A NEW DINOSAUR ASSEMBLAGE FROM THE LOWER CRETACEOUS (ALBIAN) OF SOUTHERN NEVADA

The Willow Tank Formation exposed in Valley of Fire State Park in southern Nevada consists of sediments of an alluvial system deposited in the foredeep of the Sevier foreland basin during Early Cretaceous time. Radiometric dates obtained from ashes in the upper mudrock member place the formation in the latest Albian Stage. Exploration of this mudrock member reveals a diverse dinosaurian fauna: Thyreophora, Hypsilophodontidae, Iguanodontidae, Hadrosauroidea, Titanosauriformes, Tyrannosauroidea, and Dromaeosauridae. Although most taxa are represented by teeth, additional elements include an associated femur and tibia of a dromaeosaurid; an iguanodontian pre-pubic process and other associated elements; associated vertebrae, femora, and phalanges of a hypsilophodont; and a hadrosauroid dental battery.

Early Cretaceous faunas across North America appear rather homogenous at the family level, with relatively consistent distribution of dinosaur taxa across the continent. Although tyrannosauroid teeth and hadrosauroids are known from the uppermost Cenomanian member of the Cedar Mountain Formation, the tyrannosauroid premaxillary tooth and hadrosauroid dental battery from the Willow Tank Formation represent the first report of an earliest Cretaceous occurrence for these groups in the Albian of North America. The absence of marginocephalians may represent collection bias; alternatively, it may suggest that faunal interchange between Asia and the American southwest occurred in pulses, with tyrannosauroids occurring first, followed by marginocephalians. In addition, basal iguanodontians occur stratigraphically higher in the Willow Tank Formation than hadrosauroids, suggesting temporal overlap of these families during the Early Cretaceous in North America. Although predicted by current phylogenies, co-occurrence of basal iguanodontians and hadrosauroids has not been previously documented in the Early Cretaceous of North America. Further discoveries may shed light on the dynamics of faunal interchange between North America and Asia during mid-Cretaceous times.