Evolution of Grasses and Grassland Ecosystems

Abstract

The evolution and subsequent ecological expansion of grasses (Poaceae) since the Late Cretaceous have resulted in the establishment of one of Earth's dominant biomes, the temperate and tropical grasslands, at the expense of forests. In the past decades, several new approaches have been applied to the fossil record of grasses to elucidate the patterns and processes of this ecosystem transformation. The data indicate that the development of grassland ecosystems on most continents was a multistage process involving the Paleogene appearance of (C3 and C4) open-habitat grasses, the mid-late Cenozoic spread of C3 grass-dominated habitats, and, finally, the Late Neogene expansion of C4 grasses at tropical-subtropical latitudes. The evolution of herbivores adapted to grasslands did not necessarily coincide with the spread of open-habitat grasses. In addition, the timing of these evolutionary and ecological events varied between regions. Consequently, region-by-region investigations using both direct (plant fossils) and indirect (e.g., stable carbon isotopes, faunas) evidence are required for a full understanding of the tempo and mode of grass and grassland evolution.

Decoupling the spread of grasslands from the evolution of grazer-type herbivores in South America

“…This omission differs from previous reconstructions of vegetation change in the Eocene-Miocene of Patagonia, Argentina 9,10,45 . Excluding, for example, elongate sinuous long cells, acicular hair cells and bulliforms from the sum of grass phytoliths reduces the inferred abundance of grasses relative to FIs, but does not fully explain the difference between our interpretation and that of previous authors 39 .…”

“…Our data also challenge the long-standing notion that dense concentrations of dung beetle brood balls, starting at 40 Myr ago, necessarily reflect grassland ecosystems 11 . Several phytolith assemblages described herein come from horizons with dense concentrations of brood balls yet contain very little grass, consistent with previous phytolith analyses of dung beetle brood balls from southern South America 39 . At Gran Barranca and elsewhere, dung beetle brood balls are found in strata that have yielded primate fossils, typically viewed as clear indicators of forested habitats 40 .…”

The deforestation story: testing for anthropogenic origins of Africa's flammable grassy biomes

“…There is clear evidence that aridity was a major factor in forest retreat in Eurasia and North America based on numerous pollen records (e.g. [32]) and analyses of phytoliths, the silica bodies in plant cuticles [33]. In Africa, the best fossil records are from areas that today are arid [31].…”

Continental erosion and the Cenozoic rise of marine diatoms

“…These environmental conditions became a dominant feature of the Cenozoic oceans in the late Eocene when the spin-up of the circum-Antarctic current and the growth of Antarctic ice sheets amplified the equator to pole heat gradient, and thereby intensified wind-driven ocean upwelling and nutrient supply dynamics (39,40). The second major pulse of diatom diversification began in the middle Miocene, coinciding with the expansion of Antarctic ice sheets (41) and the global rise of vast grassland ecosystems (42). Grasses contain up to 15% dry weight of silica (43), which forms micromineral deposits (phytoliths) in the cell walls.…”