A NEW SUBADULT TYRANNOSAURUS REX AND A REASSESSMENT OF ONTOGENETIC AND PHYLOGENETIC CHANGES IN TYRANNOSAUROID FORELIMB PROPORTIONS

The colossal Late Cretaceous theropod Tyrannosaurus and its close tyrannosaurid relatives are immediately recognized by their atrophied forelimbs. Substantial mystery, however, surrounds the functional utility and the evolution of this unusual feature. One pressing question is whether small forelimbs were present throughout ontogeny or only in large-bodied adults. A new specimen of a subadult Tyrannosaurus from the Late Maastrichtian Hell Creek Formation of Carter County, Montana and the compilation of an extensive database of tyrannosaurid measurements help solve this riddle. The specimen is a partial skeleton comprised of dorsal vertebrae, ribs, gastralia, and several bones of the fore and hindlimbs. Histological examination shows that the individual was a subadult (13 years old) when it died. Importantly, the forelimb morphology of the new subadult differs from that of larger and more mature adult tyrannosaurids. The humerus is longer in relation to the femur and more slender. Additionally the manual unguals are proportionally enlarged, in contrast to adults. In concert, the large humerus and unguals conclusively show that the entire forelimb was relatively longer in juveniles and subadults than in adults, demonstrating that forelimb proportions underwent negative allometry during ontogeny. Similar patterns of negative allometry are present in other Late Cretaceous tyrannosaurids such as Albertosaurus and Tarbosaurus, based on an ever-expanding collection of fossils whose measurements are incorporated into our database. The negative allometry of the tyrannosaurid forelimb lends evidence that a controversial small-bodied, but short-armed, tyrannosauroid specimen from China can be referred to its own taxon (Raptorex) and it is not a juvenile of Tarbosaurus. In sum, juvenile tyrannosaurids had larger forelimbs than adults, and the ontogenetic development of atrophied forelimbs occurred in association with the ontogenetic transition from a small, long, and shallow skull to the large, deep, and robust adult skull optimized for strong bite forces. This suggests that tyrannosaurids underwent a behavioral and dietary shift during their ontogenetic development, and perhaps that juveniles and adults employed different hunting strategies and inhabited distinct ecological niches.