Gorgosaurus femur measured long. In several smaller specimens of Gorgosaurus, the tibia was longer than the femur, a proportion typical of fast-running animals. The two bones were of equal length in the largest specimens. The long, heavy tail served as a counterweight to the head and torso and placed the center of gravity over the hips. In 2001, paleontologist Phil Currie reported skin impressions from the holotype specimen of G. libratus. He originally reported the skin as being essentially smooth and lacking the scales found in other dinosaurs, similar to the secondarily featherless skin found in large modern birds. Scales of some sort were present in this specimen, but they are reportedly widely dispersed from each other and very small. Other patches of isolated Gorgosaurus skin shows denser, and larger though still relatively fine scales (smaller than hadrosaurid scales and approximately as fine as a Gila monster's). Neither of these specimens was associated with any particular bone or specific body area. In the Encyclopedia of Dinosaurs Kenneth Carpenter pointed out that traces of skin impressions from the tail of Gorgosaurus showed similar small rounded or hexagonal scales. Classification and systematics Gorgosaurus is classified in the theropod subfamily Albertosaurinae within the family Tyrannosauridae. It is most closely related to the slightly younger Albertosaurus. These are the only two definite albertosaurine genera that have been described, although other undescribed species may exist. Appalachiosaurus was described as a basal tyrannosauroid just outside Tyrannosauridae, although American paleontologist Thomas Holtz published a phylogenetic analysis in 2004 which indicated it was an albertosaurine. More recent, unpublished work by Holtz agrees with the original assessment. 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). The close similarities between Gorgosaurus libratus and Albertosaurus sarcophagus have led many experts to combine them into one genus over the years. Albertosaurus was named first, so by convention it is given priority over the name Gorgosaurus, which is sometimes considered its junior synonym. William Diller Matthew and Barnum Brown doubted the distinction of the two genera as early as 1922. Gorgosaurus libratus was formally reassigned to Albertosaurus (as Albertosaurus libratus) by Dale Russell in 1970, and many subsequent authors followed his lead. Combining the two greatly expands the geographical and