A team of scientists has discovered in the rocks of the Hanna Formation in south-central Wyoming, in the United States – which 58 million years ago was on the edge of the ocean – the oldest known evidence of mammals gathered near the sea : a collection of thousands of footprints on the shore in what is now southern Wyoming.
Traces left by two species of mammals, one identified as Coryphodon and the other unknown, were made 58 million years ago, according to an analysis by US experts. The Coryphodon was a primitive creature with hooves and four legs the size of a brown bear, one of the largest known mammals of its time. It was a heavy, slow-moving animal with a strong constitution.
“Tracking fossils, like footprints, record interactions between organisms and their environments, providing information that body fossils alone cannot,” says Anton Wroblewski, associate associate professor in the Department of Geology and Geophysics and co-author. of the work published in the journal Scientific Reports. “In this case, fossil tracks show that large mammals regularly used marine environments only eight million years after non-avian dinosaurs became extinct.”
Among the examined and fossilized samples are footprints, impressions in soft sediments made when heavy animals walk on overlying layers of sediment, as well as footprints pressed into the surfaces of ancient tidal flats. Preserved today in sandstone, they are 1,032 meters long and were made by a minimum of two species of mammals: an animal with four fingers and another with five (the Coryphodon ).
This finding suggests that mammals would have used marine habitats for the first time at least 9.4 million years earlier than previously thought, in the late Paleocene era (66-56 million years ago), rather than the era of the Eocene (56-33.9 million years ago).
If large mammals now congregate near water to protect themselves from predators and forage for food, it is likely that these extinct mammals had the same reasons for approaching the seashore.
“Paleontologists have been working in this area for thirty years, but they have been looking for bones, leaf fossils, and pollen, so they didn’t notice the footprints,” says Wroblewski. “When I found them, it was late in the afternoon and the setting sun hit them at the right angle to make them visible on the sloping sandstone slabs. At first, he couldn’t believe what he was seeing. I had walked this outcrop for years without realizing it. Once I saw the first ones, I followed the sandstone ridge and realized that they were part of a much larger and more extensive path . “
The fossilized plants and surrounding pollen helped the team determine the age of the tracks at around 58 million years.
“The Hanna Formation footprints are the first Paleocene mammal footprints to be 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,” explains the expert. . “It is also the largest accumulation of Paleocene mammal tracks in the world, both in aerial extent and in absolute number of tracks. With at least two species leaving tracks, it is also the most taxonomically diverse.”
Referencia: Earliest evidence of marine habitat use by mammals, Scientific Reports (2021). DOI: 10.1038/s41598-021-88412-3
