The Origin of Grass-Dominated Ecosystems

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

“…Taken together, our results challenge the notion that convergent evolution necessarily reflects similar sets of environmental conditions, as has often been assumed [2][3][4][5] . Although it is still likely that environmental abrasiveness was the ultimate trigger for the evolution of hypsodonty, caution is required when using this functional trait for habitat reconstruction.…”

“…In both central North America and the eastern Mediterranean, grass-dominated habitats preceded presumed grazers by several million years (Fig. 4) 6,7 , indicating that, although delayed, hypsodonty plausibly evolved in response to the emergence of open, grassdominated habitats, in accordance with traditional views 3 . Our results from Gran Barranca call this dogma into question, showing that grass dominance and openness of vegetation are not necessary pre-conditions for favouring high-crowned dentition.…”

“…Huxley to H.F. Osborn and G.G. Simpson have viewed the Cenozoic assembly of this vitally important biome as a classic example of natural selection over macroevolutionary time scales, whereby mammals evolved hypsodont teeth and large size in response to the worldwide spread of grass-dominated vegetation 2,3 . The significance of hypsodonty in this context is that it prolongs the life of teeth in herbivores that deal with abundant abrasives, such as silica-rich grasses and wind-blown dust, in these open, arid habitats 4,5 .…”

“…Consequently, the increasing cheek-tooth crown heights in nearly all large herbivore lineages in faunas from Patagonia, southern South America, starting in the middle Eocene (B38 Myr ago) 8 (Fig. 1) are viewed as marking an unexpectedly early spread of grass-dominated habitats 3 . Consistent with this pattern, sparse phytolith records from Patagonia, Argentina, have been interpreted as showing the emergence of subtropical grasslands by 40 Myr ago 9,10 ; similarly, accumulations of dung beetle brood balls from the middle Eocene onward are likened to those found in the African savannah today 11 .…”

Dated historical biogeography of the temperate Loliinae (Poaceae, Pooideae) grasses in the northern and southern hemispheres

“…Loliinae have been traditionally considered to be of very recent, possibly late Miocene origin (Charmet et al, 1997). However, fossil studies (Linder, 1987;Jacobs et al, 1999;Prasad et al, 2005) support the past existence of open-habitat Pooideae-like grasses in the boundaries between the Mesozoic and the Cenozoic. Properly dated historical scenarios need to be reconstructed for the Loliinae that are based on more accurate nodal calibrations of the grass family.…”