By Helen Briggs
Science reporter, BBC News
An ancient ancestor of the elephant from 37 million years ago lived in water and had a similar lifestyle to a hippo, a fossil study has suggested.
The animal was said to be similar to a tapir, a hoofed mammal which looks like a cross between a horse and a rhino.
Experts from Oxford University and Stony Brook University, New York, analysed chemical signatures preserved in fossil teeth.
These indicated that the animal grazed on plants in rivers or swamps.
The study, published in Proceedings of the National Academy of Sciences, could shed light on the lifestyle and behaviour of modern elephants.
Dr Erik Seiffert, co-author of the study, told BBC News: "It has often been assumed that elephants have evolved from fully terrestrial ancestors and have always had this kind of a lifestyle.
"Now we can really start to think about how their lifestyle and behaviour might have been shaped by a very different kind of existence in the distant past.
"It could help us to understand more about the origins of the anatomy and ecology of living elephants."
DNA evidence suggests that elephants are related to seagoing manatees and dugongs, and another land-based mammal, the rabbit-like hyrax.
This led to the theory that elephants and their extinct relatives may have evolved from a water-dwelling ancestor.
Scientists in the UK and the US looked at fossil teeth of two species that belong to an extinct family of mammals related to the elephant and, more distantly, the sea cow. They lived in northern Egypt during the Eocene Epoch, about 37 million years ago.
Alexander Liu of the University of Oxford and Erik Seiffert of Stony Brook University, New York, analysed the patterns of different oxygen and carbon atoms, or isotopes, laid down in tooth enamel to investigate the lifestyle and diet of the creatures.
The isotopic signals suggest that Barytherium and Moeritherium, as they are called, were largely aquatic, feeding on freshwater vegetation in rivers or swamps.
At the time the deserts of northern Egypt, where the teeth were unearthed, were covered by sub-tropical rainforest and swamps.
Dr Erik Seiffert told BBC News: "The isotopic pattern preserved in their teeth is very similar to that of living aquatic mammals.
"It supports the hypothesis that, at some point early in the evolution of elephants, these animals were very dedicated to either a fully aquatic or amphibious lifestyle - they probably spent most of their life in water."
Co-author Alexander Liu said the animal was not completely aquatic, since it lacked adaptations like a "stream-lined body or flipper-like limbs".
He said: "It seems that [Moeritherium] was almost certainly an animal that ate freshwater plants and led a semi-aquatic lifestyle, similar to that of hippos."
It is not clear how and why the ancestor of elephants left the water for a life on land. One theory is that a cooling event at the end of the Eocene dried up swamps and rivers, forcing animals out on to the land.
"There's little real evidence yet to suggest that's true," said Alexander Liu. "We've got an awful lot of pieces in the puzzle; if we could find one more example of an aquatic or semi-aquatic elephant that would be extremely convincing."