Cambrian Period

Cambrian Period, first division of the Paleozoic Era of the geologic time scale, spanning an interval of about 46 million years, from 542 million to 488 million years before present. It was named in 1835 by the English geologist Adam Sedgwick for sedimentary rocks in Wales, which the ancient Romans knew as Cambria.

By the beginning of the Paleozoic Era, the steadily increasing oxygen content of the atmosphere and oceans (see Atmosphere) had made it possible for the marine environment to support new forms of life that could derive energy from respiration. Although life had not yet invaded dry land or the air, the seas of the Cambrian Period teemed with a great variety of marine invertebrates, including sponges, worms, bryozoans (“moss animals”), hydrozoans, brachiopods, mollusks (among them the gastropods and species ancestral to the nautilus), primitive arthropods such as the trilobite, and a few species of stalked echinoderms. The only plant life of the time consisted of marine algae. Because many of these new organisms were relatively large, complex marine invertebrates with hard shells and skeletons of chitin or lime, they had a far better chance of fossil preservation than the soft-bodied creatures of the preceding Precambrian time.

With a relatively rich fossil content, sedimentary rocks of Cambrian age are the oldest strata that lend themselves to extensive dating via stratigraphic correlation (see Dating Methods). For this reason, scientists have had far more success understanding the conditions on earth during the Cambrian and succeeding periods than in the far older, longer Precambrian time.

The Cambrian is the earliest geologic period for which science has sufficient evidence to hypothesize the existence of crustal plates and to attempt to describe them (see Plate Tectonics). Multiple collisions between these plates gave rise during this period to a vast landmass, or supercontinent, known as Gondwanaland. It incorporated the beginnings of today's four southern continents—South America, Africa, Antarctica, and Australia—and also included India, and parts of present-day Mexico and Florida, southern Europe, and possibly China.

The distribution of Cambrian continents was different from that of today. Most landmasses were situated in either the Tropics or the southern hemisphere. Evidence for a tropical location of ancestral North America and northern Europe is provided by salt deposits and coral reefs in the Cambrian rocks of those land areas. Gondwanaland, covering a much more extensive area than northern landmasses, stretched from the Tropics and the south temperate zone almost to the South Pole.