A STUDY of the ancestral cousins of modern horses has upset conventional views on what teeth can tell us about the diet of extinct animals. It was assumed that grazing animals such as horses had high-crowned teeth with enamel ridges, for eating grass. The new research suggests that this was not always the case.
Grass is one of the toughest foods there is to eat. It needs to be cut with a scissor-like action by teeth with sharp edges, which is why the high-crowned teeth and enamel ridges of grazing animals, including zebras, were seen as perfect adaptations to living on a grassland savannah.
The theory is that when modern grasses evolved, about 20 million years ago, the dentition of herbivores changed to exploit the new food. Until then herbivores were typically browsers, like modern- day deer, chewing leaves and bark off shrubs and trees. The teeth of browsers were not suited to the systematic, lawnmower-like cropping of grass. A simple way of working out the type of food eaten by an animal is to look at its dentition. However, Bruce MacFadden, a palaeontologist at the University of Florida in Gainesville, has devised a clever alternative. He grinds up small parts of a tooth to analyse its combination of carbon isotopes - carbon 12 and carbon 13. As it happens, grasses typically have a different proportion of isotopes from those of shrubs and trees and this difference is reflected in the chemical make-up of a herbivore's teeth. Using this approach, Dr MacFadden investigated the teeth of six species of prehistoric horse that lived about 5 million years ago in Florida. Each species possessed the high crowns and enamel ridges of typical grazers, suggesting they were all grass-eaters. Not so, Dr MacFadden found. Some of the horse species did eat solely grass, but others appeared to have eaten a combination of grasses and shrubs, and a couple had a diet which almost seemed not to include any grass at all. In other words, Dr MacFadden found that some prehistoric horses with the typical dentition of grazing animals, in fact behaved like browsers instead. "These techniques are revolutionising our ability to understand what prehistoric animals ate. …