The fossils of two interrelated ancestral mammals, newly discovered in China, suggest that the wide-ranging ecological diversity of modern mammals had a precedent more than 160,000,000 years ago.
With claws for climbing and teeth adapted for a tree sap diet, Agilodocodon scansorius is the earliest known tree-dwelling mammaliaform --long-extinct relatives of modern mammals. The other fossil, Docofossor brachydactylus, is the earliest-known subterranean mammaliaform, possessing multiple adaptations similar to African golden moles, such as shovel-like paws. Docofossor also has 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 were reported by international teams of scientists from the University of Chicago (Ill.) and China's Beijing Museum of Natural History.
"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, professor of organismal biology and anatomy at the University of Chicago. "The groundwork for mammalian success today appears to have been laid long ago."
Agilodocodon and Docofossor provide strong evidence that arboreal and subterranean lifestyles evolved early in mammalian evolution, convergent to those of true mammals. These two shrew-sized creatures--members of the mammaliaform order Docodonta--have unique adaptations tailored for their respective ecological habitats.
Agilodocodon, which lived roughly 165,000,000 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 adapted for feeding on the gum or sap of trees, with spade-like front teeth to gnaw into bark. This adaptation is similar to the teeth of some modern New World monkeys and is the earliest known evidence of gumnivorous feeding in mammaliaforms. Agilodocodon also had well-developed, flexible elbows and wrist and ankle joints that allowed for much greater mobility, all characteristics of climbing mammals.
'The finger and limb bone dimensions of Agilodocodon match up with those of modern tree dwellers, and its incisors are evidence it fed on plant sap," says study coauthor David Gross-nickle, graduate student at the University of Chicago. "It's amazing that these arboreal adaptions occurred so early in the history of mammals, and...