Southernmost record of the pachycephalosaurine <i>Stygimoloch spinifer</i> and palaeobiogeography of latest Cretaceous North American dinosaurs

Specimens of the pachycephalosaurine Stygimoloch spinifer from the Upper Maastrichtian Ferris Formation in south-central Wyoming’s Hanna Basin represent the southernmost record of this uncommon marginocephalian. UW 26525 is a partial, but diagnostic left squamosal that extends the geographic range of S. spinifer 100 km to the south from the Lance Formation in eastern Wyoming. It was collected from deposits that accumulated approximately 320 k.y. before the K/Pg boundary bolstering the hypothesis that S. spinifer is taxonomically distinct from Pachycephalosaurus wyomingensis and only occurred during the final 500 k.y. of the Maastrichtian. Two pachycephalosaurine teeth are also referrable to S. spinifer and were preserved approximately 370 and 457 k.y. prior to the K/Pg boundary. These specimens emphasize the value of isolated, fragmentary, but taxonomically identifiable remains from areas and horizons in which fossils are rare or unreported. Furthermore, they support the hypothesis that latest Cretaceous dinosaurian communities were generally cosmopolitan in the North American Western Interior, but the low abundance of S. spinifer in fossil collections suggests that the identification of clear palaeobiogeographic trends remains a challenge.