DEFINITION OF ORDOVICIAN

The second period of the Paleozoic era, and

the corresponding rock series, falling between the Cambrian

and the Silurian. It is commonly referred to as the age of

marine invertebrates. The base of the Ordovician is defined

on the Geological Society of America timescale (1999) as 490

million years ago, and the top or end of the Ordovician is

defined at 444 million years ago. The period was named by

Charles Lapworth in 1879 after the Ordovices, a Celtic tribe

that inhabited the Arenig-Bala area of northern Wales, where

rocks of this series are well exposed.

By the Early Ordovician, North America had broken

away from the supercontinent of Gondwana that amalgamated

during the latest Precambrian and early Cambrian period.

It was therefore surrounded by shallow water passive margins,

and being at equatorial latitudes, these shallow seas

were well suited for the proliferation of marine life-forms.

The Iapetus Ocean separated what is now the east coast of

North America from the African and South American segments

of the remaining parts of Gondwana. By the Middle

Ordovician, convergent tectonics brought an island arc system

to the North American margin, initiating the Taconic

Orogeny as an arc/continent collision. This was followed by a

sideways sweep of parts of Gondwana past the North American

margin, leaving fragments of Gondwana attached to the

modified eastern margin of North America.

During much of the Ordovician, carbonate sediments

produced by intense organic productivity covered shallow

epeiric seas in the tropical regions, including most of North

America. This dramatic increase in carbonate sedimentation

may reflect a combination of tectonic activity that brought

many low-lying continental fragments into the Tropics, high

sea-level stands related to the breakup of Gondwana, and a

sudden increase in the number of different organisms that

started to use calcium carbonate to build their skeleton or

shell structures.

Marine life included diverse forms of articulate brachiopods,

communities of echinoderms such as the crinoids

or sea lilies, and reef-building stromatoporoids, rugose, and

tabulate corals. Trilobites roamed the shallow seafloors, and

many forms emerged. The Ordovician saw rapid diversification

and wide distribution of several planktonic and pelagic

faunas, especially the graptolites and conodonts which form

useful index fossils for this period. Nautiloids floated across

the oceans, and some attained remarkably large sizes, reaching

up to several meters across. Fish fossils are not common

from Ordovician deposits but there may have been some

primitive armored types present. The end of the Ordovician is

marked by a marine extinction event, apparently caused by

rapid cooling of the shallow seas, perhaps related to continental

glaciation.

See also APPALACHIANS; CONODONT; CRINOID;

PALEOZOIC.

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