Fossil horses and carbon isotopes: new evidence for Cenozoic dietary, habitat, and ecosystem changes in North America

The transformation from low-crowned to high-crowned horse teeth during the Miocene has traditionally been interpreted as an adaptive response to the spread of savanna grasslands during the middle Miocene by about 15 m.y. ago. Carbon isotope data from 50 North American horse teeth spanning Eocene to Pleistocene age indicate that savanna grasslands, presumably dominated by C 4 grasses as they are today, first became widespread much later than do horses with high-crowned teeth. Prior to ∼ 7 m.y., horses had a C 3-based diet and after 7 m.y. horses started eating C 4 grasses. This change in diet occurred when the major drop in the diversity of horses occurs during the late Miocene. The change in vegetation reflected in horses' diet may be related to a significant reduction in atmospheric CO 2 level toward the end of the Miocene which provided the C 4 grasses with an adaptive advantage and led to their expansion at the expense of C 3 plants.