Fossil horses, carbon isotopes and global change.

Between 20 and 10 million years ago, Miocene horses demonstrate rapid dental evolution from low-crowned (brachydont) to high-crowned (hypsodont) teeth. Hypsodonty is classically interpreted as an adaptive shift from browsing to grazing to exploit the spread of savanna grasses. Recent geochemical studies allow the use of carbon Isotopes to test this hypothesis. Isotopic analysis of fossil horse teeth Indicates a predominantly C(3) diet consisting of mixed browse/grass or predominantly C(3) grasses until the latest Miocene. The advent of C(4) grassland ecosystems began about 7-8 million years ago and seems related to declining equid diversity.