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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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Author's personal copy

Triassic☆

SG Lucas, New Mexico Museum of Natural History, Albuquerque, NM, USAMJ Orchard, Geological Survey of Canada, Vancouver, BC, Canada

ã 2013 Elsevier Inc. All rights reserved.

Introduction 1Triassic Rocks 2Time-Scale 2Palaeogeography 3Tectonics and Sedimentation 4Sea-Levels 4Climate 5Extinctions 5Flora 6Shelled Marine Invertebrates 7Insects 7Fishes 8Tetrapods 8

☆C

ref

Re

GlossaryCompressional 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.

hange History: June 2013. SG Lucas introduced small edits in the text of the art

erences and added new Figure 2.

ference Module in Earth Systems and Environmental Sciences http://dx.doi.org/1

Reference Module in Earth Systems and

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.

icle mostly with regard to new numerical ages of Triassic timescale. Updated

0.1016/B978-0-12-409548-9.02872-4 1 Environmental Sciences, (2013)

http://dx.doi.org/10.1016/B978-0-12-409548-9.02872-4

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

2 Triassic


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.

Reference Module in Earth Systems and Environmental Sciences, (2013)

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

Triassic 3


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 beenmade in the past decade. Themost intensively studied is the Permian-

Triassic 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 andmarine 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.


Figure 3 Triassic Pangaea, showing major tectonic elements. After Lucas (2000) The epicontinental Triassic, an overview. Zentralblatt fur Geologieund Palaontologie Teil I 7–8: 475–496.

4 Triassic


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.


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

Triassic 5


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 landmass 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


Figure 6 Characteristic extinction/diversity patterns of marine invertebrates across the Permian–Triassic boundary. After Lucas (2000) Theepicontinental Triassic, an overview. Zentralblatt fur Geologie und Palaontologie Teil I 7–8: 475–496.

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) Theepicontinental Triassic, an overview. Zentralblatt fur Geologie und Palaontologie Teil I 7–8: 475–496.

6 Triassic


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.


Figure 7 Triassic floral provinces and floras. After Lucas (2000) The epicontinental Triassic, an overview. Zentralblatt fur Geologie und PalaontologieTeil I 7–8: 475–496.

Triassic 7


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, theDicroidium 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 domi-

nated 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.


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

8 Triassic


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 Lystro-

saurus, 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


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

Triassic 9


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, thallato-

saurs, 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. Ossterreichische 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 fur Geologie und Palaontologie 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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