In the northwestern corner of Colorado, near the small town of Rangely, a remarkable fossil has emerged from the sandstone—a relic of a mammal that once scurried through a prehistoric swamp during the Age of Dinosaurs. This tiny marvel, named Heleocola piceanus (which fittingly means “swamp dweller”), lived about 70 to 75 million years ago. A team led by Jaelyn Eberle from the University of Colorado Boulder identified the creature from its jawbone and three distinctive molar teeth, published on October 23 in PLOS ONE.

Colorado's Rare Fossil Find

While the towering dinosaurs of the Cretaceous tend to steal the spotlight, the discovery of H. piceanus sheds light on the less famous, but equally fascinating, ancient mammals that shared this world. Colorado is known for its fossil wealth, yet mammals from this era are exceptionally rare. As Eberle, a paleontologist and curator at CU Museum of Natural History, explained, “It’s really neat to see this slice of time preserved in Colorado.” Compared to the giants that roamed at the time—like tyrannosaurs and horned dinosaurs—this little mammal may seem insignificant, but it was a giant among its kind.

A Window to an Ancient Swamp

Picture a Colorado that looked more like the bayous of Louisiana than the mountainous landscapes we see today. The area was a patchwork of marshes and estuaries where land met water, providing an ideal habitat for an array of creatures. Giant crocodiles lurked in the swamps, turtles basked in the sun, and duck-billed dinosaurs wandered the wetlands. Even sharks and rays swam in the waterways. “The region might have looked kind of like Louisiana,” said ReBecca Hunt-Foster, a paleontologist based at Dinosaur National Monument.

For over 15 years, Hunt-Foster and her colleague John Foster have been searching this region for fossils, but it was in 2016 that a truly unique find emerged. The bit of jawbone they discovered seemed enormous for a mammal of the Late Cretaceous, which were typically the size of modern-day mice or rats. Foster recalls being struck by the find, saying, “I said, ‘Holy cow, that’s huge.’”

A Giant Among Ancient Mammals

Although small by today’s standards, H. piceanus was larger than most of its contemporaries, weighing around 2 pounds—about the size of a muskrat. In the era before an asteroid wiped out the non-avian dinosaurs 66 million years ago, mammals were usually small, and their fossils often consist of little more than tiny teeth. H. piceanus, in comparison, was a giant. The fossil’s teeth suggest it likely enjoyed a mixed diet of plants, insects, and perhaps other small creatures. This discovery hints that some mammals in the Late Cretaceous were larger and more diverse than previously thought.

A Treasure Trove for Paleontologists

The discovery of H. piceanus adds to the growing list of significant finds from the Mountain West, a region celebrated by scientists worldwide for its fossil wealth. “We have scientists that come from all over the world specifically to study our fossils,” Hunt-Foster noted, emphasizing the importance of protecting these ancient treasures. She advises that anyone who encounters a fossil while hiking should refrain from collecting it, as these remains hold vital scientific information. Instead, she recommends documenting the location, snapping a photo, and reporting the find to a local museum or land agency.

The Significance of Small Discoveries

While it’s easy to be dazzled by the colossal remains of dinosaurs, the discovery of H. piceanus reminds us that even the smallest creatures played crucial roles in their ecosystems. “They’re not all tiny,” Eberle remarked, reflecting on how recent findings are challenging the old image of small, unimpressive mammals scurrying around dinosaur feet. As paleontologists continue to explore the fossil-rich lands of western Colorado, more surprises are likely hiding just beneath the surface.

The discovery of this ancient “swamp dweller” not only enriches our understanding of Colorado’s prehistoric past but also adds another chapter to the story of life before the dinosaurs met their untimely end.