Gorgosaurus is classified in the theropod subfamilyAlbertosaurinae within the family Tyrannosauridae. It is most closely related to the slightly younger Albertosaurus.[6] These are the only two definite albertosaurine genera that have been described, although other undescribed species may exist.[5]Appalachiosaurus was described as a basal tyrannosauroidjust outside Tyrannosauridae,[12] although American paleontologist Thomas Holtz published a phylogenetic analysis in 2004 which indicated it was an albertosaurine.[2]More recent, unpublished work by Holtz agrees with the original assessment.[13] All other tyrannosaurid genera, including Daspletosaurus, Tarbosaurus and Tyrannosaurus, are classified in the subfamily Tyrannosaurinae. Compared to the tyrannosaurines, albertosaurines had slender builds, with proportionately smaller, lower skulls and longer bones of the lower leg (tibia) and feet (metatarsals and phalanges).[6][