SPATIAL NICHE PARTITIONING IN DINOSAURS FROM THE LATEST CRETACEOUS (MAASTRICHTIAN) OF NORTH AMERICA

We examine patterns of occurrence of associated dinosaur specimens (n=339) from the North American Upper Cretaceous Hell Creek Formation and equivalent beds, by comparing their relative abundance in sandstone and mudstone. Ceratopsians show a close association with mudstone, while hadrosaurs and the small ornithopod Thescelosaurus show a strong association with sandstone. In contrast the giant carnivore Tyrannosaurus rex shows no association with either lithology. These lithologies are used as an indicator of environment, with sandstone generally representing river environments, and fine-grained sediments typically representing floodplain environments. Given these patterns of occurrence, we argue that spatial niche partitioning helped reduce competition for resources between these herbivorous dinosaurs. Within coastal lowlands ceratopsians preferred habitats further away from rivers, while hadrosaurs and Thescelosaurus preferred habitats in close proximity to rivers, and T. rex, the ecosystem’s sole large carnivore, inhabited both palaeoenvironments. Spatial partitioning of the environment helps explain how several species of large herbivorous dinosaurs coexisted. This study emphasizes that different lithologies can preserve dramatically dissimilar vertebrate assemblages, even when deposited in close proximity and within a narrow window of time. The effects of lithology should be considered when comparing vertebrate assemblages in order to understand long-term changes in palaeocommunity structure and diversity.