The origins of primates and their earliest movements across North America remain one of the most intriguing and debated topics in paleontology. Purgatorius was a small mammal about the size of a shrew that appeared shortly after the mass extinction event that wiped out the dinosaurs roughly 65.9 million years ago. It is considered the oldest known archaic primate.

For decades, fossils of Purgatorius had only been found in present day Montana and southwestern Canada, despite the existence of similarly aged rock formations across much of North America. Meanwhile, a wider variety of related archaic primates has been discovered in the southwestern United States, but those fossils are about two million years younger. That gap in time and geography has long puzzled scientists.

Southernmost Purgatorius Fossils Ever Found

A recent study published in the peer-reviewed Journal of Vertebrate Paleontology reports the southernmost known fossils of Purgatorius. The remains were recovered from the Corral Bluffs study area in Colorado's Denver Basin.

"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 alongside 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.

Researchers had previously wondered whether the species' apparent absence south of Montana reflected real biological limits rather than a lack of fossil evidence.

"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."

Tiny Teeth Recovered Through Intensive Fossil Search

To test that idea, Chester and his colleagues used an extensive screen washing process designed to recover extremely small fossils that traditional collecting methods often miss.

The work was supported in part by a nearly $3 million collaborative grant from the National Science Foundation. The funding supports a broader project led by Dr Tyler Lyson at DMNS focused on understanding how life recovered after the mass extinction event best known for ending the age of dinosaurs.

Students and volunteers spent countless hours washing sediment and carefully sorting through the material. Their efforts uncovered numerous fossils from fish, crocodilians, and turtles. Among them were several tiny Purgatorius teeth, each small enough to fit on the tip of a baby's finger.

Possible Evidence of a New Species

According to Dr. Jordan Crowell, a postdoctoral fellow at DMNS who played a major role in the study, the teeth may represent something even more significant.

What is particularly "exciting" about these teeth, explains Dr. Crowell, is that they could in fact belong to an 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.

Why Tiny Fossils Matter

The discovery also highlights a major challenge in paleontology. The apparent absence of early primate relatives in the southern Western Interior of North America may have been influenced by sampling bias rather than actual absence.

For nearly 150 years, researchers working in this region have relied heavily on traditional surface collecting. Those methods are effective for finding larger fossils visible to the naked eye but can easily overlook tiny remains like the newly discovered teeth.

"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.