Teeth Smaller Than a Fingertip Push Earth's Oldest Primate Relative 500 Miles South of Where It Was Thought to Live

For nearly 150 years, paleontologists working the fossil-rich rock of the American West have been picking through sediments deposited just after the mass extinction that ended the Cretaceous Period, 66 million years ago. They have found fish, crocodilians, turtles, and occasional mammals. They have not, until now, found Purgatorius south of Montana and southwestern Canada.

Purgatorius holds a distinctive position in evolutionary history: it is the oldest known archaic primate relative, a shrew-sized tree-dweller that appears in the fossil record immediately after the dinosaurs' extinction. Its ankle bones suggest it lived in trees, making it an early representative of the lineage that eventually produced monkeys, apes, and humans. The fact that the entire fossil record of this animal had been confined to a relatively small area of northern North America was puzzling - and a new study published in the Journal of Vertebrate Paleontology offers a partial answer to why.

The Sampling Bias Problem

Paleontologists have been collecting fossils from the Denver Basin and other southern Western Interior localities for generations. The reason Purgatorius had not turned up had a straightforward explanation, according to the researchers: standard surface collecting techniques are biased toward specimens large enough to see with the naked eye. Purgatorius teeth fit on the tip of a baby's finger.

The research team, led by Dr. Stephen Chester of Brooklyn College and The Graduate Center at the City University of New York, in collaboration with colleagues from the Denver Museum of Nature and Science, deployed screen-washing - a technique that processes bulk sediment through progressively finer mesh sieves, concentrating small fossils that would otherwise be trampled past unnoticed. The process is time-consuming, requiring students and volunteers to systematically wash and sort through large volumes of material.

The effort was supported in part by a nearly $3 million collaborative grant from the National Science Foundation, funding a wider project led by Dr. Tyler Lyson at DMNS to study ecological recovery after the Cretaceous-Paleogene mass extinction. That broader context matters: understanding how life rebounded after the worst extinction event of the past 250 million years requires knowing where animals were, not just what they were.

What the Colorado Fossils Say About Primate Origins

The specimens - recovered from the Corral Bluffs study area in Colorado's Denver Basin - represent the southernmost Purgatorius finds ever documented, approximately 500 miles south of the previous southernmost records. Their age places them in the early Paleocene, shortly after the extinction boundary.

The discovery fills what had been a geographic gap between the northern Purgatorius record and the more diverse and geographically distributed archaic primates that appear in the fossil record roughly two million years later in the southwestern United States. The gap had been difficult to explain: the rock of the right age exists throughout North America, and early hypotheses that the absence of Purgatorius in the south reflected post-extinction forest devastation had been weakened by paleobotanical evidence suggesting plant recovery was rapid.

"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," said Dr. Chester.

A Possible New Species, Pending More Material

Dr. Jordan Crowell, a postdoctoral fellow at DMNS who played a key role in the study, notes that the Colorado specimens show a "unique combination of features" compared to known Purgatorius species. Whether they represent a distinct new species or a regional variant of a known species cannot be determined from the small number of teeth recovered so far. Additional material from the same sediment layers is needed before a formal species assessment can be made.

The study's broader methodological point is clear: the absence of a fossil from a region is not the same as evidence of its absence. When intensive screen-washing is applied to sediments where surface collecting has found nothing for 150 years, new records appear. How many other species have been missing from the record simply because no one looked carefully enough is an open question.