Fossilised footprint tracks which have been dated to 58 million years ago may represent the earliest evidence of mammals gathering by the sea, researchers say.
The findings suggest mammals may have first used marine habitats at least 9.4 million years earlier than previously thought, in the late Paleocene (66-56 million years ago), rather than the Eocene (56-33.9 million years ago).
Researchers examined and photographed more than 1,000 metres of fossilised footprints recently discovered within the Hanna Formation in Wyoming, US, in an area dated back to 58 million years ago by plant and pollen fossils.
They identified various different tracks, including one set that showed relatively large, five-toed footprints, comparable to the foot size of a modern-day brown bear, while another showed medium-sized, four-toed footprints.
The authors suggest the five-toed prints were made by Coryphodon, a type of semiaquatic Pantodont, similar to a hippopotamus.
While the four-toed prints did not match skeletal evidence of mammals known from the late Paleocene, they show similarities with artiodactyls and tapiroids, which have yet to be shown to have existed in the Paleocene.
Geologist Anton Wroblewski, an adjunct associate professor at the University of Utah, and applied biodiversity scientist Bonnie Gulas-Wroblewski of the Texas A&M Natural Resources Institute, reported their findings in Scientific Reports.
Prof Wroblewski said: “Trace fossils like footprints record interactions between organisms and their environments, providing information that body fossils alone cannot.
“In this case, trace fossils show that large-bodied mammals were regularly using marine environments only eight million years after non-avian dinosaurs went extinct.
“Paleontologists have been working in this area for thirty years, but they’ve been looking for bones, leaf fossils and pollen, so they didn’t notice footprints or trackways.”
He first saw the tracks in September 2019.
Prof Wroblewski said: “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 realised they were part of a much larger, more extensive trackway.”