Tiny fossil teeth from Colorado are revealing new clues about the very first relatives of primates, including humans.
Scientists have discovered tiny fossils of Purgatorius, the earliest known relative of all primates including humans, in a more southern part of North America than ever before – a finding that is helping researchers better understand how primates first evolved.
The origin and early geographic spread of primates has long been a topic of debate among paleontologists. Purgatorius, the oldest archaic primate, was a small mammal roughly the size of a shrew. It first appeared in North America shortly after the dinosaurs went extinct about 65.9 million years ago.
Although rocks of the right age are found across much of North America, fossils of this ancient species had previously only been discovered in present-day Montana and southwestern Canada. Later archaic primates have been found in the southwestern United States, but those fossils date to about two million years later, leaving an unexplained gap in the record that researchers have struggled to explain.
Colorado Discovery Fills a Key Fossil Gap
New research published in the peer-reviewed Journal of Vertebrate Paleontology reports the southernmost fossils of Purgatorius ever discovered. The remains were found in Colorado’s Denver Basin at a site known as Corral Bluffs.
“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.
Researchers had previously proposed that Purgatorius might have been limited to northern regions because forests farther south were devastated by the asteroid impact that occurred 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 Technique Reveals Tiny Teeth
To investigate further, Dr. Chester and his colleagues at DMNS used a careful but extensive screen washing method to sift sediments and recover extremely small fossils. The work was supported in part by a nearly $3 million collaborative grant from the National Science Foundation, which funds a larger research effort led by Dr. Tyler Lyson at DMNS to understand how life on Earth rebounded after the mass extinction that eliminated the dinosaurs.
Students and volunteers helped wash and sort large amounts of sediment from the site. Their efforts uncovered numerous fossils, including those of fish, crocodilians, and turtles. Eventually, they also recovered several tiny Purgatorius teeth small enough to rest on the tip of a baby’s finger.
Dr. Jordan Crowell, a postdoctoral fellow at DMNS who played an important role in the research, 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.
Small Fossils Often Overlooked
The discovery also highlights how easily tiny fossils can be missed. For nearly 150 years, paleontologists searching this region and time period have relied largely on traditional surface collecting, which tends to reveal larger fossils that are visible to the naked eye.
These newly recovered teeth suggest that the apparent absence of early primate relatives in the southern part of the Western Interior of North America may partly reflect a sampling bias rather than a true lack of fossils.
“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.”
Reference: “Southernmost occurrence of Purgatorius sheds light on the biogeographic history and diversification of the earliest primate relatives” by Stephen G. B. Chester, Jordan W. Crowell, David W. Krause and Tyler R. Lyson, 2 March 2026, Journal of Vertebrate Paleontology.
DOI: 10.1080/02724634.2026.2614024
The paper also includes co-author Dr. David Krause, Senior Curator of Vertebrate Paleontology at the DMNS.
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