The Triassic is a Geologic period that extends from about 251 to 200 million years (My or 'megayears') before the present. The extinction event that closed the Triassic period has recently been more accurately dated, but as with most older geologic periods, the rock beds that define the start and end are well identified, but the exact dates of the start and end of the period are uncertain by a few million years. As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both start and end of the Triassic are marked by major extinction events.
The characteristic sediments of the Triassic (red sandstones and evaporites) suggest a warm dry climate with no evidence of glaciation. As far as can be determined, there was no land or ice caps near either pole. Because of the limited shoreline of one continental mass, Triassic marine deposits are globally relatively rare despite their prominence in Western Europe. In North America, for example, marine deposits are limited to a few exposures in the West. Due to this, Triassic stratigraphy is mostly based on organisms living in lagoons and hypersaline environments, such as Estheria crustaceans.
During the Triassic both marine and continental life show an adaptative radiation, following the Permian-Triassic extinction. Corals of the hexacorallia group make their first appearance. The first Angiosperms (flowering plants) may have evolved during the Triassic as did the first flying vertebrates (pterosaurs).
Table of contents
The Triassic was named in 1834 by Friedrich Von Alberti from the three distinct layers of redbeds, capped by chalk, followed by black shales that are found throughout Germany and Northwest Europe, called the 'Trias'.
Triassic dating and subdivisions
The Triassic is usually separated into Lower, Middle, and Upper subdivisions. The faunal stages from youngest to oldest are:
|Upper Triassic (Tr3)|
|Rhaetian||(203.6 ± 1.5 – 199.6 ± 0.6 MYA)|
|Norian||(216.5 ± 2.0 – 203.6 ± 1.5 MYA)|
|Carnian||(228.0 ± 2.0 – 216.5 ± 2.0 MYA)|
|Middle Triassic (Tr2)|
|Ladinian||(237.0 ± 2.0 – 228.0 ± 2.0 MYA)|
|Anisian||(245.0 ± 1.5 – 237.0 ± 2.0 MYA)|
|Lower Triassic (Scythian)|
|Olenekian||(249.7 ± 0.7 – 245.0 ± 1.5 MYA)|
|Induan||(251.0 ± 0.4 – 249.7 ± 0.7 MYA)|
During the Triassic, almost all the Earth's land mass was concentrated into a single supercontinent, called Pangea ("all the land"). This took the form of a giant "Pac-Man" with the mouth facing East, and centered more or less on the equator. The "mouth" constitute the Tethys sea, while the rest of the ocean is known as Panthalassia ("all the sea").
The Triassic climate was generally hot and dry, forming typical redbed sandstones. It was a seasonal, continental climate, with strong monsoons. Polar regions were moist and temperate
In the Triassic fauna, three groups of organisms can be distinguished, holdovers from the Permian-Triassic extinction, some new groups which flourished briefly, and other new groups which went on to dominate the Mesozoic world.
In marine environments new, modern types of corals appeared in the early Triassic, forming small patches of reefs, nothing compared to the great reef systems of Devonian times or modern reefs. The shelled cephalopods called Ammonites recovered, diversifying from a single line that survived the Permian extinction. The fish fauna was remarkably uniform, reflecting the fact that very few families survived the Permian extinction. The first ichthyosaurs appeared in Triassic seas, and they soon diversified, some growing to large and eventually (during the late Triassic) huge size.
On land, the holdover plants included the lycophytes, cycads, and glossopterids. Seed plants dominated the land. In the northern hemisphere, conifers flourished. Glossopteris (a seed fern) was the dominant southern hemisphere tree during the Early Triassic period.
Archosauromorph reptiles – especially archosaurs – progressively replaced the synapsids that had dominated the Permian. By the end of the Triassic, synapsids played only bit parts. During the Carnian (late Triassic), while living on the fringes and grubbing out an existence, they evolved into the mammals. At the same time the Ornithodira, which until then had been small and insignificant, evolve into pterosaurs and a variety of dinosaurs. The earliest turtles, like Proganochelys, appeared during the Norian (late Triassic).
The Monte San Giorgio lagerstätte, now in the Lake Lugano region of northern Italy and Switzerland, was in Triassic times a lagoon behind reefs with an anoxic bottom layer, so there were no scavengers and little turbulence to disturb fossilization, a situation that can be compared to the better-known Jurassic Solnhofen limestone lagerstätte.
Late Triassic extinction event
Main article: Triassic-Jurassic extinction event
The Triassic period ended with a mass extinction, which was particularly severe in the oceans; the conodonts disappeared, and all the marine reptiles except ichthyosaurs and Plesiosauria. Invertebrates like brachiopods, gastropods, and molluscs were severely affected. In the oceans, 22 percent of marine families and possibly about half of marine genera went missing, according to University of Chicago paleontologist Jack Sepkoski.
Though the end-Triassic extinction event was not equally devastating everywhere in terrestrial ecosystems (see below), several important clades of Crurotarsi (large archosaurian reptiles previously grouped together as the thecodonts) disappeared, as did most of the large labyrinthodont amphibians, a number of groups of small reptiles, and some synapsids (except for the proto-mammals). Some of the early, primitive dinosaurs also went extinct, but other, more adaptive dinosaurs survived to evolve in the Jurassic. Surviving plants that went on to dominate the Mesozoic world included modern conifers and, cycadeoids.
It is not certain what caused this late Triassic extinction, which was accompanied by huge volcanic eruptions about 208–213 million years ago, the largest recorded volcanic event since the planet cooled and stabilized, as the supercontinent Pangaea began to break apart. Other possible causes for the extinction events include global cooling or even a bolide impact, for which an impact crater surrounding Manicouagan Reservoir, Quebec, Canada, has been singled out. At the Manicouagan impact crater (see illustration at the entry), however, recent research has shown that the impact melt within the crater has an age of 214±1 Ma. The date of the Triassic-Jurassic boundary has also been more accurately fixed recently, at 202±1 Ma. Both dates are gaining accuracy by using more accurate forms of radiometric dating, in particular the decay of uranium to lead in zircons formed at the impact. So the evidence suggests the Manicougan impact preceded the end of the Triassic by approximately 12±2 Ma. Therefore it could not be the immediate cause of the observed mass extinction (Hodych & Dunning, 1992).
The number of late Triassic extinctions is disputed. Some studies suggest that there are at least two periods of extinction towards the end of the Triassic, between 12 and 17 million years apart. But arguing against this is a recent study of North American faunas. In the Petrified Forest of northeast Arizona there is a unique sequence of latest Carnian-early Norian terrestrial sediments. An analysis in 2002 () found no significant change in the paleoenvironment. Phytosaurs, the most common fossils there, experienced a change-over only at the genus level, and the number of species remained the same. Some Aetosaurs, the next most common tetrapods, and early dinosaurs, passed through unchanged. However, both Phytosaurs and Aetosaurs were among the groups of archosaur reptiles completely wiped out by the end-Triassic extinction event.
It seems likely then that while there was some sort of end Carnian extinction (several herbivorous archosauromorph groups died out at this time, while the large herbivorous therapsids – the Kannemeyerid dicynodonts and the Traversodont cynodonts – were much reduced, at least in the northern half of Pangaea (Laurasia))
These extinctions allowed the dinosaurs to expand into many niches that had as a result become unoccupied. Dinosaurs would become increasingly dominant, abundant and diverse, and remained that way for the next 150 million years. The true "Age of Dinosaurs" is the Jurassic and Cretaceous, rather than the Triassic.
- Best overall introduction.
- 'The Triassic world'.
- Douglas Henderson's illustrations of Triassic animals.
- Paleofiles page on the Triassic extinctions.
|Lower Triassic||Middle Triassic||Upper Triassic|
|Induan | Olenekian||Anisian | Ladinian||Carnian | Norian | Rhaetian|