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

(Press-News.org) 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.

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