Positional Variation in Pedal Ungulas of North American Ornithomimids (Dinosauria, Theropoda): A Response to Brownstein (2017)

Abstract

Positional variation is documented in ornithomimid pedal unguals from the Dinosaur Park and Horseshoe Canyon Formations of Alberta, Canada, and characters for identifying the position of isolated ornithomimid pedal unguals are discussed. Ungual morphology has been used recently to argue for the coexistence of two distinct ornithomimosaurs, a basal taxon and distinctly more derived taxon, in the Early Cretaceous Arundel Clay of Maryland, USA. However, these conclusions are based on misconceptions of the morphology and positional variability of ornithomimosaur unguals. Some characters previously cited as diagnostic of ornithomimosaur unguals are not actually observed in this clade, or are more homoplastically distributed among theropods. Other characters proposed to distinguish between the two pedal ungual morphs in the Arundel Clay material are shown in the Albertan ornithomimid material to consistently distinguish the different ungual positions within the pes of one individual. Claims of multiple distinct ornithomimosaur taxa in the Arundel Clay are premature, as the two pedal ungual morphotypes more likely represent positional variation in a single taxon.