Extinct ancestors of elephants lived in freshwater swamps and rivers like modern-day hippos, according to a fossil study published Tuesday.

Scientists at Oxford University and Stony Brook University in the United States studied the chemical signatures of fossilized teeth of two ancient mammals that lived on the Earth about 37 million years ago.

Moeritherium, a 37-million-year-old amphibious relative of modern elephants.
(Image courtesy of Luci Betti-Nash, Stony Brook University)

They discovered chemical evidence that both mammals consumed freshwater plants, likely from grazing in swamps and rivers, and were "largely aquatic."

The researchers based their findings on the patterns of different isotopes of oxygen and carbon atoms found in the enamel of the fossilized teeth, which they were able to compare with both aquatic and terrestrial mammals from that era.

One mammal, moeritherium, is thought to be similar in size and appearance to a tapir, and could have possibly had a prehensile upper lip instead of a trunk.

Less is known about the other mammal, barytherium, other than it was much larger than moeritherium, though not quite as large as modern-day elephants.

Both mammals lived in the late Eocene epoch. Their fossils were recovered in northern Egypt, which is now a desert but was once a sub-tropical region.

The new evidence supports previous research that suggested elephants shared a common ancestry with the aquatic manatees, also known as sea cows.

The study, published in Proceedings of the National Academy of Sciences on Tuesday, could also shed light on the practices of modern-day elephants, who are capable of using their trunks as a snorkel when crossing deep rivers, for example.