A new genus of small polycotylid plesiosaur from the Upper Cretaceous of the Western Interior Seaway and a clarification of the genus Dolichorhynchops

Abstract

Polycotylids are plesiosaurian marine reptiles that evolved during the Early Cretaceous and radiated into multiple genera during the Late Cretaceous. Here we describe a small new polycotyline polycotylid, Unktaheela specta, gen. et sp. nov., from the Campanian of the Western Interior Seaway in North America. Because its specimens bear a host of features indicating they represent adults, Unktaheela is the smallest known adult polycotylid. Diagnostic features of the orbital region include a large orbit, a supraorbital ledge over the orbit's anterodorsal margin, and a broad supraorbital ridge bordering the orbit's posterodorsal rim. Other autapomorphies include an unusually short postorbital region, a uniquely wide occiput, an autapomorphic parasphenoid, unusually wide neural spines, a distinctive pubis morphology, and propodials more convex ventrally than dorsally. A new phylogenetic analysis yields a more stable polycotyline phylogeny and recovers a well-supported clade of derived polycotylines, Dolichorhynchia, clade nov. We find Dolichorhynchops herschelensis and Dolichorhynchops osborni to be united by synapomorphies not shared with other polycotylines and the only valid species of Dolichorhynchops. We erect a new genus for “Dolichorhynchops” bonneri, Martinectes, gen. nov., finding it to be united with Unktaheela and ROM 29010 by several synapomorphies not shared with Dolichorhynchops. Several plesiomorphic traits of “Dolichorhynchops” tropicensis render it basal to Dolichorhynchops and Trinacromerum, and we erect the genus Scalamagnus, gen. nov., for this taxon. Comparisons to extant tetrapods suggest that several features of Unktaheela may be adaptations for visual pursuit predation in a sunlit environment, an interpretation with ecological implications for other aquatic reptiles.