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Dinosaur Extinction Led To Mammal Growth

November 26, 2010

New research published in the journal Science this week has determined that mammals grew larger following the extinction of dinosaurs, suggesting that they needed more space and better access to food resources in order to reach their full potential.

The study, which was funded by the National Science Foundation (NSF) and led by University of New Mexico (UNM) scientists, discovered that after dinosaurs died out some 65 million years ago, mammals began to grow, ultimately reaching sizes up to 1,000 times larger than before.

According to study co-author Dr. Jessica Theodor, associate professor in the Department of Biological Sciences at the University of Calgary, the maximum pre-extinction weight of mammals was 10 kilograms, or approximately 22 pounds. Once the dinosaurs died out, however, they grew rapidly–reaching sizes of up to well over 15 tons in a span of 25 million years.

“Basically, the dinosaurs disappear and all of a sudden there is nobody else eating the vegetation,” Theodor said in a statement. “That’s an open food source and mammals start going for it, and it’s more efficient to be an herbivore when you’re big.”

“You lose dinosaurs 65 million years ago, and within 25 million years the system is reset to a new maximum for the animals that are there in terms of body size. That’s actually a pretty short time frame, geologically speaking,” she added. “That’s really rapid evolution.”

According to the researchers, the largest mammal that ever walked the Earth was a hornless herbivore that was similar to a rhinoceros, and lived in Eurasia approximately 34 million years ago. The creature, known as Indricotherium transouralicum, was about 18 foot tall at the shoulder and weighed 17 tons. It was so large, according to Lauran Neergard of the Associated Press (AP), that it made the wooly mammoth “seem a bit puny” in comparison.

“The results give clues as to what sets the limits on mammal size on land; the amount of space available to each animal and the climate they live in,” the University of Calgary said in a November 25 press release. “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.”

Image Caption: The largest land mammals that ever lived, Indricotherium and Deinotherium, would have towered over the living African elephant. The tallest on diagram, Indricotherium, an extinct rhino relative, lived during the Eocene to the Oligocene Epoch (37 to 23 million years ago) and reached a mass of 15,000 kg, while Deinotherium (an extinct proboscidean, related to modern elephants) was around from the late-Miocene until the early Pleistocene (8.5 to 2.7 million years ago) and weighed as much as 17,000 kg. Credit: Alison Boyer/Yale University

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