 Tyrannosauridae is <unk> divided into two subfamilies . Albertosaurinae comprises the North American genera Albertosaurus and Gorgosaurus , while Tyrannosaurinae includes Daspletosaurus , Teratophoneus , Bistahieversor , Tarbosaurus , <unk> , 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 . <unk> 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 . Some authors , such as George Olshevsky and Tracy Ford , have created finer subdivisions or tribes for various combinations of tyrannosaurs within the subfamilies . However , these have not been phylogenetically defined , and usually consisted of genera which are now considered synonymous with other genera or species . 
