Image: Courtesy of University of Nebraska State Museum

Today manicured vistas of peaceful farmland characterize northeastern Nebraska. Ten million years ago, though, life was decidedly wilder. Remains from the local Ashfall Fossil Beds, discovered in the 1970s, have revealed that a marvelous menagerie of prehistoric beasts--including camels, three-toed horses, saber-toothed deer and rhinoceroses (right)--once roamed the region. In recent years the rhinoceroses, which belong to the genus Teleoceras, have kicked up controversy among paleontologists. Some argue that these animals were terrestrial; others believe they led semi-aquatic lives. Now chemical studies of fossil teeth are lending support to the idea that Nebraskan rhinos actually did spend a lot of time in the water.

Mark Clementz, a graduate student at the University of California at Santa Cruz, and UCSC researcher Paul Koch presented the new evidence--an analysis of oxygen isotopes in Teleoceras teeth--yesterday in Reno, Nev., at the annual meeting of the Geological Society of America. In earlier studies, Koch noted that teeth from hippos, which are semi-aquatic, contain more lighter oxygen isotopes than those from such land animals as zebras, elephants and modern rhinos. He attributes the difference to the varying degrees of water these animals lose through evaporation, in which water containing lighter isotopes is whisked away more readily. "Hippos wouldn't evaporate much because they're in the water all day and only leave the water to graze at night when it's cool and humid," he explains. In contrast, lighter oxygen isotopes would tend to dissipate quickly from animals that spent all of their time on land.

Because the oxygen isotope composition of the body gets incorporated into the tooth enamel during tooth formation, a relatively fast process, the isotope values tend to vary. But in his analyses of living aquatic and terrestrial mammals, Clementz found that aquatic creatures display far less variability than terrestrial ones. And when he turned his attention to the prehistoric Ashfall fauna, he discovered that Teleoceras exhibited little oxygen isotope variability compared with known terrestrial beasts. Analyses of Teleoceras remains from two other sites yielded somewhat more variability. But considering this animal's broad geographic range, Clementz suspects that Teleoceras may have adapted its behavior according to different environs. "In Nebraska there's no question they were semi-aquatic," he asserts, "but they may have been more flexible in their ecological niche than modern hippos."