"Home, home on the range
Where the deer and the antelope play.
Where seldom is heard
A discouraging word
And the skies are not cloudy all day." -- John A. Lomax's 1910 version of the Dr. Brewster M. Higley poem "Oh, Give Me a Home Where the Buffalo Roam", which became the lyrics of Kansas' state song "Home on the Range"
BIG QUESTION: How did grasslands arise, and how did they affect the animal world?
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 ground cover 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. In contrast, forests are dominated by trees which are decades-to-centuries old, and thus nutrients are sequestered in the plant biomass. In addition, although a given unit area of forest might have much more volume of plant matter, the great bulk of that is in the form of woody tissue which is not directly edible to typical vertebrates; in contrast, a very high percentage of a given grass plant is edible. As a consequence of rapid turnover and high percentage of accessible food, grasslands can support vast large vertebrate biomasses.
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.
Grasses are technically called Poaceae (or sometimes Graminaea). They are a very diverse clade of angiosperms (flowering plants), with over 10,000 living species. Unlike most flowering plants, grasses have microscopic flowers and use wind-pollination (like most gymnosperms) rather than insect-aided pollination (like most angiosperms).
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:
For a long time it was thought that grasses only arose in the earliest Eocene. But in 2005 phytoliths were found in the coprolites of 67 Ma titanosaur sauropods from India. (Yes, from in dino poop...!) In 2017 grass fragments and phytoliths were found in 110 Ma teeth from an early duckbill dinosaur of China. The diversity of specimens from these fossils suggest that divergence within the grasses had already occurred, so the origin was even earlier. That said, Mesozoic grasses seem to be fairly rare. The oldest definitive grass pollen is from about 60-55 Ma South America and southern Africa, and the oldest body fossil (spikelets) from 55 Ma North America.
To vertebrate zoologists, "grazer" specifies a grass-eater. (That is in fact the etymology of the word!) (Note however that marine biologists use "grazer" for almost any animal that bites off small immobile food, such as algae, polyps, bryozoans, etc.) "Grazers" is contrasted with "browsers" (animals that eat mostly herbs or the leaves of shrubs and trees), with "mixed feeders" as having both grass and browse in their diet.
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, including buffalo, bison, cows, goats, sheep), and the like) were thought to document evolutionary responses to this environmental shift. Grazers are distinct from browsers in a number of aspects of the jaws and teeth:
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.
Some other changes from the ancestral forested home to life of the grasslands are tendencies but are not absolute distinctions: you can find some forest dwellers with these adaptations, too:
Many types of mammal (and other animals, to be fair!) diversified in the grasslands, but some of the greatest successes were among the ungulates: in particular, the Artiodactyla (the even-toed ungulates) and the Perissodactyla (the odd-toed ungulates). Indeed, within these two groups, there was huge diversifications in the Bovidae and the Equidae (respectively), both of whom were ancestrally forest-dwelling clades. The evolution of the equids in particular are an excellent example:
Although grasses are present in the Late Cretaceous, they remain a rare part of the ecosystem for a long time. The first two epochs of the Cenozoic (the Paleocene and Eocene) are largely wet, forested worlds. Even in the interior of North America (like Wyoming and Utah) there are rain forests with palm trees! Toward the end of the Eocene (around 40 Ma or so) there is a general trend towards drying, but it appears that the biomes run from forest to woodland to dry woodland to desert scrub, with still no major grasslands.
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 Miocene 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 Oligocene 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. This "grit, not grass" hypothesis is consistent with certain types of dinosaurs that show "grazer" adaptations in the world long before the grasslands: they were adapted to feeding on low, grit-covered vegetation rather than grass per se.
It is worth bringing up an issue of the geography in the mid-Cenozoic:
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.
Most land plants--including many grasses--use what is called the C3 metabolic pathway. (Do not worry: we are not going to deal with the particulars of the biochemistry here!) However, some subclades of Poaceae (and a few other plants) have evolved a different metabolic pathway called the C4 pathway. C4 plants are "turbocharged" with regards to carbon dioxide: they have specialized cells that active as chambers of concentration to increase the CO2 that reaches the photosynthesizing cells. By doing this, they are able to operate better in lower CO2 conditions than C3 plants do. Similarly, by requiring less atmospheric CO2 for the same amount amount of productivity, they can keep their stomata closed more often and risk less transpiration. Thus, C4 plants do better than C3 plants in hot and/or dry and/or well-lit (that is, open) conditions. In other words, C4 plants can do phenomenally well in grasslands!
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). This 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. Chemical weathering of rock pulls carbon dioxide out of the air, reducing the greenhouse effect and 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 in a few lectures, the spread of grasslands and reduction of forests favored the evolution of plains-adapted versions of forest animals, one lineage of which became us.