The swamp dweller: researchers discover early mammal who walked among the dinosaurs

A study published in the the journal PLOS ONE announced the discovery of a new species of prehistoric mammal named Heleocola piceanus. According to the study conducted by a team of paleontologists from the University of Colorado Boulder the ancient mammal lived approximately 70 to 75 million years ago during the Late Cretaceous period.

Heleocola piceanus is notable for its size. While most mammals of the era were small, about the size of mice or rats, this species was about the size of a modern muskrat. Lead author Jaelyn Eberle, curator of fossil vertebrates at the CU Boulder Museum of Natural History, explained, that while most mammals roaming around at that time were the size of a mouse or a rat, Heleocola had an estimated body mass similar to that of a modern muskrat. "They're not all tiny. There are some animals emerging from the Late Cretaceous that are larger than we expected 20 years ago," the curator said according to Nature.

The finding was reported by Science Daily, New York Post, and BBC News, among others.

The discovery was made in western Colorado, near the town of Rangely, where the University of Colorado team has been excavating for about 15 years. The team, led by paleontologists John Foster and ReBecca Hunt-Foster, has been digging just outside of Rangely, uncovering fossils from the Late Cretaceous period. In 2016, John Foster first noticed a piece of mammal jaw hidden beneath a slab of sandstone from the site. He exclaimed, "Holy cow, that’s huge," upon realizing the significance of the find, according to Science Daily.

Heleocola piceanus lived during a time when large portions of North America were covered by the Western Interior Seaway, a vast inland sea that split the continent into two landmasses: Laramidia to the west and Appalachia to the east. The region where Heleocola lived was near the seaway, likely comprising deltas, marshes, and swampy coastal plains. Parts of western Colorado were likely swampy during the Late Cretaceous period.

The fossil assemblage from the site includes a mixture of land-dwelling, freshwater, estuarine, and marine animals. "In the same rocks where Heleocola was found, we also find fossils of dinosaurs, turtles, crocodilians, and even fishes, including sharks and rays," added Eberle. This suggests that Heleocola piceanus lived in a diverse ecosystem, coexisting with a variety of fauna, including turtles, duck-billed dinosaurs, and giant crocodiles.

Heleocola piceanus is described as a member of metatherians, a group of mammals closely related to marsupials. The discovery is significant because mammals from this period are quite rare in Colorado. "Although Colorado is a good place to find fossils, mammals from this period are quite rare," explained Eberle.

The fossil of Heleocola piceanus was identified from a piece of jawbone and three molar teeth. The robust jaw suggests a dental system suitable for grinding and crushing, indicating that the animal was likely an omnivore, feeding on plants, insects, and small vertebrates. These dental characteristics are associated with a broader diet rich in vegetation.

"It's larger than the great majority of mammals that lived at this time, which to me is really exciting because I think it's showing us that mammals were much more diverse during the late Cretaceous than what we had initially anticipated," said Eberle.

The team emphasized the importance of continued excavation in the region. "We have scientists that come from all over the world specifically to study our fossils. We really are lucky," said Hunt-Foster.

She also highlighted the significance of the Mountain West for fossil enthusiasts and reminded visitors not to collect vertebrate fossils they find while hiking. Instead, she advised them to note the location, take a photo, and report the discovery to museum representatives or public land agencies.

This article was written in collaboration with generative AI company Alchemiq