Short in the tooth

By Kurt Kleiner in Baltimore GRITTY grass may have driven short-toothed grazers out of existence. The largest recent extinction of mammals in North America began between 6 and 7 million years ago when a climate shift encouraged the growth of prairie grass too tough for the animals’ teeth. According to Steven Stanley of Johns Hopkins University, roughly 60 kinds of North American mammals disappeared during the Late Miocene, including 9 of the 18 species of horse known to have been alive at the time. Palaeontologists had already noticed that most of the grazers that survived had longer teeth than those that became extinct. Stanley says short teeth may have become a handicap by then. Grazing animals wear down their teeth throughout their lives from continually chewing on tough plants. The shorter the teeth, the quicker they wear out. As a result, animals that found only tougher grasses could have died from malnutrition, he says. During the Miocene, grasslands became more widespread in North America, and the teeth of grazing animals grew gradually longer to take advantage of the new food supply. But then, relatively quickly, most of the animals with shorter teeth died out. “It’s a very unusual cause for a major extinction,” Stanley says, “but I think it’s what happened.” Teeth of Miocene horses are classified as “hypsodont”, meaning long, or “very hypsodont”. Of 7 horse species classified as very hypsodont, none became extinct. But of 11 species with shorter teeth, 9 became extinct. What’s more, Stanley claims, 5 new horse species arose, all of them with very long teeth. Stanley thinks the culprit was a tougher kind of grass. Palaeontologists know that C4 grasses were quickly replacing C3 grasses in North America around that time. Stanley says C4 grasses are much more abrasive because their leaves contain much more silica. Faced with the more abrasive grasses, animals with shorter teeth would have lived shorter lives, produced fewer offspring, and eventually lost out to animals with longer teeth and therefore longer life spans, he says. Bruce MacFadden, a palaeontologist at the University of Florida, whose work on horses Stanley drew on, says more abrasive grasses could be a part of the answer, but thinks it’s simplistic to attribute extinction solely to the length of teeth. “I think it’s interesting stuff, but I don’t think Stanley has a lot of data to back it up.” He says the two short-toothed species that survived show that dental abrasion alone doesn’t explain the extinctions. Researchers are still working out what caused the shift to C4 grasses. While some suspect it was a rise in carbon dioxide, Stanley blames a warming and drying of the climate. More on these topics:
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