Palaeozoic Rocks, the oldest group of fossiliferous rocks, formerly termed primary. They consist largely of sandstones and shales, with greywacke and slate in the lower part of the group, and limestone, coal, and dolomite in the upper, and numerous great thicknesses of contemporaneous volcanic rocks. The group is subdivided into six systems, as follows: -
Upper Paleozoic - Permian System. Carboniferus System. Devonian System.
Lower Paleozoic - Silurian System. Ordovian System. Cambrian System.
On account of their great relative age, Palaeozoic rocks have been more subject to disturbance than others, and are often bent, contorted, faulted, cleaved, and otherwise metamorphosed, whilst the Secondary or Mesozoic rocks (q.v.) commonly rest unconformably upon them in more horizontal positions.
The fossils of the Palaeozoic rocks are widely different from living organisms. In the lower systems there is little evidence of plants or land-animals. The plants of the upper systems include large club-mosses and horse-tails, ferns and conifers; but no known angiosperms. The corals belong mainly to the Tetra-coralla; sea-urchins are rare, and have more than twenty rows of plates; and the extinct classes of echinoderms, the Blastoidea (q.v.) and Cystoidea (q.v.), occur, as well as numerous crinoids. Graptolites (q.v.), trilobites (q.v.), and Eurypterida (q.v.) are confined to Palaeozoic rocks, and Orthoceratidae, straight allies of the pearly nautilus, are nearly so. Brachiopoda (q.v.) were more abundant than in subsequent times; Pelecypoda and Gastropoda less so. The insects all belonged to an extinct generalised sub-class, the Palaeodictyoptera (q.v.); and the fish had cartilaginous skeletons and unequally-lobed tails. No mammals or birds are known from Palaeozoic rocks. Only sixteen genera of organisms, and no species, are now alive that were living when they were deposited.