of Hainosaurus from Tylosaurus, and by the late Campanian and Maastrichtian, the transition had been completed.

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