A NEW VERTEBRATE FAUNA FROM THE EARLY CRETACEOUS WILLOW TANK FORMATION OF SOUTHERN NEVADA

With the exception of the Cedar Mountain and Cloverly formation of Montana and Wyoming, respectively, a sparse vertebrate record exists for the Early Cretaceous Sevier foreland basin. A new vertebrate assemblage recovered from the Willow Tank Formation in southern Nevada expands the biogeographic range of Early Cretaceous fauna and provides a better understanding of their paleoecology.

The Willow Tank Formation has been assigned to the Albian stage of the Early Cretaceous. The basal conglomerate of the formation records a semi-arid, Donjek-type braid plain, representing the onset of Sevier fold and thrust tectonics in the region. An abrupt transition characterizes the contact with the upper member of the formation, which was deposited within an anastomosing fluvial system, in a semi-arid environment. The Willow Tank Formation unconformably overlies the Jurassic Aztec Sandstone and is in turn overlain by the Late Cretaceous (Cenomanian) Baseline Sandstone.

The diverse vertebrate fauna recovered from the Willow Tank Formation represent a number of taphonomic modes, including microsites and localities with isolated and associated elements. With the exception of an associated ornitopod specimen, most specimens are fragmentary, thereby contributing to the difficulty of taxonomic assignment at the species level. Screen washing of microsite sediments produced a diversity of osteological remains, including those of holosteans, dipnoans, turtles, crocodylomorphs, sauropods, theropods, thyreophorans, and ornithopods. Eggshells recovered from mudstone facies and one microsite within the study area represent new and previously described ootaxa.