In rocky outcroppings near a cornfield in northern China, paleontologists have unearthed three species of squirrel-like mammals that lived at the same time as the dinosaurs.
Remarkably complete skeletons of the three new species -- Shenshou lui, Xianshou linglong and Xianshou songae -- were found in pieces of sandstone that date back 160 million years. Their discovery lends support to the theory that the population of mammals that lived among the dinosaurs was more diverse than previously thought.
"There was this idea that mammals were these miserable small little things living in the shadow of the dinosaurs, and that picture has changed quite a lot," said paleontologist Jin Meng, a curator at the American Museum of Natural History in New York.
The three new mammals, all members of the new group Euharamiyida, range in size from a house mouse to a squirrel. They had long, thin fingers, tails that were longer than their bodies, and light, fragile skeletons, which all suggest they lived in the trees of the Jurassic forest. Their diet probably consisted of insects, nuts and fruit.
A paper describing the newly discovered animals was published Wednesday in the journal Nature.
These small furry creatures belong to an extinct group called Haramiyida that is known by the strange shape of its members' molar teeth. Most mammals today are derived from an ancient mammal that had molars with three cusps arranged in a triangular shape. The Haramiyidas have two rows of cusps on their molars.
"This has puzzled paleontologists for many decades," Meng said. "They didn't know if Haramiyidas were mammals or not because their teeth were so weird."
The structure of the tooth isn't just different to look at; it also meant these animals chewed their food differently than we do. They probably moved their jaw back when they were eating, grinding the food up against the rows of cusps.
Scientists have been aware of Haramiyidas for a long time, but most of the fossilized remains of these creatures consist only of a few scattered teeth or perhaps a jawbone. With the discovery of these new mammals, paleontologists have a lot more material to work with.
"Now we have a complete skeleton that shows us the morphology of the skull, arm, leg, foot and hands," Meng said, "and all that morphology shows that these animals had mammalian features."
If Meng is correct and the Haramiyidas are mammals, that would suggest mammals started to diversify earlier than other research has suggested. Although the animals he discovered were 160 million years old, other Haramiyidas have been found that are more than 200 million years old. This means that mammals began to diverge 235 million to 201 million years ago, rather than 176 million to 161 million years ago.
"The study presents evidence that mammals were quite diverse back then," Meng said. "Our study describes animals that live in trees, but we've seen ones that swim, glide and run on the ground. This gives us a new picture of what mammals looked like when they coexisted with dinosaurs."