Other herbivorous groups like ornithomimids, therizinosaurs, pachycephalosaurs, small ornithopods, nodosaurids and ankylosaurids were also represented. Small predatory dinosaurs like oviraptorosaurs, troodonts and dromaeosaurs hunted smaller prey than the huge tyrannosaurids; Daspletosaurus and Gorgosaurus, which were two orders of magnitude larger in mass. Intervening predatory niches may have been filled by young tyrannosaurids. A Saurornitholestes dentary has been discovered in the Dinosaur Park Formation that bore tooth marks left by the bite of a young tyrannosaur, possibly Gorgosaurus. Coexistence with Daspletosaurus In the middle stages of the Dinosaur Park Formation, Gorgosaurus lived alongside a rarer species of tyrannosaurid, Daspletosaurus. This is one of the few examples of two tyrannosaur genera coexisting. Similar-sized predators in modern predator guilds are separated into different ecological niches by anatomical, behavioral or geographical differences that limit competition. Niche differentiation between the Dinosaur Park tyrannosaurids is not well understood. In 1970, Dale Russell hypothesized that the more common Gorgosaurus actively hunted fleet-footed hadrosaurs, while the rarer and more troublesome ceratopsians and ankylosaurians (horned and heavily armoured dinosaurs) were left to the more heavy built Daspletosaurus. However, a specimen of Daspletosaurus (OTM 200) from the contemporaneous Two Medicine Formation of Montana preserves the digested remains of a juvenile hadrosaur in its gut region, and another bonebed contains the remains of three Daspletosaurus along with the remains of at least five hadrosaurs. Unlike some other groups of dinosaurs, neither genus was more common at higher or lower elevations than the other. However, Gorgosaurus appears more common in northern formations like Dinosaur Park, with species of Daspletosaurus being more abundant to the south. The same pattern is seen in other groups of dinosaurs. Chasmosaurine ceratopsians and saurolophine hadrosaurs are also more common in the Two Medicine Formation of Montana and in southwestern North America during the Campanian, while centrosaurines and lambeosaurines dominate in northern latitudes. Holtz has suggested this pattern indicates shared ecological preferences between tyrannosaurines, chasmosaurines and saurolophines. At the end of the later Maastrichtian stage, tyrannosaurines like Tyrannosaurus rex, saurolophines like Edmontosaurus and Kritosaurus and chasmosaurines like Triceratops and Torosaurus were widespread throughout western North America, while lambeosaurines were rare, consisting of a few species like Hypacrosaurus, and albertosaurines and centrosaurines had gone extinct. However, in the case of the centrosaurines, they had thrived in Asia with genera like Sinoceratops. While albertosaurine remains have been found in the Hell Creek Formation, it is most likely these are indeterminate