common to the many individuals at the Albertosaurus bonebed quarry led Currie to conclude that the locality was not a predator trap like the La Brea Tar Pits in California, and that all of the preserved animals died at the same time. Currie claims this as evidence of pack behaviour. Other scientists are skeptical, observing that the animals may have been driven together by drought, flood or for other reasons. There is plentiful evidence for gregarious behaviour among herbivorous dinosaurs, including ceratopsians and hadrosaurs. However, only rarely are so many dinosaurian predators found at the same site. Small theropods like Deinonychus and Coelophysis have been found in aggregations, as have larger predators like Allosaurus and Mapusaurus. There is some evidence of gregarious behaviour in other tyrannosaurids as well. Fragmentary remains of smaller individuals were found alongside "Sue", the Tyrannosaurus mounted in the Field Museum of Natural History in Chicago, and a bonebed in the Two Medicine Formation of Montana contains at least three specimens of Daspletosaurus, preserved alongside several hadrosaurs. These findings may corroborate the evidence for social behaviour in Albertosaurus, although some or all of the above localities may represent temporary or unnatural aggregations. Others have speculated that instead of social groups, at least some of these finds represent Komodo dragon-like mobbing of carcasses, where aggressive competition leads to some of the predators being killed and cannibalized. Currie has also speculated on the pack-hunting habits of Albertosaurus. The leg proportions of the smaller individuals were comparable to those of ornithomimids, which were probably among the fastest dinosaurs. Younger Albertosaurus were probably equally fleet-footed, or at least faster than their prey. Currie hypothesized that the younger members of the pack may have been responsible for driving their prey towards the adults, who were larger and more powerful, but also slower. Juveniles may also have had different lifestyles than adults, filling predator niches between the enormous adults and the smaller contemporaneous theropods, the largest of which were two orders of magnitude smaller than adult Albertosaurus in mass. A similar situation is observed in modern Komodo dragons, with hatchlings beginning life as small insectivores before growing to become the dominant predators on their islands. However, as the preservation of behaviour in the fossil record is exceedingly rare, these ideas cannot readily be tested. In 2010, Currie, though still favouring the hunting pack hypothesis, admitted that the concentration could have been brought about