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PALAEOZOIC ERA

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Originally appearing in Volume V20, Page 592 of the 1911 Encyclopedia Britannica.
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PALAEOZOIC ERA , in See also:

geology, the See also:oldest of the See also:great See also:time divisions in which organic remains have See also:left any clear See also:record. The three broad divisions—Palaeozoic, Mesozoic, Cainozoicwhich are employed by geologists to See also:mark three stages in the development of See also:life on the See also:earth, are based primarily upon the fossil contents of the strata which, at one point or another, have been continuously forming since the very earliest times. The precise See also:line in the " record of the rocks " where the See also:chronicle of the Palaeozoic era closes and that of the Mesozoic era opens—as in more See also:recent See also:historical documents—is a See also:matter for editorial caprice. The See also:early geologists took the most natural dividing lines that came within their knowledge, namely, the line of See also:change in See also:general petrological characters, e.g. the " Ttansition See also:Series " (Ubergangsgebirge), the name given to rocks approximately of Palaeozoic See also:age by A. G. See also:Werner because they exhibited a transitional See also:stage between the older crystalline rocks and the younger non-crystalline; later in See also:Germany these same rocks were said to have been formed in the " Kohlenperiode " by H. G. See also:Bronn and others, while in See also:England H. T. de la Beche classed them as a Carbonaceous and See also:Greywacke See also:group. Finally, the divisional time separating the Palaeozoic record from that of the Mesozoic was made to coincide with a great natural break or unconformity of the strata. This was the most obvious course, for where such a break occurred there would be the most marked See also:differences between the fossils found below and those found above the See also:physical discordance. The divisions in the fossil record having been thus established, they must for convenience remain, but their artificiality cannot be too strongly emphasized, for the broad stratigraphical gaps and lithological See also:groups which made the divisions See also:sharp and clear to the earlier geologists are proved to be absent in other regions, and fossils which were formerly deemed characteristic of the Palaeozoic era are found in some places to commingle with forms of strongly marked Mesozoic type.

In See also:

short, the record is more nearly See also:complete than was originally supposed. The Palaeozoic or See also:Primary era is divided into the following periods or epochs: See also:Cambrian, Ordovician, See also:Silurian, Devonian, Carboniferous and See also:Permian. The fact that fossils found in the rocks of the three earlier epochs—Cambrian, Ordovician, Silurian —have features in See also:common, as distinguished from those in the three later epochs has led certain authors to See also:divide this era into an earlier, Protozoic (Proterozoic) and a later Deuterozoic time. The rocks of Palaeozoic age are mainly sandy and muddy sediments with a considerable development of See also:limestone in places. These sediments have been altered to shales, slates, quartzites, &c., and frequently they are found in a highly metamorphosed See also:condition; in eastern See also:North See also:America, however, and in north-See also:east See also:Europe they still maintain their horizontality and See also:primitive texture over large areas. The fossils of the earlier Palaeozoic rocks are characterized by the abundance of See also:trilobites, See also:graptolites, brachiopods, and the See also:absence of all vertebrates except in the upper strata; the later rocks of the era are distinguished by the absence of graptolites, the See also:gradual failing of the trilobites, the continued predominance of brachiopods and tabulate See also:corals, the abundance of crinoids and the rapid development of placoderm and heterocercal ganoid fishes and amphibians. The See also:land See also:plants were all cryptogams, Lepidodendron, Sigillaria, followed by Conifers and Cycads. It is obvious from the advanced stage of development of the organisms found in the earliest of these Palaeozoic rocks that the beginnings of life must go much farther back, and indeed organic remains have been found in rocks older than the Cambrian; for convenience, therefore, the See also:base of the Cambrian is usually placed at the See also:zone of the trilobite Olenellus. (J. A.

End of Article: PALAEOZOIC ERA

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