THE PHYLOGENETIC POSITION OF THE TYRANNOSAURIDAE - IMPLICATIONS FOR THEROPOD SYSTEMATICS

Tyrannosaurids are a well-supported clade of very large predatory dino saurs of Late Cretaceous Asiamerica. Traditional dinosaurian systemati cs place these animals within the infraorder Carnosauria with the othe r large theropods (allosaurids, megalosaurids). A new cladistic analys is indicates that the tyrannosaurs were in fact derived members of the Coelurosauria, a group of otherwise small theropods. Despite certain gross cranial similarities with the large predators of the Jurassic an d Early Cretaceous, the Late Cretaceous tyrannosaurids are shown to be the sister group to ornithomimids and troodontids, which share a deri ved condition of the metatarsus. This clade is found to be nested with in Maniraptora, which is a more inclusive taxon than previously recogn ized- The atrophied carpal structure found in tyrannosaurids and ornit homimids is derived from a maniraptoran condition with a large semilun ate carpal, rather than from the plesiomorphic theropod morphology. Th e taxa ''Carnosauria'' and ''Deinonychosauria'' (Dromaeosauridae plus Troodontidae) are shown to be polyphyletic, and the Late Jurassic Afri can form Elaphrosaurus is found to be the sister taxon to Abelisaurida e rather than a primitive ornithomimosaur. Purported allosaurid-tyrann osaurid synapomorphies are seen to be largely size-related, present in the larger members of both clades, but absent in smaller members of t he Tyrannosauridae. The remaining giant tetanurine theropods (Megalosa urus and Torvosaurus) were found to be progressively distant outgroups to an allosaurid-coelurosaur clade. The inclusion of the Tyrannosauri dae within Maniraptora suggests a major adaptive radiation of coeluros aurs within Cretaceous Asiamerica comparable to contemporaneous radiat ions in various herbivorous dinosaurian clades.