Tiny fossils of extinct mouse- and squirrel-like creatures unearthed in China may require a large re-thinking of when the Earth's earliest mammals came into being, scientists say.

The fossils, found in the northeastern province of Liaoning, suggest the world's earliest mammals may have arisen 200 million years ago or even before that, significantly earlier than previously believed, they say.

The creatures, known as haramiyids, were slender, graceful animals that apparently evolved for a lifetime in treetops, possessing hands and feet designed for grasping branches and prehensile tails similar to those of modern monkeys.

"The [previous] picture that Mesozoic mammals were shrew-like insectivores that lived in the shadow of the dinosaurs needs to be repainted," says paleontologist Jin Meng of the American Museum of Natural History.

The latest discovery is providing more evidence that Earth's earliest mammals took advantage of an assortment of environmental habitats.

"They walked on the ground; they also swam, dug to burrow, and glided in the forests," says Meng, co-author of a study on the new fossils.

Among the fossils were remains of three separate extinct species, ranging in size from 1 to 10 ounces, from "a house mouse to a small squirrel," Meng says.

However, they shouldn't be confused with any living species despite a mouse-like or squirrel-like appearance, he explained; they have no living descendants, having branched off long ago from the line leading to modern mammals.

An examination of their teeth suggests they were omnivorous, with a diet of fruits, nuts and insects, the researchers say.

Although haramiyids have been known of for some time in the fossil record, there has been long debate about whether they were true mammals or related but different creatures.

Though roughly mammal-like in appearance, it was long believed they fell outside of the group.

Details of the skeletons, skulls and teeth of the Chinese fossils is strong evidence for their being true mammals, the researchers say, even if they did branch off into a line that turned out to be an evolutionary dead end.

Some haramiyids have been dated to 220 to 200 million years in the past, in the Late Triassic period, suggesting mammal species are at least that old, much older than the 174 to 166 million years previous origin theories for mammals have held, they say.

The researchers acknowledge not everyone will agree with that assumption.

"I expect this will be contentious," says Anne Weil of Oklahoma State University.

"I think it's going to be part of an argument that will be going on for some time," she says, "and I expect paleontology as a whole will learn a lot from questions gleaned from these animals about the antiquity of Mammalia."