Squirrel-like Mammals Once Walked The Triassic Era—Where Dinosaurs and Mammals Met

Adjusting the evolutionary clock back quite a bit, three newly discovered fossils in China's Liaoning province reveal that mammals may have roamed the Earth with their dinosaur companions, long before previously thought.

Adapted for treetop living, far away from the predatory dangers of dinosaurs, fossils of small mouse/squirrel-like mammalian creatures suggest that the world's earliest mammals may have arisen as early as 200 million years ago. Known as "Haramiyids", the treetop dwellers appear to have had hands and feet adapted for climbing, with prehensile tails that could grasp branches much like modern monkeys.

"The [previous] picture that Mesozoic mammals were shrew-like insectivores that lived in the shadow of dinosaurs needs to be repainted" paleontologist of the American Museum of Natural History and co-author to the study published in this week's issue of the journal Nature, Jin Meng said.

Rather, the image that Meng and his fellow researchers believe should be painted is one of far more adaptability, and one that could transcend environmental nichés.

"They walked on the ground; they also swam, dug to burrow, and glided in the forests" Meng says.

Three closely related species of Haramiyids were discovered in the northeastern Chinese archaeological dig, all from a lineage of organisms that branched off long ago. As there are no living descendents of the creatures, it has long been in contention whether or not Haramiyids were in fact mammalian creatures. However, details of the skeletons, skull and dental index provide strong evidence to put questions to rest as the closely related species were truly original mammals.

And in proving that the mammalian lineage extended an approximate 34 to 54 million years prior to the 166 million-year-old fossils connected to the origin theories of mammals, the researchers are hopeful to change the view of the Triassic period and the evolutionary stresses that led to today's mammalian diversity.

"I think it's going to be part of an argument that will be going on for some time" researchers Anne Weil says. "And I expect paleontology as a whole will learn a lot from questions gleaned from these animals about the antiquity of Mammalia."