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."
Luo and others have a good idea about how the animal lived, since its fossil remains discovered in the Inner Mongolia region of northeast China, were in impressive condition. The team, led by Quang Ji of the Chinese Academy of Geological Sciences in Beijing, found not only its skeleton and teeth, but fossilized imprints of its pelt and tail scales -- even the webbing between its toes.
Thomas Martin, of the Research Institute and Natural History Museum in Senckenberganlage, Germany, points out in an accompanying Science article that the discovery pushes "back the mammalian conquest of the waters by more than 100 million years."
Previously, mammals from the Mesozoic era (248 million to 65 millions years ago) were commonly portrayed as primitive rat-size creatures that fed mainly on insects. This is because researchers' theories were based on a scant collection of fossilized remains, limited mostly to teeth and jaws. But this find, as well as other recent discoveries, including that of a large dinosaur-eating mammal from a slightly later period 128 million years ago, is blowing open scientists' understanding of the ancient mammalian groups.
"To see that kind of diversity so early on in mammal evolution is instructive," Rybczynski said. "We're seeing indications that there is a lot more diversity than people had anticipated."
And Martin believes the newly found fossil remains "may just be a glimpse of what is to come. They dramatically demonstrate how many gaps remain in our knowledge."
Still, Anne Weil, a visiting paleobiology professor at Oklahoma State University in Oklahoma City, argues, the majority of Jurassic and later mammals were likely on the small side -- and that shouldn't surprise us.
"I think because people are such large mammals, we have a skewed idea of what size a mammal should be," she said.
She points out that a third of mammals living today are rodents, which are generally not very big, and a quarter are bats, which are very small.
"It's very possible that larger Jurassic mammals will be found," she said. "But we shouldn't forget that for mammals, it's normal to be small."