A new genus and species of tyrannosauroid from the Late Cretaceous (Middle Campanian) Demopolis Formation of Alabama

ABSTRACT

The discovery of a new genus and species of tyrannosauroid from the Demopolis Formation (middle Campanian) of Alabama increases the known diversity of the clade, although it does not elucidate the place of initial dispersal. This subadult tyrannosauroid is the most complete non-avian theropod collected and described from the Cretaceous of eastern North America. In contrast to tyrannosaurids, the new taxon possesses several plesiomorphic characters, including lacrimals that lack a distinct peaked cornual process, and a dorsoventrally shallow horizontal ramus of the maxilla. Autapomorphies include a wide jugal process of the ectopterygoid, a caudal pneumatic foramen of the palatine that pierces the rostral half of the vomeropterygoid process of the bone, an articular surface for the lacrimal on the palatine that is distally positioned on the dorsolateral process, and pedal unguals that have a distinct proximodorsal lip over the articular surface. Cladistic analysis indicates the new taxon is a basal tyrannosauroid and its presence in eastern North America suggests that the recent common ancestor of Tyrannosauridae probably evolved following the transgression of the Western Interior Seaway. Cladistic analysis indicates that Dryptosaurus aquilunguis is also a basal tyrannosauroid but is less derived than the new genus.