Fossilised footprint tracks, recently discovered within the Hanna Formation in Wyoming, USA, which have been dated to 58 million years ago, may represent the earliest evidence of mammals gathering by the sea, according to a study published in Scientific Reports. The findings suggest that mammals may have first used marine habitats at least 9.4 million years earlier than previously thought, in the late Paleocene (66-56 million years ago), rather than the Eocene (56-33.9 million years ago).
Drs. Anton Wroblewski and Bonnie Gulas-Wroblewski examined and photographed over 1,000 metres of fossilised footprints in an area dated back to 58 million years ago by plant and pollen fossils. The authors identified various different tracks. One set showed relatively large, five-toed footprints, comparable to the foot size of a modern-day brown bear, another showed medium-sized, four-toed footprints. The authors suggest that the five-toed prints were made by Coryphodon, a type of semiaquatic Pantodont, similar to a hippopotamus. The four-toed prints did not match skeletal evidence of mammals known from the late Paleocene, but show similarities with artiodactyls and tapiroids (types of hoofed mammals) which have yet to be shown to have existed in the Paleocene.
The tracks led to and crossed an area which also held traces of prehistoric marine molluscs and worms, as well as sea anemones, indicating the area was once a shallow tidal lagoon or bay. The authors speculate that prehistoric mammals gathered by the sea for similar reasons as modern-day mammals, such as crossing to migrate, protection from predators and biting insects, and to access sodium minerals, which would have been limited in prehistoric North American tropical forests.
Earliest evidence of marine habitat use by mammals
University of Utah, Utah, USA
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