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OVERVIEW AND RECOMMENDATIONS 3 paleogeography, paleoceanography, and paleoclimatology. The recent surge of interest in mass extinctions has helped to promote these developments, but their roots go much deeper. The success of the Deep-Sea Drilling Program and its successor, the Ocean Drilling Program, has opened new opportunities for research, as has recent progress in plate tectonic reconstruction. Diverse new techniques have also fostered progressâimproved methods for dating strata, for example, and new techniques for studying rates of evolution and extinction, as well as innovative ways of using isotopes to evaluate changes in environments, biological activity, and biogeochemical cycles. Progress in all these areas has created a new framework for paleobiology, which entered a renaissance in the 1960s and is now well positioned to study the history of life in the context of a dynamic global environment. Patterns of evolution and extinction derived from fossil data are taking on new meaning in this context and have major implications for evolutionary biology and for studies of human-induced biotic change. The geologic record shows not only how the modern biosphere emerged in association with past global change, but also which kinds of species and biotic communities are most vulnerable to environmental change and which are most resilient. In this Overview, we offer examples of research that is emerging in the study of past changes in the global ecosystem and recommend fruitful areas for research. Detailed discussions of recent advances in understanding ancient environments, the life those environments supported, and reasons for the changes in both biotas and environments appear in the authored chapters that follow this Overview. The wide variety of methods employed to study the dynamics of ancient ecosystems illustrates the interdisciplinary nature of the subject. METHODS Functional morphology provides key biological information. For example, dental morphology reflects a mammal's diet, and the morphology of fossil leaves is an excellent indicator of ancient climatic conditions. Because these features reflect basic laws of physics, their testimony is as powerful for the past as for the future. Terrestrial pollen spectra and marine plankton assemblages offer pictures of climatic conditions that are especially detailed for the past several million years. Preservation of key materials is also of special value, as in the use of coal balls to study the fabric and composition of Pennsylvanian peat, or the use of deep-sea deposits to obtain nearly continuous records of oceanic life and environments. Stable isotopes and other geochemical signatures have been used in a variety of ways to investigate the dynamics of the oceans and climates. Oxygen isotopic composition of marine microfossils is the best indicator available for estimating ocean temperatures for the past 150 m.y. The oxygen isotope ratio (18O/16O) increases in the secreted skeletons with decreasing water temperature. Estimated ocean temperatures need to take into account that 16O evaporates from the sea surface more readily than 18O and accumulates preferentially in glacial ice. This information further allows the estimation of the volume of Cenozoic ice sheets. Oxygen isotopic ratios differ between summer-dwelling species and those representing other seasons and between surface water dwellers and forms that occupy deep, cool waters. Carbon isotopic ratios in deep-sea sediments shed light on productivity and rates of carbon burial. Concentrations of iron and manganese in deep water shales reflect degree of oxidation and, hence, ventilation of the deep-sea. Carbon isotopic ratios in paleosols appear to provide a proxy for past CO2 levels, as does the isotopic composition of specific biomarker organic molecules preserved in marine sediments. General circulation models have opened new possibilities for studying past changes in atmospheres and oceans. Models that couple oceans and atmospheres are especially valuable. Even imperfect global models can assist in simulating consequences of regional perturbations, such as the tectonic elevation of mountain systems and ocean barriers.
