TRINACROMERUM BONNERI, NEW SPECIES, LAST AND FASTEST PLIOSAUR OF THE WESTERN INTERIOR SEAWAY

The Pierre Shale represents the final days of the Western Interior Sea way before its regression at the end of the Mesozoic, and records the last of the marine reptiles that dominated the seas much as their cont emporary dinosaur counterparts dominated the land. Trinacromerum bonne ri, n. sp., is the first pliosaur (short-necked plesiosaur) to be desc ribed from this formation in the northern Great Plains; as such it rep resents the final radiation of polycotylid pliosaurs in North America. Pliosaurs have long been regarded as particularly high-speed swimmers , but T. bonneri carried this trend to an extreme. Development of the longest wingfins known in pliosaurs maximized its velocity. Unique lim b and vertebral structures resisted pressures of the surrounding water that were generated by its own swimming velocity. Such adaptations in clude tongue-and-groove articular surfaces between critical limb eleme nts and highly interlocking cervical vertebrae.