Jurassic Beaverlike Creature Found in China

Feb. 23, 2006 — -- The dinosaur's shadow just got a little smaller.

For nearly a century, scientists studying prehistoric mammals assumed that ancient creatures from the Jurassic period had never managed to evolve into a widely diverse array of animals, perhaps because they had such overwhelming neighbors to contend with in the dinosaurs.

But the discovery of a swimming, digging, beaverlike mammal that lived about 164 million years ago has laid that notion to rest.

"What's interesting about this discovery is it really breaks a stereotype perception of early mammals," said Zhe-Xi Luo of Carnegie Museum of Natural History and co-author of a study about the mammal in this week's issue of Science. "Most mammals from this period were thought to be fairly small and only capable of eating small insects and worms. This animal is a swimmer, has four limbs, and is capable of digging."

This ancient animal is the largest mammal found from its time, weighing 1 pound to 1.7 pounds and measuring just under a foot and a half long. It had curved molars, which modern-day seals and otters use for catching fish. It's also the only known mammalian swimmer from this Jurassic period.

While its broad, scaly tail and furry body resemble that of modern-day beavers, it's no relation. In fact, the species, Castoraocauda lutrasimilis, meaning "beaver tail river otterlike" in Latin, was part of a new class of animals, docodonts, which left behind no descendants. But Luo says it may have lived somewhat like the modern-day platypus.

"It probably lived along river or lake banks," Luo said. "It doggy-paddled around, ate aquatic animals and insects, and burrowed tunnels for its nests."

Natalia Rybczynski of the Canadian Museum of Nature in Ottawa specializes in the structure and function of the modern beaver tail. She points out that while the ancient mammal's tail resembles that of the beaver in shape and scaly covering, there are also some differences.

"It's more of an intermediate between the beaver and otter," she said. "Its tail is like an otter tail that has been flattened. That means it wouldn't have been as useful for pushing through water as a beaver tail, but it still probably was good for swimming."