Carbon isotopes, diets of North American equids, and the evolution of North American C4 grasslands

The evolution of North American grasslands has traditionally thought to have been documented by the evolution of equid teeth which became higher and had more pronounced crowns as grasses became abundant. The chronology of equid hypsodonty, radiation of equids, the first documented C₄ plants and the change to a C₄ diet in equids is somewhat puzzling. Hypsodonty, which is the great increase in the crown height of molars and premolars, began in the early to middle Miocene for equids and was well underway by 15 million years ago. The modern distribution of C₄ grasses in North America is characterized by abundant C₄ grasses in regions where the growing season temperatures are high; C₄ grasses are the dominant grass in the central to southern Great Plains, the Sonoran and Chihuahuan Deserts, and Florida; C₃ grasses are dominant in Canada, the Pacific Northwest, and California.