The Origin of Grass-Dominated Ecosystems

Approximately one-third of the Earth's vegetative cover comprises savannas, grasslands, and other grass-dominated ecosystems. Paleobotanical, paleofaunal, and stable carbon isotope records suggest five major phases in the origin of grass-dominated ecosystems: (1) the late Maastrichtian (or Paleocene) origin of Poaceae; (2) the opening of Paleocene and Eocene forested environments in the early to middle Tertiary; (3) an increase in the abundance of C3grasses during the middle Tertiary; (4) the origin of C4grasses in the middle Miocene; and (5) the spread of C4grass-dominated ecosystems at the expense of C3vegetation in the late Miocene. Grasses are known from all continents except Antarctica between the early Paleocene and middle Eocene. Herbivore morphology indicative of grazing, and therefore suggestive of grass-dominated ecosystems, appears in South America by the Eocene-Oligocene boundary, prior to the occurrence of grazing morphology elsewhere, and persists throughout the Cenozoic. Clear vertebrate and paleobotanical evidence of widespread grass-dominated ecosystems in northern continents does not occur until the early to middle Miocene. C4grasses are present from approximately 15 Ma and undergo a dramatic expansion in the lower latitudes of North America, South America, East Africa, and Pakistan between 9 and 4 Ma. The expansion may have taken place in a shorter interval in some regions. C4grasses are characteristic of seasonal, arid, and warm environments and are more tolerant of lower atmospheric CO2(< 400 ppmv) than C3plants. C4grass distribution, therefore, is climatically controlled. The late Miocene spread of C4grasses possibly involved a decrease in atmospheric CO2and heralded the establishment of modern seasonality and rainfall patterns.