Most people don’t think of Wyoming as oceanfront property. But go back 58 million years and the region was actually seaside-adjacent, complete with large hippo-like mammals traipsing through nearshore lagoons. And a set of tracks from those creatures—discovered recently in the Hanna Formation of Wyoming by U geologist Anton Wroblewski—are the earliest known evidence of mammals interacting with marine environments.
“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 says. He first saw the tracks in September 2019. “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.”
Now preserved in sandstone, the tracks go for more than half a mile and were made by two different animals, one with four toes and one with five. The five-toed tracks are consistent with Coryphodon, a semi-aquatic mammal similar to a hippopotamus but the size of a brown bear. The owner of the four-toed tracks remains a mystery.
Fossilized plants and pollen helped the researchers determine the tracks to be around 58 million years old, from 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. Wroblewski says that the Hanna Formation tracks are the first Paleocene mammal tracks found in the U.S. and only the fourth in the world.
Image above: The brown-bear-sized mammals called Coryphodon left thousands of tracks 58 million years ago in what is now southern Wyoming.