Ward's World Activity Guides

these include groups that are abundant, geographically wide- spread, environmentally tolerant, and rapidly evolving. Although few types of organisms share all of these traits, certain groups, such as free-swimming and planktonic organisms (for example, some foraminifera, graptolites, ammonoids, and conodonts), meet most criteria and are among the most widely used zonal fossils. The primary procedures of biostratigraphy involve es- tablishing the ranges of species and using these index fossils to correlate rocks of similar age in different locations. In some cases, the precision of correlation can be improved by using overlap- ping ranges of several species, or assemblages of fossils. Correla- tions also can be improved by methods called graphic correla- tion in which the first and last appearances of many species in two or more sections are plotted against one another. Taphonomy The field of taphonomy deals with the processes of fos- sil preservation both prior to burial (biostratinomy) or after burial (fossil diagenesis). Paleontologists pursuing studies of taphonomy may study the rates and processes of decay, disarticulation, breakage, abrasion, and disintegration of organ- isms and the compaction and mineralization of their remains. The objectives of taphonomy are twofold. First, by studying processes that affect potential fossil assemblages in modern times, it is possible to recognize the extent of biases of the fossil record in different environments. Second, fossils, as sedi- mentary particles, provide very important clues to many physical and chemical processes in ancient environments. For example, shallow-water, high- energy environments yield heavily fragmented and abraded skeletal remains, whereas muddy bottoms in deeper-water areas may preserve deli- cate shells and articulated crabs or sea urchins. Ichnology The realm of ichnology deals with trace fos- sils and their interpretation. This field (together with taphonomy) interfaces closely with sedi- mentology because most trace fossils are in one sense literally a part of the sediments or organic sedimentary structures. Trace fossils have proven to be very useful as indicators of depositional environments. The size, depth, and diversity of traces may indicate the level of oxygen in marine bottom waters; the degree of deformation or sharp- ness of burrows is an indicator of sediment firmness and the degree of compaction; and distinctive trace fossil as- semblages may provide a proxy for water depth. In addition, of course, traces provide a direct record of the behavior of ancient organisms and may be useful in identifying patterns of locomo- tion, sediment feeding, and even predator-prey relationships. Paleoecology and Paleoenvironmental Analyses Paleoecology is the ecology of ancient organisms, that is, their modes of life and interactions with physical and biotic environments. Modes of life of ancient organisms are recon- structed on the basis of (1) biological information from closely related living analogs, (2) functional morphology (function of skeletal parts inferred from detailed studies of their form) of the fossils themselves, and (3) association with sediments or other fossils, which give environmental information (Fig. 4). Environments are reconstructed using multiple sources of data from sedimentary rocks, including the sediments themselves (grain size, texture, and color) and sedimentary structures (for example, mud cracks and ripple marks). In some instances, particularly with relatively recent fossils, knowledge of the environmental tolerances of modern species can be used to infer the environments of the same or closely related fossil species. Other, less direct applications of fossil data may be ex- tremely useful in inferring ancient environmental parameters. For example, the forms of coral skeletons are commonly reflec- tive of the energy of the environment in which they grew. Paleontology (continued) + ward ' s science Fig. 4: Paleoecology of fossil organisms. A food web reconstructed for the Ordovician marine community shows inferred modes of life of various organisms found as fossils. (Credit: Adapted from an original drawing by D. R. Prothero and R. H. Dott)

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