Today, the rocks of the Hanna Formation in south-central Wyoming are hundreds of miles from the nearest ocean. But about 58 million years ago, Wyoming was a beachfront property, with large, hippo-like mammals roaming the coastal lagoons.
In a study published in Scientific Reports, geologist Anton Wroblewski, assistant associate professor in the Department of Geology and Geophysics, and Bonnie Gulas-Wroblewski, applied biodiversity researcher at the Texas A&M Natural Resources Institute, report the discovery of several sets of fossilized tracks probably the size of a brown bear Coryphodon, which represent the first known evidence of a gathering of mammals near an ocean.
“Traces of fossils like footprints record interactions between organisms and their environment, providing information that bodily fossils alone cannot,” says Wroblewski. “In this case, traces of fossils show that large mammals regularly used marine environments only eight million years after the extinction of non-avian dinosaurs.”
The tracks that Drs. The Wroblewski found in the Hanna Formation in Wyoming include subprints, impressions in soft sediment made when heavy animals walk on overlying layers of sediment, as well as indentations pressed into the surfaces of ancient mud flats. Now preserved in sandstone, the tracks are over a kilometer long and were made by two different animals, one with four toes and the other with five. Five finger tracks are consistent with Coryphodon, a semi-aquatic mammal similar to a hippopotamus. The owner of the four-toed caterpillars remains a mystery.
“Paleontologists have been working in this field for thirty years, but they look for bones, leaf fossils and pollen, so they haven’t noticed any footprints or traces,” Wroblewski says. He first saw the tracks in September 2019. “When I found them it was late afternoon and the setting sun hit them just at the right angle to make them visible on the sandstone slabs. inclined. At first I couldn’t believe what I was seeing; I had walked this outcrop for years without noticing them. Once I saw the first ones I followed the sandstone ridge and realized they were part of a much bigger and more extensive trail. “
Fossilized plants and pollen helped researchers determine the age of the traces at around 58 million years old, during the Paleocene era. Prior to this discovery, the first known evidence of mammalian interaction with marine environments came from the Eocene epoch, about 9.4 million years later. Wroblewski says the traces of the Hanna Formation are the earliest traces of Paleocene mammals found in the United States and only the fourth in the world, with two sets of traces previously found in Canada and one in Svalbard, Norway. It is also the largest accumulation of traces of Paleocene mammals in the world both in aerial extent and in absolute number of traces, he says. With at least two species leaving the trails, it is also the most taxonomically diverse.
Large mammals today congregate near marine environments for a variety of reasons, including protection from predators and biting insects, finding unique foods, and accessing sources of salt, which may have been limited. in the rainforests of North America in the Paleocene. Researchers say ancient mammals may have had similar reasons for seeking a beach day.
Research shows, says Wroblewski, that behavioral and evolutionary hypotheses based on isotopic, molecular and bodily fossil data can be tested empirically using trace fossils. “No other source of evidence directly records the behaviors of extinct organisms preserved in their preferred habitats,” he says. “There is still a lot of important information in the rocks, waiting for someone to spot it when the lighting is perfect!”
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