December 4. Rise of grasses and some coal

Everyone knows grass. Common worldwide today, grasses include the cereal grains such as rice and wheat, bamboo, swamp sedges, and of course lawns, but as common as they are today, grasses were the last major group of plants to evolve. They were certainly present but uncommon during the Cretaceous – dinosaur coprolites, or fossil excrement, are known with grass components. But grasses really began to expand during the early part of the Cenozoic, the Paleocene and Eocene epochs.

Grasses continued to evolve through the Cenozoic, inventing novel ways to fix carbon during photosynthesis. Grasses including maize or corn, sugar cane, and sorghum use a more efficient method of carbon fixation than many plants, a method that is thought to be a relatively recent development – meaning probably the past 40 million years. 

Grasses diversified a lot in the middle to late Miocene, around 6 to 10 million years ago. Their dominance in prairies and savannahs may be a result of their drought tolerance and ability to use carbon dioxide more efficiently than some other plants, even in low CO2 conditions such as were developing over the course of the Cenozoic. Another factor might be a co-evolution with hoofed animals that grazed on grasses, and helped spread them. 

Coal mining, Powder River Basin, Wyoming (USGS photo)

In the western United States during the Paleocene, as grasses were beginning to expand, swamps and lakes were forming in what are now northeastern Wyoming and southeastern Montana, the Powder River Basin. This basin was one of the low-lying areas between two of the late Cretaceous Laramide uplifts, the Black Hills and Big Horn Mountains. Those mountains were actively uplifting into the early Paleocene, and shedding sediment into the adjacent swampy basins, where extensive forests grew in Paleocene and Eocene time. The package of rocks including fluvial or river-borne sediments, lake deposits, and organic material deposited in swamps is called the Ft. Union Formation. The climate was warm temperate to subtropical but with alternating warm and cooler intervals during the Paleocene and Eocene, the first epochs of the Cenozoic. The plant matter in the Ft. Union Formation has produced thick coal beds. One individual bed, near Gillette, Wyoming, is about 110 feet thick.

Similar coals of Cenozoic age can be found throughout the Rocky Mountains, but the Wyoming-Montana coals are the most important economically. Wyoming is the leading state in the US for coal, with 338 million tons produced in 2013, 39% of the U.S. total. West Virginia, at #2, produced 11% of the total, which amounts to about 1.1 billion tons of coal for the whole United States.

In other plant-related news, just this week a new discovery was announced of a carnivorous plant in Baltic amber, dating to about 40 million years ago, the Eocene epoch. LINK

Today, December 4, is St. Barbara’s Day. She’s the patron saint of artillerymen, mathematicians, and miners. Geologists sometimes adopt her, usually as an excuse for a party.

—Richard I. Gibson

LINKS and References:
Recent evolution of grasses
Carnivorous plant

Evolution of grasses 

Cretaceous and Tertiary coals of the Rocky Mountains and Great Plains regions, by R. Flores and T. Cross, 1991, in GSA DNAG volume P-2, Economic Geology, U.S.

Photo from USGS