Paleontologists have found a fossilized jaw fragment and three isolated teeth from a new, relatively large (by Late Cretaceous standards) metatherian species in the layers of the Williams Fork Formation in northwestern Colorado, the United States.
An artist’s depiction of Heleocola piceanus in a Late Cretaceous swamp. Image credit: Brian Engh / LivingRelicProductions.com / Utah Field House of Natural History.
“The Metatheria (marsupials and their closest fossil relatives) comprise some 330 living species in 7 orders, the great majority of which inhabit the southern hemisphere,” said University of Colorado Boulder’s Professor Jaelyn Eberle and colleagues.
“However, the clade appears to have originated in the northern hemisphere during Early Cretaceous time.”
“By the end of the Cretaceous, metatherians had dispersed across Europe, Asia, and North America and were more diverse and abundant than their eutherian contemporaries.”
“Most Late Cretaceous metatherian species are represented almost exclusively by isolated teeth and jaws recovered from fossil localities in the U.S. Western Interior.”
The fossilized remains of the new metatherian species were discovered in the Williams Fork Formation on the Douglas Creek Arch, between the Uinta and Piceance Creek Basins in northwestern Colorado.
Named Heleocola piceanus, the animal lived roughly 70 to 75 million years ago (Late Cretaceous epoch) — a time when a vast inland sea covered large portions of the American West.
It weighed around 1 kg (2 pounds) and was larger than most Late Cretaceous mammals.
Based on its teeth, Heleocola piceanus likely dined on plants with a few insects or other small animals mixed in.
It co-existed with creatures like turtles, duck-billed dinosaurs and giant crocodiles.
“The region might have looked kind of like Louisiana,” said Dr. ReBecca Hunt-Foster, a paleontologist at Dinosaur National Monument.
“We see a lot of animals that were living in the water quite happily like sharks, rays and guitarfish.”
“Colorado is a great place to find fossils, but mammals from this time period tend to be pretty rare,” Professor Eberle said.
“So it’s really neat to see this slice of time preserved in Colorado.”
“Compared to much larger dinosaurs living at the time like tyrannosaurs or the horned ancestors of Triceratops, the new fossil addition to Colorado might seem tiny and insignificant. But it was surprisingly large for mammals at the time.”
The discovery is reported in a paper in the journal PLoS ONE.
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J. Eberle et al. 2024. A new Late Cretaceous metatherian from the Williams Fork Formation, Colorado. PLoS ONE 19 (10): e0310948; doi: 10.1371/journal.pone.0310948
