Dryptosaurus, AND

Brusatte, Stephen L., Benson, Roger B. J. & Norell, Mark A., 2011, The Anatomy of Dryptosaurus aquilunguis (Dinosauria: Theropoda) and a Review of Its Tyrannosauroid Affinities, American Museum Novitates 2011 (3717), pp. 1-56 : 45-46

DRYPTOSAURUS AND APPALACHIOSAURUS :

EASTERN NORTH AMERICAN TYRANNOSAUROIDS

During most of the Late Cretaceous, eastern and western North America were separated by the Western Interior Seaway, a continental sea that stretched from the present-day Gulf of Mexico to the Arctic Ocean ( Smith et al., 1994). Therefore, Dryptosaurus and its contemporary fauna would have had little, if any, faunal connection with the tyrannosaurid-dominated ecosystems of western North America during the Maastrichtian. This is also true of the slightly older (Campanian), eastern North American tyrannosauroid Appalachiosaurus . In light of this information, our current understanding of tyrannosauroid phylogeny is congruent with the physical geography of terminal Cretaceous North America: Dryptosaurus and Appalachiosaurus are not particularly close relatives of coeval western North American tyrannosaurids such as Albertosaurus and Tyrannosaurus .

The Campanian-Maastrichtian western North American tyrannosauroids form a clade, Tyrannosauridae , which also includes Asian members. It is tempting to consider whether eastern North American tyrannosauroids also form a clade. Indeed, Dryptosaurus and Appalachiosaurus are extremely similar in body size and general morphology. However, there is currently no strong evidence that they form a unique eastern clade. No such clade is recovered in the phylogenetic analysis of Brusatte et al. (2010), which instead places Dryptosaurus and Appalachiosaurus as successively closer outgroups to Tyrannosauridae . Furthermore, the two eastern tyrannosauroids are not proximal outgroups on the tyrannosaurid “stem,” but Raptorex falls out in between them.

With this said, it is notable that there is minimal overlapping material between Dryptosaurus and Appalachiosaurus , and many of the overlapping bones are poorly preserved in both taxa. Appalachiosaurus is known from a decently preserved skull, whereas little cranial material is present in Dryptosaurus . Conversely, the forelimb of Dryptosaurus is reasonably well known, but is completely unknown in Appalachiosaurus . Therefore, we consider the exact phylogenetic relationships of these two taxa as equivocal. It is clear that neither is a tyrannosaurid and that both are close outgroups to Tyrannosauridae , but whether they form a unique eastern clade can be confidently tested only with the discovery of additional material from eastern North America. It would not surprise us if such discoveries eventually illuminate a clade of tyrant dinosaurs restricted to eastern North America.