A new drepanosauromorph, Ancistronychus paradoxus n. gen. et sp., from the Chinle Formation of Petrified Forest National Park, Arizona, USA

Drepanosauromorpha is an extinct group of reptiles known from the Middle Triassic to Late Triassic (237–212 Ma). The clade currently includes seven genera (Avicranium, Dolabrosaurus, Drepanosaurus, Hypuronector, Kyrgzsaurus, Megalancosaurus, and Vallesaurus) that are known from fossils collected in Europe, North America, and Asia. These discoveries have helped shape our understanding of the biology and diversity of drepanosauromorphs. Here we describe Ancistronychus paradoxus n. gen. et sp. from the Chinle Formation in Petrified Forest National Park, Arizona based on the ungual phalanx of the second digit of the manus. A characteristic that this taxon shares with Drepanosaurus unguicaudatus is the pronounced size of the ungual relative to the penultimate element. It differs significantly from D. unguicaudatus and the Hayden Quarry Drepanosaurus in the shortened proximal dorsoventral height of the claw, its great transverse breath, the presence of both a furrow on the midline of the extensor surface and a cleft on the apex, and a broad and flattened terminus. We suggest that A. paradoxus is likely closely related to D. unguicaudatus and the Hayden Quarry Drepanosaurus, but missing phylogenetic data precludes a more definitive assessment at this point. Ancistronychus paradoxus highlights unsuspected morphological variation within Drepanosauromorpha and suggests that different drepanosauromorphs used their enlarged second manual unguals for distinct functions enabling them to fill different ecological niches