It may not have been the most opportune time to be a furry little critter, what with all those hungry dinosaurs and flying reptiles hanging around. But early mammals still managed to make their mark during the Jurassic Period.
Scientists on Thursday described fossils unearthed in China of two shrew-sized creatures that represent the oldest-known tree-climbing and burrowing mammals and show that early mammals had claimed a variety of ecological niches.
Agilodocodon scansorius, an omnivore that lived about 165 million years ago, possessed paws with curved claws for climbing, limb dimensions characteristic of other tree-dwelling mammals and flexible elbow, wrist and ankle joints good for scrambling up trees with agility.
Its spade-like front teeth, similar to some New World monkeys today, allowed it to chew into bark and eat tree gum or sap.
Docofossor, a mole-like insect-eater that lived about 160 million years ago, boasted shovel-like paws for digging, teeth similar to later burrowing mammals that forage underground and sprawling limbs ideal for underground movement.
University of Chicago paleontologist Zhe-Xi Luo called Docofossor a "dead ringer" for today's African golden mole.
Traits in Docofossor's fingers are so similar to that mole that the researchers suspect the same genes may be responsible even though the two creatures belong to different branches of the mammalian family tree and are separated by 160 million years.
The earliest mammals are thought to have appeared roughly 200 million years ago, arriving on the scene during the Mesozoic Era with dinosaurs ruling the land and flying pterosaurs keeping an eye out for a quick snack. The earliest-known bird did not appear until about 150 million years ago.
"Before the 2000s, it was generally thought Mesozoic mammals could not diversify much in the dinosaur-dominated ecosystem," Luo said.
But Luo said this notion has been undercut by discoveries in recent years of fossils of many Mesozoic mammals with numerous specializations including swimming, tree-living and digging.
"Simply put, our distant mammalian relatives explored ecological niches that were just as varied and interesting as many modern mammal groups," Luo said.
Agilodocodon and Docofossor both belonged to a long-extinct early mammalian order called docodonts. Castorocauda, another docodont living at about the same time in the Jurassic Period whose fossil was unearthed in China in 2006, had beaver-like traits tailored for an aquatic lifestyle.
The research was published in the journal Science.