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Lucas S.G., and Orchard M.J , Triassic, Reference Module in Earth Systems and Environmental Sciences, Elsevier, 2013. 27-Sep-13 doi: 10.1016/B978-0-12-409548-9.02872-4.

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Triassic☆ SG Lucas, New Mexico Museum of Natural History, Albuquerque, NM, USA MJ Orchard, Geological Survey of Canada, Vancouver, BC, Canada ã 2013 Elsevier Inc. All rights reserved.

Introduction Triassic Rocks Time-Scale Palaeogeography Tectonics and Sedimentation Sea-Levels Climate Extinctions Flora Shelled Marine Invertebrates Insects Fishes Tetrapods

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Glossary Compressional tectonics Deformation of the crust through compression (pushing together). Cycadophytes A group of gymnosperm plants with compound leaves, including the cycads. Diapsids The lizard-like and ruling reptiles, including lizards, snakes, crocodiles, and dinosaurs. Epeirogenic uplift The formation and submergence of continents by broad, relatively slow displacements of the Earth’s crust Extensional tectonism Deformation of the crust through extension (pulling apart). Fore-arc The region between an island arc and an oceanic trench. Foreland The region in front of a deformed area of the crust. Freeboard The difference between mean sea-level and mean continental altitude. Graben A block of crust dropped down along faults relative to blocks on either side. Gymnosperm A vascular plant with seeds that are not covered by an ovary, such as conifers.

Mesophytic The time of intermediate land plants, approximately equivalent to the Middle-Triassic, Jurassic, and Cretaceous. Palaeophytic The time of ancient land plants, approximately equivalent to the Palaeozoic and Early Triassic. Pteridophyte A fern-like division of vascular plants that reproduce by spores. Retroarc foreland basin A zone of thickened sediment (basin) and extensional tectonism behind an island arc, floored by continental crust. Synapsids Mammal-like reptiles, including the ancestors of mammals. Transpressional tectonics Deformation of the crust by a combination of strike-slip (horizontal) motion and oblique compression. Trough An elongate depression in the crust with gently sloping borders. Xeromorphic scale-leaved conifers A group of conifers with thick, scaly leaves that retain water and thus allow the plants to live in relatively dry climates.

Introduction In 1834, the German geologist Frederich August von Alberti coined the term ‘Triassic’ for rocks originally recognized in Germany as the Bunter, Muschelkalk, and Keuper formations. Today, the rocks of Triassic age (201 to 252 million years ago) are recognized on all continents (Figure 1). Most of these are sedimentary rocks consisting of dominantly shallow-water carbonates of marine origin and siliciclastic red beds of non-marine origin. These rocks represent a record of sedimentation on and around the vast Pangaean supercontinent and tell the tale of its final union and the initiation of its subsequent fragmentation. In this brief overview of the global Triassic, the rock record, time-scale, paleogeography, tectonics and sedimentation, sea-levels, climate, and biota of the time period are considered. ☆ Change History: June 2013. SG Lucas introduced small edits in the text of the article mostly with regard to new numerical ages of Triassic timescale. Updated references and added new Figure 2.

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Figure 1 Map of the present world, showing the distribution of Triassic rocks. After Lucas (2000) The epicontinental Triassic, an overview. Zentralblatt fu¨r Geologie und Pala¨ontologie Teil I 7–8: 475–496.

Triassic Rocks Triassic rocks are exposed on all the world’s continents (Figure 1). Estimates of their maximum thickness have been given as 9 km, and their total volume is  45 million km3. These estimates are slightly more than the values estimated for the Permian, but substantially less than estimates for the Jurassic or Cretaceous. Triassic rocks are mostly sedimentary in origin, and volcanic rocks do occur in relatively minor amounts: they have been estimated as constituting  1–2% overall of the exposed Triassic rocks in the Americas. Triassic volcanic rocks can be substantial in some regions, however, as exemplified in the Pacific north-west of North America. The Triassic was a time of great continental emergence due to a combination of widespread epeirogenic uplift and relatively low sea-level. Marine deposition was mostly confined to the Tethys, the circum-Pacific, and the circum-Arctic (Figure 1). A worldwide survey identifies 15 significant Triassic outcrop belts: (1) the Cordillera of the western United States and western Canada, which exposes significant accumulations of both marine and non-marine strata, as well as a substantial record of Triassic rocks in accreted terranes; (2) the Newark Supergroup non-marine rift basins of eastern North America; (3) extensive marine and non-marine deposits of eastern Greenland, Franz Josef Land, and Svalbard; (4) western Europe, from the dominantly non-marine deposits of the Germanic basin system to the dominantly marine strata of the northern Mediterranean; (5) the extensive, dominantly marine deposits of north-eastern Siberia; (6) shallow marine deposits in Israel; (7) marine deposits in the Transcaucasian region of Iran and Azerbaijan, which include some very fossiliferous sections of the Permian-Triassic boundary (PTB); (8) dominantly marine strata of the Caspian Basin and Mangyshlak Peninsula of western Kazakhstan; (9) the Himalayan belt from Afghanistan and Pakistan through Kashmir into Tibet, also the location of some very fossiliferous PTB sections; (10) extensive non-marine deposits of the Junggur and Ordos basins of northern China; (11) marine deposits of southern China, South-east Asia, and Indonesia, including the most well-studied PTB section at Meishan in China and the phenomenal ammonite-bearing beds of Timor; (12) mixed marine-non-marine deposits on the western and eastern coasts of Australia; (13) extensive marine deposits in New Zealand; (14) deep marine deposits in Japan; and (15) non-marine strata exposed in the Transantarctic Mountains of Antarctica.

Time-Scale The standard global chronostratigraphic scale for the Triassic is divided into seven stages. In ascending order, these are the Induan, Olenekian, Anisian, Ladinian, Carnian, Norian, and Rhaetian (Figure 2). Many other stage names of various scope and utility are employed regionally. For example, the Lower Triassic Griesbachian, Dienerian, Smithian, and Spathian stages are based on Arctic successions and have widespread currency in North America. Similarly, Tethyan substage names remain widely used in Europe. Definition and subdivision of the Triassic stages have been based largely on ammonoid biostratigraphy. At present four Global Stratotype Section and Points (GSSPs) have been defined for the Triassic time-scale (for the bases of the Induan, Ladinian, Carnian and Rhaetian), and they are based on conodont and ammonoid first occurrences. The base of the Induan defines the base of the Triassic system within the stratotype section at Meishan in southern China. International agreement and definition of the bases of the remaining three stages are under consideration.

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Figure 2 A simplified Triassic time-scale. After Lucas (2010) The Triassic Timescale. Geological Society, London, Special Publication 334.

The relatively low level of Triassic volcanism results in a dearth of numerical ages to provide geochronological control of the time-scale. Nevertheless, some important advances have been made in the past decade. The most intensively studied is the PermianTriassic boundary at Meishan, South China, where U/Pb ages measured from zircons in ash beds both above and below the defined boundary provide an age of around 252 Ma (Figure 2). The Induan Stage is very short, as recent radioisotopic ages indicate an Olenekian base of  251 Ma. The Early-Middle Triassic boundary (base of the Anisian) is bracketed by ash beds that confidently establish the boundary age at  247 Ma. One staggering implication of this is that the Early Triassic would appear to be on the order of only 5 million years long! Diverse dated tuffs associated with ammonoid biostratigraphy establish an age of the base of the Ladinian of 242 Ma. Radioisotopic ages for the Late Triassic are scarce, and the only “reliable” and biostratigraphically controlled age is from a middle Carnian tuff in Italy dated at  231 Ma. A wealth of detrital zircon ages from nonmarine Upper Triassic strata of the Chinle Group in the western USA are less precise because they are maximum ages of the sediments (based on reworked zircons) and because of debate and imprecision relating them to marine biochronology. By one analysis, these detrital zircon ages constrain the beginning of the Norian to  221 Ma. Evidence that the Rhaetian is a relatively short stage (less than 5 million years long) is complex to evaluate, involving an assessment of cyclostratigraphy, diverse biostratigraphy and detrital zircon ages in nonmarine strata. Based on such assessment, one analysis estimates the Rhaetian base as  204 Ma, but this is one of the least reliable estimates in the Triassic numerical timescale. Dated tuffs along the Pacific margin of the Americas (western Canada and Peru) provide strong support for a Triassic-Jurassic boundary age of  201 Ma. There is essentially no preserved Triassic seafloor, so there is no agreed geomagnetic polarity time-scale for the Triassic. However, a composite polarity time-scale has become available, based on successions cobbled together from non-marine and marine sections in North America, Europe, and Asia. With continued refinement, this scale will be an important supplement to that provided by biostratigraphy in both the marine and non-marine realms.

Palaeogeography At the onset of the Triassic, the world’s continents were assembled into a single supercontinent called Pangaea (Figure 3). The rest of the globe comprised a single vast ocean called Panthalassa, with a westward-extending arm called Tethys. This followed the Late Palaeozoic assembly of the continents when Laurentia, Asia, and Gondwana collided along the Alleghanian–Variscan–Ural mountain chains. The nearly hemispheric Pangaean supercontinent was encircled by subduction zones that dipped beneath the continents while the Panthalassan and Tethyan plates carried island arcs and oceanic plateaus that were destined to become accreted to the continental margins.

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Figure 3 Triassic Pangaea, showing major tectonic elements. After Lucas (2000) The epicontinental Triassic, an overview. Zentralblatt fu¨r Geologie und Pala¨ontologie Teil I 7–8: 475–496.

The supercontinent drifted northward and rotated clockwise throughout the Triassic, so there was considerable latitudinal spread to the landmass, which was nearly symmetrical about the equator (Figure 3). However, no sooner had the supercontinent been assembled than significant fragmentation began. Thus, Gondwana and Laurasia began to separate in Late Triassic time with the onset of rifting in the Gulf of Mexico basin. It was not until the Early Jurassic, though, that significant marine sedimentation took place in the nascent Atlantic Ocean basin.

Tectonics and Sedimentation At the broadest level, the tectonics of Triassic Pangaea were simple. The accreted supercontinent was simply surrounded by convergent margins (Figure 3). However, these margins were actually complex belts of magmatic arcs and terranes moving in various directions to produce compressional and transpressional tectonics. Late Triassic Pangaea was the site of widespread extensional tectonism, especially the initial opening of the Atlantic Ocean basin by rifting of the North American and African plates. During the Late Triassic, in the Tethys, North Atlantic, and Arctic, multidirectional rift systems developed (Figure 3). Rifting also took place along a zone of transforms that extended well into the Gulf of Mexico basin and, punctuated by volcanism, dominated the northern border of western Tethys. This rifting in the North Atlantic and Tethyan regions subjected western and central Europe to progressive regional extension, culminating in the development there of complex multidirectional systems of troughs and grabens. During the Early–Middle Triassic, terminal thrusting took place along the entire Gondwanan margin of Pangaea, which was followed in the Carnian by extension in southern South America and eastern Australia. Most Triassic sedimentation took place in one of three types of basins: foreland, fore-arc, or extensional. Perhaps the best example of a Triassic foreland basin is the Karoo Basin of South Africa, a retroarc foreland basin originally formed by the collision of the palaeo-Pacific and Gondwana plates during the Late Carboniferous. In the Karoo Basin, 12 km of Carboniferous-Jurassic red beds accumulated. Most of the Pangaean marginal basins were part of an array of arc-trench systems that surrounded much of the supercontinent. A good example is the complex Cordilleran basin of western North America, in which deposition took place between an offshore island arc and the continental margin. In the western United States portion of this basin, 1.2 km of siliciclastic red beds were shed to the north-west and interfinger with marine carbonates deposited in the arc-trench system. Of the (mostly Late Triassic) extensional basins, perhaps the best studied is the Newark basin in the eastern United States. This was a dip–slip-dominated half graben in which  7 km of mostly lacrustine Upper Triassic–Lower Jurassic sediments accumulated. There were also other types of Triassic extensional basins more complex than the Newark half grabens, such as those of the Germanic basin system of north-western Europe.

Sea-Levels Early Mesozoic plate reorganization was apparently associated with the development of new seafloor-spreading axes, which caused a general reduction of ocean basin volume during the Triassic. Pangaea was very emergent and, because of its high freeboard, the Triassic was a time of relatively low sea-level, which may be termed a first-order Pangaean global lowstand. After the major sea-level fall of the latest Permian, sea-level apparently rose through much of the Triassic, to peak during the Norian and then fall near the end of the period (Figure 4). There were, however, short significant falls in sea-level, especially during the Ladinian and Carnian. There are generally five second-order transgression–regression cycles recognized in the Triassic: these encompass the Lower Triassic and Middle Triassic and the Carnian, Norian, and Rhaetian.

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Figure 4 Triassic sea-level curve. After Lucas (2000) The epicontinental Triassic, an overview. Zentralblatt fu¨r Geologie und Pala¨ontologie Teil I 7–8: 475–496.

Glacio-eustasy could not have driven Triassic sea-level change, so its underlying cause must be tectonism. Indeed, a large amount of regional Triassic tectonism has been invoked to explain sea-level changes in the western Tethys and the Arctic Sverdrup Basin. Triassic sequence boundaries caused by local tectonism or global eustasy show a remarkable degree of synchrony across Pangaea, and 12 high-order boundaries of global extent have been identified and attributed to episodic, major plate tectonic reorganizations.

Climate Triassic climates marked the transition from the Late Palaeozoic ice house to the Mid-Late Mesozoic greenhouse (Figure 5). During the Triassic, there were no glacial ages, and there is no evidence of pack ice in the boreal or austral realms. The Triassic was thus a time of increased warmth with relatively wide subtropical dry (desert) belts at 10 to 30 latitude, as attested to by the broad latitudinal distribution of Triassic evaporites. There was also strong east–west climatic asymmetry across Pangaea, with eastern Pangaea (at least between latitudes 40 S and 40 N) being relatively warmer and wetter because of the presence of Tethys and the absence of an Atlantic Ocean to facilitate oceanic heat exchange. With the Pangaean landmass centered near the equator during the Triassic, and a prominent Tethyan bight, climate models suggest that seasonality was monsoonal. Hence, there were only two seasons, wet and dry. The abundant rainfall was concentrated in the summer months, and there was little annual temperature fluctuation. During the Northern Hemisphere summer, the northern landmass would have been relatively hot, whereas the southern land mass would have been relatively cool. Moisture from Tethys would have been pulled into the Northern Hemisphere low-pressure cell, producing extensive rains, whereas the Southern Hemisphere high-pressure cell would have remained relatively dry. During the Southern Hemisphere summer, this process would have occurred in reverse. Thus, seasonality across Triassic Pangaea would have been alternating hemisphere-wide wet and dry seasons. The warm and highly seasonal climates (wet–dry) of Triassic Pangaea are reflected in its biota. The Triassic saw an increase in the diversity of gymnosperms, particularly of xeromorphic scale-leaved conifers and seed ferns and cycadophytes with thick cuticles. Similarly, during the Triassic, in the evolution of reptiles, more water-efficient (putative uric-acid-excreting) diapsids diversified at the expense of less water-efficient (probably urea-excreting) synapsids.

Extinctions The Permian ended with the greatest biotic extinction of Phanerozoic history (here termed the PTB biotic crisis). This extinction is best documented in the marine realm (Figure 6), where it is estimated that  90% of the species, and more than half of the families

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Figure 5 Triassic climate was a transition between the Late Palaeozoic ice house and the Late Mesozoic greenhouse. pC, Precambrian; C, Cambrian; O, Ordovician; S, Silurian; D, Devonian; Ca, Carboniferous; P, Permian; T, Triassic; J, Jurassic; K, Cretaceous; Cen, Cenozoic. After Lucas (2000) The epicontinental Triassic, an overview. Zentralblatt fu¨r Geologie und Pala¨ontologie Teil I 7–8: 475–496.

Figure 6 Characteristic extinction/diversity patterns of marine invertebrates across the Permian–Triassic boundary. After Lucas (2000) The epicontinental Triassic, an overview. Zentralblatt fu¨r Geologie und Pala¨ontologie Teil I 7–8: 475–496.

of shelled marine invertebrates, became extinct. The magnitude and synchrony of the terrestrial extinction are much less clear. The Triassic records the recovery of the global biota from this massive extinction. The period also bore witness to further marine extinctions within the Late Triassic, and was terminated by a series of Late Triassic marine and non-marine extinctions. The cause of the PTB biotic crisis remains uncertain. Some workers have identified a complex and interrelated group of terrestrial events as a possible cause: (1) major marine regression that reduced marine shelf habitat areas and increased climatic variability, (2) eruption of Siberian flood basalts, (3) release of gas hydrates and erosion/oxidation of marine carbon due to the regression, and (4) elevated atmospheric CO2 due to all of these phenomena resulting in ocean anoxia and global warming. During the Carnian, there was a substantial marine extinction of many kinds of conodonts, ammonoids, bivalves, echinoids, and reef organisms, although the impact on land was less obvious, with evolutionary turnover occurring throughout the Late Carnian and Early Norian. A further extinction has been identified at the Norian–Rhaetian boundary with the nearly total disappearance of the ubiquitous flat clam Monotis. This, in fact, was part of a series of Late Triassic extinctions that included the disappearance of the conodonts, near extinction of the ammonites, decimation of about half of the marine bivalves, and collapse of the reef ecosystem. On land, there were also profound extinctions of tetrapods between the end of the Triassic and sometime in the middle Early Jurassic (Sinemurian), but it has been difficult to establish the exact timing. A major carbon isotope anomaly has been identified in both marine and terrestrial environments at the end of the Triassic. This major perturbation in the global carbon cycle has been variously linked to a significant fall in sea-level, extraterrestrial impact, flood-basalt volcanism, and/or methane release.

Flora During the Permian and Triassic, there was a complex and prolonged replacement of the palaeophytic flora by the mesophytic flora. This was the global change from pteriodophyte-dominated floras of the Palaeozoic to the gymnosperm-dominated floras that characterized much of the Mesozoic. Thus, the arborescent lycopods and sphenopsids gave way to Triassic floras dominated by seed ferns, ginkgophytes, cycads, cycadeoids, and conifers. Distinct Gondwanan and Laurasian floras can be recognized, and within Laurasia two or three provinces are recognized – more boreal Siberian and more equatorial Euramerican provinces (Figure 7). However, the endemism of these floral provinces was not great. Triassic Laurasian floras were dominated by primitive conifers, ferns, cycads, bennettitaleans, and sphenopsids. Conifers were the dominant large trees, whereas the other plant types formed the understory. In coastal settings, stands of the lycopsid Pleuromeia were dominant.

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Figure 7 Triassic floral provinces and floras. After Lucas (2000) The epicontinental Triassic, an overview. Zentralblatt fu¨r Geologie und Pala¨ontologie Teil I 7–8: 475–496.

Gondwanan floras of the Triassic were dominated by a wide range of seed ferns, especially the genus Dicroidium. These floras were generally composed of only a few (no more than 10) genera. Dicroidium was dominant in a variety of vegetation types, from heath to broad-leaved forest to dry woodland. Other important elements of Gondwanan floras were conifers and some Laurasian groups of cycadaleans and ginkgoes. Near the end of the Triassic, the Dicroidium flora declined and was replaced by a cosmopolitan conifer–benettitalean flora. From the end of the Permian until the Middle Triassic, there is a global coal discontinuity – there are no Early Triassic coal beds. This has been attributed to either an extinction of peat-forming plants at the PTB or to unfavorable tectonic conditions for coal preservation, though unfavorable climatic conditions for coal formation and preservation may also have been a factor.

Shelled Marine Invertebrates Late Palaeozoic seas were dominated by pelmatozoans, brachiopods, and bryozoans, but molluscs dominated the Triassic seas (Figure 8). Most prominent of the Triassic molluscs were ammonoid cephalopods and their rapid diversification during the Triassic provides a fossil record by which Triassic time has long been measured. Most Triassic ammonoids were ceratitidans with relatively simple suture lines. These were descended from only two ammonoid stocks that survived the PTB crisis: the otoceratids and the xenodiscids. Triassic ammonoid genera define three broad marine palaeobiogeographic provinces – Tethyan, Boreal, and Notal, the last of which is not well differentiated. The ammonoid paleobiogeography of Triassic Panthalassa was complex and remains little understood. Triassic nautiloid cephalopods appear to have undergone relatively little change at the PTB, but reached great diversity in the Triassic, only to suffer an extensive (but not complete) extinction near the end of the period. Bivalves were common Triassic molluscs and underwent a substantial diversification. Earliest Triassic assemblages are dominated by epifaunal pteriomorphs and detritus-feeding nuculoids, and they are very abundant as fossils. The Middle–Late Triassic saw a diversification of arcoid, mytiloid, trigonioid, and veneroid genera. The thin-shelled bivalves (so-called flat clams) Claraia, Daonella, Halobia, and Monotis are characteristic Triassic forms widely used in biostratigraphy. In contrast to ammonoids and bivalves, Triassic gastropods are relatively uncommon and not particularly diverse. A well-described Early Triassic (Smithian) gastropod assemblage from the western United States contains many genera that are also known from the Permian. Younger Triassic gastropod faunas are more diverse, but still contain numerous Permian holdover genera. The major Mesozoic change in gastropods took place after the end of the Triassic. Brachiopods, bryozoans, and crinoids did not suffer total extinction at the PTB, although their numbers were greatly reduced (Figure 6). They were relatively minor, but persistent, components of Triassic marine faunas. More interesting is the distribution of corals and other reef-building organisms, which are virtually unknown in the Early Triassic (an exception are basal Triassic Renalcis biostromes in south China). In Middle Triassic time, Permian-type reef communities were re-established by Tubiphytes, bryozoans, calcisponges, and calcareous algae. The Carnian–Norian marine extinction was followed by a rapid turnover of the reef-building organisms, so that Norian reefs were characterized by abundant scleractinian corals, probably a result of the evolution of coral– zooxanthellae symbiosis. This presaged the extensive radiation of, and reef building by, corals that typified the Jurassic.

Insects There was a major turnover in insect orders during the PTB biotic crisis, followed by a Triassic adaptive radiation, especially of beetles and cockroaches. A review of the Gondwanan Triassic record of plants and insects supports the concept of a co-evolution that led to the establishment of most modern insect orders by the end of the period.

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Figure 8 Reconstruction of (a) a characteristic Permian seafloor dominated by brachiopods, bryozoans, and crinoids and (b) a Triassic seafloor dominated by molluscs. After Lucas (2000) The epicontinental Triassic, an overview. Zentralblatt fu¨r Geologie und Pala¨ontologie Teil I 7–8: 475–496.

Fishes Fishes underwent a significant diversification during the Triassic. This is particularly evident in the appearance of new kinds of primitive actinopterygians, lungfishes, hybodontid sharks, and coelacanths. The extent of the extinction of fishes at the PTB biotic crisis is uncertain, but it appears to have been more significant within the marine realm. Conodonts, regarded by many now as a primitive fish group, were relatively unaffected by the end-Permian extinction. For them, the major biotic turnover occurred at the end of the Griesbachian, and this was followed by an explosive radiation at the start of the Olenekian. By the end of the Early Triassic, stocks had dwindled and there was a paucity of genera for the remainder of the period. The conodonts, nevertheless, continued to evolve rapidly, and their tooth-like elements now provide very useful biostratigraphic markers. After a long record, stretching throughout the Palaeozoic, conodonts became extinct at the end of the Triassic.

Tetrapods Tetrapod vertebrates dominated Triassic landscapes and underwent at least two successive evolutionary radiations and extinctions during the period. Early Triassic tetrapod faunas were very similar to those of the Late Permian in being dominated by a relatively low diversity of dicynodont therapsids and capitosauroid/trematosaurid temnospondyls. Most notable is the dicynodont Lystrosaurus, the broad geographic distribution of which has provided classic evidence of the integrity of Triassic Pangaea (Lystrosaurus fossils have been found in Antarctica, South Africa, India, China, and Russia). Middle Triassic tetrapod faunas remained dicynodont- and temnospondyl- dominated. However, by this time, the shift toward archosaur domination of the terrestrial tetrapod fauna had begun. By Late Triassic time, dicynodonts were rare, and the

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Figure 9 Dinosaurs appeared during the Triassic. These 3-m-long skeletons of the Late Triassic theropod Coelophysis are from Ghost Ranch in northern New Mexico.

temnospondyls were greatly reduced in diversity. Instead, archosaurs were the most abundant terrestrial tetrapods. It was at this time that new groups of tetrapods appeared – turtles, dinosaurs (Figure 9), crocodiles, pterosaurs, and mammals – making the Late Triassic one of the most significant junctures in the history of vertebrate life. Indeed, it is fair to say that during the Late Triassic, the Mesozoic tetrapod fauna was born. Marine reptiles also had their highest diversity during the Triassic. These reptiles were huhpehsuchians, nothosaurs, thallatosaurs, and placodonts, groups for which the entire diversification period was confined to the Triassic, and ichthyosaurs and plesiosaurs, groups that became prominent marine predators throughout much of the Jurassic and Cretaceous. There appears to have been a substantial extinction of marine reptiles (loss of 64% of families) at about the Middle–Late Triassic boundary. Apparently, the Reptilia successfully and explosively invaded the marine realm after the PTB crisis, but their diversity diminished rapidly, possibly due to the overall Late Triassic lowering of the sea, which reduced the epicontinental seaways in which most of the marine reptiles lived.

Further Reading von Alberti F (1834) Beitrag zu einer monographie des bunten sandsteins, muschelkalks und keupers, und die verbindung dieser gebilde zu einer formation. Stuttgart: Cotta. Benton MJ (2003) When life nearly died the greatest mass extinction of all time. London: Thames & Hudson, Ltd. Benton MJ (ed.) (1993) The fossil record 2. London: Chapman & Hal. Callaway JM and Nicholls EL (eds.) (1997) Ancient marine reptiles. San Diego: Academic Press. Dobruskina IA (1994) Triassic floras of Eurasia. O¨ssterreichische Akademie Wissenschaften Schriftenreihe Erdwissen Kommission 10: 1–422. Embry AF (1988) Triassic sea level changes: evidence from the Canadian arctic archipelago. SEPM Special Publication 42: 249–259. Embry AF (1997) Global sequence boundaries of the Triassic and their identification in the western Canada sedimentary basin. Bulletin of Canadian Petroleum Geology 45: 415–433. Erwin DH (1993) The great Paleozoic crisis: life and death in the Permian. New York: Columbia University Press. Hauschke N and Wilde V (eds.) (1999) Trias eine ganz andere welt. Munich: Verlag Dr. Friedrich Pfeil. Kummel B (1979) Triassic. Treatise on invertebrate paleontology, part a, introduction. Fossilization (taphonomy) biogeography and biostratigraphy. Lawrence, KS: Geological Society of America and University of Kansas Press, pp. 351–389. Lucas SG (1998) Global Triassic tetrapod biostratigraphy and biochronology. Palaeogeography, Palaeoclimatology, Palaeoecology 143: 347–384. Lucas SG (ed.) (2010) The Triassic timescale. London: Geological Society, Special Publication, p. 334. Lucas SG (2000) The epicontinental Triassic, an overview. Zentralblatt fu¨r Geologie und Pala¨ontologie Teil I 7–8: 475–496. Lucas SG, Tanner LH, Kozur HW, Weems RE, and Heckert AB (2012) The late Triassic timescale: Age and correlation of the carnian-norian boundary. Earth-Science Reviews 114: 1–18. Ogg J (2012) Triassic. In: Gradstein FM, Ogg JG, Schmitz MD, and Ogg M (eds.) The geologic time scale, pp. 681–730. Amsterdam: Elsevier. Sherlock RL (1948) The permo-triassic formations. London: Hutchinson’s Scientific and Technical Publ. Sues H-D and Fraser NC (2010) Triassic life on land: the great transition. New York: Columbia University Press. Tozer, E.T. (1984). The Trias and its ammonoids: the evolution of a time scale. Geological Survey Canada, Miscellaneous Report 35. Ottawa: Geological Survey Canada. Yin H (ed.) (1996) The palaeozoic-mesozoic boundary. Candidates of the global stratotype section and point of the Permian-Triassic boundary. Wuhan, China: University of Geosciences Press. Ziegler PA (1989) Evolution of laurussia. Dordrecht: Kluwer Academic Publ.

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