Squirrel-like Jurassic Critters Shed Light on Mammal Origins

By Will Dunham 11 September 2014

WASHINGTON — It may not have been the friendliest place for furry little creatures, but three newly identified squirrel-like mammals thrived in the trees of the Jurassic Period, with dinosaurs walking below and flying reptiles soaring above.

Scientists announced on Wednesday the discovery in China of fossils belonging to three critters in a find that sheds light on a poorly understood collection of ancient mammals, and indicates that mammals as a group appeared earlier than some experts thought.

The three species come from a group called haramiyids that previously had been known only from isolated teeth and fragmented jaws. Scientists had not even been sure they were mammals at all.

The nicely preserved fossils from Liaoning Province in northeastern China proved definitively they were mammals, in part because of the presence of three bones of the middle ear characteristic of all mammals from shrews to whales to people.

The three species—whose scientific names are Shenshou lui, Xianshou linglong and Xianshou songae—date from about 160 million years ago, a time when dinosaurs ruled the land. But a number of recent fossil discoveries have shown that mammals were far more diverse during that period than previously recognized.

The three species likely looked like small squirrels, with slim bodies and elongated fingers in the hands and feet, indicating they were dedicated tree dwellers. They had long and probably prehensile, or grasping, tails, another feature that helped them stay in the tree branches.

“I would predict that they spent even more time in the trees than squirrels,” said Jin Meng, a vertebrate paleontologist at the American Museum of Natural History in New York, who led the study published in the journal Nature.

Based on the shape of their teeth, they probably were omnivorous, eating insects, nuts and fruit, Meng said. The remains were so well preserved that they showed more than just the hard parts such as teeth and bones that commonly fossilize, but also soft parts such as fur and the animal’s guts, he added.

The three species had an estimated weight ranging from about that of a mouse, one ounce, to that of a small squirrel, about 10 ounces. While they may have looked and acted like today’s squirrels, they were only very distantly related to them.

The researchers said these fossils, along with other evidence, suggests that the first true mammals that evolved from mammal-like ancestors appeared perhaps 208 million years ago during the Triassic Period. Some scientists have contended that mammals entered the picture millions of years later than that.