PALEOZOIC ERA

Table 4: Geologic time scale. To see more information about a period, select one from the chart. also spelled Palaeozoic, major interval of geologic time that began about 540 million years ago with an extraordinary diversification of marine animals and ended about 245 million years ago with the greatest extinction event in Earth history. The major divisions of the Paleozoic Era, from oldest to youngest, are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. Some geologists recognize the Mississippian and Pennsylvanian periods in place of the Carboniferous (see Table). The Paleozoic takes its name from the Greek for ancient life. Paleozoic rocks are widely distributed on all continents. Most are of sedimentary origin, and many show evidence of deposition in or near shallow oceans. Among the more useful guide fossils for correlation are trilobites (Cambrian to Ordovician), graptolites (Ordovician to Silurian), conodonts (Ordovician to Permian), ammonoids (Devonian to Permian), and fusulinids (Carboniferous to Permian). On a global scale, the Paleozoic was a time of continental assembly. Cambrian continents were scattered, but none covered either pole, and the average world climate was probably warmer than it is today. The continent of Laurentia, composed mostly of present-day North America and Greenland, lay across the equator and remained there even after joining with other continents. By Ordovician time the large continent of Gondwana, consisting primarily of present-day Africa, Antarctica, Australia, South America, southern Europe, much of the Middle East, and India, began to move over the South Pole. The distribution of extensive glacial deposits has been used to track the movement of parts of Gondwana over and around the South Pole during the remainder of the Paleozoic. Parts of Gondwana, because of its large size, also extended into tropical latitudes. Siberia, essentially the large Asian part of what is now Russia, was a separate continent during the early and middle Paleozoic, when it moved from equatorial to north temperate latitudes. Baltica, composed mostly of present-day northern Europe (including Scandinavia), moved across the equator from southern cool temperate into northern warm latitudes during the Paleozoic, and it collided with and joined Laurentia during early Devonian time. Continued tectonic plate movements resulted in the final assembly of the supercontinent of Pangaea by the end of the Paleozoic. Such mountainous regions as the Appalachians, Caledonides, and Urals were originally deformed by the Paleozoic collision of the lithospheric plates. Large areas of all continents were episodically inundated by shallow seas, with the greatest inundations during the Ordovician and early Carboniferous periods. At the beginning of the Paleozoic, animals were restricted to the oceans, and land plants had not appeared. About half of all animal phyla, especially those with hard shells and mineralized skeletons, originated during the early and middle Cambrian. The biota rapidly diversified throughout the Cambrian and Ordovician as life-forms adapted to virtually all marine environments. In numbers of described marine species, trilobites are the dominant kind of fossil in Cambrian rocks, whereas brachiopods predominate in Ordovician to Permian rocks. Several different kinds of organisms independently adapted to living on land, primarily during the middle Paleozoic. Leafless vascular plants (psilophytes) and invertebrate animals (centipede-like arthropods) were both established on land at least by Silurian time. Vertebrate animals made the transition to land via evolution of amphibians from air-breathing crossopterygian fish during the Devonian. Further conquest of the land became possible during the Carboniferous as dependence on moist environments for depositing spores and shell-less eggs was overcome, as plants evolved seeds (seed-fern origin), and as animals evolved amniote eggs with protective shells (reptile origin). Flight was first achieved also during the Carboniferous, as insects evolved wings. The great extinction event at the end of the Paleozoic Era eliminated such major invertebrate groups as the blastoids, fusulinids, and trilobites. Other major groups, as, for example, the ammonoids, brachiopods, bryozoans, corals, and crinoids, were severely decimated but managed to survive. It has been estimated that as many as 95 percent of the marine invertebrate species perished during the late Permian. Extinction rates were much lower among vertebrates, both aquatic and terrestrial, and among plants. Causes of the extinction are not clear, but they may be related to changing climate and exceptionally low sea level. Although of lesser magnitude, other important mass extinctions occurred at the end of the Ordovician and during the late Devonian. Richard A. Robison also spelled Palaeozoic, major interval of geologic time extending from 540 to 245 million years ago. It is the first era of the Phanerozoic Eon. Eras have traditionally been named to reflect major changes in the development of life on Earth. The Paleozoic, from the Greek for ancient life, is followed by the Mesozoic (intermediate life) and Cenozoic (recent life). The Paleozoic is divided into six periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian (qq.v.). The early Paleozoic (roughly the first 130 million years) was characterized by the opening of the Iapetus Ocean and the formation of extensive shallow-water continental margins, especially between North America and Europe. Much of North America was covered by a warm shallow sea with many coral reefs. In the Ordovician and subsequent periods, episodes of mountain building followed as the Iapetus Ocean shrank and eventually disappeared. Fossils from the early Paleozoic include such invertebrates as worms and primitive invertebrate shells like Volborthella, Hyolithes, and others. Indeed, it was during the Cambrian Period that many of today's invertebrate groups first came into being. In the Cambrian Period the trilobites, marine arthropods whose skeletons are divided into three longitudinal lobes, were the most abundant faunal group and are among the most frequently found fossil forms today. By the end of the Late Cambrian several marine life-forms were abundant: graptolites, cephalopods, brachiopods, and trilobites were the four dominant groups that gave rise to a flourishing fauna of bryozoans (marine colonial animals), mollusks such as the earliest gastropods (snails and slugs), ostracods (small bivalved crustaceans), eurypterids (large arthropods, now extinct), and others. Primitive fish also appeared during the early Paleozoic. The plants of this time were predominantly algae, with some mosses and ferns. The late Paleozoic, which extended from about 410 to 245 million years ago, was a time when tremendous changes were wrought in the Earth. Both plant and animal life flourished in the great warm, shallow seas, and the various convolutions of the Earth laid down extensive mineral deposits. Much of the copper, gold, lead, zinc, and other minerals mined today derive from Devonian times in the late Paleozoic; and mining activities in modern times have greatly expanded geologic knowledge about this time interval. Huge, swampy forest regions covered much of the northern continents, and these were repeatedly and suddenly invaded by the seas, which buried the vegetation, then covered it with silt. When the sea subsequently withdrew, the forests revived and were again buried in rhythmic cycles that are now evident in deposits called cyclothems. Heat and pressure transformed the buried vegetation into the oil and coal so important to the modern world. During the Devonian Period animal life emerged from the ocean, and various species adapted themselves to breathing air and moving about on land. This happened by way of the amphibians, which evolved with considerable diversity in the Carboniferous and Permian periods. The amphibians ranged in length from several centimetres to 3 m (a few inches to 10 feet). When desert conditions began to prevail again in the Permian, the amphibians were succeeded by a new form, the reptiles, who could lay eggs in the desert that would hatch and bring forth young quite independent of water. The reptiles then developed rapidly into a number of diversely adapted species. The late Paleozoic was also when insect life began. More than 500 species are known from fossils found in Carboniferous coal deposits, including dragonflies more than 60 cm (2 feet) across and winged cockroaches measuring up to 30 cm (1 foot). Thus the first steps were taken for life-forms to invade the air, but it would be some time yet before the reptiles grew wings, which led eventually to that dominant aerial form, the birds. Fishes underwent rapid development in the late Paleozoic. Sharks and sharklike forms were common; the largest, Dinichthys (now extinct), was more than 6.8 m (22 feet) long, with massive bony armour plates protecting its head and hinged to body armour around the neck region. The Sarcopterygii, the fleshy-finned bony fishes, developed the ability to breathe air, which enabled them to survive in deserts in times of drought: five genera of these Dipnoi still survive. The crossopterygian fishes gave rise to amphibians. Late Paleozoic flora saw a profusion of land plants. Ferns grew to tree size in the Carboniferous forests. The cordaiteans, the precursors of the conifers, first appeared in the Lower Carboniferous. Another development of the late Paleozoic was the occurrence of several major glaciations. South America, Africa, Antarctica, and Australia show early glacial deposits; such conditions threatened much warmwater life with extinction. A second and a third glaciation followed, and then, at the end of the Paleozoic Era, came one of the greatest crises in the history of life. The climate warmed, and there was no further glaciation for many millions of years. Of the life-forms adapted to cold water, most were nearly halved, both in variety and in numbers. On land, as well, some 75 percent of the amphibian families and more than 80 percent of the reptiles were extinct by the end of the Permian. Additional reading Summaries of Paleozoic geologic history and life are included in Robert H. Dott, Jr., and Roger Lyman Batten, Evolution of the Earth, 4th ed. (1988); Steven M. Stanley, Earth and Life Through Time, 2nd ed. (1989); and Reed Wicander and James S. Monroe, Historical Geology: Evolution of the Earth and Life Through Time (1989). Richard A. Robison