Nov. 21, 2000 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.
"Morphologically, Teleoceras looked a lot like modern hippos, with large, squat bodies and short legs, and we now have evidence from isotope analysis that they were semi-aquatic like hippos," said Mark Clementz, a graduate student in Earth sciences at the University of California, Santa Cruz. Clementz presented his findings at the Geological Society of America meeting on November 14.
Clementz and Paul Koch, an associate professor of Earth sciences at UCSC, analyzed oxygen isotopes in fossil Teleoceras teeth for clues to the animals' habits. Oxygen occurs in nature as three different isotopes. The two heavy isotopes are very rare (together they make up less than 0.5 percent of all oxygen atoms), but their greater mass has interesting consequences. 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.
"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," Koch said. "When we analyzed oxygen isotope compositions in fossil hippo teeth, we saw the same thing."
But when Bruce MacFadden at the University of Florida applied this technique to Teleoceras several years ago, he found no significant difference in oxygen isotope compositions between Teleoceras and other animals in the same ecosystem. MacFadden concluded that Teleoceras were terrestrial grazers.
In the new study, Clementz took a different approach, looking at the variability of isotope compositions within populations rather than averaging all the measurements for a given species. He analyzed teeth from a variety of modern terrestrial and aquatic mammals before applying the technique to Teleoceras.
"We found very low variability in aquatic mammals, including dolphins, seals, and river otters, versus much higher variability in land mammals," Clementz said.
The researchers attribute the low variability in aquatic mammals to the fact that all the water they take in comes from the same source, so the oxygen isotope composition in their bodies remains relatively stable. In land mammals, however, isotope composition varies between individuals and even within the same animal from hour to hour, Koch said.
"In a population of deer, for example, one deer might drink from a puddle that's been evaporating in the sun, another from a creek, and some live in hotter environments than others, so their isotope compositions will vary," Koch said.
Since a tooth forms within a relatively narrow window of time, the oxygen isotope composition in the body at that time gets locked into carbonate molecules in the tooth's enamel. Variability among samples can make it hard to obtain an accurate average value, but the variability itself turns out to be a useful source of information, Koch noted.
Clementz analyzed fossil teeth of Teleoceras and other mammals from the Ashfall Fossil Beds in Nebraska, one of the most spectacular fossil sites in the world. Whole herds of Teleoceras and other animals died and were buried by volcanic ash at the Ashfall site, where paleontologists have uncovered numerous intact skeletons of North America's ancient fauna. Clementz found much less variability in the oxygen isotope compositions of Teleoceras teeth than in other animals, as would be expected for a semi-aquatic mammal.
At two other sites, Clementz found somewhat more variability in Teleoceras, although it was still relatively low compared to known land mammals. This finding, together with the broad geographic range of Teleoceras, suggests they may have adapted their behavior to different environments. "In Nebraska, there's no question they were semi-aquatic, but they may have been more flexible in their ecological niche than modern hippos," Clementz said.
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