72-million-year-old fossil found near Utah leads to discovery of muskrat-sized prehistoric mammal

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VERNAL — A previously unknown muskrat-sized mammal species that once roamed this area when the Intermountain West looked more like a swampland millions of years ago has been discovered through a serendipitous fossil discovery near the Utah-Colorado border.

Paleontologists from the University of Colorado, Utah Division of State Parks, Dinosaur National Monument and two other institutions say the mammal, which they've dubbed "Heleocola piceanus," likely lived in the forests of the river delta that existed in northeast Utah and northwest Colorado between 75 million and 70 million years ago.

"Mammals from this time period tend to be pretty rare, so it's really neat to see this slice of time preserved in Colorado," said Jaelyn Eberle, a professor of geological sciences at the University of Colorado, in a statement.

The team's findings, published Wednesday in the journal Plos One, come from the analysis of fossilized teeth and a jaw that was unwittingly collected in 2016 from a sandstone block near Rangely, Colorado, close to Dinosaur National Park and a few miles east of the Utah-Colorado border.

While turtle shell fragments were visible in the block, the new mammal discovery wasn't. It wasn't discovered until a Museums of Western Colorado volunteer noticed a peculiar set of fossils as he prepared the rock in 2018.

A team of researchers, led by Eberle, began studying what was determined to be a jaw fragment with three molars. They were curious because the teeth were almost five times the size of mammal teeth typically found in similar sandstone blocks in an era when most mammals were relatively small, like a chipmunk.

"The jaw, the teeth, everything was so much bigger than most mammal fossils you see out of the Cretaceous rocks around there. I thought, 'That's huge,'" said John Foster, curator of collections at the Utah Field House of Natural History State Park Museum, and the study's co-author.

This sparked a more extensive review of the fossil, which was determined to be 72 million years old.

Eberle had identified previous prehistoric mammals through teeth, which are sometimes all that's left from mammals during this era. The team determined that they had stumbled across a species that hadn't been recorded before. They believe it weighed at least 2 pounds and was a cousin to today's marsupials.

Researchers settled on the name "Heleocola," or "swamp dweller," because it likely lived by rivers and swampy locations scattered across the Western Interior Seaway and areas east of it, back when the region looked more like the modern-day Mississippi River delta in Louisiana.

The finding helps researchers better understand the ecosystem that existed at the time. Foster's previous work includes all sorts of prehistoric fish, dinosaur, reptile and mammal species that would have lived in the region around the same time as Heleocola.

"This delta seems to have been packed with a wide variety of animals on land and in the water," he said.

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Intermountain WestUtahOutdoorsScienceHistoric

Carter Williams

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Carter Williams is an award-winning reporter for KSL.com. He covers Salt Lake City news, as well as statewide transportation issues, outdoors, environment and weather. Carter has worked in Utah news for over a decade and is a graduate of Southern Utah University.