BALTIMORE--Ten million years ago, camels, rhinos, and as many as 20 different species of horses roamed North America. Most of these grazers died out about 6 million years ago, probably because of a sudden climate shift. At the Ecological Society of America's annual meeting here earlier this month, paleontologist Steven Stanley of Johns Hopkins University aired a provocative view as to what may have caused the extinction: scratchy grass. In the new climate, Stanley suggests, softer grasses gave way to more abrasive plants, so that only certain horses and other grazers with teeth able to withstand tough chewing survived.
Stanley's idea attempts to explain a mammalian mystery: The species that survived the Miocene epoch are almost all animals with very high-crowned molars, while those that died out have lower crowned teeth. Stanley put this puzzling extinction data together with other recent clues to the vegetation changes of the time. As Thure Cerling of the University of Utah and his colleagues reported in Nature last September, the composition of fossil teeth from ancient horses and other grazers reveals a major shift in late-Miocene plants: Wheat, bluegrass, and other plants known as C3 (because of their photosynthetic pathway) gave way to tougher C4 plants, such as crab-grass and Bermuda grass, which prefer a drier, more tropical setting.
Stanley now links this switch to C4 grasses to the mammal extinctions. Paleontologists generally think that grazers evolved high-crowned teeth so they could tolerate the wearing effects of eating grass. Stanley's hunch: That horses with shorter teeth died young because their teeth were worn down by the even more abrasive C4 grasses. He points to botany papers showing that C4 grasses, which have more veins than C3 plants, produce more than five times as many abrasive silica bodies by leaf area. The grass change doomed animals that were not long in the tooth, Stanley concludes. "If you need long teeth to eat grass with silica," he says, "then you need longer teeth to eat grass with more silica."
Stanley's idea is "plausible," says Richard Hulbert of Georgia Southern University, but he and others are not yet convinced. One strategy to test the hypothesis, Hulbert says, might be to study tooth wear-down rates of modern species--for example, by comparing the teeth of U.S. bison to those in Canada, where there is more C3 grass. These modern grazers might offer clues to why their ancestral cousins weren't so lucky.