Penny-sized ankles hint first primates lived in trees

Scientists believe Purgatorius looked similar to Dryomomys szalayi (above), another primitive primate discovered near Yellowstone National Park by co-author Jonathan Bloch. (Credit: Illustration courtesy of Doug Boyer)

Newly-discovered ankle bone fossils smaller than a penny show that our earliest primate ancestors most likely lived in trees.

Found in the same area of Montana that yielded the massive Tyrannosaurus rex, the new bones from a squirrel-like creature that weighs no more than a deck of playing cards suggest a tree-dwelling existence.

That’s important because living in trees gave those early primates access to food sources that other species lacked—likely a critical factor in why primates succeeded in evolution where others may have failed.

Purgatorius bones
Lead researcher Stephen Chester holds the tiny ankle bones of Purgatorius, which scientists believe weighed about 3.5 ounces. (Credit: Stephen Chester)

Unique ankle bones

Published online in the Proceedings of the National Academy of Sciences, the study describes the first bones below the skull of Purgatorius—previously known only by its teeth. The shape of the teeth helped paleontologists determine that the shrew-like animal ate insects and plants, but researchers knew little else about how the creature lived.

The wide range of mobility in the joints of the ankle bones suggests that tiny Purgatorius—estimated to have weighed about 3.5 ounces—spent its time climbing trees and reaching for fruit at the edge of limbs, says lead author Stephen Chester, assistant professor of anthropology at Brooklyn College, City University of New York, who received a bachelor’s degree in anthropology from University of Florida.

“These ankle bones have really unique characteristics that indicate a specific kind of mobility that we only find in primates and their closest relatives today. Early primates were using this high degree of mobility to access resources that other animals on the ground couldn’t reach.”

Evolutionary success

The adaptations Purgatorius had for climbing support the idea that the earliest primates diversified at the same time flowering plants became widespread, says coauthor Jonathan Bloch, curator of vertebrate paleontology at the Florida Museum of Natural History.

Unique adaptations to a changing environment lead to the evolutionary success of the oldest primate ancestors.

“While Purgatorius is found just after a dramatic extinction that includes all non-avian dinosaurs, the new fossils suggest that the divergence of primates from other mammals was a more subtle event,” Bloch says.

“The beginning of primate evolution involved small modifications of the skeleton, making it easier to move through trees and eat the fruits, flowers, and leaves that they encountered.”

T. rex’s neighbor

The ankle bones were found among boxes full of small and fragmented unidentified fossils recovered from the Garbani Channel sites in Montana by paleontologist and study coauthor William Clemens, professor emeritus of integrated biology at University of California, Berkeley.


The area is famous for producing the holotype specimen of T. rex. The ankle bones were found in the same area as the teeth of Purgatorius and compared with the ankles of later primates.

“This discovery gives us an idea of what the rest of the animal looked like and what it could do,” Clemens says. “The take-home message is that arboreal life evolved early in the history of primates and is shared by most of their descendants.”

As you dig deeper back in time, you start finding animals with fewer characteristics that people think of as primate-like when they think of today’s primates, Chester says, which can make it difficult to distinguish the early ancestors of modern primates.

Bloch and Douglas Boyer, assistant professor of anthropology at Duke University and study coauthor, have worked throughout their careers to identify and uncover fossilized skeletons of many species of early primates.

“You couldn’t possibly have predicted this animal if you only looked at the types of mammals that are alive today,” Bloch says. “This is something you only find in the fossil record.”

Source: University of Florida