Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria)

A study of ontogenetic variation is used to clarify aspects of tyrannosauri d taxonomy and investigate the supposed phenomenon of dwarfism in the clade . A hypothetical ontogenetic trajectory is described for the relatively wel l-represented taxon Albertosaurus libratus. The type specimen of the purpor ted "pygmy" tyrannosaurid Nanotyrannus lancensis was compared with specimen s of A. libratus and found to share many morphological characters that exem plify immature specimens of the latter taxon. Most of the cortical surface of the Cleveland skull of N. lancensis has immature bone grain. Also, the s kull shares unique derived characters with mature specimens of Tyrannosauru s rex, suggesting that the specimen is a young T, rex and not a dwarf tyran nosaurid. An increase in tooth width, accompanied by loss of tooth position s, and a global shift from an immature gracile to a mature robust morphotyp e in the craniofacial skeleton typifies the ontogenetic changes in T. rex. Similarly, on the basis of immature characters, Maleevosaurus novojilovi is considered to be an immature Tarbosaurus bataar.