Did dinosaurs suppress the evolution of large mammals, or was early mammal size and diversity more related to the environmental conditions? Mammals first appeared in the Late Triassic, and lived alongside dinosaurs during the rest of the Mesozoic (Jurassic and Cretaceous), during which time they were all small and generalized in their ecologies. Only after dinosaur extinction (at the end of the Mesozoic) did mammals diversify into larger and more specialized forms, and so suppression by dinosaur competition is the usual explanation for this evolutionary pattern. I propose that the vegatational habitat was the more important factor. The angiosperms (flowering plants) that make up the majority of today’s vegetation did not appear until the Cretaceous, and did not become the dominant flora until the Cenozoic. Angiosperm forests are multi-strated with a large diversity of fruits and seeds, and provide a more suitable habitat for mammals than the gymnosperm (e.g.. conifers) forests of the Mesozoic. Towards the end of the Cretaceous, angiosperm leaf morphology had evolved such that the trees would be able to create their own microhabitats, making rain forests possible. This time period provides a window of opportunity to see if mammal evolution changed in response to the encroaching angiosperm dominance. There is evidence of dietary change at this time (to more herbivory), but was there also a change in substrate use; i.e., in the proportions of tree-dwelling versus ground-dwelling mammals? Many isolated skeletal elements can be used in the determination of locomotor behavior, but while they can be found in fossil assemblages they are usually not described. Examination of such elements will enable the determination of whether or not fossil mammal communities underwent an ecological shift in substrate use at the end of the Cretaceous, and will answer the question of whether mammal evolution is influenced more by the floral environment than by the presence of dinosaurs.