Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria)

ABSTRACT A study of ontogenetic variation is used to clarify aspects of tyrannosaurid taxonomy and investigate the supposed phenomenon of dwarfism in the clade. A hypothetical ontogenetic trajectory is described for the relatively well-represented taxon Albertosaurus libratus. The type specimen of the purported "pygmy" tyrannosaurid Nanotyrannus lancensis was compared with specimens of A. libratus and found to share many morphological characters that exemplify immature specimens of the latter taxon. Most of the cortical surface of the Cleveland skull of N. lancensis has immature bone grain. Also, the skull shares unique derived characters with mature specimens of Tyrannosaurus rex, suggesting that the specimen is a young T. rex and not a dwarf tyrannosaurid. An increase in tooth width, accompanied by loss of tooth positions, and a global shift from an immature gracile to a mature robust morphotype 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.