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Paleobotany

species pollen plant fossil

Paleobotany endeavors to reconstruct past climates and regional vegetation systems by studying the fossilized remains of plants or preserved pollen samples. Such studies have yielded information regarding global climate change, both natural and man-made, and its effects on specific environments. Paleobotanists aid in the identification of various climatic episodes. By collaborating geological evidence of glacial periods, or ice ages, with changes in regional flora, scientists have been able to create a more detailed picture of the development, course, and effects of such episodes. Paleobotany is an essential branch of research on evolution. As early as 1790, some of the seminal research in evolutionary theory included botanical studies. Today, paleobotany is utilized in a multitude of scientific settings, from archaeology to natural resource acquisition.

Paleobotany is essentially the study of the plant life of the geological past. Paleobotany is pursued most often through the study of fossils, or impressions of plant parts that have been preserved in sedimentary rocks, coal, or other geological deposits. The most ancient plant fossils are older than one billion years, as is the case of microscopic impressions of Precambrian algae. There are also much younger fossils, as is the case of pollen in recently deposited lake sediments.

The primary goals of paleobotany are to discover the earliest appearances of various groups of plants, and to understand the evolutionary relationships among these taxa. Other objectives of paleobotany include the use of knowledge about fossil plants to infer the likely characteristics of their environment, including the type of climatic conditions under which they grew. Paleobotanists are also interested in the nature of the communities of fossil plants, and the species of animals with which they may have lived. Sometimes paleobotanical knowledge can be used for more practical purposes, such as assisting in the discovery of underground reserves of fossil fuels.

Paleobotanists commonly collect and identify microscopic spores, pollen, and bits of larger tissues. They also may identify larger, macroscopic plant remains such as leaves and even fossil tree trunks. Often, only the major plant group to which these plant parts belong, such as order or family, can be identified. In the case of more recent plant fossils that represent species that are still extant (not extinct), the remains may even be identifiable down to genus or species. Sometimes, the age of samples is known quite accurately. Paleobotanical studies of some recent lake sediments have shown that sediment layers sometimes develop as annual accumulations. The total number of layers can be subtracted from the current year to determine an age for the sequence or any layer within.

Palynology (the study of fossil spores and pollen) is an important sub-discipline of paleobotany, and can be used to illustrate the nature and breadth of paleobotanical research. Palynologists search samples of lake sediment, river sediment, or a bog peat of known age, carefully identifying and counting the microscopic pollen. Identification serves to place each specimen into whatever fossil group it belongs, down to the most specific level possible, which is often to the species.

From the assemblages of fossil pollen, palynologists make inferences about the types of forests or other plant communities that may have occurred in the local environment. These interpretations must be made carefully, however, because species are not represented in the pollen record in ways that directly reflect their abundance as mature plants. For example, pollen of wind-pollinated species is relatively abundant in lake sediments, whereas species that are insect pollinated are not well Fossilized Alethopteris. A seed fern from the Pennsylvanian period. JLM Visuals. Reproduced with permission.
represented. Therefore, palynological studies of lake sediment might indicate that about 15,000 years ago the local environment around a particular lake in Minnesota used to support species that are now typical of northern tundra, while 10,000 years ago the vegetation was a boreal forest of spruces and fir. More recently, the pollen assemblage may be dominated by species such as oaks, maples, basswood, chestnut, and other species of trees from more temperate climates. Combining these sorts of observations and knowledge of the present, climaticallyinfluenced distributions of these species, scientists can come to insightful conclusions about both the historical plant communities and past climates that occurred after the most recent glaciation ended in the region in which the lake occurs.

Palynologists sometimes work with archaeologists to study plants represented in archaeological deposits. Pollen samples are collected from geological strata or from artifacts, such as charcoal from the inside face of pottery, and is then analyzed to determine what types of plants are present. Samples taken from geological strata yield clues about the environment in which prehistoric people lived. Similar samples taken from artifacts give researchers clues about prehistoric subsistence and farming patterns. For example, palynological studies provided the first scientific evidence of crop domestication, especially corn, in the Americas. Archaeologists have also used palynological research on past climates to determine which species of plants or crops were imported to certain areas through trade or conquest. This area of paleobotany is often known as paleoethnobotany, a special sub-discipline interested in the way past communities interacted with local flora and climates.

Bill Freedman

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