Researchers from the University of Chicago and China's Beijing Museum of Natural History have discovered fossils of two mammals that lived more than 160 million years ago and which exhibited characteristics that demonstrate their ability to adapt to their ecological habitats since the early stages of their evolution.

The fossils, belonging to Agilodocodon scansorius, the oldest tree-dwelling mammal discovered so far, and Docofossor brachydactylus, the earliest known subterranean mammal, were both found in China, according to an article published this week in the journal Science.

The two new species, that belong to two extinct groups of the first mammals, showed unique adaptations for their respective habitats.

According to Zhe-Xi Luo, from the University of Chicago, who led the research team, these special characteristics were present millions of years earlier than calculated by scientists.

"We know that modern mammals are spectacularly diverse, but it was unknown whether early mammals managed to diversify in the same way," explained Luo.

"These fossils help demonstrate that early mammals did indeed have a wide range of ecological diversity," he added.

Luo and his colleagues described the Docofossor brachydactylus as a much smaller burrowing animal similar to present-day moles in many respects although with smaller paws and longer limbs similar to the golden mole of Africa.

Among the striking features of the Agilodocodon scansorius are its teeth that show that it fed on the gum and sap of trees much like New World monkeys and other small primates.

"We consistently find with every new fossil that 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," he added.