Teeth smaller than a fingertip reveal the first primate ancestor

Scientists have uncovered tiny new fossils of Purgatorius, the earliest known relative of all primates, including humans, in a more southern part of North America than ever before. The discovery is offering fresh insight into how primates first evolved and spread across the continent.

The origins and early geographic history of primates have long sparked debate among researchers. Purgatorius, a small mammal about the size of a shrew, appears in the fossil record shortly after the dinosaurs went extinct around 65.9 million years ago. It is widely considered the oldest archaic primate.

Although rock layers from that time period are found across North America, fossils of this species had previously been limited to Montana and parts of southwestern Canada. Other early primate relatives have been discovered in the southwestern United States, but they date to roughly two million years later – creating a gap in the fossil record that puzzled scientists… until now.

According to findings published in the peer-reviewed Journal of Vertebrate Paleontology, the newly discovered fossils represent the southernmost record of Purgatorius. They were found in Colorado’s Denver Basin at the Corral Bluffs study area.

Evidence of Southward Spread After the Dinosaur Extinction

“The discovery helps fill the gap in understanding the geography and evolution of our earliest primate relatives,” explains lead author Dr. Stephen Chester, associate professor at Brooklyn College and The Graduate Center, City University of New York (CUNY), who led the study with colleagues from the Denver Museum of Nature & Science (DMNS).

“The presence of these fossils in Colorado suggests that archaic primates originated in the north and then spread southward, diversifying soon after the mass extinction at the end of the Cretaceous Period,” adds Dr. Chester.

Fossilized ankle bones show that Purgatorius likely lived in trees. Because of this, researchers once suspected its absence south of Montana might have been linked to widespread forest destruction caused by the asteroid impact 66 million years ago.

“Ankle bones of Purgatorius exhibit features that indicate it lived in trees, so we initially thought its absence south of Montana could be related to the sweeping devastation of forests from the asteroid impact 66 million years ago.

“However, our paleobotanical colleagues suggested the recovery of plants in North America was fast leading us to believe that Purgatorius should also be in more southern regions and perhaps we simply hadn’t looked hard enough.”

Screen Washing Reveals Tiny Teeth

To search more thoroughly, Dr. Chester and his colleagues at DMNS used an intensive screen washing process to sift through sediment for extremely small fossils. This effort was supported in part by a nearly $3 million collaborative grant from the National Science Foundation, which funds a broader research initiative led by Dr. Tyler Lyson at DMNS focused on how life recovered after the mass extinction that ended the age of dinosaurs.

Students and volunteers painstakingly washed and sorted through large amounts of sediment. Their work uncovered numerous fossils of fish, crocodilians, and turtles and eventually several tiny Purgatorius teeth small enough to sit on the tip of a baby’s finger.

Dr. Jordan Crowell, a postdoctoral fellow at DMNS and key member of the research team, says what makes these teeth especially “exciting” is the possibility that they belong to an even earlier species of Purgatorius.

“The specimens have a unique combination of features compared to known species of Purgatorius, but we are awaiting the recovery of additional material to assess whether these fossils represent a new species,” he adds.

Overlooked Fossils and New Insights

The discovery also suggests that the apparent absence of early primate relatives in more southern parts of the Western Interior may have been influenced by sampling bias. For nearly 150 years, paleontologists in this region have relied largely on surface collecting methods, which tend to yield larger fossils visible to the naked eye while tiny specimens remain hidden.

“Thanks to our long-term partnership with the City of Colorado Springs who own the land where the fossils were collected, as well as countless hours of work by our volunteers and interns picking through the dirt for the precious vertebrate fossils, we are building some incredible datasets that provide insights on how life including our earliest primitive primate ancestors, rebounded after the single worst day for life on Earth,” adds co-author Dr. Lyson.

“Our results demonstrate that small fossils can easily be missed,” concludes Dr. Chester. “With more intensive searching, especially using screen-washing techniques, we will undoubtedly discover many more important specimens.”

The study also includes co-author Dr. David Krause, Senior Curator of Vertebrate Paleontology at the DMNS.