This is in Science for November 26.
SOCIETY OF VERTEBRATE PALEONTOLOGY MEETING:
Timing Complicates History of Horses
Erik Stokstad
DENVER, COLORADO--Almost 1000 paleontologists and enthusiasts met here from 3 to 6 November for the 64th annual meeting of the Society of Vertebrate Paleontology.
It's a classic story of evolution. About 18 million years ago in North America, horses, camels, and other groups of herbivores independently evolved high-crowned cheek teeth. This condition, called hypsodonty, has long been considered a response to a changing environment: During this time, the Miocene Epoch, the climate was cooling, and grasses--which contain abrasive silica--began to spread and replace leafy woodlands. Tall teeth that last longer would have provided an immediate advantage.
The tale is not so straightforward, it turns out. At the meeting, Caroline Strömberg of the Swedish Museum of Natural History in Stockholm reported that it took 4 million years after the grass began to dominate the Great Plains for hypsodonty to appear--a puzzling lag. "It really does raise questions," says Christine Janis of Brown University. Yet not all was quiet on the western front: Janis and colleagues presented evidence that at about this time horses were developing legs more efficient at moving, which may have allowed them to range more widely for tender grass in the open landscape. Strömberg charted changes in vegetation by examining the tiny bits of silica, called phytoliths, contained in grasses, palms, and many other kinds of plants. She collected 99 samples from rocks across the central Great Plains, spanning roughly 31 million years (from the middle Eocene, through the Oligocene and Miocene) until about 9 million years ago. The relative amounts of various kinds of phytoliths revealed whether the habitat was open grassland resembling the modern savanna, woodland, or forest. The work paints the first high-resolution picture of vegetation for this time period. "It's an excellent, well-constrained study," says Bruce MacFadden of the University of Florida, Gainesville.
Because Strömberg collected the samples from the same rock formations that had yielded fossils, she could compare the changes in vegetation with known shifts in tooth height. In the late Eocene and early Oligocene, the area was forested. Grasses replaced the trees in the central Great Plains by at least 22 million years ago, but full-blown hypsodonty didn't take root in horses for another 4 million years. "This is a significant lag," Strömberg says. "It weakens the argument for coevolution, in lockstep, of horses and grasses."
Then why the lag? One possible reason could be that there was weak or no pressure to adapt to the new vegetation. But Strömberg points out that when the savanna first appeared, the closest relative to hypsodont horses, which belong to the genus Parahippus, evolved slightly higher teeth than its ancestors had. It may also be that some animals compensated by learning new behaviors to cope, such as feeding on grasses only in the spring, when they are tender, as red deer do.
Clues may come from elsewhere in the skeleton. Janis and Manuel Mendoza and Paul Errico of the University of Rhode Island have examined horses' limbs, for example. During the Miocene, horses and camels were evolving longer limbs, but apparently not to escape accelerating predators--which evolved longer limbs some 20 million years later. Instead, Janis proposed, the limbs first evolved to be more efficient at walking. In a preliminary analysis, Janis measured the limbs of fossil horses at the American Museum of Natural History in New York City. Compared with their ancestors, the advanced horses of the Miocene had knees and ankles with features suggesting that the limbs would have been more constrained to move in a fore and aft plane and hence more efficiently. "I think they're increasing their foraging radius," Janis says. High-crowned teeth might not be the only way to make life on the grasslands less of a grind.