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OVERVIEW AND RECOMMENDATIONS 2 ancient oceans. Information about biotic productivity and other aspects of ancient ecosystems contributes to the understanding of secular changes in geochemical cycles. In all large-scale studies of ancient ecosystems, high-resolution stratigraphy is essential for establishing time scales. (Table 1 offers a simplified geologic time scale, which is designed to assist readers who are nongeologists.) INTRODUCTION The past few years have seen the emergence of a new interdisciplinary field of earth science that addresses the impact of large-scale environmental changes on ancient life. Exemplifying this development has been the maturation of the overlapping disciplines of TABLE 1 Simplified Geologic Time Scale Era Period Epoch Time (m.y. ago)* Holocene Past 10,000 years Pleistocene 1.6-0.01 Cenozoic Neogene Pliocene 5.3-1.6 Miocene 23.7-5.3 Oligocene 34-23.7 Paleogene Eocene 55-34 Paleocene 65-57.8 Cretaceous 144-65 Mesozoic Jurassic 208-144 Triassic 245-208 Permian 286-245 Carboniferous Pennsylvanian 320-286 Mississippian 360-320 Paleozoic Devonian 408-360 Silurian 438-408 Ordovician 505-438 Cambrian 544-505 Proterozoic 2,500-544 Precambrian Archean Prior to 2,500 NOTE: The time scale was initially devised based on paleontologic evidence, with each period and epoch representing a significant paleontologic change. Each of the epochs can be further subdivided (e.g., the Cenomanian age that is in the Cretaceous Period, with ages ranging from about 97.5 to 91 million years (m.y.) ago). * The relative numerical ages, based largely on radiometric determinations, are mostly from the Decade of North American Geology (1983) time scale issued by the Geological Society of America, with more recent modifications for the Cenozoic part of the record and for the Cambrian-Precambrian boundary 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.
