What kind of plant is that?

Marilyn chasing emu eating plants, 2009

C-3 and C-4 plants have distinct isotope compositions

C-3 and C-4 Plants

Another set of plants turn the carbon dioxide in air into a 4-carbon sugar and so we call them C-4 plants (C4). C4 plants evolved well after C-3 plants and this adaptation allows them to thrive in hot, dry environments, such as the outback of Australia. C4 plants absorb the lighter carbon-12 isotope significantly slower than the C3 plants, because the protein that uses the carbon dioxide works differently. Because these proteins in plants that take up carbon dioxide are different, their carbon isotopes are different as well. Scientists now use these differences to determine whether a plant is a C-3 or C-plant.

The shrubs in the photo--chenopods-have isotope signals between C-3 and C-4 plants. Emus eat their flowers and seeds.

Why is this important? Well, C-4 plants are usually grasses, especially in Australia. Grasses of the C-4 type mean that the climate was warmer and drier. If we know that an animal was eating C-4 plants we know that it lived at a time when the climate was warm and dry. This is important when we examine fossils from animals that went extinct years ago. When we measure their carbon isotopes, we can tell if they ate grass (usually C-4) or leaves of trees (usually C-3). Once we know this, we can predict what the climate must have been like many years ago.