from the slightly older Dinosaur Park Formation in Alberta. Lawrence Lambe named this dinosaur Gorgosaurus libratus in 1914. Other specimens were later found in Alberta and the US state of Montana. Finding—largely due to a lack of good Albertosaurus skull material—no significant differences to separate the two taxa, Dale Russell declared the name Gorgosaurus a junior synonym of Albertosaurus, which had been named first, and G. libratus was renamed Albertosaurus libratus in 1970. A species distinction was maintained because of the age difference. The addition extended the temporal range of the genus Albertosaurus earlier by several million years and its geographic range southwards by hundreds of kilometres. In 2003, Philip J. Currie, benefiting from much more extensive finds and a general increase in anatomical knowledge of theropods, compared several tyrannosaurid skulls and came to the conclusion that the two species are more distinct than previously thought. As the two species are sister taxa, they are more closely related to each other than to any other species. Recognizing this, Currie nevertheless recommended that Albertosaurus and Gorgosaurus be kept as separate genera, as he concluded that they were no more similar than Daspletosaurus and Tyrannosaurus, which are almost always separated. In addition, several albertosaurine specimens have been recovered from Alaska and New Mexico, and Currie suggested that the Albertosaurus-Gorgosaurus situation may be clarified once these are described fully. Most authors have followed Currie's recommendation, but some have not. Other species In 1930, Anatoly Nikolaevich Riabinin named Albertosaurus pericolosus based on a tooth from China, that probably belonged to Tarbosaurus. In 1932, Friedrich von Huene renamed Dryptosaurus incrassatus, not considered a nomen dubium by him, to Albertosaurus incrassatus. Because he had identified Gorgosaurus with Albertosaurus, in 1970, Russell also renamed Gorgosaurus sternbergi (Matthew & Brown 1922) into Albertosaurus sternbergi and Gorgosaurus lancensis (Gilmore 1946) into Albertosaurus lancensis. The former species is today seen as a juvenile form of Gorgosaurus libratus, the latter as either identical to Tyrannosaurus or representing a separate genus Nanotyrannus. In 1988, Gregory S. Paul based Albertosaurus megagracilis on a small tyrannosaurid skeleton, specimen LACM 28345, from the Hell Creek Formation of Montana. It was renamed Dinotyrannus in 1995, but is now thought to represent a juvenile Tyrannosaurus rex. Also in 1988, Paul renamed Alectrosaurus olseni (Gilmore 1933) into Albertosaurus olseni; this has found no general acceptance. In 1989, Gorgosaurus novojilovi (Maleev 1955) was renamed by Bryn Mader and Robert