The Full Wiki

Triassic System: Wikis

Advertisements

Note: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article! This article doesn't yet, but we're working on it! See more info or our list of citable articles.

Encyclopedia

(Redirected to Triassic article)

From Wikipedia, the free encyclopedia

Triassic Period
251 - 199.6 million years ago
T
LateTriassicGlobal.jpg
Mean atmospheric O2 content over period duration ca. 16 Vol %[1]
(80 % of modern level)
Mean atmospheric CO2 content over period duration ca. 1750 ppm[2]
(6 times pre-industrial level)
Mean surface temperature over period duration ca. 17 °C [3]
(3 °C above modern level)

The Triassic is a geologic period that extended from about 250 to 200 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 flying vertebrates, the pterosaurs, evolved during the Triassic.

Contents

Dating and subdivisions

Key events in the Triassic
view • discuss • edit
-255 —
-250 —
-245 —
-240 —
-235 —
-230 —
-225 —
-220 —
-215 —
-210 —
-205 —
-200 —
Full recovery of woody trees[4]
Coals return[5]
Scleractinian
corals & calcified sponges[6]
T
r
i
a
s
s
i
c
An approximate timescale of key Triassic events.
Axis scale: millions of years ago.

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:

Upper/Late Triassic (Tr3)
  Rhaetian (203.6 ± 1.5 – 199.6 ± 0.6 Ma)
  Norian (216.5 ± 2.0 – 203.6 ± 1.5 Ma)
  Carnian (228.0 ± 2.0 – 216.5 ± 2.0 Ma)
Middle Triassic (Tr2)
  Ladinian (237.0 ± 2.0 – 228.0 ± 2.0 Ma)
  Anisian (245.0 ± 1.5 – 237.0 ± 2.0 Ma)
Lower/Early Triassic (Scythian)
  Olenekian (249.7 ± 0.7 – 245.0 ± 1.5 Ma)
  Induan (251.0 ± 0.4 – 249.7 ± 0.7 Ma)

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"). From the east a vast gulf entered Pangaea, the Tethys sea. It opened farther westward in the mid-Triassic, at the expense of the shrinking Paleo-Tethys Ocean, an ocean that existed during the Paleozoic. The remaining shores were surrounded by the world-ocean known as Panthalassa ("all the sea"). All the deep-ocean sediments laid down during the Triassic have disappeared through subduction of oceanic plates; thus, very little is known of the Triassic open ocean. The supercontinent Pangaea was rifting during the Triassic—especially late in the period—but had not yet separated. The first nonmarine sediments in the rift that marks the initial break-up of Pangaea—which separated New Jersey from Morocco—are of Late Triassic age; in the U.S., these thick sediments comprise the Newark Group.[7] 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. Thus Triassic stratigraphy is mostly based on organisms living in lagoons and hypersaline environments, such as Estheria crustaceans.

Advertisements

Africa

At the beginning of the Mesozoic Era, Africa was joined with Earth's other continents in Pangaea.[8] Africa shared the supercontinent's relatively uniform fauna which was dominated by theropods, prosauropods and primitive ornithischians by the close of the Triassic period.[8] Late Triassic fossils are found through-out Africa, but are more common in the south than north.[8] The boundary separating the Triassic and Jurassic marks the advent of an extinction event with global impact, although African strata from this time period have not been thoroughly studied.[8]

Climate

Middle Triassic marginal marine sequence, southwestern Utah

The Triassic climate was generally hot and dry, forming typical red bed sandstones and evaporites. There is no evidence of glaciation at or near either pole; in fact, the polar regions were apparently moist and temperate, a climate suitable for reptile-like creatures. Pangaea's large size limited the moderating effect of the global ocean; its continental climate was highly seasonal, with very hot summers and cold winters.[9] It probably had strong, cross-equatorial monsoons.[9]

Life

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

Three categories of organisms can be distinguished in the Triassic record: holdovers from the Permian-Triassic extinction, new groups which flourished briefly, and other new groups which went on to dominate the Mesozoic world.

Flora

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.

Marine fauna

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.

Terrestrial fauna

Reconstruction of Proterosuchus, a genus of carnivorous reptile, classified under Archosauromorpha, that existed in the Early Triassic period.
Coelophysis, one of the first Dinosaurs, appeared in the mid-Triassic.

The Permian-Triassic extinction devastated terrestrial life. Biodiversity rebounded with the influx of disaster taxa, however these were short lived. Diverse communities with complex trophic structures took 30 million years to reestablish.[10]

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, characterized by the first frogs, 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.

Archosauromorph reptiles — especially archosaurs — progressively replaced the synapsids that had dominated the Permian. Although Cynognathus was a characteristic top predator in earlier Triassic (Olenekian and Anisian) Gondwana, and both kannemeyeriid dicynodonts and gomphodont cynodonts remained important herbivores during much of the period. By the end of the Triassic, synapsids played only bit parts. During the Carnian (early part of the Late Triassic), some advanced cynodont gave rise to the first mammals. At the same time the Ornithodira, which until then had been small and insignificant, evolved into pterosaurs and a variety of dinosaurs. The Crurotarsi were the other important archosaur clade, and during the Late Triassic these also reached the height of their diversity, with various groups including the phytosaurs, aetosaurs, several distinct lineages of Rauisuchia, and the first crocodylians (the Sphenosuchia). Meanwhile the stocky herbivorous rhynchosaurs and the small to medium-sized insectivorous or piscivorous Prolacertiformes were important basal archosauromorph groups throughout most of the Triassic.

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.[11] 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.[12]

Coal

At the start of the Triassic period coal is noticeable by its absence throughout the world. This is known as the "coal gap" and can be seen as part of the Permian–Triassic extinction event.[5] Sharp drops in sea level across the Permo Triassic boundary may be partially to blame.[13] During the preceding Permian period the hot desert conditions had contributed to the evaporation of many inland seas and the inundation of these seas, perhaps by a number of tsunami events may have been responsible for the drop in sea level.[14] There are large salt basins in the southwest United States and a very large basin is suspected in central Canada.[15]

Immediately above the boundary the glossopteris flora was suddenly[16] 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.[17][18] It has been suggested that there was a climate barrier for the conifers.[19] 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 Strait, however. There was a spike of fern and lycopod spores immediately after the close of the Permian.[20] In addition there was also a spike of fungal spores immediately after the Permian-Triassic boundary.[21] 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.[22].

Lagerstätten

Triassic sandstone near Stadtroda, Germany.

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. The remains of fish and various marine reptiles (including the common pachypleurosaur Neusticosaurus, and the bizarre long-necked archosauromorph Tanystropheus), along with some terrestrial forms like Ticinosuchus and Macrocnemus, have been recovered from this locality. All these fossils date from the Anisian/Ladinian transition (about 237 million years ago).

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[23])], forming the Central Atlantic Magmatic Province [(CAMP)],[24] 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.[25]

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.[26] 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

Notes

  1. ^ Image:Sauerstoffgehalt-1000mj.svg
  2. ^ Image:Phanerozoic Carbon Dioxide.png
  3. ^ Image:All palaeotemps.png
  4. ^ McElwain, J.C.; Punyasena, S.W. (2007). "Mass extinction events and the plant fossil record". Trends in Ecology & Evolution 22 (10): 548-557. doi:10.1016/j.tree.2007.09.003. 
  5. ^ a b Retallack GJ Veevers JJ & Morante R (1996). "Global coal gap between Permian–Triassic extinctions and middle Triassic recovery of peat forming plants". GSA Bulletin 108 (2): 195–207. http://bulletin.geoscienceworld.org/cgi/content/abstract/108/2/195. Retrieved 2007-09-29. 
  6. ^ Payne, J.L.; Lehrmann, D.J.; Wei, J.; Orchard, M.J.; Schrag, D.P.; Knoll, A.H. (2004). "Large Perturbations of the Carbon Cycle During Recovery from the End-Permian Extinction". Science 305 (5683): 506. doi:10.1126/science.1097023. http://www.sciencemag.org/cgi/content/abstract/305/5683/506. 
  7. ^ Lecture 10 - Triassic: Newark, Chinle
  8. ^ a b c d Jacobs, Louis, L. (1997). "African Dinosaurs." Encyclopedia of Dinosaurs. Edited by Phillip J. Currie and Kevin Padian. Academic Press. p. 2-4.
  9. ^ a b Stanley, 452-3.
  10. ^ Sahney, S. and Benton, M.J. (2008). "Recovery from the most profound mass extinction of all time" (PDF). Proceedings of the Royal Society: Biological 275: 759. doi:10.1098/rspb.2007.1370. http://journals.royalsociety.org/content/qq5un1810k7605h5/fulltext.pdf. 
  11. ^ Tanner LH, Lucas SG & Chapman MG (2004). "Assessing the record and causes of Late Triassic extinctions" (PDF). Earth-Science Reviews 65 (1-2): 103–139. doi:10.1016/S0012-8252(03)00082-5. http://nmnaturalhistory.org/pdf_files/TJB.pdf. Retrieved 2007-10-22. 
  12. ^ Ruben, J.A., and Jones, T.D. (2000). "Selective Factors Associated with the Origin of Fur and Feathers". American Zoologist 40 (4): 585–596. doi:10.1093/icb/40.4.585. http://icb.oxfordjournals.org/cgi/content/full/40/4/585. 
  13. ^ 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
  14. ^ Knauth LP 1998 Salinity history of the earth's early ocean, Nature 395; 554-555.
  15. ^ Dott, R.H. and Batten, R.L. (1971) Evolution of the Earth, 4th ed. McGraw Hill, NY.
  16. ^ Hosher WT Magaritz M Clark D 1987 Events near the Permian-Triassic boundary. Mod. Geol. 11; 155-180, on p173-174.
  17. ^ Florin R (1963) The distribution of Conifer and Taxad genera in time and space. Acta Horti Bergiani. 20, 121-312.
  18. ^ Melville R (1966) Continental drift, Mesozoic continents, and the migrations of the angiosperms. Nature 211, 116.
  19. ^ Darlington PJ, (1965) Biogeography of the southern end of the world. Harvard University Press, Cambridge Mass., on p168.
  20. ^ Retallack GJ (1995) Permian -Triassic life crises on land. Science 267, 77-79.
  21. ^ 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.
  22. ^ 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.
  23. ^ Nomade et al.,2007 Palaeogeography, Palaeoclimatology, Palaeoecology 244, 326-344.
  24. ^ Marzoli et al., 1999, Science 284. Extensive 200-million-year-old continental flood basalts of the Central Atlantic Magmatic Province, pp. 618-620.
  25. ^ Hodych & Dunning, 1992.
  26. ^ 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) Stratigraphy.org, 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

Triassic Period
Lower/Early Triassic Middle Triassic Upper/Late Triassic
Induan | Olenekian Anisian | Ladinian Carnian | Norian
Rhaetian
Preceded by Proterozoic Eon 542 Ma - Phanerozoic Eon - Present
542 Ma - Paleozoic Era - 251 Ma 251 Ma - Mesozoic Era - 65 Ma 65 Ma - Cenozoic Era - Present
Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Paleogene Neogene Quaternary

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

TRIASSIC SYSTEM, in geology, the lowest or youngest system of the Mesozoic era; it occupies a position above the Permian and below the Jurassic system of rocks. The principal formations of the type region, Germany, are the Bunter, Muschelkalk and Keuper; these were for the first time grouped together under the systematic name " Trias " by F. von Alberti (1834). A description of the rocks in these formations will be found under their respective headings. For a long time this German development of the strata was regarded as typical of the period; later, however, the discovery of another more fossiliferous phase in the Alps and Mediterranean region, and subsequently in Asia and elsewhere, led geologists to take a different view of the system as a whole. It was clearly seen that there existed two distinct phases of Triassic rock-building, the one continental (terrestrial and lagoonal), the other marine (pelagic).

Missing image
Triassicsystem-1.jpg

Triassic Period Hypothetical distribution of Land & Sea f -4 The original Trias of the " Germanic " area (including Great Britain) must be understood as a special local expression of the continental Trias, while the thoroughly marine type represents the normal aspect of sedimentation. Similarly, the fauna of the mr; x ne Trias is the standard for comparison with the life of other geoligical systems. The term Trias - indicative of the threefold grouping in Germany - thus loses its original significance when applied to the world-wide deposits of the period; its use, however, is continued by general consent.

Table of contents

Continental Trias

The records of the terrestrial and lagoonal conditions during this period are to be found in the coarse conglomerates, red and mottled sandstones, marls and clays with their accompanying beds of dolomite and limestone, and layers of gypsum, anhydrite, rock-salt and coal. The coarser breccias and conglomerates appear to represent ancient screes and shore deposits, and in part at least their formation may have been due to torrential action. The remarkable oblique bedding in many of the sandstones, coupled with the fact that the sand grains are often very perfectly rounded, points to the transporting action of wind. Even the pebbles occasionally exhibit the dreikanter form, familiar in our modern deserts. But the marls, muds and many sandy beds were certainly deposited in sheets of water, which were evidently shallow and subject to frequent periods of desiccation. Of this we have evidence in the great abundance of reptilian foot-prints, of rain pits, ripple marks, and sun cracks upon what were once surface muds and sands. That the drying up of the water sheets repeatedly produced a highly saline condition is shown by the common occurrence of rock-salt, gypsum and anhydrite. In short, the physical conditions under which the continental Trias was formed appear to have been similar to those obtaining at the present day in the Caspian region.

In Europe the earlier deposits of the continental Trias occupy a compact area covering nearly the whole of Germany, whence they may be followed into central and northern England, Heligoland, Upper Silesia and the Vosges. Another tract lay over what are now the western Alps and south-east France; also in the Pyrenees, Balearic Islands, Sardinia, Sicily and southern Spain, and on to the north coast of Africa. In the Carpathians the same rocks appear, and they cover a large area in north-east Russia (Tartarian), and north-west Siberia. Later, the Muschelkalk limestones point to a temporary influx of the sea involving most of the above regions except Britain and Russia. Three encroachments of the sea are indicated, each followed by a period of excessive evaporation and contraction; these happened in the time of the Roth, the Lower and the Upper Muschelkalk. Finally the last influx, that of the Rhaetic Sea, not only spread much beyond the limits of the earlier incursions but remained as the forerunner of the succeeding Jurassic waters. In North America the continental Trias appears with a close resemblance to that of western Europe along the Atlantic coastal strip from Prince Edward's Island, through New Brunswick, Nova Scotia, Connecticut, New York, Pennsylvania, Virginia, to North Carolina. These are the rocks of the Newark series. Southwards it may be traced in Honduras, the Andes, Brazil, Argentina and Chile. Another large area in the western interior, Wyoming and New Mexico, is occupied by " red beds " (600-2000 ft., in part Permian) with gypsum and rock-salt. In southern Africa the upper part of the Karoo formation appears to represent Triassic time - the Stormberg beds (Permo-Trias) and the Beaufort beds (Rhaetic). In India the Panchet beds of the Gondwana system and in New South Wales the Hawkesbury series (Wianametta shales with coals and iron-stone, Hawkesbury sandstone, and at the base the Narraburra beds) belong to about the same horizon. In New Zealand the Otapiri, Wairoa and Oreti series appear to contain fossils indicating a transition from Permian to Rhaetic.

The Marine or Open-sea Trias

This type of Triassic deposit is frequently referred to under the titles " Alpine," " Mediterranean " or " Pelagic." It first came into notice through the discovery of fossils in the neighbourhood of Recoaro and St Cassian on the southern side of the Alps, and these rocks were subsequently correlated with those at Hallstatt on the northern side. On both sides of the Alps rocks of this age flank the central core, but they are better developed, thicker and less altered towards the east than towards the west. In the western Alps Triassic beds can be only dimly recognized amongst the masses of schists called the Schistes-lustres and Bundnerschiefer. In the eastern Alps, however, although there are sandy and conglomeratic members, such as the Werfen beds and Lunz sandstone, yet the most striking feature, in contrast with the continental Trias, is the prevalence of calcareous and dolomitic strata, to which must be added the enormously greater abundance of organic remains. The Alpine Trias varies in lithological character so rapidly from point to point, and has furthermore been subjected to so much dislocation, that great difficulty has been experienced in correlating the beds in different areas and in placing them in their proper order of sequence. The result of this difficulty has been the production of a nomenclature so unwieldy that no attempt at a detailed exposition is possible in the space here available. The principal members of the Alpine Trias will be found in their correct relative positions in the table. One of the most striking aspects of the Alpine Trias, on both the northern and southern sides, is the great development of dolomite which is so prominent a feature in the scenery of southern Tirol (Drei Zinnern, &c.). Some of these rocks contain-the remains of corals, still more bear the fossils of calcareous algae, and although the view originally advanced by F. v. Richthofen that they represent Triassic coral reefs has been strongly opposed, it still seems to be the most reasonable explanation of their origin. The rocks of the marine Trias generally are argillaceous beds and dark limestones; in the Alpine regions many of the latter have been marmorized. The well-known white marble of Carrara in the Apuan Mountains is a metamorphosed Triassic limestone. The same type of Trias occurs also in south Italy (Longobardian), in Sicily, Barcelona, Balearic Islands, Crete, Bosnia, East Hungary, and the Carpathian Mountains by Bukovina and Dobrudja.

The Alpine-Mediterranean Trias sea evidently had a prolongation into Western Asia, for in Asia Minor, Armenia and Bokhara rocks with closely related fossils have been found. In Central Asia Triassic rocks are known in Afghanistan (sandstones with coal), Russian Turkestan, and in the Pamir. In India the lower Trias of the Salt Range presents the most typical example of the marine deposits of this stage. The Himalayan Trias more perfectly represents the upper portion of the system. Triassic limestones are found also in Kashmir and Hazara, and shales in Baluchistan. The marine Trias is known in Burma, Tongking, China and north-east Tibet; also in Japan, Siberia and in the arctic regions of Spitsbergen and Bear Island. In the Australasiatic region the marine Trias is found in the Sunda Islands, Sumatra, Roth and Timor and in New Caledonia.

Climate, Vulcanism

There seems little room for doubt that the climate of Triassic times was, over large tracts of the northern continental region, dry and arid in character, certain features in the flora tending to support this view. On the other hand, the southern continental deposits, with Glossopteris and its allies, is more suggestive of a moist climate. There is no evidence of the glacial condition of the preceding Permian period. The Triassic period was one of rest so far as crustal movements were concerned. Volcanic activity, however, was exhibited on a large scale in the north-western part of North America, the great batholith of the Coast Range being nearly moo m. long; in British Columbia and Alaska large bodies of igneous rock are supposed to belong to this period. On the eastern side of the continent the diabase and dolerite lava flows, veins and sills of the famous Palisades of the Hudson valley belong to the Newark system. In Europe and Asia igneous rocks are scarce, but tuffs, porphyrites, &c., occur in the Schlern district (Upper Cassian age) and at Falzarego Strasse, Trarenanzes (Wengen horizon), in the Alpine region.

Life of the Triassic Period

The plant life of this period exhibits on the whole a closer relationship with the Jurassic than with the preceding Palaeozoic formations. Flowering plants are unknown in the Triassic deposits and the dominant forms are all gymnosperms, the prevailing types being ferns and fern-like plants. cycadeans, conifers and equisetums. The Palaeozoic calamites, sigillarias and lepidodendrons became extinct early in this period; but in the southern hemisphere the Glossopteris flora still held on in considerable force. Amongst the ferns were Lepidopteris, Sagenopteris, Danaeopteris, with the Carboniferous genera Sphenopteris, Pecopteris and others. Equisetites and Schizoneura became common. Characteristic conifers were Voltzia, Araucarites, Brachyphyllum. The Cycadeans were represented by Pterophyllum, Cycadites, Podozamites, &c. Baiera was the representative of the ginkgos. Calcareous algae were important rock builders in some of the Triassic seas (Gyroporella, Diplopora). Fish remains are not generally common in the Trias; teeth and scales are crowded together in the " bone beds " in the Rhaetic and between the Keuper and Muschelkalk; in the marine Trias of the Alpine region skeletons are much more common. They are abundant also in the bituminous shales of the Connecticut Valley and in the Hawkesbury series of New South Wales. Selachians are represented by species of Hybodus, A crodus and Palaeobates; dipnoids by Ceratodus and Gosfordia. The ganoids, with Palaeozoic as well as younger forms, include Gyrolepis, Semionotus, Dictyopyge, Graphiurus, Belonorhynchus and Pholidopleuras. Bony fish were very feebly represented. The amphibian labyrinthodonts (Stegocephalia) were numerous, their bones being found in the " bone beds " and in the Bunter and Keuper sandstones and their equivalents in North America, South Africa and India (Labyrinthodont, Mastodonsaurus, Trematosaurus, Capitosaurus). Their footprints are often very abundant, e.g. Cheirotherium. The reptiles of the Triassic deposits, unlike the amphibians, which are Permian in character, show a closer relationship with Jurassic forms; one of the most interesting facts in the life-history of the group is the development during this period of sea-going forms such as at a later geological period played so prominent a part. Early crocodilian reptiles are represented by Belodon, Mystriosuchus, Stagonolepis, Parasuchus; and Rhyncocephalia by Telerpeton and Hyperodapedon. Ichthyopterygians were represented by Mixosaurus, Nothosaurus, Cymatosaurus; early dinosaurs (carnivorous) by Zanclodon, Anchisaurus, Thecodontosaurus, Palaeosaurus; the remarkable theromorphs (anomodonts), by Elginia, Dicynodon, Geikia, Gordonia. Turtles became well established during this period (Psammochelys, Chelyzoon). Of great interest is the discovery of the earliest traces of mammals in the Trias of Europe, South Africa and North America. The imperfect remains (teeth and jaw-bones) do not admit of any certainty in deciphering their relationships. Microlestes from the Rhaetic of England and Wurttemberg and Dromatherium from North America are perhaps the best known; Tritylodon from South Africa may also be added. Among the lower forms of marine life foraminifera and sponges play a subordinate part. Corals, which with the calcareous algae built considerable reefs in some regions, at this time began to assume a modern aspect, and henceforth the Hexacorallids took the place of the Palaeozoic Tetracorallid forms (Stylophyllum, Pinacophyllum, Thecosmilia). Crinoids were locally very numerous individually (Encrinus liliiformis, Dadocrinus gracilis). Urchins were not very common, but an important change from the Palaeozoic to the Mesozoic type of shell took place about thisftime. Brachiopods were important; rostrate forms like Terebratula and Rhynchonella from this time onward became more prevalent than broad hinged genera. Pelecypods were abundant, Myophoria, Halobia, Daonella, Pseudomonotis, Avicula, Gervillia and many others. Gasteropods also were numerous; at the beginning of the period, as in other groups, many Palaeozoic forms lingered on, but one of the main changes about this time was the development and expansion of siphonostomous forms with canaliculate shells. Quite the most important Mollusca were the Cephalopods. In the early Trias there still remained a few of the Palaeozoic genera, Orthoceras, Continental Trias.

Hungarites, and forms which linked up the goniatites with the ammonites, which henceforth took the lead in numbers and variety. Prionolobus, Aspidites, Celtites, Meekoceras, Tirolites, Ptychites, Tropites, Ceratites, Arcestes, Psiloceras and Flemingites are a few of the prominent Triassic genera. The nautiloids were fairly well represented, but they exhibit no such marked development from Palaeozoic to Mesozoic types as is shown among the ammonoid In the tabulated synopsis of the Triassic system given bel it has been impossible to include many of the names of groups and subordinate divisions. Some of these, such as the term " Noric " (Norian), have been used in a variety of ways. A clear account of the history of the study of the Trias will be found in K. A. von Zittel's History of Geology and Palaeontology (Eng. trans., London 1901). 1901).

REFERENCES. - The literature of the Trias is very voluminous. A full account, with full references as to date of publication, in Lethaea Geognostica, ed. by F. Frech, Theil II.; Das Mesozoicum, Bd. i. " Einleitung des Mesozoicum and der Trias " (F. Frech); ' ` Continentale Trias " (E. Philippi and J. Wysogorski), 1903; 2nd Lieferung, " Die asiatische Trias " (F.:4'Noetling), 1905; 3rd Lieferung, " Die Alpine Trias des Mediterran-Gebietes " (G. von Hathaber), Stuttgart, 1905. (J. A. H.)


<< Triangle (Music)

Triazines >>


Advertisements






Got something to say? Make a comment.
Your name
Your email address
Message