In 2016, John Foster was walking through the sandstone countryside that characterizes northwestern Colorado when he noticed something strange etched into the rocks.
It was a fossil — a jaw about an inch long, Foster recalled in a recent news release.
“What a big cow,” he thought.
Not huge to the untrained eye, but certainly huge to eyes accustomed to Late Cretaceous mammal fossils.
Foster is a paleontologist based across the Colorado border at the Utah Natural History Fieldhouse State Park Museum. The area is home to Dinosaur National Monument, which features fossils much larger than its inch-long jaws, the apex beasts of its time.
But for Foster and his colleagues, this finding is no small matter.
They described the previously unknown mammal in their findings published in the journal PLOS ONE. They call this species Heleochora piceanus, which roughly translates to “swamp dweller.”
About 70 million years ago, the muskrat-like animals roamed this part of Colorado, which, like much of the West, was covered by an inland sea.
Get the latest local updates on arts, entertainment and life in Colorado Springs every week. Delivered to your inbox every Thursday.
success! Thank you for subscribing to our newsletter.
“This region may have resembled Louisiana,” Rebecca Hunt-Foster, a paleontologist at Dinosaur National Monument and co-author of the study, said in a news release. “We see a lot of animals like sharks, rays and guitarfish that are very happy in the water.”
Now we can picture Heleochora piceanus.
Based on part of its jawbone and three molar teeth, scientists estimate that the creature may have weighed more than 2 pounds. As CU Boulder Today points out, this is not the size of the badger-like didelphodons ever found and thought to weigh up to 11 pounds. But Heleochora piceanus outnumbered most recorded Late Cretaceous mammals, many of which were about the size of a mouse.
“They’re not all small,” says Jaylin Eberle of the University of Colorado Boulder, lead author of the discovery paper. “Some of the animals that emerged in the Late Cretaceous are larger than we expected 20 years ago.”
For about 15 years, part of the Heleochora piceanus research team has been excavating fossils across rocks in northwestern Colorado known as the Williams Fork Formation. The PLOS ONE paper describes several fish, salamanders, frogs, lizards, and turtles discovered over the years.
“Mammalian fossils from the Williams Fork Formation are rare and represented almost entirely by isolated teeth,” the paper says. “Our report is the first jaw fragment of a therian (mammal group) from this rock unit.”
The teeth of Heleochora piceanus suggest that it ate plants and insects. The paper added that “its large size means it has an increased need for nutritious foods,” including small vertebrates, roots, fruits and nuts. It points out that there is a possibility.
Colorado has a wealth of discovery potential and is “a great place to find fossils,” Eberle said. “But mammals from this era tend to be quite rare, so it’s really amazing to see fragments from this era preserved in Colorado.”
