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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)a

Cenozoic

Neogene

Holocene

Past 10,000 years

Pleistocene

1.6-0.01

Pliocene

5.3-1.6

Miocene

23.7-5.3

 

Paleogene

Oligocene

34-23.7

 

Eocene

55-34

 

Paleocene

65-57.8

Mesozoic

Cretaceous

144-65

 

Jurassic

208-144

 

Triassic

245-208

Paleozoic

Permian

286-245

Carboniferous

Pennsylvanian

320-286

Mississippian

360-320

 

Devonian

408-360

 

Silurian

438-408

 

Ordovician

505-438

 

Cambrian

544-505

Precambrian

Proterozoic

2,500-544

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).

a 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 progress—improved 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.



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