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.

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