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Tracks show ancient hippos wandered the beaches — yes beaches — of Wyoming

University of Utah discovery is first of its kind in USA

A reconstruction of the brown bear-sized mammals Coryphodon that made thousands of tracks in a 58-million-year-old, brackish water lagoon in what is now southern Wyoming.
Anton Wroblewski

University of Utah researchers from Salt Lake City working in south central Wyoming were as startled as anyone when light from the sun illuminated fossilized tracks from a large hippo-style animal that wandered the Earth 58 millions of years ago when that state included beachfront property.

The discovery of several sets of the tracks, likely from a brown bear-sized Coryphodon, represent the earliest known evidence of mammals gathering near an ocean.

University of Utah geologist Anton Wroblewski, an adjunct associate professor in the Department of Geology and Geophysics, and applied biodiversity scientist Bonnie Gulas-Wroblewski, of the Texas A&M Natural Resources Institute, detailed their findings in a study published in Scientific Reports Thursday.

Anton Wroblewski points to an underprint made 58 million years ago by a heavy mammal (likely Coryphodon) walking on the deltaic deposits above. Underprints form when sediment is displaced downward by footsteps from heavy animals.
Anton Wroblewski

“Trace fossils like footprints record interactions between organisms and their environments, providing information that body fossils alone cannot,” Wroblewski said. “In this case, trace fossils show that large-bodied mammals were regularly using marine environments only 8 million years after non-avian dinosaurs went extinct.”

The tracks they found in the Hanna Formation of Wyoming are preserved in sandstone and cover more than a half mile. They were made by two different mammals, with the five-toed tracks consistent with Coryphodon, a semiaquatic mammal similar to a hippopotamus. The owner of the four-toed tracks remains a mystery.

Wroblewski said that the Hanna Formation tracks are the first Paleocene mammal tracks found in the United States and only the fourth in the world, with two sets of tracks previously found in Canada and one in Svalbard, Norway. It’s also the largest accumulation of Paleocene mammal tracks in the world in both aerial extent and the absolute number of tracks, he added. With at least two species leaving the tracks, it’s also the most taxonomically diverse.

What makes this find especially notable is how the tracks were discovered, with the researchers getting an assist from the sun.

“Paleontologists have been working in this area for 30 years, but they’ve been looking for bones, leaf fossils and pollen, so they didn’t notice footprints or trackways,” Wroblewski said.

He said he first saw the tracks in September 2019 by accident.

Section of the 58-million-year-old tracksite demonstrating near-vertical tilting of the originally horizontal bedding with three separate trackways made by five-toed mammals walking in parallel.
Anton Wroblewski

“When I found them, it was late afternoon and the setting sun hit them at just the right angle to make them visible on the tilted slabs of sandstone. At first, I couldn’t believe what I was seeing; I had walked by this outcrop for years without noticing them. Once I saw the first few, I followed out the ridge of sandstone and realized they were part of a much larger, more extensive trackway.”

Fossilized plants and pollen helped the researchers determine the age of the tracks to be around 58 million years old, during the Paleocene epoch. Before this finding, the earliest known evidence of mammals interacting with marine environments came from the Eocene epoch, around 9.4 million years later.

Modern-day mammals hang out in marine environments to escape the heat and biting insects, and to forage for food and find protection from predators.

Researchers believe ancient mammals had a similar motivation to spend a day at the beach.

The research shows, Wroblewski said, that theories about ancient mammal behavior can be tested using these sort of trace fossils.

“No other line of evidence directly records behaviors of extinct organisms preserved in their preferred habitats,” he said. “There’s still a lot of important information out there in the rocks, waiting for somebody to spot it when the lighting is just right.”

Utah researchers have an accomplished track record in the field of discovering ancient creatures, and the Grand Staircase-Escalante National Monument has literally been described as a treasure trove of fossilized remains.

The Natural History Museum of Utah includes an impressive display of dinosaurs and in its lab, critical research goes on the field of paleontology.

This latest find is just another example of peeling back the layers of the world mankind inhabits.