The newly-discovered fossilized footprints were made by at least two mammalian species around 58 million years ago in a brackish water lagoon in what is now southern Wyoming, the United States, and may represent the earliest evidence of mammals gathering by the sea.
“Trace fossils like footprints record interactions between organisms and their environments, providing information that body fossils alone cannot,” said Dr. Anton Wroblewski, a geologist in the Department of Geology and Geophysics at the University of Utah.
“In this case, trace fossils show that large-bodied mammals were regularly using marine environments only 8 million years after non-avian dinosaurs went extinct.”
Dr. Wroblewski and his colleague, Dr. Bonnie Gulas-Wroblewski from the Texas AM Natural Resources Institute, examined and photographed several sets of fossilized footprints in the Hanna Formation in southern Wyoming.
The tracks include underprints, impressions in soft sediment made when heavy animals walk on overlying sediment layers, as well as prints pressed into the surfaces of ancient tidal flats.
Now preserved in sandstone, they are 1,032 m long and were made by a minimum of two mammalian species: one associated with relatively large, narrow-gauge, five-toed tracks, and the other with medium-sized, four-toed tracks.
“Candidates for the five-toed tracemakers are pantodonts such as Titanoides primaevus, Barylambda faberi, or Coryphodon proterus. The owner of the four-toed tracks remains a mystery,” the researchers said.
Fossilized plants and pollen helped the team determine the age of the tracks to be around 58 million years old (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.
“The Hanna Formation tracks are the first Paleocene mammal tracks found in the United States and only the fourth in the world, with two sets of tracks previously found in Canada and one in Svalbard, Norway,” Dr. Wroblewski said.
“It’s also the largest accumulation of Paleocene mammal tracks in the world in both aerial extent and the absolute number of tracks.”
“With at least two species leaving the tracks, it’s also the most taxonomically diverse.”
The team’s paper was published in the journal Scientific Reports.
A.FJ. Wroblewski B.E. Gulas-Wroblewski. 2021. Earliest evidence of marine habitat use by mammals. Sci Rep 11, 8846; doi: 10.1038/s41598-021-88412-3