Scientists have discovered an extinct animal the size of a small squirrel that lived in China at least 125 million years ago and soared among the trees. It is the earliest known example of gliding flight by mammals, and the scientists say it shows that mammals experimented with aerial life about the same time birds first took to the skies, perhaps even earlier.

From an analysis of the fossil, the researchers concluded that this gliding mammal was unrelated to the modern flying squirrel and unlike any other animal in the Mesozoic, the period best known for dinosaurs living in the company of small and unprepossessing mammals. They announced today that the species qualified as a member of an entirely new order of mammals.

Richard L. Cifelli, a paleontologist at the Oklahoma Museum of Natural History in Norman who reviewed the findings for publication, said this “wholly unexpected diversity of something adapted for gliding at this early time is absolutely astonishing.”

Until a couple of years ago, Dr. Cifelli said, most scientists held the view that such early mammals were simple shrew-like creatures that cowered in the shadows of the dominant dinosaurs, and now “this adds a new dimension to our knowledge of early mammals.”