The ancient kin of modern elephants may have spent much of their time in lakes, rivers or swamps.

Creatures in the proboscidean genus Moeritherium have been known for more than a century, but scientists have never agreed about how the animals lived, says Alexander G.S.C. Liu, a paleontologist at the University of Oxford in England.

Liu and his team analyzed fossils from 37-million-year-old rocks southwest of Cairo. Other fossils from the same sediments include terrestrial animals along with sharks and marine fish. The shapes of Moeritherium skulls, placement of their eyes and other features hint that the 300-kilogram animals lived aquatic or semiaquatic lifestyles. However, he notes, the arrangement of bones in their inner ears doesn’t suggest that the creatures could have heard well underwater. They likely were not full-time swimmers.

Liu and his colleagues weighed in on the debate by analyzing the ratio of oxygen-16 and oxygen-18 isotopes preserved in the teeth of Moeritherium fossils. Most atoms incorporated in tooth enamel when a creature is alive don’t readily swap out with those in the environment during fossilization. Therefore, that isotope ratio, among others, can provide clues to various characteristics of a creature’s environment, its metabolism or its dietary preferences.

The main clue was that the variation in oxygen isotope ratios among Moeritherium samples was extremely low. Creatures that spend much or all of their time on land get their water from a variety of sources, so their populations often have a much broader variation in ratios, Liu notes. The new evidence suggests that Moeritherium spent much of its time in or around freshwater habitats, the researchers reported in the April 15 Proceedings of the National Academy of Sciences.

“When the morphology, geology and isotope evidence come together and tell the same story, it’s really nice,” says Bill Sanders, a paleontologist at the University of Michigan in Ann Arbor.

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