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FHSM VP-5515 is a medium-sized russellosaurine mosasaur collected in the 1970s in Logan County, western Kansas, USA. A suite of cranial features are unique to this specimen at the species level. One is a conspicuous lack of a predental rostrum on the premaxilla, whose dentigerous portion is spatula shaped in dorsoventral aspect. Furthermore, the frontal of FHSM VP-5515 exhibits the following unique combination of features: (1) the median dorsal keel absent; (2) straight, long preorbital borders converging anteriorly at 30° from each other; and (3) supraorbital border thick and extensive, clearly separating the prefrontal and the postorbitofrontal above the orbit. The fifth character pertains to the uniquely square outline of the intramandibular joint surface of the splenial, resulting from a robust dorsomedial flange. Finally, hitherto unreported on any mosasaur jaws, a deep notch excavates the dorsal border of the articular immediately posterior to the glenoid fossa. Examination of preexisting Ectenosaurus specimens suggests that this articular notching diagnoses the genus. Based on the unique suite of characteristics including the articular notch, we consequently hypothesize the following for FHSM VP-5515: (1) it is most comparable with the enigmatic russellosaurine Ectenosaurus clidastoides (Merriam, 1894); and (2) it represents a new species of Ectenosaurus. [ABSTRACT FROM AUTHOR]

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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]