A study published recently in the journal Science demonstrates that the extinction of dinosaurs 65 million years ago paved the way for mammals to get bigger — about 1,000 times bigger — than they had been.
The study is the first to quantitatively explore the patterns of mammalian body sizes across the globe after the demise of the dinosaurs.
The research, funded by a National Science Foundation Research Coordination Network grant, was led by Felisa Smith from the University of New Mexico and brought together an international team of paleontologists, evolutionary biologists and macroecologists from universities around the world, including Mason professor Mark D. Uhen.
To document what happened to mammals after the extinction of dinosaurs, researchers collected data on the maximum size for major groups of land mammals on each continent, including Perissodactyla, which are odd-toed ungulates such as horses and rhinos; Proboscidea, which includes elephants, mammoths and mastodons; Xenarthra, which are anteaters, tree sloths and armadillos; as well as a number of other extinct groups.
The researchers spent three years assembling the data and comparing mammals on all continents since the extinction of the dinosaurs.
“We support the idea that after the dinosaurs and many other large organisms went extinct at the Cretaceous-Paleogene boundary, mammals ‘filled up’ the ecological space left behind, and that it happened in similar ways on separate continents due to similar ecological factors,” says Uhen, an assistant professor in the Department of Atmospheric, Oceanic and Earth Sciences and co-author on the paper.
Uhen helped assemble the data on the body sizes of fossil mammals. He checked whether these trends were likely to be driven by an increase in diversity over time and determined they were not. Researchers found that mammals grew from a maximum size of about 10 kilograms — 22 pounds — when they were sharing the earth with dinosaurs to a maximum of 17 tons afterwards.
Moreover, the pattern was surprisingly consistent across space, time and trophic groups (carnivore, herbivore, etc.) and lineages.
The maximum size of mammals began to increase sharply about 65 million years ago, peaking about 34 million years ago in Eurasia and again about 10 million years ago in Eurasia and Africa.
The largest mammal that ever walked the earth — Indricotherium transouralicum, a hornless rhinoceros-like herbivore that weighed approximately 17 tons and stood about 18 feet high at the shoulder — lived in Eurasia almost 34 million years ago.
The results give clues as to what sets the limits on maximum body size on land: the amount of space available to each animal and the climate they live in.
The colder the climate, the bigger the mammals seem to get, as bigger animals conserve heat better.
The study also shows that no one group of mammals dominates the largest size class — the absolute largest mammal belongs to different groups over time and space.
Other team members on the project were Alison Boyer, James Brown, Daniel Costa, Tamar Dayan, Morgan Ernest, Alistair Evans, Mikael Fortelius, John Gittleman, Marcus Hamilton, Larisa Harding, Kari Lintulaakso, Kathleen Lyons, Christy McCain, Jordan Okie, Juha J. Saarinen, Richard Sibly, Patrick Stephens and Jessica Theodor.