Earliest primates were tree dwellers

by Mary Caperton Morton
Monday, April 20, 2015

The ankle bones of the 65-million- year-old primate Purgatorius, shown here in an artist's rendition, reveal that the earliest primates likely lived in trees. Credit: Patrick Lynch/Yale University.

The first primates evolved shortly after the mass extinction of the dinosaurs about 66 million years ago. But whether these small mammals lived on the ground or in trees has puzzled paleontologists, who only had fossil teeth and jaws to examine, which left much of the animals' appearance and behavior a mystery.

Now, a set of 65-million-year-old ankle bones found in Mongolia and belonging to Purgatorius, part of an extinct group of rodent-like primates called plesiadapiforms, has revealed that our ancestors likely took to the trees very early in the evolutionary history of primates.

“The ankle bones have diagnostic features for mobility that are only present in those of primates and their close relatives today,” said lead author Stephen Chester, in a statement provided by Yale University, where Chester, now at Brooklyn College in New York, conducted the research. The bones provide the oldest fossil evidence to date that arboreality played a key role in primate evolution, Chester and his colleagues reported in Proceedings of the National Academy of Sciences.

Particular features of the bones, such as greater rotational flexibility between neighboring ankle bones, would have allowed Purgatorius to adjust its feet to grab branches while moving through trees, Chester said. Ground-dwelling mammals generally lack these features; their joints are better suited for propelling themselves forward in a more restricted, fore-and-aft motion.

Modern textbooks still portray Purgatorius walking on the ground, Chester noted in the statement. “Hopefully, this study will place Purgatorius in the trees where it rightfully belongs.”

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