Ancient Swamp-Dwelling Mammal Discovery Surprises Scientists In Colorado

In northwest Colorado a new prehistoric mammal species, Heleocola piceanus has risen to the surface. Lurking in ancient swamps, paleontologists uncovered fossils and teeth of the ancient creature. This breakthrough pulls back the curtain on a little known part of North American history. Finally we can know what the everyday mammal was up to during the Late Cretaceous, a period often overshadowed by dinosaurs.

Missing Mammal Pieces

The fossilized remains of Heleocola piceanus were found near the small town of Rangely in Colorado. These remains—consisting of a piece of jawbone and three molar teeth—were dated to 70 to 75 million years ago. The species is described by researchers \ for the first time in the journal PLOS ONE, giving it a name that translates to “swamp dweller” in Latin. According to Jaelyn Eberle, lead author of the study, this discovery fills a gap in knowledge about mammals from that era in North America.

Life Along the Western Interior Seaway

During the Late Cretaceous, an inland sea known as the Western Interior Seaway split North America into two landmasses: Laramidia to the west and Appalachia to the east. The seaway stretched from the Arctic Ocean to the Gulf of Mexico, submerging much of eastern Colorado under shallow waters. Western Colorado, on the other hand, stayed above sea level as part of Laramidia. Made up of coastal plains, swamps, marshes, and river deltas, the land was rich in water.. These swampy regions along the seaway’s western edge provided the perfect habitat for Heleocola piceanus, Shrek and other creatures.

Teenage Years of North American Mammal Evolution

Mammals were typically small during this transitional era, living under the shadow of enormous dinosaurs. However, Heleocola piceanus weighed around 2 pounds making it notably larger than many mammals of its time. “Whereas most mammals scurrying around were mouse to rat-sized, Heleocola had an estimated body mass similar to today’s muskrat,” Eberle explained. Its relatively large size challenges previous assumptions about mammalian evolution during this period.

Fossils in the Williams Fork Formation

The fossils of Heleocola piceanus were unearthed from the Williams Fork Formation. Despite the name this geological area in western Colorado t’s not known for an outstanding collection of cutlery but rather its ancient river channels, swamps, and deltas. Eberle noted that the diverse fossil assemblage found there suggests a mix of land-dwelling, freshwater, and marine animals. This diversity indicates a thriving ecosystem likely due to the region shifting between dry land and swampy environments. Based on the 3 teeth and jawbone found, Heleocola piceanus was a herbivore. Fossil evidence also points to the mammal’s proximity to both land and water, making it well-suited for a semi-aquatic lifestyle.

What Lurked Beneath

Rebecca Hunt-Foster, a paleontologist involved in the study, compared the ancient Colorado landscape to modern-day Louisiana, with its swamps, rivers, and deltas. It’s the kind of wild, swampy world Heleocola piceanus called home. But the region also supported turtles, duck-billed dinosaurs, and giant crocodiles that could eat a car tire whole. Together these critters surfed through the same estuaries and coastal plains.

A Bombastic Baby Name

The name Heleocola piceanus is not something you should name an infant, but it’s surely one rich in meaning.  “Heleocola” hints at the creature’s swampy lifestyle, while “piceanus” nods to the fossil’s discovery in western Colorado. By naming the species after its swampy home, paleontologist Matt Eberle underscores its important role in this ancient ecosystem.

Mysterious Mammals of North America

“There are a few animals emerging from the Late Cretaceous that are bigger than what we anticipated 20 years ago,” Eberle said, reflecting on prehistoric mammal diversity. Which isn’t surprising since the Late Cretaceous period is not well-documented in North American mammalian history. Researchers believe that more discoveries like Heleocola piceanus could change how scientists understand the evolution of mammals during this era. 

Bigger Than Expected

Scientists have long believed that most mammals of the Late Cretaceous were no larger than today’s mice or rats. However, Heleocola piceanus demonstrates that bigger mammals existed much earlier than we initially thought. Mammals were beginning to diversify not just in size but also ecological roles well before the global wipeout of the dinosaurs.

Outside of The Ancient Ecosystem

The swamp-dwelling Heleocola piceanus from Colorado provides new insights into ancient ecosystems along the Western Interior Seaway. Before this I had no idea about the mammals maximizing their body weight during the Late Cretaceous. While researchers continue to uncover new fossils, I hope these discoveries will help reshape our understanding of the past and highlight the diversity of life that existed millions of years ago. With each find, paleontology brings us closer to uncovering the full story of Earth’s prehistoric history.