 In the middle stages of the Dinosaur Park Formation , Gorgosaurus lived alongside a rarer species of the tyrannosaurine , 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 . 
