Among the first things that often come to mind when one thinks of the Jurassic Era are the gigantic dinosaurs. While these large reptiles did indeed dominate the Earth during this period, they also lived in the same landscape with small rodent-like animals the size of the shrew 165 million years ago.

Fossils of two new species of these ancient mammals have been found in China, providing evidence that regardless of their small size, the earliest mammals were just as diverse as the present-day mammals.

"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 Zhe-Xi Luo from the University of Chicago, who co-authored two separate studies that described the fossils. Both studies were published in the journal Science. "The groundwork for mammalian success today appears to have been laid long ago."

One of these newly discovered ancient mammalian species was a tree dweller called Agilodocodon scansorius, which had a number of features, such as long claws as well as flexible elbows and ankles, which equipped it with the ability to climb. The mammal, which likely weighed about 40g, also had spade-like front teeth that it may have used to chew into the bark of the trees.

This new finding is currently tagged the earliest tree-dwelling mammal discovered.

The second mammalian fossil that was discovered was the Docofossor brachydactylus. Standing at 3.5 inches and weighing up to 17g, this creature was smaller than the Agilodocodon. It is currently the earliest known mammal that may have dwelled underground, and researchers think it might have had short, wide digits that it could have used for digging.

The two creatures belonged to the order Docodonta, which is now extinct and had a common ancestor with the modern-day mammals.

The researchers said that the fossils suggest that early mammalian groups filled diverse ecological niches regardless of the presence of the dinosaurs millions of years ago and that these animals were able to adapt to their environment in the same way as modern mammals. It was previously believed that these animals had limited ecological opportunities that would have allowed them to diversify.

"We now know that they [early mammals] could not only climb trees, but also dig holes," said study researcher Ji Qiang of the Chinese Academy of Geological Science. "It means that they occupied a variety of ecological niches millions of years ago."