The Triassic is a geologic period and system that extends from about 251 to 199 Ma (million years ago). As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events. 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.

During the Triassic, both marine and continental life show an adaptive radiation beginning from the starkly impoverished biosphere that followed the Permian-Triassic extinction. Corals of the hexacorallia group made their first appearance. The first flowering plants (Angiosperms) may have evolved during the Triassic, as did the first flying vertebrates, the pterosaurs.

Dating and subdivisions

The Triassic was named in 1834 by Friedrich Von Alberti from the three distinct layers (Latin trias meaning triad) —red beds, capped by chalk, followed by black shales— that are found throughout Germany and northwest Europe, called the 'Trias'.

The Triassic is usually separated into Early, Middle, and Late Triassic Epochs, and the corresponding rocks are referred to as Lower, Middle, or Upper Triassic. The faunal stages from the youngest to oldest are:

Paleogeography

230 Ma plate tectonic reconstruction

Climate

Middle Triassic marginal marine sequence, southwestern Utah

Life

Triassic flora as depicted in Meyers Konversations-Lexikon (1885-90)

In marine environments, new modern types of corals appeared in the Early Triassic, forming small patches of reefs of modest extent 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. There were also many types of marine reptiles. These included the Sauropterygia, which featured pachypleurosaurs and nothosaurs (both common during the Middle Triassic, especially in the Tethys region), placodonts, and the first plesiosaurs; the first of the lizardlike Thalattosauria (askeptosaurs); and the highly successful ichthyosaurs, which appeared in Early Triassic seas and soon diversified, some eventually developing to huge size during the late Triassic.

On land, the holdover plants included the lycophytes, the dominant cycads, ginkgophyta (represented in modern times by Ginkgo biloba) and glossopterids. The spermatophytes, or seed plants came to dominate the terrestrial flora: in the northern hemisphere, conifers flourished. Glossopteris (a seed fern) was the dominant southern hemisphere tree during the Early Triassic period.

Temnospondyl amphibians were among those groups that survived the Permian-Triassic extinction, some lineages (e.g. Trematosaurs) flourishing briefly in the Early Triassic, while others (e.g. capitosaurs) remained successful throughout the whole period, or only came to prominence in the Late Triassic (e.g. plagiosaurs, metoposaurs). As for other amphibians, the first Lissamphibia are known from the Early Triassic, but the group as a whole did not become common until the Jurassic, when the temnospondyls had become very rare.

Middle and Upper Triassic shallow marine sequence, Makhtesh Ramon, Israel.

Among other reptiles, the earliest turtles, like Proganochelys and Proterochersis, appeared during the Norian (middle of the Late Triassic). The Lepidosauromorpha—specifically the Sphenodontia—are first known in the fossil record a little earlier (during the Carnian). The Procolophonidae were an important group of small lizard-like herbivores.

Archosaurs were initially rarer than the therapsids which had dominated Permian terrestrial ecosystems, but they began to displace therapsids in the mid-Triassic.^[4] This "Triassic Takeover" may have contributed to the evolution of mammals by forcing the surviving therapsids and their mammaliform successors to live as small, mainly nocturnal insectivores; nocturnal life probably forced at least the mammaliforms to develop fur and higher metabolic rates.^[5]

Coal

When the Triassic commenced a coal hiatus (no coal) appeared simultaneously all over the world at the Permian-Triassic boundary ^[6] Probably a sudden large drop in sea level permitted whatever caused the hiatus, and thus accounts for the sudden appearance, for at the close of the Permian there was an even quicker drop in sea level than the slower drop that had occurred in its last half, the sharpest in history ^[7]. There had been many salt deposits in Permian basins in the last half ^[8]. There are large salt basins in the southwest United States and a very large basin is suspected in central Canada, now eroded away ^[9]. Possibly a tsunami opened up some of these basins, evaporation from which would have previously delayed the sea level decline, and thus account for that quicker drop at the end. This or something like this could account for a subsequent rapid rise when the inland sea created evaporated again after barriers were reestablished. Glaciers can be safely ruled out because there is no evidence of glaciers anywhere during the Triassic. Immediately above the boundary the glossopteris flora was suddenly ^[10] largely displaced by an Australia wide coniferous flora containing few species and containing a lycopod herbaceous under story. Conifers became common in Eurasia also. Each of these groups of conifers arose from endemic species because conifers are very poor at crossing ocean barriers and they remained separated for hundreds of millions of years, largely to the present. Podocarpis was south and Pines, Junipers, and Sequoias were north, for instance. The dividing line ran through the Amazon Valley, across the Sahara, and north of Arabia, India, Thailand, and Australia ^{[11] [12]}. It has been suggested that there was a climate barrier for the conifers ^[13], although water barriers are more plausible. If so, something that can cross at least short water barriers must have been involved in producing the coal hiatus. Hot climate could have been an important auxiliary factor across Antarctica or the Bering Straights , however. There was a spike of fern and lycopod spores immediately after the close of the Permian ^[14]. In addition there was also a spike of fungal spores immediately after the Permian-Triassic boundary ^[15]. This spike may have lasted 50,000 years in Italy and 200,000 years in China and must have contributed to the climate warmth. If so, something besides an instant catastrophe must have been involved to cause the coal hiatus because fungi would surely have removed all dead vegetation and coal forming detritus in a few decades in most tropical places. Besides, the fungal spores rose gradually and declined similarly. There was also much woody debris. Each phenomenon would hint at widespread vegetative death. Whatever caused the coal hiatus must have started in North America 25 million years sooner ^[16]. Weesner believes that Mastotermitidae termites may go back to the Permian ^[17] and fossil wings have been discovered in the Permian of Kansas which have a close resemblance to wings of Mastotermes of the Mastotermitidae, which is the most primitive living termite and which is thought to be the descendant of Cryptocercus genus, the wood roach. This fossil is called Pycnoblattina. It folded its wings in a convex pattern between segments 1a and 2a. Mastotermes is the only living insect that does the same ^[18], so it is possible that they are responsible for the coal hiatus. This is plausible because termites attack the trunk, which is the most vulnerable part. Modern termites also eat detritus. If parasitoids were what brought back the coal after about 10 million years past the opening, their initial evolution must have taken place in or near Australia because the coal reappeared there first by several million years ^[19]. Ancestors of the Evaniidae, which parasitize roach egg sacs ^[20], could have been the ones involved, and this may explain why termites evolved separated eggs except in Mastotermitidae. During the Triassic coal hiatus in the beginning of the Triassic it was possible to find stump impressions up to 45 cm (17.7 in) and root impressions up to 18 cm (7 in) in south Australia, but no roots or logs. The soil was extremely low in organic matter and there was no detritus at all ^[21].

Lagerstätten

Triassic sandstone near Stadtroda, Germany.

Late Triassic 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 plesiosaurs. Invertebrates like brachiopods, gastropods, and molluscs were severely affected. In the oceans, 22% 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, several important clades of crurotarsans (large archosaurian reptiles previously grouped together as the thecodonts) disappeared, as did most of the large labyrinthodont amphibians, 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.

What caused this Late Triassic extinction is not known with certainty. It was accompanied by huge volcanic eruptions that occurred as the supercontinent Pangaea began to break apart about 202 to 191 million years ago [(40Ar/39Ar dates^[22])], forming the Central Atlantic Magmatic Province [(CAMP)]^[23], one of the largest known inland volcanic events since the planet cooled and stabilized. Other possible but less likely causes for the extinction events include global cooling or even a bolide impact, for which an impact crater containing Manicouagan Reservoir in Quebec, Canada, has been singled out. At the Manicouagan impact crater, 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 201.58±0.28 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 Manicouagan impact preceded the end of the Triassic by approximately 10±2 Ma. Therefore it could not be the immediate cause of the observed mass extinction. ^[24]

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.^[25] 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 there was some sort of end-Carnian extinction, when several herbivorous archosauromorph groups died out, while the large herbivorous therapsids— the kannemeyeriid dicynodonts and the traversodont cynodonts— were much reduced in the northern half of Pangaea (Laurasia).

These extinctions within the Triassic and at its end allowed the dinosaurs to expand into many niches that had become unoccupied. Dinosaurs became 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.

See also

• Geologic timescale
• List of fossil sites (with link directory)
• Paleorrota (Triassic place in Brazil)

Notes

1. ↑ Lecture 10 - Triassic: Newark, Chinle
2. ↑ Stanley, 452-3.
3. ↑ Stanley, 452-3.
4. ↑
5. ↑
6. ↑
7. ↑ Holser WT Schonlaub H_P,Moses AJr Boekelmann K Klein P Magaritz MOrth CJ Fenninger A Jenny C Kralik M Mauritsch EP Schramm J_M Sattagger K Schmoller R 1989 A unique geochemical record at the Permian/Triassic boundary. Nature 337; 39, on p42
8. ↑ Knauth LP 1998 Salinity history of the earth's early ocean, Nature 395; 554-555.
9. ↑ Dott, R.H. and Batten, R.L. (1971) Evolution of the Earth, 4th ed. McGraw Hill, NY.
10. ↑ Hosher WT Magaritz M Clark D 1987 Events near the Permian-Triassic boundary. Mod. Geol. 11; 155-180, on p173-174.
11. ↑ Florin R (1963) The distribution of Conifer and Taxad genera in time and space. Acta Horti Bergiani. 20, 121-312.
12. ↑ Melville R (1966) Continental drift, Mesozoic continents and the migrations of the angiosperms. Nature 211, 116.
13. ↑ Darlington PJ (1965) Biogeography of the southern end of the world. Harvard University Press, Cambridge Mass., on p168.
14. ↑ Retallack GJ (1995) Permian -Triassic life crises on land. Science 267, 77-79.
15. ↑ Eshet Y Rampino MR (1995) Fungal event and palynological record of ecological crises and recovery across Permian-Triassic boundary. Geology 23, 967-970, on p969.
16. ↑ Retallack GJ Veevers JJ Morante R (1996) Global coal gap between Permian-Triassic extinctions and middle Triassic recovery of peat forming plants (review). Geological Society Am. Bull. 108, 195-207.
17. ↑ Weesner FM (1960) Evolution biology of termites. Annual Review of Entomology. 5; 153-170.
18. ↑ Tilyard RJ (1937) Kansas Permian insects.. Part XX the cockroaches, or order BlattariaI, II Am. Journal of Science 34; 169-202, 249-276.
19. ↑ Retallack GJ Veevers JJ Morante R (1996) Global coal gap between Permian-Triassic extinctions and middle Triassic recovery of peat forming plants (review). Geological Society Am. Bull. 108, 195-207, on p196.
20. ↑ Godfrey HCJ (1994) Parasitoid's Behavioral and Evolutionary Ecology. Princeton University Press, Princeton.
21. ↑ Retallack G (1997) Paleosols in the upper Narrabeen group of New South Wales as evidence of early Triassic paleoenvironments without exact modern analogs (review) Australian Journal of Earth Sciences 44; 185-281.
22. ↑ Nomade et al.,2007 Palaeogeography, Palaeoclimatology, Palaeoecology 244, 326-344.
23. ↑ Marzoli et al., 1999, Science 284. Extensive 200-million-year-old continental flood basalts of the Central Atlantic Magmatic Province, pp. 618-620.
24. ↑ Hodych & Dunning, 1992.
25. ↑ No Significant Nonmarine Carnian-Norian (Late Triassic) Extinction Event: Evidence From Petrified Forest National Park

References

• Emiliani, Cesare. (1992). Planet Earth : Cosmology, Geology, & the Evolution of Life & the Environment. Cambridge University Press. (Paperback Edition ISBN 0-521-40949-7)
• Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006
• Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6
• van Andel, Tjeerd, (1985) 1994, New Views on an Old Planet : A History of Global Change, Cambridge University Press

External links

• Overall introduction
• 'The Triassic world'
• Douglas Henderson's illustrations of Triassic animals
• Paleofiles page on the Triassic extinctions
• Examples of Triassic Fossils

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