Wednesday, March 11, 2026

Miniscule fossil discovery reveals fresh clues into the evolution of the earliest-known relative of all primates

Purgatorius had previously only been found in the upper regions of North America, this discovery, 500 miles south, suggests they diversified soon after the mass extinction at the end of the Cretaceous

Taylor & Francis Group

image:
Purgatorius upper molar from Corral Bluffs Denver Basin CO
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Credit: Dr Stephen Chester

New, miniscule fossils of the earliest-known relative of all primates, including humans, Purgatorius, have been unearthed in a more southern region of North America than ever before – and the breakthrough is providing paleontologists fresh clues about evolution.

The origin and early biogeographic history of primates is a fascinating, albeit controversial topic. The oldest archaic primate, Purgatorius, is a small, shrew-sized mammal that first appears in North America immediately after the extinction of the dinosaurs around 65.9 million years ago.

While fossil bearing rock of the right age exists throughout North America, to-date this mammal had previously only been found in present day Montana and southwestern Canada.
The next set of archaic primates include a diversity of relatives in southwestern USA, but these date to some two million years later – which has left somewhat of a puzzle… until now.

As findings, published today in the peer-reviewed Journal of Vertebrate Paleontology, report the southernmost discovery of Purgatorius fossils ever unearthed – uncovered in Colorado’s Denver Basin, at the Corral Bluffs study area.

“The discovery helps fill the gap in understanding the geography and evolution of our earliest primate relatives,” explains lead author Dr. Stephen Chester, associate professor at Brooklyn College and The Graduate Center, City University of New York (CUNY), who led the study alongside colleagues from the Denver Museum of Nature & Science (DMNS).

“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,” adds Dr. Chester.

“Ankle bones of Purgatorius exhibit features that indicate it lived in trees, so we initially thought its absence south of Montana could be related to the sweeping devastation of forests from the asteroid impact 66 million years ago.

“However, our paleobotanical colleagues suggested the recovery of plants in North America was fast leading us to believe that Purgatorius should also be in more southern regions and perhaps we simply hadn’t looked hard enough.”

To enable this deeper dive, Dr. Chester and colleagues from DMNS, deployed a careful, but extensive screen-washing technique. It was used, thanks, in part, to the support of a nearly $3 million collaborative grant from the National Science Foundation, which has funded a wider project – led by Dr Tyler Lyson at DMNS – to understand how life on Earth recovered following the mass extinction, best known for the demise of the dinosaurs.

The extensive screen-washing of sediments and picking was carried out by students and volunteers. It resulted in countless fossils of fish, crocodilians, turtles, and… eventually, a few tiny Purgatorius teeth that would fit on the tip of a baby’s finger.

What is particularly “exciting” about these teeth, explains Dr. Jordan Crowell, a postdoctoral fellow at the DMNS who also played a key role in the study, is that they could in fact belong to an earlier species of Purgatorius.

“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 adds.

These tiny teeth also demonstrate that the previously presumed absence of early primate relatives in more southern states of the Western Interior of North America was at least partly due to a sampling bias. Paleontologists have been finding fossils from this region and time interval using traditional surface collecting techniques for nearly 150 years, which mostly results in the collection of large fossils that are apparent to the naked eye.

“Thanks to our long-term partnership with the City of Colorado Springs who own the land where the fossils were collected, as well as countless hours of work by our volunteers and interns picking through the dirt for the precious vertebrate fossils, we are building some incredible datasets that provide insights on how life including our earliest primitive primate ancestors, rebounded after the single worst day for life on Earth,” adds co-author Dr Lyson.

“Our results demonstrate that small fossils can easily be missed,” concludes Dr. Chester. “With more intensive searching, especially using screen-washing techniques, we will undoubtedly discover many more important specimens.”

The paper also includes co-author Dr. David Krause, Senior Curator of Vertebrate Paleontology at the DMNS.

Journal

Journal of Vertebrate Paleontology

DOI

10.1080/02724634.2026.2614024

Method of Research

Observational study

Subject of Research

Animal tissue samples

Article Title

Southernmost occurrence of Purgatorius sheds light on the biogeographic history and diversification of the earliest primate relatives

Article Publication Date

3-Mar-2026

Purgatorius original by Andrey Atuchin

Brooklyn College Undergraduate Research Assistants

Credit

Stephen Cheste

DMNS interns and volunteers

Credit

Dr Tyler Lyson

“Peculiar” ancient ancestor of the crocodile started life on four legs in adolescence before it began walking on two

Newly discovered Late Triassic reptile was among creatures that had physical features mimicking the late-evolving dinosaurs it lived beside

Taylor & Francis Group

Sonselasuchus cedrus in its environment (now known as Petrified Forest National Park) — image:
Artist's reconstruction of Sonselasuchus cedrus in its environment in what is now Petrified Forest National Park, 215 million years ago.
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Credit: Artwork by Gabriel Ugueto

A “peculiar” ancient relative of the crocodile which experts believe began life on four legs before, in adulthood, it learnt how to walk on just two has been revealed in a new study.

Named Sonselasuchus cedrus, this archaic reptile was part of the shuvosaurid group, most of which had an appearance mimicking that of the ornithomimid dinosaurs that it shared the landscape with during Late Triassic time (approximately 225-201 million years ago).

In peer-reviewed findings, published today in the Journal of Vertebrate Paleontology, experts from University of Washington Department of Biology and Burke Museum reveal that unusual proportions of some of the fossils led them to believe that this poodle-sized creature had to learn how to walk on two feet.

“By analyzing the proportions of the limb skeletons of different animals, they determined its bipedal stance (standing on two feet) may have been the result of a differential growth pattern,” explains lead author Elliott Armour Smith.

“We think that Sonselasuchus had more proportional forelimbs and hindlimbs as young, and their hindlimb grew longer and more robust through adulthood.

“Essentially, we think these creatures started out their lives on four legs… they then started walking on two legs as they grew up.

“This is particularly peculiar.”

Armour Smith, a graduate student, carried out the study alongside Burke Museum colleague Professor Christian Sidor.

Professor Sidor was among the dig team that unearthed the 950 Sonselasuchus fossils, in 2014, from Arizona’s Petrified Forest National Park – an extraordinary fossil site which in 10 years of excavation and preparation has revealed more than 3,000 fossil bones.

Sonselasuchus’ fossils also reveal many clues about its appearance and 25-inch tall size. It had a toothless beak, a large eye socket, hollow bones, the experts believe.

“Although similar to the ornithomimid dinosaurs these features would have evolved separately,” explains Armour Smith, “and this similarity was probably due to the fact that croc-line and bird-line archosaurs evolved in the same ecosystems and converged upon similar ecological roles.

“Also, despite the fact that features like bipedalism, a toothless beak, hollow bones and a large orbit are characteristic of ornithomimid theropod dinosaurs, shuvosaurids like Sonselasuchus show that these features evolved on the croc-line as well.”

Sonselasuchus would have lived in the forest, and its name cedrus represents the cedar tree, an evergreen conifer similar to those of Late Triassic forests.

The Sonselasuchus part of the name (pronounced “sawn-SAY-la-SOOK-us”) is in recognition of the geologic unit (the Sonsela Member of the Upper Triassic Chinle Formation) from which the animal originates.

This bedrock has presented many finds to-date.

For Professor Sidor, this project is a culmination of over a decade of fieldwork in collaboration with the National Park Service.

“Since starting fieldwork at Petrified Forest in 2014, we have collected over 3,000 fossils from the Sonselasuchus bonebed, and it doesn’t seem to show any signs of petering out,” Professor Sidor states.

“In addition to Sonselasuchus, the bonebed has yielded fossils of fish, amphibians, as well as dinosaurs and other reptiles. Over 30 University of Washington students and volunteers have been involved over the years. It’s exciting to see that the site continues to produce new and interesting fossils.”