DEFINITION OF SILURIAN

The third period of Paleozoic time ranging from

443 Ma to 415 Ma, falling between the Ordovician and

Devonian periods, and the corresponding system of rocks.

From base to top it is divided into the Llandoverian and

Wenlockian ages or series (comprising the Early Silurian),

and the Ludlovian and Pridolian ages or series (comprising

the Late Silurian). The period is named after a Celtic tribe

called the Silures, who inhabited a part of Wales where rocks

of the Silurian System are well exposed. The Silurian is also

known as the age of fishes.

Rocks of the Silurian System are well exposed on most

continents, with carbonates and evaporites covering parts of

the mid-west of North America, the Russian platform, and

China. Silurian clastic sequences form thick orogenic wedges

in eastern and western North America, central Asia, western

Europe, China, and Australia. Much of Gondwana was

together in the Southern Hemisphere and included the present-

day landmasses of South America, Africa, Arabia, India,

Antarctica, Australia, and a fragmented China. North America,

Baltica, Kazakhstan, and Siberia formed separate landmasses

in equatorial and northern latitudes. Much of

Gondwana was bordered by convergent margins, and subduction

was active beneath the Cordillera of North America.

Baltica and Laurentia had collided during early stages of the

Acadian-Caledonian orogeny, following an arc-accretion

event in the Middle to Late Ordovician, known as the Taconic

orogeny in eastern North America.

Land plants first appeared in the Early Silurian and were

abundant by the middle of the period. Scorpion-like

eurypterids and arthropods inhabited freshwater environments

and may have scurried across the land. In the marine

realm trilobites, brachiopods, cephalopods, gastropods, bryozoans,

crinoids, corals, and echinoderms inhabited shallow

waters. Conspicuous reefs were built by stromatoporoids,

rugose and tabulate corals, while jawed fish fed on plankton

and nekton.

See also PALEOZOIC; PHANEROZOIC.

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