UNUSUAL OCCURRENCE OF GASTROLITHS IN A POLYCOTYLID PLESIOSAUR FROM THE UPPER CRETACEOUS TROPIC SHALE, SOUTHERN UTAH

A polycotylid plesiosaur (Dolichorhynchops sp.), recently discovered in the Upper Cretaceous (Cenomanian-Turonian) Tropic Shale in Glen Canyon National Recreation Area, southern Utah, is associated with 289 gastroliths. This specimen is significant due to the general lack of gastroliths associated with most short-necked plesiosaur skeletons. The skeleton (MNA V10046) was excavated from a stratigraphic interval of marine shale that does not contain coarser-grained material. The stones were generally concentrated in one area of the skeleton, with the majority situated near the back of the skull. The stones are mainly composed of dark grey chert, are smooth and well rounded, and have varying degrees of polish. The majority of the stones are spherical in shape an are likely fluvial in origin. The gastroliths from this animal are similar in shape and number to those documented from elasmosaurid plesiosaurs. The gastroliths from MNA V10046, however, are much smaller and weigh considerably less than most elasmosaurid stomach stones. Given the lack of comparative material from other short-necked plesiosaurs, the size and mass differences are attributed to differences in function of gastroliths between long- and short-necked plesiosaurs.