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Paleozoic

A major division of time in geologic history, extending from about 540 to 250 million years ago (Ma). It is the earliest era in which significant numbers of shelly fossils are found, and Paleozoic strata were among the first to be studied in detail for their biostratigraphic significance. Western Europe, especially the British Isles, was the cradle of historical geology. Early work with rock strata and their fossils was strictly practical; the relative ages of rock units were essential for correlating scattered outcrops to search for natural resources—particularly coal—in the early part of the nineteenth century.

During its first four decades, natural groupings of strata were studied and named for easy reference. Thus the several subdivisions of the Paleozoic, ultimately the six standard systems, were established. The original basis for establishing sequence was superposition. The operational stratigraphic hypothesis is that, in most instances, the strata at the bottom of a sequence are the oldest and the overlying beds are progressively younger. Thus, the basal system of the Paleozoic, in which primitive shelly fossils are found, is the Cambrian. As younger and younger layers were studied, their fossils collected, and the biological affinities suggested, the concept of evolution from simpler to more complex life forms took shape in the minds of the paleontologists and geologists who were studying the rocks. This process did not take place in an orderly way, from oldest to youngest strata, but rather as a consequence of fulfilling a need of the moment, whether to complete a geologic map or to solve a problem of stratigraphic correlation. Consequently, the first Paleozoic system to be named and studied in some detail was the Carboniferous—the great “coal-bearing” sequence—given that name by W. D. Conybeare and W. Phillips in 1822. These strata were to provide the world's major energy resources during the next century and a half. Most of the Northern Hemisphere's coal fields, and much of its oil and gas as well, were produced from Carboniferous rocks.  See also: Superposition principle

In the 1830s and 1840s two British geologists, R. Murchison and A. Sedgwick, studied and named the natural groupings of rock strata in the British Isles. Sedgwick named the Cambrian System in 1835, for a sequence of strata that overlies the Primordial (Precambrian) rocks in northwest Wales. Four years later, Murchison gave the name Silurian to the early Paleozoic rocks found in the Welsh borderland. However, there was an almost complete overlap of the Cambrian by Murchison's Silurian. It was not until 1879, when C. Lapworth named the Ordovician System for rocks intermediate between the Cambrian and the “upper” Silurian, that the three early Paleozoic systems were sorted out in the correct order. In the meantime, Murchison and Sedgwick managed to agree on the rocks above the Silurian and, in 1839, they named the Devonian System for rocks exposed in Devonshire, England. The final Paleozoic system, the Permian, was named by Murchison in 1841, after an expedition to Russia, where he recognized the youngest Paleozoic fossil assemblages in the carbonate rocks exposed in the province of Perm. See also: Precambrian

Subdivisions

The Paleozoic Era is divided into six systems; from oldest to youngest they are Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Carboniferous is subdivided into two subsystems, the Mississippian and the Pennsylvanian which, in North America, are considered systems by many geologists. The Silurian and Devonian systems are closer to international standardization than others; all the series and stage names and lower boundaries have been agreed upon, and most have been accepted. Despite continuing revisions, the major subdivisions of the geologic time scale have been relatively stable for nearly a century. See also: Cambrian; Carboniferous; Devonian; Ordovician; Permian; Silurian

Paleotectonics

The Paleozoic oceans, just as those today, surrounded a series of landmasses that formed the cores of ancient plates, always in motion as are their modern counterparts. Sediments were supplied to the seas through a network of river drainage systems and distributed in the oceans, by currents and gravity, very like today. Clastic sediments were supplied by the mountainous regions that were uplifted and eroded in cyclic patterns as the major plates collided and parted; and subduction at the leading edges of some plates produced volcanic highlands. The plate tectonic theory provides a template for sorting out the periods of mountain building during the Paleozoic. Like the discovery of the stratigraphic systems, periods of orogeny, with their concurrent volcanic and intrusive igneous activities, were revealed by field studies. Tectonic effects (folding and faulting) were analyzed by geologic mapping, as were crosscutting igneous relations and unconformities in the sedimentary sequence. Regional orogenic terranes were named and the general time sequence assigned; these were sharpened as the use of isotopic age analyses of the igneous components became possible in the twentieth century.

Because Alpine and Appalachian mountain chains were among the first studied in detail, orogenies were first named there. In eastern North America, mountain-building effects during the early Paleozoic were ascribed to the Taconic orogeny (Middle and Late Ordovician); middle Paleozoic events were assigned to the Acadian orogeny (Middle and Late Devonian); and late Paleozoic movements were called Appalachian (more accurately Alleghenian) for Permian and, perhaps, Triassic events. Similar, but not precisely correlative, orogenic episodes in western Europe are ascribed to the early Paleozoic Caledonian and the late Paleozoic Variscan (or Hercynian) orogenies. This regionalization, overlapping of timing of events and lack of correlation of intrusive phases, tectonics, and sedimentation cycles emphasize the universality of the ever-moving plates as the global mechanism responsible for all tectonic events. See also: Dating methods; Isotope; Orogeny; Plate tectonics; Unconformity

Fig. 1  Paleogeography of the Cambro-Ordovician (Tremadoc) showing most of the northern plates spread east-west in the equatorial regions. (After W. S. McKerrow and C. R. Scotese, eds., Paleozoic Palaeogeography and Biogeography, Geol. Soc. Mem. 12, Geological Society, London, 1990)

Fig. 2  Paleogeography of Old Red Sandstone continent (outlined in color) in Late Devonian. (After R. Goldring and F. Langenstrassen, Open shelf and near-shore clastic facies in the Devonian, in M. R. House, C. T. Scrutton, and M. G. Bassett, eds., The Devonian System, Spec. Pap. Palaeont. 23, Palaeontological Association, London, 1979)

Fig. 3  Paleogeography of the Early Carboniferous (Viséan), showing the Euro-American megaplate centered in equatorial region. (After W. S. McKerrow and C. R. Scotese, eds., Paleozoic Palaeogeography and Biogeography, Geol. Soc. Mem. 12, Geological Society, London, 1990)

Fig. 4  Paleogeography at the end of the Permian showing the consolidation into Pangaea preparatory to the onset of the Mesozoic tectonic cycle. (After W. S. McKerrow and C. R. Scotese, eds., Paleozoic Palaeogeography and Biogeography, Geol. Soc. Mem. 12, Geological Society, London, 1990)

Lithofacies

The major changes in lithofacies during the Paleozoic were also effected by biotic evolution through the era. Limestone facies became more abundant and more diversified in the shallow warm seas as calcium-fixing organisms became more diverse and more widespread. Sediment input from the land was modified as plants moved from the seas to the low coastal plains and, eventually, to the higher ground during the Devonian. Primitive vertebrates evolved during the Cambro-Ordovician, but true fishes and sharks did not flourish until the Devonian. Amphibians invaded the land during the Late Devonian and early Carboniferous at about the same time that major forests began to populate the terrestrial realm. These changes produced an entirely new suite of nonmarine facies related to coal formation, and the Carboniferous was a time of formation of major coal basins on all continental plates.

Climate continually influenced depositional patterns and lithofacies both on land and in the seas. In the major carbonate basins and platforms, particularly from the Late Ordovician onward, cyclic climatic changes resulted in changes from calcitic to magnesian carbonates and, ultimately, to various saline deposits as the basins dried up. There were great salt deposits in several systems, but spectacular thicknesses developed in many basins during the Silurian and Permian. Major cycles of cold and warm climates were overlaid on depositional and evolutionary patterns, producing periods of continental glaciation when large amounts of the Earth's water were tied up in ice during the Late Ordovician, the Late Devonian, and the late Permian. During the earliest and latest of these periods, icesheets were concentrated in the Southern Hemisphere on a single large Paleozoic continental mass—Gondwana. See also: Depositional systems and environments; Facies (geology); Paleoclimatology

Paleogeography

Paleogeographic changes naturally followed the shifting of plates on a megacyclic scale during the Paleozoic. In general, the Paleozoic featured a single southern landmass (Gondwana) for most of the era. This megaplate moved relatively sedately northward during this entire time interval (540–250 Ma) and always contained the magnetic and geographic south poles. Consequently, many of the facies and biologic provinces in the Gondwanan region were influenced by the cooler marine realms and continental and mountain glaciers in nearly every Paleozoic period. Most of the tectonic action that produced major periods of collision, mountain building, carbonate platform building, back-arc fringing troughs with their distinctive faunas and lithofaces, and formation of coal basins and evaporites took place in the Northern Hemisphere. These pulsations produced combinations of Laurentian (North American), Euro-Baltic, Uralian, Siberian, and Chinese plates at various times during the Paleozoic; and these combined units, in turn, moved slowly across the latitudes, producing climatic change; lithofacies changed in response to both the climate and the plate tectonics.

Fig. 5  Major fossil groups used for detailed biostratigraphy of the Paleozoic and younger strata. These are not total ranges of all groups. (After J. T. Dutro, R. V. Dietrich, and R. M. Foose, eds., AGI Data Sheets, 3d ed., American Geological Institution, 1989)

Representative geographies that show the range of change have been deduced (Figs. 1–4). The map for the Cambro-Ordovician portrays the general early Paleozoic patterns (Fig. 1); these hold for the entire span of time from the Early Cambrian (about 540 Ma), through the Cambrian, Ordovician, and Silurian, into the early Devonian (about 400 Ma). There were consolidations in the Northern Hemisphere in the Devonian, leading to a northern landmass—the so-called Old Red Continent (Fig. 2)—the forerunner of the Euro-American megaplate of the Carboniferous (Fig. 3), and culminating at the end of the Permian in the Pangaean continental mass (Fig. 4). This, in turn, set the stage for the breakup of Pangaea during the subsequent Mesozoic tectonic megacyle. See also: Paleogeography

Biogeography and biostratigraphy

The complexities of evolution from relatively simple forms at the beginning of the era to more advanced faunas and floras at the beginning of the Mesozoic produced a web of distributions in both time and space during the Paleozoic. In general terms, there were fewer and simpler life forms in the Cambrian—often termed the Age of Trilobites. All groups of invertebrates and plants became more numerous through geologic time. For example, 7 major invertebrate animal groups at the beginning of the Cambrian doubled to 14 by the end of the period, 20 by the end of the Ordovician, 23 at the end of the Devonian, and 25 at the end of the Paleozoic. The pattern for plant diversification, although starting later, is similar. Three simple plant groups became 5 by the end of the Silurian, 7 at the end of the Devonian, and 13 at the end of the Paleozoic. The vertebrates also diversified very slowly. From one or two groups in the Cambro-Ordovician (conodonts are now considered primitive vertebrates), the number of major kinds rose to 6 at the end of the Devonian and 8 at the end of the Paleozoic.

Biostratigraphic usefulness of fossils varies widely. Certain groups have been shown empirically to be more useful than others, and the abundance and diversity within these groups change from system to system during the Paleozoic. Groups with wide dispersal, occurrences in several facies, and rapid rates of evolution have proved most useful (Fig. 5). In the Paleozoic, trilobites are most valuable in the Cambrian and Ordovician; conodonts are more widely studied and are providing detailed biochronologic control for many system, stage, and zonal boundaries. Graptolites are indispensible in the deeper-water facies of the Ordovician through Early Devonian; goniatite cephalopods provide standards in the Devonian through the Permian; and fusulinids have long been essential for detailed work in the Carboniferous and Permian. Of course, all groups are useful for other kinds of paleobiologic research. Paleoenvironmental, paleoecological, and paleobiogeographic reconstructions use all appropriate biologic, chemical, and physical data in developing models of ancient Paleozoic worlds. See also: Biogeography; Cephalopoda; Conodont; Fusulinacea; Geologic time scale; Graptolithina; Index fossil; Paleoecology; Stratigraphy; Trilobita