Size-mediated competition and community structure in a Late Cretaceous herbivorous dinosaur assemblage

It has been argued that, throughout the Mesozoic, the immature growth forms of megaherbivorous dinosaurs competitively excluded small herbivorous dinosaur species, leading to the left-skewed species richness-body mass distributions of their fossil assemblages. By corollary, where large and small herbivores coexisted over a geologically significant period of time, they must have exhibited niche partitioning. We use multivariate ecomorphological analysis of the Late Cretaceous ornithischian dinosaur assemblage of North America to examine this prediction. Our results indicate good ecomorphological separation of most, but not all, species at small body size, although more work is required to demonstrate that these patterns were adaptive. Calculation of browse profiles using corrected abundance data and bracketed estimates of energy requirements suggests that immature megaherbivores – most particularly hadrosaurids – outstripped coexisting small ornithischian species in their control of the resource base.