The family tree of the largest living land animal may have its roots deep in the water, a new study suggests.

Chemical signatures from fossil teeth reveal that at least one species of proboscidean, an ancient elephant relative, lived in an aquatic environment.

The teeth of the ancient animal, which belonged to a genus called Moeritherium, suggest that it ate freshwater plants and dwelled in swamps or river systems, said Alexander Liu of Oxford University's department of earth sciences.

"Essentially it's a hippo-like mode of life. That's the closest animal that we can think of today," said Liu, lead author of recent research on the teeth.

Living elephants and their extinct relatives share a common ancestor with manatees, dugongs and the other aquatic mammals known as sirenians.

Moeritherium lived some 37 million years ago, many millions of years after the genetic lineages of elephants and sirenians split, Liu said.

Teeth Solve Mystery

Moeritherium didn't much resemble modern elephants. It was probably about the size of a tapir—29 to 42 inches (74 to 107 centimeters) tall at the shoulder.

It seems to have lacked a trunk but may have had a prehensile upper lip.

The animal's teeth were unearthed in northern Egypt's Faiyum region, which in ancient times was a shallow estuary or coastal system where the environment changed often.

The Moeritherium fossils were found in rock containing strong evidence of swamp and river ecosystems. But it was difficult for scientists to tell whether the ancient animals had actually lived in such an environment or whether their bodies had washed up there after their deaths.

In the end, the teeth told the tale.

Carbon isotopes in tooth enamel retained signatures of Moeritherium'sdiet, while oxygen isotopes evidenced the local water sources from which they originated.

By comparing variations in the ratios of these isotopes with those of terrestrial animals that lived during the same period, the team determined that the proboscidean was likely semi-aquatic.

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Their research appears in this week's issue of the Proceedings of the National Academy of Sciences.

From Land to Sea and Back Again?

William Sanders of the University of Michigan's Museum of Paleontology said he found the new study to be "first rate," providing convincing evidence that Moeritherium was indeed semi-aquatic.

"Paleontologists have thought for nearly a century that moeritheres were at least semi-aquatic, hippo- or sea cow-like in their overall adaptations and lifeways," he said.

Yet Sanders cautioned against assuming an aquatic ancestry for modern elephants or even suggesting that all early proboscideans were aquatic.

"[Moeritherium is a] very specialized animal that may have been off the main line of evolution from that which led to elephants," he said.

The creatures also lived long before the first modern elephants appeared about seven million years ago. Thus Sanders noted that if elephants did have an aquatic past, some 20 million years of terrestrial evolution would have left few traces today.

"A popular myth about elephants, for example, is that their trunk evolved as a sort of snorkel in more aquatic settings," he said.

"The truth is that early proboscideans lacked a projecting proboscis, and that the development of trunks has more to do with the hypertrophy [enlargement] of the tusks and feeding adaptations on land."

Oxford's Liu hopes to tackle such questions by testing the teeth of even more primitive elephants to discover more about when their lifestyle shifts occurred and when the sirenians may have split from their relatives.

He's also intrigued by the possibility that proboscideans will provide the first evidence of terrestrial mammals becoming aquatic and then returning to land.

"The first mammals were terrestrial," he explained.