Carbon isotopes in tooth enamel retained signatures of Moeritherium's diet, 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.
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.
(See related photo: "Loch Ness Monster Was an Elephant?" [March 9, 2006].)
"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.
"For a mammal group to have been terrestrial and then to have moved into an aquatic habitat, as [Moeritherium] seems to have done—and then at some point reverted back from being aquatic, as our data suggest happened—that's a very intriguing possibility."
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