The Triassic is the first geological period in the Mesozoic era and the seventh period of the Phanerozoic eon.  It lasted 50.6 million years.  The period began 251.9 million years ago, and ended 201.3 million years ago.

The period before the Triassic was the Permian period, the last period in the Palaeozoic era.  The Jurassic period came after the Triassic.

Many new groups arose during the Triassic period, including the dinosaurs and the first mammals.

Extinction events

Great extinction events happened at both the beginning and end of the Triassic period. The Triassic began after the great extinction at the end of the Palaeozoic era, the Permian/Triassic extinction event. Nobody really knows the exact cause of the Permian/Triassic extinction, but many experts have different theories. Generally, experts agree that many things caused the extinction.

The Triassic had several more extinction events, whose causes are also unknown. The most significant of these took place at the end of the Triassic, which was one of the 'big five' Phanerozoic marine extinctions.

Geology

Name 
The name 'Triassic' comes from the three rock strata that formed during the Triassic period ("tri" means "three"). Three layers of rock strata formed during this period:
Red beds, laid down in desert conditions;
Limestone, laid down in seas, mostly over continental shelves; and
Black slate, laid down in rich organic conditions such as river deltas.

Overall climate 
On average, the Triassic's climate was very different from today's:
It had about 80% of today's oxygen levels
There was about six times as much carbon dioxide in the air as there was before the Industrial Revolution
The Earth's average surface temperature was about 3o C hotter than it is now

Paleogeography 

During the Triassic period, there were no separate continents like there are today. Almost all the Earth's land mass was together in a single supercontinent called Pangaea ("all the land"). Pangaea was centered more or less on the equator. All the deep-ocean sediments (the things that settled at the bottom of the ocean) had disappeared.  This happened because oceanic plates moved sideways and downward until they were finally on top of each other, or under each other.

The supercontinent Pangaea was rifting during the Triassic – especially late in the period – but had not yet separated into different continents.

Being a super-continental land mass, Pangaea had limited shoreline.  Because of this, Triassic marine deposits  fossils from Triassic ocean life  are rare in most of the world.  In North America, for example, marine deposits are limited to a few exposures in the west.  However, they are common in Western Europe, where the Triassic was first studied.

Major adaptive radiations 

The first part of the Triassic had much less variety than the Permian, and showed signs of a deteriorated environment. This situation lasted for about 5 million years, then steadily improved. Into the vacant ecological niches, new forms evolved, replacing old with new. This rapid adaptive radiation happened to the reptiles on land, the fish in the seas, and a number of other groups, like the insects. Diverse communities with complex ecology took 30 million years to re-establish.

Fish 
The group of bony fish known as the teleosts first appeared during the Triassic period. The group now includes over 80% of all fish and 95% of all bony fish. The only other common group of fish are the Chondrichthyes (the sharks and rays). The rays also appeared during the Triassic.

Reptiles 
Many reptile groups first appeared during the Triassic period, possibly because so many niches were vacant at the time. Some of these new groups died out in the End–Triassic extinction event, but those that survived ruled the Earth for the rest of the Mesozoic. Examples include:
The Archosaurs, including dinosaurs and crocodiles
Pterosaurs
Ichthyosaurs
Turtles
Plesiosaurs

This was a major change, because in the middle Triassic period, the Synapsids (mammals' ancestors) were still dominant.

The end-Triassic extinction 

Many reptile groups became extinct during the Triassic, including:p. 41
Crocodilian Archosaurs:
 Ornithosuchids
 Aetosaurs
 Rauisuchians
Other Archosaurs:
 Euparkeria
 Erythrosuchids
 Proterosuchids
Proto-Archosaurs
 Rhyncosaurs
 Trilophosaurs
Prolacertiforms
Anapsids
 Protocolophonids

Also, amongst the Synapsids (which used to be called mammal-like reptiles), the Dicynodonts died out during the end-Triassic extinction.p. 41

The Triassic was a period of great change in tetrapods: many new and important groups appeared, but many others became extinct. Of all the periods in the Mesozoic era, there are the least number of fossils from the Triassic period. This makes it difficult for scientists to explain these events.

Related pages
 Mesozoic era
 Geological period
 Pangaea
 Extinction events
 Permian/Triassic extinction event
 End-Triassic extinction event

References