Tiny Fossil Teeth in Colorado Expand the Range of the Earliest Known Primate

Hours into washing sediment from Colorado’s Denver Basin, the trays looked much like the others — fish bones, turtle fragments, small bits of crocodilians. Then a few teeth with a familiar shape began to appear.

They belonged to Purgatorius, considered the oldest known archaic primate, a shrew-sized mammal that first appeared in North America about 65.9 million years ago, just after the asteroid impact that ended the dinosaurs. Fossil-bearing rocks of that age stretched across much of the continent, yet Purgatorius had previously been found only in Montana and parts of Canada.

The new specimens from Corral Bluffs in Colorado, described in the Journal of Vertebrate Paleontology, mark the southernmost discovery of the species so far and hint that its absence elsewhere may have had more to do with how fossils were collected than where the animal actually lived.

“The discovery helps fill the gap in understanding the geography and evolution of our earliest primate relatives,” said lead author Stephen Chester in a press release.

The Early Paleocene Spread of Purgatorius in North America

That northern concentration had shaped how researchers thought about the animal’s early history. Slightly younger archaic primates appear in the Southwest roughly two million years later. If close relatives were already present there, why wasn’t Purgatorius?

One line of reasoning focused on habitat. Ankle bones attributed to Purgatorius show features linked to climbing, suggesting it lived in trees. If forests south of Montana had been slow to recover after the asteroid impact, that might have limited where the animal could survive.

But studies of fossil plants from the same time period show that vegetation rebounded quickly across large parts of North America. If forests quickly returned, there was little reason to think the species stayed confined to the north for long.

“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,” added Chester in the release.

Rather than showing a delayed move south, the pattern may just reflect missing fossils from that small slice of time.

How Screen-Washing Revealed Early Primate Fossils

Fossils from the earliest Paleocene have been collected in North America for nearly 150 years. Much of that work relied on surface collecting, scanning exposed rock for visible bones and teeth.

Purgatorius upper molar found in Corral Bluffs Denver Basin CO — Upper molar from Purgatorius found in Corral Bluffs, in Denver Basin, CO.
(Image Credit: Dr Stephen Chester/CC BY)

That approach works well for larger animals. It is less effective for mammals represented primarily by teeth just a few millimeters wide.

To look more closely, the team relied on screen-washing. Sediment was soaked and passed through a fine mesh to isolate the smallest fragments. Students and volunteers spent months sorting the residue grain by grain.

“Small fossils can easily be missed,” explained Chester. “With more intensive searching, especially using screen-washing techniques, we will undoubtedly discover many more important specimens.”

Early Primate Diversity in Purgatorius

The newly recovered teeth differ in small ways from previously described Purgatorius material. They show a combination of features not seen together before. More fossils will be needed to figure out whether the differences reflect a distinct and possibly earlier species or fall within known variation.

Either possibility adds clarity. The fossils point to greater diversity among the earliest archaic primates or confirm a wider geographic range than previously documented.

The change is small, measured in millimeters of enamel, but it sharpens both the timeline and the map of the oldest known archaic primate, and highlights how much of early primate history depends on what researchers are able to see.

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