In the conventional view, the earliest mammals were small, primitive shrewlike creatures that did not begin to explore the world's varied environments until the dinosaurs died out 65 million years ago.
But scientists are reporting that they have uncovered fossils of a swimming, fish-eating mammal that lived in China fully 164 million years ago, well before it was thought that some mammals could have spent much of their lives in water.
The extinct species appears to have been an amalgam of animals. It had a broad, scaly tail, flat like a beaver's. Its sharp teeth seemed ideal for eating fish, like an otter's. Its likely lifestyle — burrowing in tunnels on shore and dog-paddling in water — reminds scientists of the modern platypus.
Its skeleton suggests that it was about 20 inches long, from snout to the tip of its tail, about the length of a small house cat.
The surprising discovery, made in 2004 in the abundant fossil beds of Liaoning province, China, is being reported in the journal Science by an international team led by Qiang Ji of Nanjing University.
In the article, Ji and other researchers from the Chinese Academy of Geological Sciences in Beijing and the Carnegie Museum of Natural History in Pittsburgh said the fossil skeleton showed that some mammals occupied more diverse ecological niches than had been suspected in the Jurassic Period<, in the middle of the age dominated by dinosaurs.
Thomas Martin, an authority on early mammals at Senckenberg Research Institute in Frankfurt, Germany, said the new find pushed back "the mammalian conquest of the waters by more than 100 million years" and "impressively contradicts" the conventional view.
"This exciting fossil," he wrote in a commentary accompanying the report, "is a further jigsaw puzzle piece in a series of recent discoveries, demonstrating that the diversity and early evolutionary history of mammals were much more complex than perceived less than a decade ago."
Despite similarities with some modern animals, the Jurassic mammal has no modern descendants and is not related to any existing species. The discoverers have given it the name Castorocauda lutrasimilis, Latin for beaver tail and similarity to the otter.
Zhe-Xi Luo, one of the discoverers and the curator of vertebrate paleontology at the Carnegie museum, said the specimen was well preserved, unlike the fragments of bone and tooth that survive of most mammals from the dinosaur age.
The skeleton is accompanied by fur and scale imprints and the suggestion of soft-tissue webbing in the hind limbs.
Luo said the fur was to keep water from the animal's skin. It is the most primitive known mammal to be preserved with hair, evidence that for its evolution before the appearance of more complex mammals.
The scientists said the tail and limbs of the newly found specimen were well developed for aquatic life. They surmised that like the platypus, Castorocauda swam in rivers and lakes, ate aquatic animals and insects and built nests in burrows along the shore. The animal had molars specialized for feeding on small fish and small aquatic invertebrates.
"So far, it is the only semiaquatic mammal from the Jurassic," Luo said.
The skeleton was found by peasants in Liaoning, the province in northeast China that in recent years has produced several notable discoveries of mammal diversity.
The semiaquatic mammal was uncovered in the same hilly country where paleontologists have collected fossils of feathered dinosaurs and two 130-million-year-old animals did not fit the lowly image of mammals of that period.
One of them, the size of an opossum, had feasted on a small dinosaur just before dying.
Jin Meng of the American Museum of Natural History in Manhattan, one of the discoverers of previous Liaoning mammals but who was not involved in the most recent one, said in a telephone interview that more than a dozen new mammals from that area had recently produced "real evidence to show the diversity of lifestyles and behaviors of mammals" in the age of dinosaurs.
"We have been seeing mammals at that time that were larger than a mouse or rat, some that climbed trees and now we see some that could swim in water," Meng said.
Martin, the German mammal specialist, said the potential of these fossil-rich deposits in China "is only just beginning to be exploited."