The Cambrian Period, from about 570 to 500 million years ago, is the first period of the Paleozoic Era of geologic time. The name, derived from Cambria, the Roman name for Wales, was given (1835) by the English geologist Adam Sedgewick to a sequence of sandstones and shales in northwestern Wales. The Cambrian is a key period in the geologic record because its advent coincides with the first appearance of abundant fossils, a characteristic of all succeeding periods of geologic time.
PALEOTECTONICS AND GEOGRAPHY
During the Cambrian Period the Iapetus Ocean formed. This ancestor of the Atlantic separated the embryonic North American and Eurasian continents, although not precisely along modern boundaries. A portion of eastern Canada and New England were part of ancestral Europe, and northern Scotland was attached to America. In addition, the area of the present-day southeastern coast of the United States was appended to Africa. Also in the Cambrian, continental drift resulted in the final consolidation of Gondwanaland, a southern supercontinent including most of present South America, Africa, Arabia, Madagascar, India, Australia, and Antarctica, plus portions of the present-day southeastern United States, southern Europe, and, probably, China. The Pan-African orogeny, a great mountain-building event that shook much of Gondwanaland during the beginning of the period, may have been the final collision of the several pieces of the supercontinent.
Cambrian continents had a significantly different orientation from present ones. North America was situated in the tropics, as were the present areas of eastern Europe and Siberia, then on separate continents. India, Antarctica, southern Africa, and southwestern Australia were along the northern edge of Gondwanaland; they were also in the tropics, as were the various fragments of China. In the south temperate zone were present-day western Europe and that portion of Gondwanaland consisting of what is now central Africa and South America. Northern Australia, a projection from Gondwanaland, was the only major land area in the north temperate zone. Present-day northwestern Africa and southern Europe, then adjacent, were in the south polar region, and the north polar area was in a vast ancestral Pacific Ocean, sometimes called Panthalassa.
The North American craton (Continental Shield) was flooded by a major inland sea, the Sauk Sea, which expanded throughout the Cambrian and culminated at the end of the period in a vast transgression that covered virtually the entire continent. The basal sandstone deposited in the Sauk Sea is Lower Cambrian at the margins and Upper Cambrian in the heart of the continent. Overlying it is a great thickness of limestones, the characteristic marine sediments of shallow tropical seas with restricted circulation. The European continent was also extensively flooded in the Cambrian, and ancestral Siberia was inundated by seas, as in all the early Paleozoic periods. In Gondwanaland, flooding was less extensive, although the areas of central Australia, northern India, and the Amazon River valley were covered by seas.
The locations of tectonic plate margins are not well known, but some can be identified (plate tectonics). For example, geologists believe that a trench ran along the edge of Gondwanaland in present-day eastern Australia and Antarctica. A spreading ridge (seafloor spreading) must have existed between ancestral North America and Europe, as it otherwise would have been impossible for the Iapetus Ocean to open. Some sort of plate boundary or boundaries also probably existed in the Theic Ocean, a broad seaway separating North America, Europe, and Siberia from Gondwanaland.
Economic resources are not extensive in Cambrian rocks. Some lead has been mined in southwestern Missouri, and several sedimentary units are quarried as building stones.
Large, shelled organisms first occur as fossils in the Cambrian, in contrast to the vast Precambrian, when only delicate microorganisms and algal mounds occur. This is the most noteworthy change in the geologic record of life; although a number of explanations have been proposed, no entirely satisfactory answer has been found. Most specialists believe that it was only at the beginning of Cambrian time that the Earth's Atmosphere contained sufficient oxygen to sustain the metabolism of complex organisms.
The fossil record of the Cambrian is dominated by the two ancient groups of marine invertebrates: trilobites and brachiopods. Trilobites are primitive three-lobed arthropods. Unlike other members of the phylum, such as crustaceans, arachnids, and insects, trilobites had simple, unspecialized appendages. They reached their zenith in the Cambrian and declined thereafter, becoming extinct in the Permian Period. Brachiopods, or lamp shells, are primitive two-valved shellfish that superficially resemble clams. Although most later brachiopods had hinged shells of lime, most Cambrian ones had unhinged phosphatic shells rotated by muscles. Another notable group, largely confined to the Early Cambrian, is the archaeocyathids. These extinct marine organisms had conical or cylindrical shells with double porous walls. Sharing features of both sponges and corals, they built small patch reefs in the Cambrian tropics--present-day Siberia, Australia, and the western United States. Except possibly for the bryozoans (moss animals) and vertebrates, all phyla of modern animals had evolved by the end of the Cambrian; plants were still limited to primitive algae.
Leigh W. Mintz