FIG. 33.22

When did grasslands expand over the land surface?

BACKGROUND Today, prairies, steppes, and other grasslands occur widely in the interiors of continents. Grasses, however, do not fossilize readily. How, then, can we understand how grasslands developed through time?

HYPOTHESIS Grasslands expanded as climate changed over the past 50 million years.

OBSERVATIONS AND EXPERIMENTS Grasses commonly make phytoliths, small structures of silica (SiO₂) in their cells. These preserve well, providing a direct record of grass expansion. Moreover, mammals that feed on grasses evolved high-crowned teeth, which also preserve well, giving us an indirect record of grassland history. Finally, C₄ grasses, which are adapted to hot sunny environments with limited rainfall, have a distinctive carbon isotopic composition imparted by the initial fixation of CO₂ by PEP carboxylase (Chapter 29). Measurements of ¹³C and ¹²C in mammal teeth, soil carbonate minerals, and more recently, tiny amounts of organic matter incorporated into phytoliths, allow scientists to track C₄ grasslands through time.

RESULTS Studies of phytoliths, mammal tooth structure, and carbon isotopic composition of teeth enamel from the North American midcontinent clearly show that grasslands expanded 20 to 15 million years ago, and C₄ grasslands expanded later, about 8 to 6 million years ago.

CONCLUSION In North America, grasslands expanded as atmospheric CO₂ levels declined and climates became drier. Other continents show evidence of a similar linkage of grassland expansion to climate change.

FOLLOW-UP WORK At present, atmospheric CO₂ levels are increasing rapidly, which may affect the competitive abilities of C₃ and C₄ grasses. Scientists are working to understand how global change will influence grasslands and other vegetation.

SOURCE Stömberg, C. A. E. 2011. “Evolution of Grasses and Grassland Ecosystems.” Annual Review of Earth and Planetary Sciences 39: 517–544.