The recently discovered mammal lived in Colorado around 70 to 75 million years ago, during a time when much of the state was covered by a huge inland sea teeming with sharks, turtles, and massive crocodiles.
A team of paleontologists near Rangely, Colorado, has found a remarkable ancient resident: a fossil mammal about the size of a muskrat that likely roamed through swamps during the Age of Dinosaurs.
The researchers, led by Jaelyn Eberle from the University of Colorado Boulder, published their findings on October 23 in the journal PLOS ONE.
Eberle and her team named the new species Heleocola piceanus, identified from a jawbone fragment and three molar teeth. This creature lived in Colorado approximately 70 to 75 million years ago, a period when a massive inland sea spanned much of the American West. Interestingly, “Heleocola” translates to “swamp dweller” in Latin.
“Colorado is known for its impressive fossil finds, but mammals from this era are quite rare,” said Eberle, who is both a curator of fossil vertebrates at the CU Museum of Natural History and a professor in the Department of Geological Sciences. “It’s fascinating to have a glimpse of this era preserved in Colorado.”
In contrast to the much larger dinosaurs like tyrannosaurs or the horned ancestors of Triceratops, the newly discovered fossil may seem insignificant. However, Eberle pointed out that it was surprisingly large for mammals at that time.
She expressed excitement over Rangely, positioned in the northwestern part of the state near Dinosaur National Monument, finally getting recognition.
“It may be a small town, but from my experience as a paleontologist, many exciting discoveries come from rural areas,” Eberle remarked. “It’s wonderful to see western Colorado getting attention for a remarkable find.”
Where Land Meets Water
This fascinating discovery helps to enhance our understanding of a vastly different Colorado that would be unrecognizable to today’s residents.
Paleontologists John Foster and ReBecca Hunt-Foster, co-authors of the study, have been digging for fossils in this region every summer for the last 15 years. Seventy million years ago, this area featured a landscape where land met water. Creatures such as turtles, duck-billed dinosaurs, and enormous crocodiles likely thrived in and around marshes and estuaries, feasting on wetland plants, fish, and more.
“The area might have resembled modern-day Louisiana,” ReBecca Hunt-Foster noted, a paleontologist at Dinosaur National Monument in Utah and western Colorado. “We find many animals that thrived in water, like sharks, rays, and guitarfish.”
John Foster recalls first encountering the small piece of mammal jawbone from a chunk of sandstone collected in 2016. The fossil was approximately an inch long.
“I was shocked; that’s impressive,” Foster, a scientist at the Utah Field House of Natural History State Park Museum in Vernal, Utah, remarked.
A Notable Mammal
Eberle explained that prior to the asteroid impact that wiped out non-avian dinosaurs 66 million years ago, mammals were generally small, many comparable in size to modern mice or rats. Eberle typically identifies them by their small teeth.
In comparison, H. piceanus was quite large for its time. Eberle estimates that this animal, a relative of modern marsupials, weighed around 2 pounds or more, making it larger than most Late Cretaceous mammals. While not a record-holder—another mammal from that era, known as Didelphodon, could weigh up to 11 pounds—H. piceanus likely thrived by eating plants along with some insects or small animals.
While dinosaurs often steal the spotlight, this new discovery highlights why ancient mammals should not be overlooked by paleontologists. Regardless of their size, they played a vital role in the ecosystems of Late Cretaceous Colorado.
“Not all of them are tiny,” Eberle noted. “There are more animals emerging from the Late Cretaceous that are larger than what we expected two decades ago.”
Hunt-Foster emphasized that the Mountain West is a unique location for fossil enthusiasts. She also encouraged visitors to public lands to refrain from collecting vertebrate fossils, such as those of dinosaurs, to avoid disrupting valuable scientific evidence. Instead, they should record the location, capture a photo, and report it to a local museum or land agency representative.
“We have scientists coming from all over the globe specifically to study our fossil findings,” she said. “We are truly fortunate.”
