How do scientists learn about plants that are extinct?
For a plant to become fossilized, it first has to be buried underneath mud or sediment quickly (so it doesn’t start to decay) and gently (so it doesn’t fall apart). As minerals accumulate around the plant structure and form into rock over time, the shape of the plant is preserved in the rock. Depending on how the plant gets preserved, different types of fossils form. In some fossils, called impressions or compressions, the plant gets flattened out, allowing scientists to use high-powered microscopes to observe detailed features about the shape of the plant. Sometimes a three-dimensional plant is preserved, leaving behind a mold, or cast, as the original plant material decays and is filled in by minerals. This type of fossil enables scientists to see the whole structure of the plant. In rare cases, minerals can penetrate into the plant tissue and preserve the cells in great detail. This process is called permineralization, and it’s how petrified wood is formed. Studying different types of fossils of the same kind of plant helps paleobotanists get a more complete picture of the extinct plant.
Using the physical characteristics they observe, paleobotanists can try to match fossils to a living species to hypothesize about the type of climate in which the extinct plant lived. For example, if the fossil shows a leaf with a thick, waxy structure, the plant probably lived in a dry environment where it had to conserve water, similar to living plants with those traits. Scientists also use radioactive techniques to determine the age of the rocks in which fossils are found. By comparing fossils of different ages, paleobotanists can identify when different traits first appeared, allowing us to understand how plants today might have evolved.
Although studying plant fossils often involves looking at what’s left in the rocks around them, sometimes in very cold or dry environments, the actual plant material can be preserved. Analysis of spores, pollen, or amber from these specimens is often used to determine past environmental conditions, an important aspect of studying climate change. Finally, with modern techniques of extracting DNA, scientists can obtain much more accurate evidence of the relationships between extinct and living plants that they can use to complement their observations of fossils.
For more information:
http://www.fossils-rocks-minerals.com/plant-fossils/common-plant-fossils.htm -- A brief overview of how plant fossils are formed
http://museumvictoria.com.au/prehistoric/time/plant.html -- This site from the Museum Victoria in Australia shows some comparisons of fossil plants to living species
http://www.paleobotanyproject.org/ -- The Paleobotany Project at the Denver Museum of Nature and& Science has an extensive image collection of fossil plants and explains the characteristics used to classify them
http://paleoplant.blogspot.com/ -- Jamie Boyer at the New York Botanical Garden blogs about recent research in paleobotany and the evolution of plants