The world's oldest known primate had upward mobility, according to new research.
For decades, a few teeth and jaw fragments were the only clues available about a mouse-size creature called Purgatorius, which lived some 65 million years ago. Now paleontologists have identified ankle bones from the species, and the wide range of motion in the joints suggests that the animal was a nimble tree climber.
"Even though these are isolated ankle bones, we are extremely confident they belong to Purgatorius," paleontologist Stephen Chester of Yale University said at a Society of Vertebrate Paleontology meeting last week in Raleigh, North Carolina. "It's the first direct evidence that these primates spent most of their time in trees."
Purgatorius lived during the early Paleocene epoch, right after the demise of dinosaurs. By then, plants were beginning to bear fruit. The revelation that the earliest primate climbed trees supports the theory that primates and flowering plants co-evolved in ways that benefited both groups, Chester said. (See our prehistoric time line.)
"Plants were starting to produce something that was attractive to primates, as a way of spreading their seeds. And at the same time, primates were becoming more specialized for life in the trees, allowing them to climb out on a branch and collect that fruit," he said.
Tree-climbing would have given the species a competitive edge in gathering food resources, since many other mammals at the time were land-dwelling, Chester said. "It probably contributed to the evolutionary success of these early primates."
Unlocking the Past
The ankle bones are among the many fossils collected in northeastern Montana over the past four decades by William Clemens of the University of California Museum of Paleontology in Berkeley. But the Purgatorius remains—found on Montana's Purgatory Hill—could not have been identified earlier, because there was little to compare them to in the fossil record, Chester said.
"Previously, people didn't really know much about these early primates—they had their teeth, but that was about it," Chester explained. He pointed to the work of co-researcher Jonathan Bloch, a paleontologist at the Florida Museum of Natural History, as key to decoding the ankle bones.
By painstakingly dissolving blocks of surrounding limestone with formic acid, Bloch has revealed the fossilized skeletons of many other early primates.
"The unbelievably complete skeletons that Jon has been uncovering are kind of like a Rosetta stone that no one had before," Chester said. "Now, we can go into collections where there are just isolated bones and be able to positively identify them as early primates."
Now that they know what they're looking for, the team intends to keep searching through existing fossil collections to identify other bones from Purgatorius that might provide information about the animal's posture and how it moved.
Calling the ankle bones "an extraordinary discovery," UC Berkeley's Clemens said the fossil site may have more to reveal about the dawn of the age of mammals. "We plan to continue research in Montana in order to address other critical questions about causes of the mass extinction [that preceded the Paleocene], as well as the origin of primates."