In 2003, Greg Wilson Mantilla, who at the time was a Doctor of Philosophy candidate at UC Berkeley, was examining fossils from excavations in Hell Creek, Montana, which had been directed by his advisor William Clemens. Initially labeled as mammal, the fossils had “slipped past careful study.”
“At that time, no one was studying them, so they were just put in a drawer and they weren’t identified,” Wilson Mantilla, now a professor in the department of biology and a curator of vertebrate paleontology at the Burke Museum at the University of Washington, said. “I recognized them as Purgatorius.”
That work — which resumed in 2016 and is a collaboration by the family of John and Cathy McKeever, on whose property the fossils were collected, the Berkeley Geochronology Center and the late Clemens — resulted in the identification of a new species, since named Purgatorius mckeeveri.
Their work also included the discovery of the fossils of the oldest recorded relatives of modern humans and resulted in new understandings of the origins of primates at the end of the Cretaceous Period.
Two species of Purgatorius were identified in fossils that were excavated at the site, according to Wilson Mantilla. One was identified as Purgatorius janisae, an existing species, while Purgatorius mckeeveri was identified as a new species due to its variation from previous fossils.
Hell Creek is one of the world’s best places for collecting fossils from the end of the Cretaceous Period, a time when a meteor strike and volcanic activity caused the mass extinction of dinosaurs, according to Wilson Mantilla and Ronald Mundil, a researcher at Berkeley Geochronology Center.
By studying radioisotopes found in volcanic ash surrounding the fossils, researchers at the Geochronology Center found that those fossils were from roughly 105,000 to 139,000 years after the end of the Cretaceous Period — 65.9 million years ago — suggesting that these fossils are the earliest known ancestors of modern primates, including humans.
“These are the oldest fossils of our lineage,” Wilson Mantilla said. “They go back to immediately after the mass extinction event. Those extend the range back 200,000 years further than they were before.”
The fossils themselves tell a story, according to Wilson Mantilla. Purgatorius’ body type and diet can be inferred, creating a better image of humanity’s tree-dwelling, mouse-sized ancestor.
The presence of the two species at that time paints an even larger picture, Wilson Mantilla added.
“The idea of two species implies that there was an ancestor to them because they’re in the same genus, and that ancestor has to have been older than those two different species,” Wilson Mantilla said.
This suggests that primates or their earlier ancestors lived alongside dinosaurs during the late Cretaceous Period and shows how mass extinction events affect ecology and evolution.