Osteology of a new plesiosaur from the lower Cretaceous (Albian) Thermopolis Shale of Montana

Abstract

ABSTRACT A new taxon of Cretaceous short-necked plesiosaur, Edgarosaurus muddi, gen. et sp. nov., is described. The specimen consists of a complete skull, 34 vertebrae including the entire series of 26 cervicale, and a nearly complete forelimb. The new taxon occurs in marine sediments of the Thermopolis Shale (late Albian), deposited during an early transgression of the Western Interior Seaway. The skull of Edgarosaurus is the oldest, and one of the best preserved plesiosaur specimens from the Cretaceous Western Interior of North America described to date. Edgarosarus differs from the morphologically and stratigraphically close short-necked taxon Plesiopleurodon wellesi in a number of cranial characters. In the absence of an established phylogenetic framework, the new plesiosaur is placed in the Polycotylidae, as recently defined by Carpenter in 1996. It is plesiomorphic with respect to other polycotylids in that it possesses a relatively robust and moderately elongated rostrum, a pineal foramen, caniniform teeth, a shorter mandibular symphysis, and a relatively high number of cervical vertebrae.