Fossils of two interrelated ancestral mammals, one a tree climber and one an underground burrow dweller, shows the ecological diversity of today's mammals began at least 160 million years go, paleontologists say.

Recently discovered in China, the fossils -- two tiny shrew-sized creatures -- are evidence of a surprising ability of so-called mammaliaformes, long-extinct relatives of modern mammals, to fill different ecosystem niches.

One fossil, Agilodocodon scansorius, with claws adapted for climbing and teeth evolved to take advantage of a diet of tree sap, is the earliest-known tree-dwelling mammaliaform, researchers from the University of Chicago and the Beijing Museum of Natural History report in the journal Science.

Another fossil, described in a second Science paper and dubbed Docofossor brachydactylus, is the earliest known subterranean mammaliaform, displaying adaptations similar to modern-day African golden moles including shovel-like paws, researchers say.

The fossils -- one tree dweller and one burrower -- strengthen the conviction that the worldwide success of mammal species has been aided largely by the ability to evolve early and quickly to suit any new environment or ecosystem, they say.

This contradicts long-held assumptions that early mammals would have had limited ecological chances at diversification in a Mesozoic era dominated by dinosaurs, they point out.

"We consistently find with every new fossil that the earliest mammals were just as diverse in both feeding and locomotor adaptations as modern mammals," says Zhe-Xi Luo, a professor of organismal biology and anatomy at the University of Chicago and an author on both papers. "The groundwork for mammalian success today appears to have been laid long ago."

In 2006 Luo and his colleagues described a beaver-like, swimming early mammal Castorocauda lutrasimilis that also lived 164 million years ago, which challenged the prevailing assumption that mammals might have been primitive creatures that were confined to land.

In 2005 another fossil of an early mammal about the size of a honey badger suggested is was a predator of baby dinosaurs.

All these fossil discoveries are evidence of an explosion of mammal evolution early on that made them worthy competitors to the large prehistoric dinosaurs once considered kings of their age, Luo says.

"These fossils help demonstrate that early mammals did indeed have a wide range of ecological diversity. It appears dinosaurs did not dominate the Mesozoic landscape as much as previously thought," he says.

Although the Chinese fossil creatures shared a common ancestor with extant mammals, they have no direct descendants living today, the researchers say.