Earth's earliest primates lived on trees, according to a new study of 65 million year old ankle bones, discovered from the sites in northeastern Montana, by paleontologists at Yale University. Researchers found that Purgatorius, a small mammal that lived on a diet of fruit and insects and is a part of an extinct group of primates called plesiadapiforms that first appears in the fossil record shortly after the extinction of non-avian dinosaurs, was a tree dweller.
Lead author Stephen Chester said, "The textbook that he is currently using in his biological anthropology courses still has an illustration of Purgatorius walking on the ground. Hopefully this study will change what students are learning about earliest primate evolution and will place Purgatorius in the trees where it rightfully belongs. The ankle bones have diagnostic features for mobility that are only present in those of primates and their close relatives today and these unique features would have allowed an animal such as Purgatorius to rotate and adjust its feet accordingly to grab branches while moving through trees. In contrast, ground-dwelling mammals lack these features and are better suited for propelling themselves forward in a more restricted, fore-and-aft motion."
The research provides the oldest fossil evidence that arboreality played a key role in primate evolution. Researchers said, "It implies that the divergence of primates from other mammals was not a dramatic event. Rather, primates developed subtle changes that made for easier navigation and better access to food in the trees."