Science.- Fossil footprints of mammals on the seashore of 58 million years

05/13/2021 A reconstruction of the brown bear-sized mammals (Coryphodon) that left thousands of footprints in a 58-million-year-old brackish water lagoon in what is now southern Wyoming. RESEARCH AND TECHNOLOGY POLICY ANTON WROBLEWSKI


Several new sets of fossilized footprints, probably of the brown bear-sized ‘Coryphodon’, constitute the oldest known evidence of mammals gathered near an ocean.

Today, the rocks of the Hanna Formation in south-central Wyoming in the United States are hundreds of kilometers from the nearest ocean. But about 58 million years ago, Wyoming was on the ocean’s edge, with large hippo-like mammals roaming the lagoons near the coast.

In their study, published in Scientific Reports, geologist Anton Wroblewski, an associate professor in the Department of Geology and Geophysics, and applied biodiversity scientist Bonnie Gulas-Wroblewski of the Texas A&M Institute of Natural Resources, report the discovery of several sets of fossilized footprints, probably from Coryphodon, representing the first known evidence of mammals near an ocean.

“Trace fossils, like footprints, record interactions between organisms and their environments, providing information that body fossils alone cannot,” says Wroblewski. “In this case, footprint fossils show that mammals large-bodied dinosaurs regularly used marine environments only eight million years after non-avian dinosaurs became extinct. “

The tracks that Drs Wroblewski found in Wyoming’s Hanna Formation include subprints, impressions in soft sediment made when heavy animals walk over overlapping layers of sediment, as well as tracks pressed into the surfaces of old marshes.

The footprints, which are now preserved in the sandstone, are over a kilometer long and were made by two different animals, one with four toes and the other with five. The five-toed prints correspond to the ‘Coryphodon’, a semiaquatic mammal similar to the hippopotamus. The owner of the four-toed prints remains a mystery.

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“Paleontologists have been working in this area for thirty years, but they have been looking for bones, leaf fossils and pollen, so they did not notice the tracks or traces,” says Wroblewski.

The first time he saw the footprints was in September 2019. “When I found them, it was late afternoon and the setting sun was hitting them at just the right angle to make them visible on the sloping sandstone slabs. At first, I couldn’t believe what I was seeing, I had been through this outcrop for years without noticing them, “he recalls. Once I saw the first ones, I followed the sandstone ridge and realized that they were part of a much larger and more extensive footprint.”

The fossilized plants and pollen helped the researchers determine that the age of the tracks was about 58 million years, during the Paleocene epoch. Before this find, the first known evidence that mammals interacted with marine environments came from the Eocene epoch, some 9.4 million years later.

Wroblewski states that the Hanna Formation footprints are the earliest Paleocene mammalian footprints found in the United States and only the fourth in the world, with two sets of footprints previously found in Canada and one in Svalbard, Norway.

It is also the largest accumulation of Paleocene mammal footprints in the world, both in aerial extent and in absolute number of footprints, he says. With at least two species leaving tracks, it is also the most taxonomically diverse.

Today’s large mammals congregate near marine environments for a variety of reasons, such as protection from predators and biting insects, searching for unique foods, and access to sources of salt, which may have been limited in tropical forests. of North America during the Paleocene. The researchers say that ancient mammals might have similar reasons for looking for a day at the beach.

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The research shows, according to Wroblewski, that hypotheses about behavior and evolution based on isotopic, molecular and body fossil data can be empirically tested using trace fossils.

“No other line of evidence directly records the behavior of extinct organisms preserved in their preferred habitats,” he says. “There is still a lot of important information out there, on the rocks, waiting for someone to discover it when the lighting is right.”

Myrtle Frost

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