Among the large mammals that roamed prehistoric North America was a type of rhinoceros that seems to have lived in the water, much like a modern hippopotamus. The extinct rhinoceros, known as Teleoceras, ranged from Florida to the West Coast from about 17 million years ago until about 4.5 million years ago. Its semi-aquatic lifestyle, first suggested by its body shape, has been disputed by some researchers. But evidence preserved in fossil teeth now indicates that in some areas, at least, Teleoceras did spend much of its life in the water. Clementz and Paul Koch, an associate professor of Earth sciences, analyzed oxygen isotopes in fossil Teleoceras teeth for clues to the animals' habits.
Oxygen occurs in nature as three different isotopes.
Evaporation, for example, acts preferentially to remove water molecules containing the common, light isotope of oxygen. The water left behind, whether in a puddle or in the body of an animal, ends up with a higher proportion of heavy oxygen isotopes. The Teleoceras study grew out of work Koch had done on modern mammals in East Africa comparing different species within the same ecosystem. Koch observed that hippo teeth contain a higher proportion of the light isotope of oxygen than the teeth of land animals, such as rhinos, zebras, and elephants. He attributed this to differences in the amount of water the animals lost through evaporation.