A new polycotylid plesiosaur with extensive soft tissue preservation from the early Late Cretaceous of northeast Mexico

A nearly complete skeleton of a polycotylid plesiosaur is described from the early Late Cretaceous laminated limestones at Vallecillo, northeast Mexico. It shows extensive soft tissue preservation. In some exceedingly well preserved areas there are transversely elongate rectangular to trapezoid millimetric scale-like structures arranged in longitudinal rows. The trailing edge skin flap preserves fibers and scale rows perpendicular to the trailing edge. A thick layer of subdermal tissue is present, especially along the tail base. It was responsible for the hydrodynamic drop-shaped body contour, with the body and tail forming a single unit. The body shape determined from the preserved soft tissues suggests a swimming speed similar to extant leatherback turtles. Based on the unique osteology of the palate, which is intermediate between Dolichorhynchops and Trinacromerum, and according to the unique morphology of the girdles and propodials as well as the medially converging gastralia, the new specimen is placed in a new genus and species of Polycotylidae, Mauriciosaurus fernandezi gen. et sp. nov.