Triassic

Eon	Era	Period	Epoch	Start Million years ago
Phanerozoic	Cainozoic	Palaeogene	Palaeocene	66
	Mesozoic	Cretaceous	Upper Cretaceous	100.5
			Lower Cretaceous	145
		Jurassic	Upper Jurassic	163.5
			Middle Jurassic	174.1
			Lower Jurassic	201.3
		Triassic	Upper Triassic	237
			Middle Triassic	247.2
			Lower Triassic	252.17
	Palaeozoic	Permian		298.9

220 million years ago during the Late Triassic

The Triassic period is the first period of the Mesozoic era. It lasted from about 252.17 to 201.3 million years ago. Both the start and the end of the Triassic are marked by great extinction events. The period began after the great extinction at the end of the Palaeozoic era, the P/Tr extinction event. Many new groups arose during its 50 million years, including the dinosaurs.

Nobody really knows the exact cause of the Permian/Triassic extinction, but it has been discussed by many experts. Generally, it is agreed that many things caused it. The Triassic had several more extinction events, whose causes are also obscure. 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 way that some rock in Europe is arranged: called a "rock sequence." Because "tri" means "three," there are three layers: red bed, limestone, and black slate.

Dates, epochs and stages

Period	Epoch	Age	Start
Triassic	Lower Triassic	Induan	252 ± 0.5 mya - 251.2 million years ago
		Olenekian	251.2 - 247.2 million years ago
	Middle Triassic	Anisian	247.2 - c 242 million years ago
		Ladinian	c 242 - c 235 million years ago
	Upper Triassic	Carnian	c 235 - c 228 million years ago
		Norian	c 228 - c 208.5 million years ago
		Rhaetian	208.5 - 201.3 ± 0.2 mya million years ago

Overall climate

On average, the Triassic was:

1. 80% of modern oxygen level
2. 6 times pre-industrial CO₂ level
3. Average surface temperature +3^oC compared with present

Paleogeography

230 Ma plate tectonic reconstruction

During the Triassic, almost all the Earth's land mass was concentrated into a single supercontinent centered more or less on the equator, called Pangaea ("all the land"). All the deep- ocean sediments, or the things that settled at the bottom of the ocean, have disappeared because oceanic plates moved sideways and downward until they finally were on top of or under each other. The supercontinent Pangaea was rifting during the Triassic–especially late in the period–but had not yet separated. Because of the limited shoreline of one super-continental mass, Triassic marine deposits are globally relatively rare, despite their prominence in Western Europe, where the Triassic was first studied. In North America, for example, marine deposits are limited to a few exposures in the west.

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 and the fish in the seas, and to a number of other groups, such as the insects. Diverse communities with complex ecology took 30 million years to reestablish.

Fish

The group of bony fish known as the teleosts first appeared in the Triassic. 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, and the rays are also a Triassic 'invention'.

Reptiles

Many reptile goups originated in the Triassic, possibly because so many niches were vacant at the time. Some of these new groups were caught in the end-Triassic extinction, but those that survived ruled the Earth for the rest of the Mesozoic. examples are:

• The Archosaurs, including:
  • Dinosaurs
  • Crocodiles
• Pterosaurs
• Ichthyosaurs
• Turtles
• Plesiosaurs

This was an extaordinary turn-around, for in the middle Triassic the ancestors of mammals, the Synapsids, were still dominant. All the same, many reptile groups went extinct in the Triassic:^p41

• Crocodilian Archosaurs:

  Ornithosuchids

  Aetosaurs

  Rauisuchians

• Other Archosaurs:

  Euparkeria

  Erythrosuchids

  Proterosuchids

• Proto-Archosaurs

  Rhyncosaurs

  Trilophosaurs

  Prolacertiforms

• Anapsids

  Protocolophonids

And, amongst the Synapsids (formerly Mammal-like reptiles), the End-Triassic extinction saw the last of the Dicynodonts. Thus the Triassic is seen to be a period of great turnover in tetrapods: many new and important groups, but many extinctions. The fossil record of the Triassic is the poorest in the Mesozoic, which makes it difficult to explain these events.

References

Geologic History

Source

International Chronostratigraphic Chart 2013. International Commission on Stratigraphy, retrieved 8 April 2013. Divisions of geologic time – major chronostratigraphic and geochronologic units USGS, retrieved 8 April 2013.