The team found that, in current communities of present-day mammals, ecological richness is primarily driven by vegetation type, with more small mammals filling eco-cells (41 percent) compared to the paleocommunities (16 percent). The five mammal paleocommunities were also ecologically distinct from modern communities and pointed to important changes through evolutionary time. Locomotor diversification occurred first during the Mesozoic, possibly due to the diversity of microhabitats, such as trees, soils, lakes, and other substrates to occupy in local environments. It wasn’t until the Eocene that mammals grew larger and expanded their diets from mostly carnivory, insectivory and omnivory to include more species with diets dominated by plants, including fruit. The team determined that the rise of flowering plants, new types of teeth, and the extinction of dinosaurs likely drove these changes.
Before the rise of flowering plants, mammals likely relied on conifers and other seed plants for habitat, and their leaves and possibly seeds for food. By the Eocene, flowering plants were both diverse and dominant across forest ecosystems. Flowering plants provide more readily available nutrients through their fast-growing leaves, fleshy fruits, seeds, and tubers. When becoming dominant in forests, they fundamentally changed terrestrial ecosystems by allowing for new modes of life for a diversity of mammals and other forest-dwelling animals, such as birds.
“Flowering plants really revolutionized terrestrial ecosystems,” Strömberg said. “They have a broader range of growth forms than all other plant groups―from giant trees to tiny annual herbs―and can produce nutrient-rich tissues at a faster rate than other plants. So when they started dominating ecosystems, they allowed for a wider variety of life modes and also for much higher ‘packing’ of species with similar ecological roles, especially in tropical forests.”
Tribosphenic molars (complex multi-functional cheek teeth) became prevalent in mammals in the Late Cretaceous. Mutations and natural selection drastically changed the shapes of these molars, allowing them to do new things like grinding. In turn, this allowed small mammals with these types of teeth to eat new kinds of foods and diversify their diets.
Lastly, the K-Pg mass extinction event that wiped out all dinosaurs except birds 66 million years ago provided an evolutionary and ecological opportunity for mammals. Small body size is a way to avoid being eaten by dinosaurs and other large vertebrates. The mass extinction event not only removed the main predators of mammals, but also removed small dinosaurs that competed with mammals for resources. This ecological release allowed mammals to grow into larger sizes and fill the roles the dinosaurs once had.
“The old theory that early mammals were held in check by dinosaurs has some truth to it,” Wilson said. “But our study also shows that the rise of modern mammal communities was multifaceted and depended on dental evolution and the rise of flowering plants.”
Study Information: Assembly of modern mammal community structure driven by Late Cretaceous dental evolution, rise of flowering plants, and dinosaur demise. Meng Chen, Caroline A. E. Strömberg, and Gregory P. Wilson. Proceedings of the National Academy of Sciences. Published on April 29, 2019.https://doi.org/10.1073/pnas.1820863116
Article Source: Burke Museum