A large tyrannosaurid from the Late Cretaceous (Campanian) of North America

The Tyrannosauridae emerged as the dominant large predators in Laurasia during the Late Cretaceous. Their evolution in North America culminated with the replacement of Albertosaurinae, Daspletosaurinae, and Teratophonei, with masses of 2-3 tonnes, by the giant Tyrannosaurus in the late Maastrichtian, with a mass approaching or exceeding 10 tonnes. The origin of Tyrannosaurus remains enigmatic, but fossils suggest an origin in the Campanian-Maastrichtian of southern Laramidia. Here we describe an unusually large and robust tyrannosaurid tibia from the late Campanian aged Hunter Wash Member of the Kirtland Formation, New Mexico, ~ 74 Ma. The tibia measures 960 mm in length and 128 mm in diameter, ~84% and 78% the dimensions of the largest known Tyrannosaurus, suggesting a mass approaching 5 tonnes, larger than any contemporary tyrannosaur. The Hunter Wash tyrannosaur could potentially represent (i) an unusually large and robust Bistahieversor, (ii) a previously unknown lineage of giant tyrannosaur, or (iii) an early representative of Tyrannosaurini. The tibia's large size, robust proportions, and shape of the distal shaft are most consistent with referral to Tyrannosaurini. The Hunter Wash tyrannosaur emphasizes the marked endemicity of Laramidian dinosaurs; while smaller Albertosaurinae and Daspletosaurini inhabited the north, giant tyrannosaurins occurred in the south.