66 Million Years Ago, a Small Mammal Moved South. Fossils in Colorado Are Finally Telling That Story.

Sixty-six million years ago, the world was reorganizing itself. The asteroid impact that ended the Cretaceous had killed the non-avian dinosaurs and roughly three-quarters of all species on Earth, and in the ecological wreckage that followed, mammals were beginning their slow ascent. Among them was Purgatorius - a small, tree-dwelling creature from what is now Montana that most paleontologists consider the earliest known relative of the primate lineage that would eventually produce monkeys, apes, and humans.

New fossil evidence, published in the Journal of Vertebrate Paleontology, has turned up Purgatorius remains in Colorado - the southernmost location the animal has ever been recorded. The discovery, led by Stephen Chester of CUNY Graduate Center and Brooklyn College, adds a crucial data point to one of paleontology's most interesting open questions: where did the earliest primates originate, and how did they spread?

Teeth in Colorado, Questions About Continents

Fossil mammals from this period are known almost entirely from their teeth - tiny, durable structures that preserve far better than bones over geological timescales. Purgatorius teeth are distinctive enough that paleontologists can identify them with confidence when they find them, which makes range mapping possible even from fragmentary material.

Until this discovery, Purgatorius fossils had been confined to the northern regions of what is now North America - primarily Montana and Wyoming. Finding them in Colorado's Denver Basin pushes the known southern boundary of the animal's range considerably, and Chester's team argues that the geographic pattern supports a specific biogeographic model: Purgatorius originated in the north and migrated southward as the post-extinction world opened up and temperatures allowed tree cover to expand into lower latitudes.

That model is cleaner than the alternative - that primates had multiple independent origin points, or that they originated somewhere other than North America and dispersed in complex patterns. The southernmost Colorado occurrence fits a simple dispersal narrative: start in the north, spread south as the world recovers and the climate warms.

Small Fossils, Easy to Miss

Chester emphasized something that anyone who has not done paleontological fieldwork might underestimate: how much of what we do not know is simply a function of where people have looked. Mammal teeth from the Paleocene - the geological epoch immediately following the mass extinction, roughly 66 to 56 million years ago - are often no larger than a grain of rice. Finding them requires intensive, systematic searching of sediment that looks, to the untrained eye, identical to sediment that contains nothing at all.

"Small fossils can easily be missed," Chester noted, adding that more intensive searching in understudied regions would yield additional discoveries. This is less a complaint than a research program: the Colorado find came partly from applying rigorous field methods to a region that had not previously received that level of attention.

A five-year NSF grant of $3 million is supporting the broader research program, which involves undergraduate students in the fieldwork - an investment in both the science and the next generation of paleontologists who will carry it forward.

Why Primate Origins Still Matter After All This Time

The question of where and when primates originated sits at the foundation of understanding human evolutionary history. Modern molecular biology has given us rough timelines - divergence estimates based on the rate at which DNA accumulates mutations - but the fossil record provides the physical evidence that can anchor those estimates in actual time and place.

Purgatorius is not a primate in the modern sense. It belongs to a group called plesiadapiforms, which share a common ancestor with primates but branched off before the characteristics that define the primate order - forward-facing eyes, grasping hands, enlarged brains - had fully evolved. What makes these animals important is their temporal position: they are the earliest fossil evidence of a lineage that connects directly to primates, and by extension to us.

The Denver Basin, where the new fossils were found, preserves sediment from precisely the period when this evolutionary story was beginning. The basin accumulated material continuously across the Cretaceous-Paleogene boundary, creating a geological record of the world before and after the extinction event. For paleontologists studying the mammalian recovery, it is an unusually complete archive.

The Case for Understudied Terrain

Chester's broader argument - that understudied regions of the fossil record are likely hiding significant discoveries - is one that resonates through paleontology. The classic Badlands sites of Wyoming and Montana have been worked for over a century, and while they continue to yield important specimens, the low-hanging fruit is largely gone. Regions like Colorado's Denver Basin, or international sites in Africa, Asia, and South America where Paleocene mammals are less thoroughly documented, may hold the next generation of key fossils.

That is partly a logistical argument and partly a scientific one. Documenting how early primate relatives spread across North America requires knowing their full geographic distribution, not just the distribution of sites that happened to attract collectors in the nineteenth and twentieth centuries. The southernmost Purgatorius is a reminder of how much the map is still being drawn.