The chewing surface of a fossilised upper molar from an equid - an ancestor of zebras - that lived in East Africa 9.6 million years ago. Researchers studied 452 fossil teeth from nine families of animals to show how different herbivores changed at different times from a diet of trees and shrubs to a diet of warm-season grasses.
Credit: Kevin Uno, University of Utah
BRISTOL: When grasses first appeared in East Africa 10 to 15 million years ago, herbivores quickly shifted from browsing trees and shrubs. But how swiftly, which species and when?
In a new study, scientists have studied 452 fossil teeth from nine families of animals.
They examined the dietary carbon incorporated into tooth enamel of the ancient ancestors of zebras, rhinos, giraffes and other animals that roamed the region 3-10 million years ago to determine if and when herbivores began to prefer grazing grasses over browsing trees and shrubs.
"This has implications for the future of our planet as climate and ecology change as a result of human activities – not only climate change, but land-use change such as agriculture and desertification," said senior author Thure Cerling from the University of Utah, USA.
The difference a carbon makes
Trees, shrubs and other cool, moisture-loving plants use a different photosynthetic pathway than warm, arid adapted grasses. The first organic compound produced in photosynthesis by trees and shrubs contains three carbon atoms and they are therefore grouped as C3 plants.
Warm-season grasses produce a compound with four carbon atoms so are termed C4 plants.
This one carbon difference enables researchers to determine whether herbivores were eating C3 or C4 plants based on the carbon isotope ratio found in the enamel of the now-fossilised teeth, a technique used by the international team who published their findings in Proceedings of the National Academy of Science.
Herbivores responded very differently
The analysis spans the time period after grasses first appeared in east Africa but long before they became the dominant vegetation. Using this data, the researchers were able to build a picture of how different herbivore groups responded to the emergence of this new resource.
The researchers measured the carbon isotope ratio from 452 fossilised teeth from nine families collected in three locations in Kenya.
The result is a seven million year record of how different groups of herbivores responded very differently to a change in the ecological landscape. Some took advantage of the new resource very quickly, while others took millions of years before they started to incorporate C4 plants into their diet.
