DEFINITION OF TRIASSIC

The oldest of three Mesozoic periods, and the corresponding

system of rocks. The Triassic period is bounded

below by the Permian period of the Paleozoic era, and above

by the Jurassic period. The time span by the Triassic ranges

from 248 million years ago to 206 million years ago, divided

into Early (248–242 Ma), Middle (242–227 Ma), and Late

(227–206 Ma) epochs, and seven ages including, from oldest

to youngest, the Induan, Olenekian, Anisian, Ladinian, Carnian,

Norian, and Rhaetian.

The base of the Triassic is also the base of the Mesozoic,

meaning middle life, so this boundary is marked by a profound

change in the fossil biota in the stratigraphic sequence.

The end of the Permian saw the extinction of 90 percent of

marine organisms, followed by a re-radiation of the pelecypods,

sea urchins, lobsters, and ammonoids. The ammonoids

have proven very useful in subdividing the Triassic in dozens

of biozones, and the ages of the periods are under consideration

for revision based on these higher resolution divisions.

The first hexacorals appeared in the Early Triassic, whereas

oysters appeared in the Ladinian, and the first dinosaurs are

known from the Carnian. The first mammals, turtles, and

crocodiles all appeared in the Late Triassic. Many extinctions

occurred throughout the Triassic, including the loss of dozens

of families of marine gastropods, cephalopods, brachiopods,

bivalves, and sponges. Terrestrial extinctions include numerous

families of insects, freshwater fish, and reptiles. Some

models have suggested that these numerous extinctions created

opportunities for mammals to radiate and succeed in a previously

hostile world. The cause of so many extinctions is not

well known, but several seem to be grouped at the end of the

Early Triassic, in the Carnian, and at the end of the Triassic.

The triggering mechanism for the first two events is unknown,

but some models suggest that a meteorite impact, identified as

the 50-mile (80-km) wide Manicuagan structure in Ontario,

may have been responsible for the end-Triassic event.

The supercontinent of Pangea stretched nearly from pole

to pole in the Triassic, surrounded by the Panthalassa Ocean,

and partly surrounding the wedge-shaped Tethys Ocean that

contained numerous reefs and carbonate platforms. The western

margin of Pangea was dominated by convergent margin

activity with active volcanism stretching from Alaska through

the North and South American cordillera and into Antarctica.

Numerous flood basalts were erupted and swarms of

diabase dikes intruded during the Triassic, including the

Karoo of South Africa, the Permo-Triassic Siberian traps, and

along the eastern coast of North America, western North

Africa, and northeastern South America.

Triassic climates were generally warm, and sea levels

started low, fluctuated in the Middle Triassic, and were generally

about 300 feet (100 m) higher than the present level in

the Late Triassic. Deserts covered much of inland Pangea,

with extensive evaporite basins, red beds, and coal swamps

forming in different locations on land, and marine carbonates

deposited in much of the Tethys Ocean.

See also FLOOD BASALT; MESOZOIC; PANGEA; PANTHALLASA;

TETHYS.

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