Fossils of earliest tree-dwelling, subterranean mammals found in China

Two 160 million-year-old fossils from the dinosaur-dominated Mesozoic era suggested that mammals had adapted to wide-ranging environments even at this early stage in their evolution, researchers from China and the United States said Thursday.

One of them, Agilodocodon scansorius, which was discovered in north China's Inner Mongolia, is the earliest-known tree-dwelling mammal with claws for climbing and teeth adapted for a tree sap diet. The other one, Docofossor brachydactylus, found in Hebei Province, also in north China, is the earliest-known subterranean mammal possessing multiple adaptations similar to African golden moles such as shovel-like paws.

The two new species, from an extinct group of early mammals called docodontans, also showed off distinct skeletal features that resemble patterns shaped by genes identified in living mammals, suggesting these genetic mechanisms operated long before the rise of modern mammals.

These discoveries are reported by international teams of scientists from the University of Chicago and Beijing Museum of Natural History in two separate papers, which appeared in the U.S. journal Science.

"We know that modern mammals are spectacularly diverse, but it was unknown whether early mammals managed to diversify in the same way," said Zhe-Xi Luo, professor of organismal biology and anatomy at the University of Chicago and an author on both papers.

"These new 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," Luo said.

These two shrew-sized creatures have unique adaptations tailored for their respective ecological habitats, providing strong evidence that arboreal and subterranean lifestyles evolved early in mammalian evolution, the researchers said.

Agilodocodon, which lived roughly 165 million years ago, had hands and feet with curved horny claws and limb proportions that are typical for mammals that live in trees or bushes.

It is also adapted for feeding on the gum or sap of trees, with spade-like front teeth to gnaw into bark, an adaptation similar to the teeth of some modern New World monkeys and the earliest-known evidence of gumnivorous feeding in ancient mammals.

In addition, Agilodocodon had well-developed, flexible elbows and wrist and ankle joints that allowed for much greater mobility, all characteristics of climbing mammals.

Docofossor, which lived around 160 million years ago, had a skeletal structure and body proportions strikingly similar to the modern day African golden mole. It had shovel-like fingers for digging, short and wide upper molars typical of mammals that forage underground, and a sprawling posture indicative of subterranean movement.

Docofossor also had reduced bone segments in its fingers, leading to shortened but wide digits, which the researchers said is an evolutionary advantage for digging mammals.

This feature is due to the fusion of bone joints during development, a process influenced by the genes BMP and GDF-5. The researchers hypothesized that this genetic mechanism may have played a comparable role in early mammal evolution.

They also found the spines and ribs of both Agilodocodon and Docofossor showed evidence for the influence of genes seen in modern mammals.

"We can now provide fossil evidence that gene patterning that causes variation in modern mammalian skeletal development also operated in basal mammals all the way back in the Jurassic," Luo said.

Early mammals were once thought to have limited ecological opportunities to diversify during the dinosaur-dominated Mesozoic era, but the new findings and numerous other fossils, including Castorocauda, a swimming, fish-eating mammal described by Luo and colleagues in 2006, suggested widespread ecological diversity among them.

"The earliest mammals were just as diverse in both feeding and locomotor adaptations as modern mammals," said Luo. "The groundwork for mammalian success today appears to have been laid long ago."