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The Cretaceous Pierre Shale Group along the Missouri River has produced numerous mosasaur specimens since the western fossil discoveries of Lewis and Clark in 1804 that included a 45-foot “fish.” Many of these marine reptile specimens represent the largest of mosasaurs, the tylosaurines. In 1990 the largest mosasaur heretofore recorded along the Missouri River was discovered near Nicholas Creek, Lyman County, central South Dakota. The specimen was recovered from a lag deposit representing an intra–Pierre Shale Group unconformity and consists of vertebrae, ribs, paddle elements, and a partial skull. The partial skeleton is referable to the subfamily Tylosaurinae, cf. Tylosaurus sp., based on large size, tooth structure, and long pre-dental rostrum. Further identification must await resolution of the taxonomy of the Tylosaurinae. A lower jaw measures 1.6 m, indicating a projected body length of 11.5 m. Therefore, the large “fish” described by Lewis and Clark may have been a tylosaurine mosasaur.

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