RESUMO
Although vertebrate herbivory has existed on land for about 300 million years, the grazingadaptation, principally developed in mammals, did not appear until the middle Cenozoic about 30 million years ago. Paleontological evidence indicates that grazing mammals diversified at the time of the spread of grasslands. Recently revised fossil calibrations reveal that the grazing mammal guild originated during the early Miocene in South America about 10-15 million years earlier than it did during the late Miocene in the northern hemisphere. Carbon isotopic analyses of extinct grazers' teeth reveal that this guild originated predominantly in C(3) terrestrial ecosystems. The present-day distribution of C(3) and C(4) grasslands evolved on the global ecological landscape since the late Miocene, after about 7 million years ago.
RESUMO
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.