LOGAN – An international research team co-directed by Utah State University biologist Morgan Ernest found that the extinction of dinosaurs some 65 million years ago paved the way for mammals to get bigger – much bigger.”The demise of dinosaurs provided vast evolutionary opportunities for mammals,” says Ernest, associate professor and co-director of graduate programs in USU’s Department of Biology. “Mammals evolved to sizes nearly thousand times larger than had been seen before.”The team’s study, published in the Nov. 26, 2010, issue of the journal Science, is the first to quantitatively explore the pattern of body size of mammals after the end of the Dinosaur Age. Funded by a National Science Foundation Research Coordination Network grant, the research brought together paleontologists, evolutionary biologists and macroecologists from 13 universities and institutions throughout the world.”It is well known in biology that size profoundly influences everything from how quickly a species reproduces to its vulnerability to extinction,” Ernest says. “The aim of our group is to bring together a diverse group of scientists to investigate unanswered questions about the evolution of size in mammals.”She and team members found that mammals grew from a size of about 22 lbs. when sharing the earth with dinosaurs to a whopping maximum of 17 tons afterwards. Moreover, the pattern was surprisingly consistent across space, time and trophic groups and lineages.”In addition to the strong consistency in the patterns across continents, I was particularly struck by the relationship between the evolution of size in carnivores and herbivores,” Ernest says. “The evolution of maximum size in carnivores tracked increases in size in herbivores, though carnivores remained ten times smaller than their potential prey.”To document what happened to mammals after the extinction of dinosaurs, she and the team spent three years collecting 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, anteaters, tree sloths and armadillos; as well as a number of other extinct groups. “These sorts of comprehensive datasets are important because they allow us to ask questions on a scale not previously possible,” Ernest says.The maximum size of mammals began to increase sharply about 65 million years ago, peaking about 34 million years ago in Eurasia during the Oligocene Epoch and again about 10 million years ago in Eurasia and Africa during the Miocene Epoch.Indricotherium transouralicum, a hornless rhinoceros-like herbivore believed to be the largest mammal that ever walked the earth, weighed in at approximately 17 tons. It stood about 18 feet high at the shoulder and lives in Eurasia some 34 million years ago.”The results of the study give clues as to what sets the limits on maximum body size on land; namely, the amount of space available to each animal and the climate they live in,” Ernest says.The colder the climate, the bigger the mammals seem to get as larger animals conserve heat better.”The results also show that no one group of mammals dominates the largest size class,” she says. “The absolute largest mammal belongs to different groups over time and space.”
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