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LACM 128319, which was collected in western Kansas, U.S.A., and is assignable to Platecarpus tympaniticus (Mosasauridae, Plioplatecarpinae), represents arguably one of the most exquisite mosasaur specimens known to date. Measuring 5.67 m from the tip of the snout to the end of the tail, it comprises an exceptionally well-articulated skeleton, accompanied by soft-tissue remains, such as skin impressions and tracheal cartilage. P. tympaniticus is one of the most numerously collected mosasaur taxa in North America, but as most specimens are fragmentary or reconstructed to various degrees, LACM 128319 provides a unique opportunity to document the taxon's osteology from a single skeleton. In this study, we first present a detailed osteological description of LACM 128319. Following this, we present an analysis of the evolution of a streamlined body shape in P. tympaniticus, specifically by comparing the length distribution of the dorsal ribs in relevant anguimorphan taxa. We conclude that both an anterior migration of the rib cage and an increasing regionalization within the dorsal vertebral series are key features contributing to formation of a streamlined body profile in P. tympaniticus, and probably in many other hydropedal members of mosasaurs. [ABSTRACT FROM AUTHOR]