In order to make a more useful and practical time-scale the Ordovician has been sub-divided into distinctive, well-defined intervals of time often referred to as series and stages. The Ordovician, like most periods, has traditionally been sub-divided into Early, Middle and Late epochs, and Lower, Middle and Upper Series, but in practice, this classification is of little use except for the application of broad general patterns. The Ordovician is now dated between 488.3 and 443.7 Ma (million years ago), which makes its duration 44.6 million years (Webby, 2003). The Ordovician Period is stratigraphically younger than the Cambrian Period and stratigraphically older than the Silurian, which overlies it. These dates are used to study, among other things, the tempo or rates of environmental and biologic change occurring on Earth. Using a variety of techniques and dating methods, geologists have been able to ascertain the age of the Earth, as well as major eras, periods, and epochs within Earth's history. The modern Geologic Time Scale as shown above is a compendium of both relative and absolute age dating and represents the most up-to-date assessment of Earth's history. Much of the relative geologic time scale had been constructed prior to the 20th century, but much progress has been made in the last half-century toward defining the absolute ages of the rocks on which the relative time scale is based. These eras were referred to as the Paleozoic (meaning ancient life), the Mesozoic (meaning middle life), and the Cenozoic (meaning recent life) based on their relative similarity with modern taxa. The majority of fossil organisms, however, did not match with modern groups this led to the classification of three major eras within the period of time when the Earth's surface was populated with advanced life forms. Moreover, these geologists recognized that the fossilized biota demonstrated rather large changes in overall composition and showed both similarities with, and differences from living taxonomic groups. Within the context of the Phanerozoic Eon, geologists beginning in the late 1700's recognized that fossils appeared in an orderly fashion in stratigraphic units. the Paleozoic, Mesozoic and Cenozoic Eras) representing the period of Earth's history with advanced life forms, and the Pre Cambrian (or Proterozoic and Hadean Eras) representing the period before advanced life. In the Geologic Time Scale, time is generally divided on the basis of the earth's biotic composition, with the Phanerozoic Eon (i.e. In addition to the relative dating of periods in Earth's history for which we have rocks preserved, geologists are now able to assign absolute age dates to critical intervals. The Geologic Time Scale, as shown above, documents intervals of geologic time relative to one another, and has been continuously developed and updated over the last two centuries. Diagram from the United States Geological Survey
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