Fossil Ankles Shows Early Primates Were Tree-Dwellers

History textbooks have told us that some of Earth's earliest primates walked on the ground like the rest of us, but newly discovered fossilized ankles turns this theory upside down, so to speak, indicating that they in fact were tree-dwellers, according to recent research. 

Purgatorius, a small mammal that lived on a diet of fruit and insects, was part of an extinct group of primates called plesiadapiforms. They first appeared in the fossil record shortly after the extinction of non-avian dinosaurs, supposedly walking around on Earth.

But 65-million-year-old ankle bones recently discovered at sites in northeastern Montana, alongside Purgatorius  teeth fossils, shows that these earliest of primates were not terrestrial, but in fact preferred tree canopies instead.

"The textbook that I am currently using in my 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 Purgatoriusin the trees where it rightfully belongs," study lead author Stephen Chester of Yale University said in a statement.

Paleontologists' view of Earth's early primates like Purgatorius  had been until now based on only fossils of the animal's teeth and jaws, leaving the majority of its appearance and behavior to the imagination. But with this latest discovery of its ankle bones, the picture is becoming clearer.

"The ankle bones have diagnostic features for mobility that are only present in those of primates and their close relatives today. 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," Chester explained. "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 findings, described in the journal Proceedings of the National Academy of Sciences, also provide a breakthrough in primate evolution. Thanks to this research, it now seems that the divergence of primates from other mammals was not so dramatic, but rather a gradual process starting with the ankles.

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