Carl Zimmer in The New York Times:
The Mesozoic Era, from 252 million years ago to 66 million years ago, is often called the Age of Dinosaurs. To generations of paleontologists, early mammals from the period were just tiny nocturnal insect-eaters, trapped in the shadows of leviathans. In recent years, scientists have significantly revised the story. Mammals already had evolved into a staggering range of forms, fossil evidence shows, foreshadowing the diversity of mammals today. In a study published on Wednesday, a team of paleontologists added some particularly fascinating new creatures to the Mesozoic Menagerie. These mammals did not lurk in the shadows of dinosaurs. Instead, they glided far overhead, avoiding predatory dinosaurs on the ground — essentially flying squirrels of the Jurassic Period, from an extinct branch of mammals that probably still laid eggs.
The fossils “are most primitive-known mammal forerunners that took to air,” said Zhe-Xi Luo, a paleontologist at the University of Chicago who led the research. The first Mesozoic mammal fossils came to light in the early 1800s, but for generations, paleontologists struggled to find more than teeth and bits of bone. In the late 1990s, they hit the jackpot. At a site in northeastern China, hillside after hillside turned out to contain stunning mammal fossils, most dating back about 160 million years. Researchers were suddenly able to examine entire skeletons, some still bearing impressions of skin and hair.