Paleozoic Era
Paleozoic Era, 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.
A brief treatment of the Paleozoic Era follows. For full treatment, see Geochronology: Paleozoic Era .
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. (see also Index: Ordovician Period)
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.
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
Permia.
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