June 17, 2008 -- The world's smallest prehistoric beaver was the size of a tiny mouse but directly related to an 8-foot-long, 480-pound Ice Age giant, suggests a new study of fossils found in the American Midwest.
Much of the beaver family tree's bizarre history played out at the Valentine Formation in northern Nebraska, where paleontologist William Korth has studied the skeletons of big and little beavers that lived as long as 23 million years ago.
"The world's tiniest beaver turns out to have been related to the world's largest," Korth, of the Rochester Institute of Vertebrate Paleontology, told Discovery News.
He came to that conclusion after recently identifying the most primitive member of a beaver subfamily -- technically called a "tribe"-- known as the Nothodipoidini. Members of this now-extinct branch on the beaver family tree tended to be on the small side, but their huge front teeth often dwarfed the rest of the body.
Such was the case for the newly identified beaver Temperocastor valentinensis, which had large, fast-growing incisors that Korth says are "typical of tooth-digging rodents."
The findings are published in the latest issue of the journal Acta Paleontologica Polonica.
Today's beavers "do it all" by digging, swimming, cutting wood with their teeth and building dams. Their ancient relatives, however, seemed to be divided up into those that were digging and burrowing specialists and those that spent more of their time swimming and munching vegetation.
The diggers often evolved into smaller species with big front teeth, while the swimming experts could get "bigger and bigger," said Korth. No one is certain why beavers became so big, but Jon Kramer of the Minnesota Department of Natural Resources thinks it's simply "because they could."
Kramer said that as long as the food supply is plentiful, "animals may evolve bigger and bigger until nature eventually balances their size against what the environment can provide."
The largest beaver was the giant Castoroides ohioensis, which emerged in North America about three million years ago. This distant cousin of today's modern beavers was the size of black bear. It had 6-inch ridged teeth, a chubby-looking round head and a muskrat-like tail.
"Giant beavers evidently died out near the close of the last glaciation about 10,000 years ago," said Richard Harington, a researcher emeritus at the Canadian Museum of Nature. "Because they coexisted with early humans in North America, it seems unusual that there is no evidence that people hunted them."
Harington added, "Surely a Castoroides pelt would have made an excellent coat or sleeping robe!"
Being such a large target might have ushered in the demise of the large beavers. Korth theorizes smaller species began to emerge on swamp and riverbanks, where they could dig to escape predators. Animals with enhanced adaptations for digging underground then gradually replaced their larger relatives.
Modern beavers, however, represent an entirely different subfamily of animals that split from their giant relatives millions of years ago. Like the Nothodipoidine, they too evolved buckteeth for digging, an adaptation that emerged three separate times during the family history of the semi-aquatic animals.
Korth, one of the world's leading experts on ancient rodents, says he's often asked why he chose to work in this field.
"I tell them that rodents like beavers are fascinating creatures," he said. "Remember too that among mammals, rodents make up half of all species. As a scientist, it's hard to ignore them."
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