Arranged as they were in life, they would fit comfortably on the tip of your finger.
But this assortment of tiny bones — excavated from the same deposit in Montana that has produced mighty specimens of Tyrannosaurus rex — holds hefty implications for our evolutionary history, paleontologists say.
The few fossil bones, originally excavated from the Tullock Formation in eastern Montana, are the first complete bones ever found from the earliest known primate on Earth.
Previous evidence of the creature had been hinted at by just a few minuscule teeth and jaw fragments found in the same deposit in 1965, at a site known as Purgatory Hill.
The shew-like teeth were familiar enough to paleontologists to suggest that the animal was a member of an extinct group of very early primates called plesiadapiforms.
But after analyzing other fossils turned up by that dig, which languished for 50 years in museum storage, a team of researchers discovered that the diminutive bones were from the ancient animal’s ankle.
The ancestral primate — named Purgatorius, after the site of its discovery — was indeed the earliest known primate. And what’s more, the bones showed that it was built for climbing.
Our oldest evolutionary ancestor lived in trees.
“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,” said Stephen Chester, an anthropologist at Brooklyn College, City University of New York, in a press statement.
“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.”
Along with the teeth, these newly described bones suggest that Purgatorius resembled a shrew or a mole more than a monkey — let alone a human — and probably weighed no more than 100 grams, making its living on a diet of fruits and insects. [Read about a similarly tiny ancient mammal found not far away: “Tiny, Toothy Mammal Discovered in Wyoming Fossil Named for Lady Gaga“]
And the researchers note that the primate’s remains were found in deposits dating back 65 million years, immediately following the mass-extinction event that wiped out the non-avian dinosaurs.
Although mammals appeared on the scene millions of years before the dinosaurs vanished, the unique features found in Purgatorius suggest that its adaptations ended up giving primates a small, but ultimately important, evolutionary advantage in a rapidly changing world.
“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,” said Jonathan Bloch, curator of vertebrate paleontology at the Florida Museum of Natural History. [Watch the video: “What Really Killed the Dinosaurs?“]
“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.”
The physical resemblance is surely difficult to spot, the researchers concede.
“You couldn’t possibly have predicted this animal if you only looked at the types of mammals that are alive today,” Bloch said.
And yet, this new evidence of the animal’s stature and arboreal lifestyle make a certain, perfect kind of sense, given what we see in modern primates, like monkeys and lemurs.
“The take-home message is that arboreal life evolved early in the history of primates and is shared by most of their descendants,” said UC Berkeley paleontologist William Clemens, an expert in early primates.
“The textbook that I am currently using in my biological anthropology courses still has an illustration of Purgatorius walking on the ground,” Chester added.
“Hopefully this study will change what students are learning about earliest primate evolution and will place Purgatorius in the trees where it rightfully belongs.”
The team report their findings in the Proceedings of the National Academy of Sciences.
Chester, S., Bloch, J., Boyer, D., & Clemens, W. (2015). Oldest known euarchontan tarsals and affinities of Paleocene to PrimatesProceedings of the National Academy of Sciences DOI: 10.1073/pnas.1421707112