Two new plioplatecarpine mosasaurs (Mosasauridae; Plioplatecarpinae) of the genus <i>Ectenosaurus</i> from the Upper Cretaceous of North America

Abstract

Two new species of the rare mosasaur Ectenosaurus are reported from the Upper Cretaceous of North America. Ectenosaurus tlemonectes sp. nov. (YPM VP4673) consists of a largely complete skull and some associated post-cranial elements that were derived from an unknown level within the Smoky Hill Member of the Niobrara Chalk (upper Coniacian-lower Campanian), of Kansas, USA. Ectenosaurus shannoni sp. nov. (ALMNH:Paleo:5452) is described from a much more fragmentary specimen collected from the unnamed member of the Mooreville Chalk (upper Santonian-lower Campanian) of Alabama, USA. These new taxa can be distinguished from the other members of the genus, E. clidastoides (Merriam, 1894) and E. everhartorum Willman et al. (2021), by clear morphological differences in the skull and jaws. Two parsimony analyses of a data matrix consisting of 98 characters and 20 terminal taxa were carried out, the first without constraints and the second constrained by the assumption of monophyly in Ectenosaurus. Both analyses resulted in 30 equally parsimonious trees of 255 steps. Neither analysis yielded definitive information about the position of Ectenosaurus within Plioplatecarpinae, suggesting the need for an expanded data matrix. Although Ectenosaurus is an extremely rare component of the mosasaur assemblages from which it is found, it is also a diverse genus, with now at least four recognized species. This raises questions about the paleoecology and paleobiogeography of this genus and requires additional investigation.