Abstract

Based on the type material and a new, virtually intact skull from the mid-Maastrichtian part (probably Tierra Loma Shale) of the Moreno Formation, San Joaquin Valley, central California, the cranial osteology of the enigmatic mosasaurine Plesiotylosaurus crassidens (Squamata, Mosasauridae) is here described in detail. Plesiotylosaurus is characterized by a protruding rostrum on the snout and mandibles, a wide and virtually unconstricted internarial bar, large and robust pterygoid teeth, and quadrates with distally fused suprastapedial and infrastapedial processes. Distinctly obtruding predental prows, along with a number of elaborate cranial features, suggest a morphological convergence with tylosaurine mosasaurs.

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