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Encyclopedia Britannica



PALAEOZOIC ERA

This article appears in Volume V20, Page 592 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: PAI-PAS
PALAEOZOIC ERA , in geology, the oldest of the great time divisions in which organic remains have left any clear record. The three broad divisionsPalaeozoic, Mesozoic, Cainozoicwhich are employed by geologists to mark three stages in the development of life on the 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 line in the " record of the rocks " where the chronicle of the Palaeozoic era closes and that of the Mesozoic era opensas in more
recent
  historical documentsis a matter for editorial caprice. The early geologists took the most natural dividing lines that came within their knowledge, namely, the line of
change
  in general petrological characters, e.g. the " Ttansition
Series
  " (Ubergangsgebirge), the name given to rocks approximately of Palaeozoic age by A. G. Werner because they exhibited a transitional stage between the older crystalline rocks and the younger non-crystalline; later in Germany these same rocks were said to have been formed in the " Kohlenperiode " by H. G.
Bronn
  and others, while in England H. T. de la Beche classed them as a Carbonaceous and Greywacke 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 differences between the fossils found below and those found above the 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 groups which made the divisions
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 short, the record is more nearly complete than was originally supposed.
The Palaeozoic or Primary era is divided into the following periods or epochs: Cambrian, Ordovician,
Silurian
 , Devonian, Carboniferous and Permian. The fact that fossils found in the rocks of the three earlier epochsCambrian, Ordovician,
Silurian
  have features in common, as distinguished from those in the three later epochs has led certain authors to 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 limestone in places. These sediments have been altered to shales, slates, quartzites, &c., and frequently they are found in a highly metamorphosed condition; in eastern North America, however, and in north-
east
  Europe they still maintain their horizontality and primitive texture over large areas. The fossils of the earlier Palaeozoic rocks are characterized by the abundance of trilobites, graptolites, brachiopods, and the absence of all vertebrates except in the upper strata; the later rocks of the era are distinguished by the absence of graptolites, the
gradual
  failing of the trilobites, the continued predominance of brachiopods and tabulate corals, the abundance of crinoids and the rapid development of placoderm and heterocercal ganoid fishes and amphibians. The land 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 base of the Cambrian is usually placed at the zone of the trilobite Olenellus. (J. A. H.)


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