A Newly Discovered Prehistoric Mammal That Lived Among Dinosaurs May Have Been A Swamp-Dweller

Discovered in Colorado, a new prehistoric mammal species that lived during the age of the dinosaurs may have been a swamp-dweller.

A team of paleontologists found the fossils near Rangely, Colorado, in the northwest of the state, and dated them to about 70 million to 75 million years ago during the Late Cretaceous period.

The team identified the new species from the remains of a jawbone and three molars. It has been named Heleocola piceanus.

“Heleocola, which was an ancient opossum-like animal and a distant cousin to today’s marsupials, is important, in part, because the time slice it comes from is not well documented in North America—and we just don’t know a lot about mammals from this time,” said Jaelyn Eberle, a lead author of the study from the University of Colorado Boulder.

The discovery helps piece together what Colorado’s ancient landscape looked like. At the time the creature was alive, there was a vast inland sea called the Western Interior Seaway, and it divided North America into two masses of land—Laramidia to the west and Appalachia to the east.

The seaway stretched from the Arctic Ocean to the Gulf of Mexico. A large portion of eastern Colorado was submerged underwater, while the western part stayed above sea level.

During the Cretaceous period, some areas of western Colorado were likely swampy and would’ve included deltas, marshes, and coastal plains.

The region where H. piceanus was found was one of the areas where land met water 70 million years ago or so.

It would’ve been home to creatures such as turtles, duck-billed dinosaurs, and giant crocodiles that lived in and around wetlands and estuaries.

Olha Rohulya – stock.adobe.com – illustrative purposes only

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

By today’s standards, H. piceanus is relatively small. But back then, it was large for mammals of that era.

Before non-avian dinosaurs went extinct about 66 million years ago, mammals were roughly the size of modern mice or rats.

However, the researchers estimated that H. piceanus weighed two pounds or more with a body mass similar to today’s muskrat.

Its teeth suggest that it ate a diet of mostly plants but would sometimes consume insects or other small animals.

While dinosaurs receive most of the attention, this discovery has shown that ancient mammals are important for understanding the ecosystems of the Late Cretaceous.

“They’re not all tiny,” said Eberle. “There are a few animals emerging from the Late Cretaceous that are bigger than what we anticipated 20 years ago.”

The study was published in the journal PLOS One.

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