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

摘要

The Pierre Shale represents the final days of the Western Interior Seaway before its regression at the end of the Mesozoic, and records the last of the marine reptiles that dominated the seas much as their contemporary dinosaur counterparts dominated the land. Trinacromerum bonneri, n. sp., is the first pliosaur (short-necked plesiosaur) to be described from this formation in the northern Great Plains; as such it represents 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 limb and vertebral structures resisted pressures of the surrounding water that were generated by its own swimming velocity. Such adaptations include tongue-and-groove articular surfaces between critical limb elements and highly interlocking cervical vertebrae.