The origin of grass-dominated ecosystems

Approximately one-third of the Earth's vegetative cover comprises savannas, grasslands, and other grass-dominated ecosystems. Paleobotanical, paleofau nal, and stable carbon isotope records suggest five major phases in the ori gin of grass-dominated ecosystems: (1) the late Maastrichtian (or Paleocene ) origin of Poaceae; (2) the opening of Paleocene and Eocene forested envir onments in the early to middle Tertiary (3) an increase in the abundance of C-3 grasses during the middle Tertiary; (4) the origin of C-4 grasses in t he middle Miocene: and (5) the spread of C-4 grass-dominated ecosystems at the expense of C-3 vegetation in the late Miocene. Grasses are known from a ll continents except Antarctica between the early Paleocene and middle Eoce ne. Herbivore morphology indicative of grazing, and therefore suggestive of grass-dominated ecosystems, appears in South America by the Eocene-Oligoce ne boundary, prior to the occurrence of grazing morphology elsewhere, and p ersists throughout the Cenozoic. Clear vertebrate and paleobotanical eviden ce of widespread grass-dominated ecosystems in northern continents does not occur until the early to middle Miocene. C-4 grasses are present from appr oximately 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. C-4 grasses are characteristic of seasonal, arid, and warm environments and are more tolerant of lower atmospheric CO2 (< 400 ppmv) than C-3 plants. C -4 grass distribution, therefore, is climatically controlled. The late Mioc ene spread of C-4 grasses possibly involved a decrease in atmospheric CO2 a nd heralded the establishment of modern seasonality and rainfall patterns.