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George Mason University Researcher Explores Evolution of the World’s Mammals over the Past 100 Million Years

Nov. 29, 2010

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FAIRFAX, Va.Researchers have demonstrated that the extinction of dinosaurs 65 million years ago paved the way for mammals to get bigger—about one thousand times bigger than they had been. The study, released Nov. 25 in the prestigious journal Science, is the first to quantitatively explore the patterns of body size of mammals 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 George Mason University 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, odd-toed ungulates such as horses and rhinos; Proboscidea, which includes elephants, mammoths and mastodons; Xenarthra, the anteaters, tree sloths, and armadillos; as well as a number of other extinct groups.


The researchers spent 3 years assembling the data and compared 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, a coauthor 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 it was not. 

Researchers found that mammals grew from a maximum size of about 10 kilograms 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 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 seventeen tons and stood about eighteen 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. It 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 include 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.