The Time Lines
GENERAL CONSIDERATIONS The Precambrian is defined as the period of time that extends from a little more than 3.9 billion years ago, which is the approximate age of the oldest known rocks, to the beginning of the Cambrian Period, roughly 540 million years ago. The Precambrian era thus represents more than 80 percent of the whole of geologic time. It has long been known that the Cambrian marks the earliest stage in the history of the Earth when many varied forms of life evolved and were preserved extensively as fossil remains in sedimentary rocks. Detailed mapping and examination of Precambrian rocks on most continents have since revealed that primitive life-forms already existed more than 3.5 billion years ago. The original terminology to distinguish Precambrian from all younger rocks, nevertheless, is still used for subdividing geologic time.
Paleozoic Era
Paleozoic Era, also spelled PALAEOZOIC, major interval of geologic time extending from 540 to 245 million years ago. It is the first era of the Phanerozoic Eon.
A brief treatment of the Paleozoic Era follows. For full treatment, see Geochronology: Paleozoic Era .
Eras have traditionally been named to reflect major changes in the development of life on Earth. The Paleozoic, from the Greek for "ancient life," is followed by the Mesozoic ("intermediate life") and Cenozoic ("recent life"). The Paleozoic is divided into six periods: the Cambrian , Ordovician , Silurian , Devonian , Carboniferous , and Permian (qq.v.).
The Mesozoic (from the Greek for middle life) began about 245 million years ago and ended 66.4 million years ago (see Table 4). The major divisions of the era, from oldest to youngest, are the Triassic, Jurassic, and Cretaceous periods. The Mesozoic was a time of heightened tectonic activity during which the supercontinent of Pangaea fragmented into separate continents that were gradually scattered across the Earth in a nearly modern geographic distribution. It also was a time marked by a distinct modernization of life-forms; the ancestors of the major plant and animal groups that exist today first made their appearance.
The Cenozoic began about 66.4 million years ago and extends to the present (see Table 4). The term, originally spelled Kainozoic, was introduced by John Phillips in an 1840 Penny Cyclopaedia article to designate the most recent of the three major subdivisions of the Phanerozoic. Today, the Cenozoic is internationally accepted as the youngest of the three subdivisions of the fossiliferous part of Earth history. The Cenozoic Era is generally divided into two periods, the Tertiary and the Quaternary. In 1856 Moritz Hörnes introduced the terms Paleogene and Neogene, the latter encompassing rocks equivalent to those described by Charles Lyell as Miocene and older and newer Pliocene (which included what he later called the Pleistocene; see above Completion of the Phanerozoic time scale ). Subsequent investigators have determined that the designation Neogene correctly applies to the rock systems and corresponding time intervals delineated by Lyell, though some authorities prefer to exclude the Pleistocene from the Neogene. The Paleogene encompasses the Paleocene, Eocene, and Oligocene. (The terms Paleocene and Oligocene were coined subsequent to Lyell's work and inserted in the lower part of the Cenozoic stratigraphic scheme.)
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