The Purgatorius is an extinct primate belonging to a group called plesiadapiforms. It first appeared on record after non-avian dinosaurs were wiped out and have always been believed to be terrestrial, simply moving on to trees later on. Researchers challenged this belief, pointing out that the early primate was actually a tree-dweller.

In a study published in the Proceedings of the National Academy of Sciences, researchers noted that ankle bones from a Purgatorius featured certain characteristics that shed light on how the primate lived. Previous research done only examined jaws and teeth, so a lot about the Purgatorius' behavior and appearance was left a mystery.

"The ankle bones have diagnostic features for mobility that are only present in those of primates and their close relatives today," explained Stephen Chester, a curatorial affiliate for the Yale Peabody Museum of Natural History and lead author of the study.

Chester added that these features unique to ankle bones would provide animals like the Purgatorius with the ability to rotate and adjust their feet as needed to be able to grab branches as they moved through trees. Animals dwelling on the ground simply don't have these abilities. Instead, they are physically equipped to propel themselves forward in a more restricted manner.

The idea that the Purgatorius was a land animal had persisted so much that even Chester's book for biological anthropology courses (he's also an assistant professor from City University of New York's Brooklyn College) still had illustrations of the primate walking on the ground. He is hopeful that because of their study, information on early primate evolution would be changed to rightfully place the Purgatorius on trees.

Thanks to utilizing the oldest fossil evidence available, the study also showed how tree-living played a crucial role in the evolution of primates. This also implies that the moment primates diverged from other mammals was not a drastic event. Instead, primates developed bit by bit, incorporating subtle changes that allowed for easier navigation and quicker access to food up on trees.

Other authors for the study include Jonathan Bloch from the Florida Museum of Natural History, who is a Yale Institute for Biospheric Studies' Edward P. Bass Distinguished Visiting Environmental Scholar; William Clemens from the University of California Museum of Paleontology; and Doug Boyer from Duke University. Clemens was responsible for collecting Purgatorius fossils and associated geological data on the primate over the last 40 years with his field crews in Montana.