Paleobotany is the study of plants that lived in prehistoric times. It largely involves the study of fossils, that is, impressions of plant parts that have been preserved in sedimentary rock or coal. The most ancient plant fossils are of microscopic algae that lived more than one billion years ago, during Precambrian times. Paleobotanists also study much younger plant fossils, such as pollen in recent lake sediment.
One of the goals of paleobotany is to discover the earliest occurrences of different kinds of plants in the geological record. This knowledge of sequential occurrence of taxa is then used to develop an understanding of the evolutionary relationships among groups of plants. Other paleobotanists are interested in determining what fossil plants were like, and the kinds of animals that utilized them as food and habitat. This information can also be used to infer the characteristics of the ancient environment, including the type of climatic conditions in which the plants grew. Sometimes paleobotanical knowledge can be used for quite practical purposes, such as the development of fossil-plant indicators that can be used to help locate underground deposits of coal or petroleum.
The most common samples studied by paleobotanists are microscopic spores, pollen, and bits of tissues. They also may identify larger, "macroscopic" remains, such as foliage and even substantial parts of fossil tree trunks. These various plant parts can usually be classified to their major group, such as an order or family. Plant fossils of species that still exist may be classified to the actual genus or even the species.
In addition to classifying the remains of their fossil samples, paleobotanists must know the context of fossils in terms of geological time. The relative age of fossil deposits can be inferred from geological stratigraphy (that is, the layers of rock or mud from which they were retrieved). An even more accurate process to measure age calculates the degree of radioactive decay of isotopes of certain elements.This carbon-dating method is based on the measurement of the amount of carbon-14 (compared with that of carbon-12) in an isotope, and is commonly used to date organic material up to 40-50 thousand years old. In some cases, recent fossils retrieved from deep lakes can be dated quite accurately, by calculating their position in annual layers of sediment (the highest, most recent layer corresponds to the present time).
Palynologists are paleobotanists whoexamine samples of lake sediment or bog peat of known age for fossil pollen (). They carefully enumerate the pollen and classify it to the degree possible, which is often to species or genus. Palynologists use information on fossil pollen to make inferences about the types of local forest and other plant communities that may have occurred near where the fossils were found. Such interpretations must be made carefully, because species are not represented in the pollen record in the same proportions that they occurred in the local vegetation.The pollen of wind-pollinated species is much more abundant in lake sediment than that of insect-pollinated species. Reconstructions of local paleovegetation also allows inferences to be made about local climatic conditions at the time. For example, a typical palynological study might discover the following about the local environment around a lake in Wisconsin: about 15 thousand years ago the site supported plant species that are now typical of northern tundra; 10 thousand years ago the vegetation was a boreal forest of spruces and fir; and during the most recent centuries the pollen assemblage was dominated by oaks, maples, basswood, chestnut, and other relatively southern trees. These kinds of observations, coupled with knowledge of the present, climatically influenced distributions of these tree species, allows palynologists to make reasonable inferences about the historical plant communities and climates that occurred since the glaciers retreated from the region just over 15 thousand years ago.
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