Paleontologists have found the southernmost fossils of Purgatorius, the earliest known relative of primates including humans, in Colorado's Denver Basin. These tiny teeth, smaller than a fingertip, suggest the shrew-sized mammal spread southward soon after the dinosaur extinction 66 million years ago. The discovery fills a gap in the early geographic history of primate ancestors.
The fossils of Purgatorius, a small tree-dwelling mammal about the size of a shrew, appear in the record shortly after the asteroid impact that ended the Cretaceous Period around 65.9 million years ago. Previously, such remains were limited to Montana and southwestern Canada, with other early primate relatives found further south but dating two million years later. This created a puzzle for scientists studying primate origins.
The new specimens, uncovered at the Corral Bluffs study area in the Denver Basin, represent the southernmost record of Purgatorius to date. Lead author Dr. Stephen Chester, associate professor at Brooklyn College and The Graduate Center, City University of New York, stated, “The discovery helps fill the gap in understanding the geography and evolution of our earliest primate relatives.” He added that the presence of these fossils suggests archaic primates originated in the north and diversified southward soon after the mass extinction.
Fossilized ankle bones indicate Purgatorius lived in trees, leading earlier researchers to link its absence in southern regions to forest devastation from the asteroid. However, paleobotanical evidence points to a rapid plant recovery, prompting more intensive searches. Dr. Chester and colleagues at the Denver Museum of Nature & Science employed screen-washing techniques to sift sediment, supported by a nearly $3 million National Science Foundation grant led by Dr. Tyler Lyson.
The effort yielded tiny teeth, possibly from an even earlier Purgatorius species, according to Dr. Jordan Crowell, a postdoctoral fellow at the museum. “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 said.
The findings, published in the Journal of Vertebrate Paleontology, highlight sampling biases in past collections, which favored larger fossils. Co-author Dr. Lyson noted the partnership with the City of Colorado Springs enabled the work, building datasets on post-extinction recovery. Dr. Chester concluded, “Our results demonstrate that small fossils can easily be missed,” emphasizing the value of thorough methods for future discoveries. Co-author Dr. David Krause, senior curator at the museum, contributed to the study.