In 2016, John Foster was walking around the sandstone countryside that defines northwest Colorado when he stopped at something peculiar ingrained in the rock.

It was a fossil — a jaw about an inch long, Foster recalled in a recent news release.

“Holy cow,” he thought, “that’s huge.”

Not so huge to an untrained eye, but certainly so to an eye familiar with mammal fossils from the Late Cretaceous era.

Foster is a paleontologist based at the Utah Field House of Natural History State Park Museum, across the Colorado state line. The region is home to Dinosaur National Monument, which displays fossils much bigger than that inch-long jaw — remains of apex beasts of the day.

But for Foster and fellow researchers, the discovery is no small matter.

They have described a previously unknown mammal in findings published in the journal PLOS ONE. They are calling the species Heleocola piceanus, roughly translating to “swamp dweller.”

Resembling a muskrat, the animal about 70 million years ago skittered around this part of Colorado that was largely covered by an inland sea, like much of the West.

“The region might have looked kind of like Louisiana,” ReBecca Hunt-Foster, a paleontologist at Dinosaur National Monument and co-author of the research, said in a news release. “We see a lot of animals that were living in the water quite happily like sharks, rays and guitarfish.”

Now we can picture Heleocola piceanus.

From that piece of jawbone and three molar teeth, scientists suspect the creature might have weighed more than 2 pounds. That’s not the size of the previously uncovered, badger-looking Didelphodon, thought to weigh up to 11 pounds, as CU Boulder Today notes. But Heleocola piceanus outweighs most Late Cretaceous mammals on record, many of them the size of mice.

“They’re not all tiny,” said University of Colorado Boulder’s Jaelyn Eberle, the lead author on the discovery paper. “There are a few animals emerging from the Late Cretaceous that are bigger than what we anticipated 20 years ago.”

For about 15 years now, some from the Heleocola piceanus research team have been digging for fossils across the northwest Colorado rock known as the Williams Fork Formation. The PLOS ONE paper notes several fish, salamanders, frogs, lizards and turtles found over the years.

“Fossil mammals from the Williams Fork Formation are rare and almost entirely represented by isolated teeth,” the paper reads. “Our report is the first jaw fragment of a therian (mammal group) from this rock unit.”

The teeth of Heleocola piceanus suggest a diet of plants and bugs. “Its larger size implies a greater need for highly nutritious foods,” the paper adds, noting the possibility of small vertebrates, roots, fruits and nuts.

The possibilities for discovery are abundant in Colorado — “a great place to find fossils,” Eberle said, “but mammals from this time period tend to be pretty rare. So it’s really neat to see this slice of time preserved in Colorado.”