First Primate's Lifestyle All In The Ankles

Patrick Lynch/Yale University. Artist's impression of a Purgatorius, possibly the ancestor of all primates.

The discovery of fossilized ankle bones from the oldest known primate has confirmed that our ancestors were tree-dwellers sixty-five million years ago.

The Cretaceous/Paleogene extinction event that wiped out most of the dinosaurs spelled doom for many mammals. However, for the survivors it provided unprecedented opportunities. Among the chief beneficiaries were the plesiadapiforms, who are considered to be either the predecessors of primates, or very close relatives. They appear in the fossil record for the first time soon after the Chicxulub impact.

Among these was the delightfully named Purgatorius, a genus of at least four species that some consider the first primates. However, not that much is known about any of the Purgatorius species, which until now had only been examined via their teeth and jaws. 

“The textbook that I am currently using in my biological anthropology courses still has an illustration of Purgatorius walking on the ground,” says Stephen Chester, who is studying for his Ph.D. at Brooklyn College. Chester is lead author of the paper published in the Proceedings of the National Academy of Scienceswhich details the first Purgatorius tarsals, or ankle bones, to have been found.

Purgatorius was around the size of a rat, leading to drawing such as this, and perhaps encouraging the notion of a ground-dwelling creature.

Credit: Nobu Tamura via wikimedia commons. Artist's rendition based on teeth and jaws alone.

The discovery of tarsals thought to have come from Purgatorius sparked speculation that primates were arboreal from the beginning, but it has taken Chester a further three years to confirm their tree-dwelling life.

"The ankle bones have diagnostic features for mobility that are only present in those of primates and their close relatives today," Chester says. "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."

Animals take to the trees for safety or in search of food. With the ecosystem cleared of larger predators as a result of the dinosaur extinction, members of the research team think hunger is a more likely explanation than fear, noting that their teeth indicate they were plant eaters.

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