Home on the Range: The Evolution of Grasslands
Modern Grassland Biomes
Grasslands are one of the most important modern biomes. There are many types and names for these: savannas and veldts of Africa, prairies of North America, pampas of South America, steppes of Eurasia, rangelands in general, etc. (Generally, savanna is used for grasslands with scattered trees, and some other terms for those few or no shrubs and trees.) (Incidentally, woodlands are generally biomes where there is >40% woody plant cover, typically smaller trees where the crowns of the trees do not overlap; while forests are biomes here there is >>40% wood plant cover, where the trees can be more than 20 m tall, and where the crowns of the trees do overlap (and thus small groundcover plants receive very little sunlight).
Approximately 40% of the modern land surface is covered by some form of grassland. Grasslands typically have very deep soil (in large part because of the rapid turnover of plants). Grasslands are typically associated with dry but not desert conditions: only 500-900 mm/yr rain.
Most of the grassland plants (both grasses and herbs) are annuals (live for just one year), so there is very rapid turnover of plants and thus VERY high productivity, allowing grasslands to support extremely tremendous animal biomass. (In contrast, forests are dominated by trees which are decades to centuries old, and thus nutrients are sequestered in the plant biomass.).
Fires are very, VERY common in grasslands. In fact, fires promote the maintenance and spread of grasslands, because grass can recover easily from fires, but trees and shrubs typically will need to recolonize in order to be present.
There is far more to the grasses than just lawn grass, though. For example, all the cereals (wheat, barley, corn, millet, rice, etc.) are grasses: indeed, nearly every human being in the world gets much of their nutrients from grasses! Also, bamboo is a clade of extremely tall grass. Grass is capable of easily growing back after grazing and burning, for reasons detailed below.
Grasses are characterized by:
The oldest fossil evidence of grasses are from phytoliths of the late Maastrichtian (around 66.5-67 Ma) of India, within the coprolites of titanosaurian sauropod dinosaurs! (Yes, the oldest known grasses are in dino poop...). The diversity of phytolith shapes in these coprolites show that the grasses had already undergone considerable phylogenetic diversification at this point, so the origins of grasses almost certainly predate the Maastrichtian. (In fact, there are possible grass phytoliths from 70 Ma, but these might be from some other phytolith-bearing plant group.) The oldest definite grass pollen is from about 60-55 Ma South America and southern Africa, and the oldest body fossil (spikelets) from 55 Ma North America.
It had long been observed that the Cenozoic saw a drying of the planet, and a shift from a forest-dominated to a grass-dominated world. Certain adaptations in mammal lineages (such as the evolution of equids (horses), bovids (antelopes), and the like) were thought to document evolutionary responses to this environmental shift. These changes include such aspects as:
There are other proxies for grazing vs. browsing. For example, if you look at the surface of teeth under the microscope, browsers typically have fewer scratches and more pits, while grazers have more scratches than pitting. Also the isotopic signal of C4 grasses (about which more below) show up in the chemical composition of the bones and teeth of grazers.
The Oligocene (33.9-23.03 Ma) saw the rise of grasslands, at least in the form of desert grasslands (primarily taking over from both desert scrub and dry woodlands.) During the Miocene (23.02-5.333 Ma) short grasslands with deep soils really start to take over, and in the late Miocen and Pliocene (5.333-2.588 Ma) we see the spread of tall grasslands with exceedingly deep soils.
For some time it was thought that South America was precocial in terms of grasslands, with true desert grasslands in the Eocene. It is true that some of the native placental herbivores (such as the notohippids) show hypsodonty early than their North American equivalents (the equids). However, the idea of Eocene/Oligocene pampas dominating South America is an over-exaggeration: new study of the phytoliths used to suggest this model shows that they are mostly phytoliths of palms or other forest plants. (That said, there are a lot more Eocene grass phytoliths in South America than anywhere else!) So the latest evidence suggest that the oldest well-documented true grasslands are the desert grasslands of late Oligoence North America. The hypsodont mammals of South America seem to be primarily munching phytolith-rich forest plants.
More nuanced examination of equid and bovid and the like evolution show that browsing and mixed-feeding forms dominate these lineages until the Miocene (although there were still many browsing and mixed-feeding horse and bovid genera during the Miocene.) Furthermore, it has been recognized that grit (from the ground in dry areas) may be more important with hypsodonty than phytoliths: the height of the crowns of ALL feeders (grazers, mixed, and browser) are higher in the prairie/steppe than the equivalent modes in the savanna, and these higher than woodland equivalents, and these higher than forest equivalents.
With the huge expansion of tall grasslands in the 10-4 Ma range, we see a huge increase in highly hyposodont forms. What might have triggered this change? The clue is in the type of grasses that spread.
The particular turbocharger cells (the "Kranz anatomy") of C4 plants have been identified in leaf fossils from the middle Miocene (around 12.5 Ma). However, as internal leaf anatomy is preserved only in Lagerstätten, there might be much older plants using the turbocharged method. However, it has been recognized that the isotopic signature of C3 and C4 plants is very distinctive, as are the signatures in bones and teeth of animals which eat these different plant types, and even the animals at eat the carnivores.
Because we can recover the influence of C3 vs. C4 in the isotopes of bones and teeth (and the soil derived from plants), it is possible to see the contribution of the these different grass types at different moments in geologic time. One of the great discoveries of the last few decades in Cenozoic paleontology is the recognition of a C3/C4 transition worldwide between about 8 and 4 Ma (with the main shift between 7-5 Ma). Thius is recognized in the grasslands of North America, Asia, South America, Africa, and nearly everywhere studied so far.
This definitely seems to be associated with an increase in aridity throughout the world. With an decrease in humidity and/or increase in seasonality, there is a favoring of grass over trees and shrubs, which itself turns a feedback into decreasing humidity. Other feedbacks in this system:
So what might be the trigger? In fact, it looks as if the primary trigger for this is the final phase of uplift of the Himalayas. This enormous mountain range was built up for 10s of millions of years as India slammed into southern Asia. But the final phase (starting in the Miocene) saw enormous amounts of erosion as the mountains get pushed up and weathered away. As you recall from early in the course, chemical weathering of rock pulls carbon dioxide out of the water, causing the world to cool. At the same time, disruption of the climate system from this caused a general drying of the world. As a consequence, the cooler, drier world favored grasslands at the expense of forests.
And as we will see next lecture, the spread of grasslands and reduction of forests favored the evolution of plains-adapted versions of forest animals, one lineage of which became us.
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