Ordovician Period

Ordovician Period, second division of the Paleozoic Era of the geologic time scale, spanning a period from about 488 million to 444 million years ago. It was named for an ancient Welsh tribe, rocks of this age having first been studied systematically in Wales. The Ordovician was preceded by the Cambrian Period and followed by the Silurian Period.

North America and Europe, separated by water during the Cambrian Period, collided in Ordovician time, crumpling between them a great thickness of accumulating sediments of the Appalachian geosyncline and lifting these rocks to form a mountain range, the Taconics, remnants of which are visible today in eastern New York State. Shallow seas that covered much of North America at the beginning of the period withdrew, leaving behind thick deposits of limestone; returning later in the Ordovician, the seas deposited thick blankets of quartz sand and additional limestone. Europe and Asia were separated by a long, narrow sea in which sediments of the Uralian geosyncline accumulated. Asia itself was fragmented, with Siberia and China separated by marine waters. In the southern hemisphere, the supercontinent of Gondwanaland, encircled by a belt of geosynclines, encompassed South America, Antarctica, Africa, India, and Australia, as well as portions of continental crust—Mexico and Florida—that would not become attached to North America until the Carboniferous period. See Plate Tectonics.

The climate of Ordovician time was warm and humid in much of present-day North America and Eurasia but colder in the southern continents, the South Pole of the time being centered in what is now Algeria. Marine invertebrates were still the predominant life forms. On land, a few forms of primitive life may have appeared—plants and burrowing invertebrates similar to millipedes. The seas now harbored the earliest vertebrates, primitive fish with bony armor plates. Graptolites (extinct colonial organisms), corals, crinoids, bryozoans, and clams first made their appearance in this period. See Paleontology.