In the Hell Creek Formation there are many different animals from Tyrannosaurus rex to Edmontosaurus annectens to Triceratops horridus. All of the fossils of these animals are fairly common occurences in the formation, but there are also more rare animals, such as Ankylosaurus magnivetris, Dakotaraptor steini, and Pachycephalosaurus wyomingensis. These fossils are rarer, but why? I have concocted a theory of why. Maybe these animals lived in the mountains. In mountainous environments fossils hardly form, so this could explain why their fossils are so rare. In a mountainous ecosystem, huge parliaments of pachycephalosaurs could form, while Dakotaraptor would reign supreme as the apex predator. Solitary ankylosaurs would forage on the slopes, their armour making them almost impenetrable. This is what a late Cretaceous Larimidian mountain could be like.