For decades, a few teeth and jaw fragments were the only clues available about a mouse-size creature called Purgatorius, which lived some 65 million years ago. Now paleontologists have identified ankle bones from the species, and the wide range of motion in the joints suggests that the animal was a nimble tree climber.

"Even though these are isolated ankle bones, we are extremely confident they belong to Purgatorius," paleontologist Stephen Chester of Yale University said at a Society of Vertebrate Paleontology meeting last week in Raleigh, North Carolina. "It's the first direct evidence that these primates spent most of their time in trees."

"Plants were starting to produce something that was attractive to primates, as a way of spreading their