Phylogeny of Tyrannosauroidea (Dinosauria: Coelurosauria) with special reference to North American forms

The interrelationships of tyrannosauroids are unclear owing to a proliferation of taxa and an incomplete knowledge of variation in the clade. A review of the taxonomy, based principally on the skulls of North American forms, is necessary because multiple new specimens have been collected since the publication of the initial taxonomic works. These fossils include diagnostic specimens collected from the southeastern and southwestern United States, regions where tyrannosauroid diversity was previously unknown. Restudy of the holotype and referred specimens of North American forms allowed for a taxonomic revision and permitted the discovery of new phylogenetically informative characters.; A cladistic hypothesis of tyrannosauroid phylogeny is proposed, based on the taxonomic revision and new characters. The results consist of three successively remote basal taxa and a monophyletic Tyrannosauridae that is comprised by two lineages, Albertosaurus and Tyrannosaurinae . The latter clade includes a new genus from Utah, and a clade formed by Daspletosaurus and a monophyletic Tyrannosaurus .; Tyrannosauroids are among the few large theropods for which segments of growth series or adults and juveniles are known for several species. This relative wealth of data allows for the integration of ontogeny and phylogeny. The neontological method called "event-pair cracking" is applied to identify shifts in the sequence of developmental events between the growth series of Albertosaurus libratus and Tyrannosaurus rex. Five events were shifted early in T. rex relative to A. libratus. Also, the juvenile and adult states of four tyrannosaurids (A. libratus, Daspletosaurus sp., T. bataar, T. rex) are compared to identify general heterochronic changes that underlie evolutionary transformations; evidence of heterochronic change was found in six characters. The evolution of two characters show evidence of peramorphosis, three show paedomorphosis, and one displays isomorphosis. Since event pair cracking provides evidence of the rate of growth and the comparison of juvenile and adult states provides evidence of the onset and offset of the growth for a given character, the results of these analyses were combined to identify the specific heterochronic processes that produced evolutionary novelties.