Southernmost Purgatorius Fossils Unearthed in Colorado, Extending the Range of Earth's Earliest Known Primate Relative by 800 Kilometers
Paleontologists have recovered tiny fossilized teeth of Purgatorius from Colorado's Denver Basin, marking the southernmost occurrence ever documented and suggesting that archaic primates dispersed rapidly across western North America within hundreds of thousands of years of the dinosaur extinction.
Overview
Three minuscule fossilized teeth recovered from Colorado’s Denver Basin have extended the known geographic range of Purgatorius — the earliest documented relative of all primates, including humans — roughly 800 kilometers south of any previous find. The discovery, published in the Journal of Vertebrate Paleontology on 2 March 2026, indicates that archaic primates spread across western North America far more rapidly after the end-Cretaceous mass extinction than paleontologists had assumed.
The Discovery
The fossils were recovered from the Corral Bluffs study area in the Denver Basin through an intensive screen-washing campaign — a technique that passes ancient sediment through fine mesh to capture specimens too small for traditional surface collection. Each tooth measures roughly two millimeters across, small enough to fit on the tip of a baby’s finger.
The research team was led by Dr. Stephen Chester, an associate professor at Brooklyn College and The Graduate Center of the City University of New York (CUNY), alongside co-authors Dr. Jordan Crowell, a postdoctoral fellow at the Denver Museum of Nature & Science (DMNS), Dr. Tyler Lyson and Dr. David Krause, both also of DMNS. A collaborative National Science Foundation grant of nearly three million dollars supported the fieldwork.
What Purgatorius Is
Purgatorius was a small, shrew-sized mammal that first appeared in North America roughly 65.9 million years ago — within a few hundred thousand years of the asteroid impact that ended the Cretaceous Period and wiped out the non-avian dinosaurs. Ankle bone morphology from previously known specimens indicates that Purgatorius was adapted to life in trees, making it the earliest known arboreal mammal in the primate lineage. Before this study, all documented Purgatorius fossils came from sites in Montana and southwestern Canada.
Why It Matters
The Colorado specimens sit approximately 65.4 million years in age — roughly 600,000 years after the mass extinction event — and display a unique combination of dental features compared to known Purgatorius species. According to the researchers, the teeth may represent a previously unknown species, though additional material will be needed to make a formal determination.
The southward range extension carries two significant implications. First, it fills a roughly two-million-year gap in the fossil record between Purgatorius in the northern plains and later archaic primates documented in the southwestern United States. Second, it challenges the long-held assumption that forest devastation caused by the end-Cretaceous impact would have prevented early primates from migrating southward so soon after the extinction.
“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,” Dr. Chester stated. Dr. Crowell noted that the specimens possess “a unique combination of features” that sets them apart from previously described species.
A Lesson in Sampling Bias
The team emphasized that the previous absence of Purgatorius from southern localities likely reflected a sampling problem rather than a real distributional boundary. For more than 150 years, paleontologists collected fossils at Corral Bluffs using surface-walking methods, which reliably recover larger bones but routinely miss teeth smaller than a grain of rice. The NSF-funded screen-washing effort changed that calculus. As Dr. Chester observed, “small fossils can easily be missed” — and those small fossils can carry outsized evolutionary significance.
The Corral Bluffs site has already proven one of the most important Cretaceous-Paleogene boundary localities in the world, previously yielding detailed records of how mammals, plants and reptiles recovered in the first million years after the asteroid impact. The addition of Purgatorius to its fauna further cements its status as a key window into the post-extinction world that gave rise to the primate order.