Paleozoic

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Palaeozoic The Palaeozoic era and rocks are named on account of the fossils of ‘ancient life’ (from the Greek) within them. The term was first used by Professor Adam Sedgwick in 1838 for his series of somewhat deformed rocks lying beneath the Old Red Sandstone in Wales. It was soon afterwards used by John Phillips to include all the rocks that Sedgwick had described plus the Old Red Sandstone, the Carboniferous, and the Magnesian Limestone of northern England. Today it encompasses the systems from Cambrian to Permian inclusive, and spans geological time from 545 Ma to 250 Ma. Conventionally, the Cambrian, Ordovician, and Silurian systems constitute the Lower Palaeozoic and the Devonian, Carboniferous, and Permian comprise the Upper Palaeozoic.

The base of the Cambrian system is thus also the base of the Palaeozoic erathem. For many years the lowest occurrence of trilobites was taken as the first indication of Palaeozoic life, but the International Commission on Stratigraphy regarded a properly defined and internationally recognized base of the Cambrian system as a priority, and set up a working group to find one. This has now been done (see Cambrian).

The lowest Cambrian rocks are not everywhere very fossiliferous, but the record shows that by Mid-Cambrian time there was a very diverse marine ecosystem. Faunal communities, largely of animals lacking hard parts, were superseded by those in which well-skeletonized creatures were prominent. This probably indicates that animals evolved to colonize the more turbulent and shallow reaches of the seas and inland waters during early Cambrian times.

The biostratigraphy of this immense sector of geological time is highly varied, with many fossil groups providing zonal schemes. All modern animal phyla and a large part of the plant kingdom are represented. Habitats ranged from the terrestrial to the bathyal. There were several groups of animals that were highly successful during this era but became extinct before its end. Certain facies in each system can be correlated and dated with great precision on the basis of zonal fossils. The development of terrestrial vegetation, culminating in the great forests of the Carboniferous period, had profound effects upon ecology. Not only were new habitats provided and habitable space on the planet increased, but the spread of vegetation from water to land affected rates of rock weathering, erosion, and sedimentation. Terrestrial tetrapods had appeared well before the end of Devonian time. Marine fossil evidence tells of numerous extinctions, global mass extinction, and geochemical events. At the end of the Palaeozoic era the greatest extinction event of all time took place, providing the classical catastrophe of early nineteenth-century writers.

Palaeozoic geography progressed through an initial dispersal of cratons from a late Proterozoic supercontinent in the southern hemisphere, followed by a progressive regrouping near to the equator by late Carboniferous times. The result was the supercontinent Pangaea with a widespread continental climate. During this long period of time, plate-tectonic margin activity was vigorous and collision tectonics common. At various times parts of the great southern continental area (Gondwanaland) experienced the growth of ice sheets of enormous proportions. Evidence for former glaciation occurs in the Silurian of North Africa and the Devonian of South America, but in the Carboniferous and Permian rocks of all the southern continents there are abundant signs of a very long glacial period, with many phases of growth and decline of an ice mass of unrivalled size. These glacial oscillations constantly affected global sea level and sedimentation at the continental margins.

Palaeozoic volcanicity was conspicuous along active continental margins especially where there was subduction. All traces of Palaeozoic ocean floor are presumed to have been subducted. There were numerous polar reversals, and mantle activity appears to have been an underlying cause of these events.

D. L. Dineley

Bibliography

Hailwood, E. A. and Kidd, R. B. (eds) (1993) High resolution stratigraphy. Geological Society of London Special Publication No. 70.
Harland, W. B. et al. (eds) (1967) The fossil record. Geological Society, London.
McKerrow, W. S. and and Scotese, C. R. (1990) Palaeozoicpalaeogeography and biogeography. Geological Society of London Memoir No. 12.
Moullade, M. and Nairn, A. E. M. (eds) (1991) The Phanerozoic geology of the world. I. The Palaeozoic. Elsevier, Amsterdam.

Palaeozoic(Paleozoic) The first of the three eras of the Phanerozoic, about 570–248 Ma ago. The Cambrian, Ordovician, and Silurian Periods together form the Lower Palaeozoic Sub-Era; the Devonian, Carboniferous, and Permian the Upper Palaeozoic Sub-Era. It was an era of great evolutionary change among plants, which began to invade the land at its beginning. By the end of the era, amphibians and reptiles were major components of various communities and giant tree-ferns, horsetails (Calamitaceae), and cycads gave rise to extensive forests. The faunas of the Palaeozoic are noted for the presence of many invertebrate organisms including trilobites, graptolites, brachiopods, cephalopods, and corals. The name is derived from palaeo- and the Greek zoe, meaning ‘life’.

Palaeozoic The first (570–248 Ma) of the three eras of the Phanerozoic. The Cambrian, Ordovician, and Silurian Periods together form the Lower Palaeozoic sub-era; the Devonian, Carboniferous, and Permian the Upper Palaeozoic sub-era. During the Palaeozoic, two major orogenies occurred: the Caledonian during the Lower Palaeozoic, and the Variscan in late Palaeozoic times. The faunas of the Palaeozoic are noted for the presence of many invertebrate organisms, including trilobites (Trilobita), graptolites (Graptolithina), brachiopods (Brachiopoda), cephalopods (Cephalopoda), and corals. By the end of the era, amphibians and reptiles were major components of various communities and giant tree-ferns, horsetails, and cycads gave rise to extensive forests.

Palaeozoic (Paleozoic) The first of the 3 eras of the Phanerozoic, about 590–248 Ma ago. The Cambrian, Ordovician, and Silurian Periods together form the Lower Palaeozoic Sub-Era; the Devonian, Carboniferous, and Permian the Upper Palaeozoic Sub-Era. It was an era of great evolutionary change among plants, which began to invade the land at its beginning. By the end of the era, amphibians and reptiles were major components of various communities and giant treeferns, horsetails (Calamitaceae), and cycads (Cycadaceae) gave rise to extensive forests. The name is derived from palaeo- and the Greek zoe, meaning ‘life’.

Palaeozoic (Paleozoic) The first of the three eras of the Phanerozoic, about 542–251 Ma ago. The Cambrian, Ordovician, and Silurian Periods together form the Lower Palaeozoic Sub-Era; the Devonian, Carboniferous, and Permian the Upper Palaeozoic Sub-Era. The faunas of the Palaeozoic are noted for the presence of many invertebrate organisms including Trilobitomorpha, Graptolithina, Brachiopoda, Cephalopoda, and corals. By the end of the era, amphibians and reptiles were major components of various communities and giant tree-ferns, horsetails, and cycads gave rise to extensive forests.

Palaeozoic Second era of geological time, after the Precambrian era, lasting from 590 million to 248 million years ago. It is sub-divided into six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian. Invertebrate animals evolved hard skeletons capable of being preserved as fossils in the Cambrian; fish-like vertebrates appeared in the Ordovician; amphibians emerged in the Devonian; and reptiles in the Carboniferous.

Palaeozoic The first era of Phanerozoic time. It follows the Precambrian and is subdivided into the Lower Palaeozoic, comprising the Cambrian, Ordovician, and Silurian periods, and the Upper Palaeozoic, comprising the Devonian, Carboniferous, and Permian periods, It extended from about 590 million years ago to about 248 million years ago, when it was succeeded by the Mesozoic era.

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