The Anatomy of Dryptosaurus aquilunguis (Dinosauria: Theropoda) and a Review of Its Tyrannosauroid Affinities

Although among the first theropod dinosaurs known to science, and an iconic taxon in the history of dinosaur paleontology, the large carnivore Dryptosaurus aquilunguis from the Late Cretaceous of New Jersey remains poorly understood. Its anatomy has been described only in brief and its phylogenetic relationships have long been the subject of debate, although recent work proposes Dryptosaurus as a member of the tyrannosauroid clade. Here we present a thorough osteological description of the holotype of Dryptosaurus aquilunguis, supplemented with photographs of all the material, and provide extensive comparisons with other theropods, especially tyrannosauroids. In concert with recent phylogenetic analyses, our description confirms the tyrannosauroid affinities of Dryptosaurus and supports its placement as an intermediate taxon bracketed between small, basal forms (e.g., Guanlong, Dilong) and the derived, Late Cretaceous tyrannosaurids (e.g., Albertosaurus, Tyrannosaurus). We identify several autapomorphies of Dryptosaurus, including the combination of a reduced humerus and an enlarged hand. These forelimb proportions, which differ from the uniformly large arms of basal tyrannosauroids and uniformly atrophied arms of tyrannosaurids, suggest that forelimb reduction in tyrannosauroids may not have proceeded in a uniform fashion. Functionally, Dryptosaurus may have used both its skull and arms as weapons for prey acquisition and processing.