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Earth's earliest primates have taken a step up in the world, now that researchers have gotten a good look at their ankles.

A new study has found that Purgatorius, a small mammal that lived on a diet of fruit and insects, was a tree dweller. 

Paleontologists made the discovery by analyzing 65-million-year-old ankle bones collected from sites in northeastern Montana.

A new study has found that Purgatorius, a small mammal that lived on a diet of fruit and insects, was a tree dweller.

Purgatorius, part of an extinct group of primates called plesiadapiforms, first appears in the fossil record shortly after the extinction of non-avian dinosaurs. 

Some researchers have speculated over the years that primitive plesiadapiforms were terrestrial, and that primates moved into the tree canopy later. 

These ideas can still be found in some textbooks today.

'The textbook that I am currently using in my biological anthropology courses still has an illustration of Purgatorius walking on the ground,' said Stephen Chester of Brooklyn College, City University of New York. the paper's lead author.

'Hopefully this study will change what students are learning about earliest primate evolution and will place Purgatorius in the trees where it rightfully belongs,' 

Until now, paleontologists had only the animal's teeth and jaws to examine, which left much of its appearance and behaviour a mystery. 

The identification of Purgatorius ankle bones, found in the same area as the teeth, gave researchers a better sense of how it lived.

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

'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 to date that arboreality played a key role in primate evolution. 

Previous images of Purgatorius show it on the ground

In essence, said the researchers, 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.

 'These new fossils support the idea that the first 10 million years of primate evolution happened in the context of an intense period of similar diversification in flowering plants, including the ability to climb in branches and collect fruits and other products of the trees at the very beginning,' the team has said previously.

Purgatorius lived during the Paleocene, shortly after the extinction of non-avian dinosaurs.

Given the end of the Age of Dinosaurs, the new era began the mammal-dominated era, which we are still in.

This mammal is generally believed to have been small and brown, and had a bushy tail.

THE MINI PRIMATES OF MONTANA 

The tiny mammal is generally believed to have been brown and had a bushy tail.

It weighed about 1.3 ounces, making it roughly the size of the smallest living primates: the mouse lemurs of Madagascar.

Purgatorious had a lot of teeth, including relatively low-crowned molars, which were idea for eating fruit, although it probably ate other things as well.

Researchers believe it spent much of its time eating fruit and climbing trees.

Remains were originally discovered in what is now eastern Montana's Purgatory Hill (hence the animal's name) in deposits believed to be about 66 million years old. 

The researchers liken it to another early primate, Dryomomys, for which more fossil material is available. 

Based on that and the newly found bones, Purgatorius weighed about 1.3 ounces, making it roughly the size of the smallest living primates: the mouse lemurs of Madagascar.

The mammal had a lot of teeth, including relatively low-crowned molars, which were specialized for eating fruit, although it probably ate other things too.

Tree living served this and other primates well, such that all but a few existing species remain at least partly arboreal. 

Humans are part of the rare exceptions, since our more recent ancestors left the trees some 60 million years after Purgatorius' lifetime.