First report of a giant neosuchian (Crocodyliformes) in the Williams Fork Formation (Upper Cretaceous: Campanian) of Colorado

Abstract

A large osteoderm found in a channel sandstone in the Williams Fork Formation (“Mesaverde Group”) of northwestern Colorado represents the first reported evidence of a large neosuchian crocodyliform in the formation in northwestern Colorado. The osteoderm is of a size and pit pattern that resembles the large alligatoroid Deinosuchus but the thickness and shape of the bone suggest possible affinities with the goniopholidid or pholidosaurid crocodyliform material known from the Campanian of southern Utah; smaller, similar osteoderms from the formation may suggest that this form was relatively abundant as juveniles. Overall faunal similarities of the Williams Fork Formation are closest to the Kaiparowits Formation of southern Utah also, but few lower level taxa can be confirmed from the Williams Fork that are not also known from other areas north and south, suggesting that the Williams Fork vertebrate fauna comes from the southern end of a transition zone between possible latitudinal provinces of Laramidia.