Index fossils are sometimes used by Geologists to determine stratigraphic data about Earth's surface. This is true in the case of spores and pollen grains, the reproductive materials found in plants. By doing so provides the palyntologist means to determine relative age and position of rock, containing the spores. When studying drill cores, this kind of information can be useful to miners searching for fossil fuels.
Palynologists have used index fossils for practical applications, including understanding relationship between major groups of plants, specifically, gymnosperms and angiosperms.
A good index fossil is considered to be one that is easily identifiable, with wide horizontal distribution, and vertical range of approximately one million years. Traditionally, due to scarcity and difficulty in identification, plant megafossils were rarely used as index fossils. Though possessing wide geographical distribution in various sedimentary rocks, the vertical range spans millions of years. Assemblages of megafossils used as indices (or, indexes), accompanied by palynological information, scientists have been able to characterize restrictive stratigraphic units in rock units containing megafossils.
Extant organisms in their structure and distribution, reflect the composition of their environments. We assume extinct organisms also adapted to their environment in the same way. If this assumption is true, then it is possible to determine seasonal variations using growth rings from petrified wood, including paleo-environmental availability of water and temperature changes. Fossilized wood which reflects lack of growth rings, indicate a continuous supply of water and uniform temperature, just as thickened cuticles and sunken stomata of fossilized leaves indicate a lack of water, while roots and spongy stem tissue suggest a swampy or aquatic paleo-environment. With such information extracted from morphology and anatomy of fossil plants, provides in part, the basis for paleoecology and paleoclimatology. Further studies are taken into consideration, such as those on sedimentary materials which naturally occur with the fossils and, how the fossil became preserved, all play into better understanding the paleoenvironment. Specialists caution however, conclusions should never be based entirely upon extant organisms and how they interact with present environments. What we observe occuring today, is not necessarily the key to the past. (Paleobotany and the Evolution of Plants, Stewart and Rothwell, 1993; DiMichele & Wing, 1987.)
Paleofloristics, which specializes in assemblages of fossil plants, provides insight into Earth's restricted and widescale climate. On the worldwide scale, such studies have provided scientists with insight into plate tectonics and continental drift. These studies can also provide further insight into plant distribution, population, migration and significant changes in early environments. Studies on succession of plants in the geological column have became popular among paleoecologists, and also useful in studies on extant organisms and plant successions in natural history.
Successional changes in organisms throughout natural history are the basis of studies related to the evolution of life. Most paleontologists focus more on relative stratigraphic positioning of fossils, than absolute ages. It remains more important to most in the field, to determine how a fossil relates to other specimen with objective to un-ravel evolutionary patterns and origins.
Bringing Fossils to Life, An Introduction to Paleobiology, McGraw Hill Publishers, Donald R. Prothero
Paleobotany and the Evolution of Plants, by Cambridge University Press; 2 edition, Wilson N. Stewart, Gar W. Rothwell
Atlas of the Prehistoric World, by Discovery Channel Books, Douglas Palmer
Kingfisher Illustrated Dinosaur Encyclopedia, Kingfisher Publishers, David Burnie
FURTHER SUGGESTED READING
Adrienne Mayor's books
1) The First Fossil Hunters (Princeton 2000) explains how ancient Greek and Roman discoveries of mysterious petrifed bones of extinct dinosaurs and mastodons led to myths about griffins, giants, and monsters. Watch for "Ancient Monster Hunters" on the History Channel.
2) Fossil Legends of the First Americans (Princeton 2005) gathers exciting Native American discoveries and myths about fossils, from tiny shells to enormous dinosaur bones, with stories from more than 45 different tribes, beginning with the Aztecs & Incas.
Stephen Meyer's article, "Are Dinosaurs Mentioned in the Bible?"