Tyrannosauridae is uncontroversially divided into two subfamilies. Albertosaurinae comprises the North American genera "Albertosaurus" and "Gorgosaurus", while Tyrannosaurinae includes "Daspletosaurus", "Teratophoneus", "Bistahieversor", "Tarbosaurus", "Nanuqsaurus", "Zhuchengtyrannus", and "Tyrannosaurus" itself. Some authors include the species "Gorgosaurus libratus" in the genus "Albertosaurus" and "Tarbosaurus bataar" in the genus "Tyrannosaurus", while others prefer to retain "Gorgosaurus" and "Tarbosaurus" as separate genera. Albertosaurines are characterized by more slender builds, lower skulls, and proportionately longer tibiae than tyrannosaurines. In tyrannosaurines, the sagittal crest on the parietals continues forward onto the frontals. In 2014, Lü Junchang "et al." described the Alioramini as a tribe within the Tyrannosauridae containing the genera "Alioramus" and "Qianzhousaurus". Their phylogenetic analysis indicated that the tribe was located at the base of the Tyrannosaurinae. Some authors, such as George Olshevsky and Tracy Ford, have created other subdivisions or tribes for various combinations of tyrannosaurids within the subfamilies. However, these have not been phylogenetically defined, and usually consisted of genera that are now considered synonymous with other genera or species.