Ancient mammals gathered by the ocean

US scientists have uncovered hundreds of fossilised footprints stretching out over a kilometre in Wyoming – and it’s the earliest evidence of mammals hanging out by the ocean.

Today the tracks are hundreds of kilometres from the coast, but the team used fossilised plants and pollen to date the footprints to 58 million years ago – back when the environment would have been a brackish nearshore lagoon.

Orange rock with white dots
Section of the 58-million-year-old tracksite demonstrating near-vertical tilting of the originally horizontal bedding with three separate trackways made by five-toed mammals walking in parallel. Credit: Anton Wroblewski

Several sets of tracks traipsed across the ancient tidal flats, which have now hardened into sandstone. They were made by two different animals: five-toed footprints are suspected to have been made by a large hippo-like, semi-aquatic mammal called Coryphodon, while four-toed footprints are more of a mystery. They haven’t been matched up to any animal in the fossil record, but they resemble types of hoofed mammals that haven’t yet been shown to exist in this era.

“Trace fossils like footprints record interactions between organisms and their environments, providing information that body fossils alone cannot,” says Anton Wroblewski from the University of Utah, who co-authored the paper published in Scientific Reports.

“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.”

66 million years ago, the K-T mass extinction event wiped out a significant chunk of species, including most dinosaurs, allowing mammals to proliferate and inherit the Earth.

These are the first mammal tracks from the Paleocene found in the USA, Wroblewski says, and only the fourth in the world.

Three animals on four legs and a person.
Reconstructed track maker candidates for size comparison. (A) Paleocene Coryphodon proterus; (B) Titanoides primaevus; (C) Barylambda faberi. Credit: Wroblewski et al. 2021.

So why did these ancient mammals congregate by the seaside?

The authors say they were likely drawn to the ocean for the same reason that modern mammals are – including protection from predators and biting insects, migration, and access to sodium and other minerals.

In their paper, they conclude that these footprints provide “a glimpse into the lives of megafauna mucking through the brackish-water tidal flats of the late Paleocene”.

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