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Mesozoic Article by: Dubiel, Russell F. U.S. Geological Survey, U.S. Department of the Interior, Denver, Colorado. Publication : 2014 DOI: http://dx.doi.org/10.1036/1097­8542.417200 (http://dx.doi.org/10.1036/1097­8542.417200)

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Triassic Bibliography Additional Readings

The middle of the three major divisions of the Eon (, Mesozoic, and ) of geologic , encompassing an interval from 251 to 65 million ago (Ma) based on various isotopic­ dates. The Mesozoic Era is known also as the Age of the and the interval of middle life. The Mesozoic (the largest recognized time­stratigraphic unit) encompasses all sedimentary rocks, body and trace of organisms preserved, metamorphic rocks, and intrusive and extrusive igneous rocks formed during the Mesozoic Era.

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The Mesozoic Era was originally named for one of three principal divisions of the record, or of life, that was bounded before and after by significant mass that dramatically changed the biotic composition of the world. In during the early 1840s, geologist John Phillips introduced the terms Mesozoic Era and Cenozoic Era, in conjunction with geologist Adam Sedwick's Paleozoic Era, proposed in 1838, to denote the widespread observation that three successive and distinct biotic assemblages were preserved in the fossil record. The Mesozoic Era comprises life intermediate in kind between ancient life­forms (Paleozoic Era) and recent life­ forms (Cenozoic Era). See also: Cenozoic (/content/cenozoic/118600); Paleozoic (/content/paleozoic/484300)

The Mesozoic Era records dramatic changes in the geologic and biologic history of the Earth. At the beginning of the Mesozoic Era, all the were amassed into one large , . Both the marine and continental biotas were impoverished from the mass that marked the boundary between the and periods, and the end of the Paleozoic Era. This mass extinction was responsible for the loss of over 90% of the on Earth. During the Mesozoic Era, many significant events were recorded in the geologic and fossil record of the Earth, including the breakup of Pangaea and the of modern basins by , the rise of the dinosaurs, the ascension of the angiosperms (flowering ), the diversification of the and , and the appearance of the and . The end of the Mesozoic Era is marked by a major mass extinction at the Cretaceous­ boundary that records several impacts, the extinction of the dinosaurs, the rise to dominance of the mammals, and the beginning of the Cenozoic Era and the life­forms dominant today. See also: Continental drift (/content/continental­drift/159000); Plate (/content/plate­tectonics/527000)

http://www.accessscience.com/content/mesozoic/417200 2/11 5/10/2016 Mesozoic ­ AccessScience from McGraw­Hill Education The Mesozoic Era comprises three periods of geologic time: the Triassic Period (251–200 Ma), the Jurassic Period (200–146 Ma), and the Cretaceous Period (146–65 Ma) [Fig. 1]. These periods are each subdivided into epochs, formal designations of geologic time described as Early, Middle, and Late (except for the Cretaceous, which has no middle designated yet). The packages of rock themselves are subdivided into designated Lower, Middle, and Upper (except for Cretaceous). Each epoch is subdivided into ages. Likewise, each series is subdivided into stages, which are time­stratigraphic units whose boundaries are based on , hiatuses, or erosional surfaces, on correlations to a type section (where rocks are first described), or preferably on changes in the biota that depict true measurable time (for example, evolutionary changes). See also: (/content/unconformity/720400)

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Fig. 1 Subdivisions of the Mesozoic Era, including the best age estimates and the eustatic sea­level curve depicted as relative change in coastal onlap as the shoreline moved landward (sea­level rise) or seaward (sea­level fall).

http://www.accessscience.com/content/mesozoic/417200 4/11 5/10/2016 Mesozoic ­ AccessScience from McGraw­Hill Education The correlations of time equivalency and age dating in the Mesozoic Era have been accomplished by utilizing biostratigraphic zones based on individual fossil groups or by an acme or composite zonal assemblage based on numerous fossil groups. Marine and continental fossil groups used to describe chronologically Mesozoic rocks include marine and nannofossils (shelled protozoa), ammonites (), and inoceramids (mollusks); continental spores and (palynology); dinosaurs; and mammals. Correlations based on these and other organisms in the Mesozoic Era depend on the faunal and floral succession through origination and extinction of species. See also: Cephalopoda (/content/cephalopoda/120800); Foraminiferida (/content/foraminiferida/267800); (/content/mollusca/431300); Palynology (/content/palynology/484800)

The organization of subdivisions based on physical and biological evidence allows geologists and paleontologists to describe both rocks and fossils in specific intervals of time and space. Thus, earth scientists can communicate effectively with one another and characterize more precisely the physical and biotic changes during the Mesozoic Era, as well as the other eras in geologic history. See also: (/content/paleontology/484100); Stratigraphy (/content/stratigraphy/659000)

Triassic

The Mesozoic Era begins with the Triassic Period, which constitutes nearly one­third the total time of the era and is well exposed especially in and , with other important outcrops in , , Argentina, and South . The Triassic Period was named originally the Trias in in 1834 by Friedrich August von Alberti for its unique and natural division into three distinct stratigraphic units.

As a result of the unique geography of the single Pangaean landmass, the alteration in oceanic currents produced around one , and the monsoonal climatic setting, life changed substantially in both marine and continental ecosystems. The marine ecosystems witnessed the addition of large and the modern ­building corals, the reemergence and diversification of the mollusks, and the emergence of pelagic life in the form of planktonic organisms. Ray­finned and bony and dominated the seas. Placodonts and were aquatic marine reptiles that fed on mollusks and other marine . appeared in the for the first time. Fresh­water and terrestrial ecosystems were marked by the emergence and diversification of the dinosaurs, flying reptiles, , , terrestrial , and birds; the appearance of the mammals, though quite small in size; the emergence of freshwater and terrestrial ; and the emergence of new insects, such as the Isoptera (), Diptera (), and the (, , and possibly ), appearing earlier in the Mesozoic than previously thought. evidence for these new insects indicates the advent of social behavior in termites and in primitive bees, prior to the appearance of angiosperms in the Cretaceous. In terrestrial ecosystems and seed ferns were abundant, but floras continued to dominate the landscape. rediversified after the Permo­Triassic extinctions, and thecodonts gave rise to the crocodiles and to the first dinosaurs, which were small in stature.

During the Triassic Period the continents were amassed tectonically into one great landmass, the supercontinent Pangaea, that was distributed equally across the paleoequator in both the Northern and Southern hemispheres (Fig. 2a). Since the majority of the enormous Pangaean landmass was inland from the influence of the ocean, and its configuration distributed equally across the , a worldwide monsoonal pattern dominated during

http://www.accessscience.com/content/mesozoic/417200 5/11 5/10/2016 Mesozoic ­ AccessScience from McGraw­Hill Education the Triassic that created alternating wet and dry in many regions. Areas landward of the coasts experienced increased continentality of the climate and produced more pronounced wet and dry seasons. See also: (/content/paleoclimatology/483500); Paleogeography (/content/paleogeography/483800)

Fig. 2 Schematic reconstruction showing paleogeography of continents, epicontinental seas, and ocean basins (arrows denote ocean currents) on Pangaea in the Mesozoic Era from the (a) Triassic (220 Ma), (b) Jurassic (155 Ma), and (c) Cretaceous (70 Ma) periods.

At the end of the Triassic, Pangaea began to break apart and the monsoonal climate pattern began to disintegrate. Evidence for the breakup of Pangaea and the eventual formation of the northern Ocean is the presence of basins along the east coast of North America and the northwest coast of Africa. A mass extinction defines the boundary between the end of the Triassic and the beginning of the Jurassic. This mass extinction was responsible for the loss of about 60% of the species on Earth. The mass extinctions in the marine and continental realms affected the ocean ecosystem by eliminating the marine and placodont reptiles, and many species of bivalves, ammonoids, plesiosaurs, and ichthyosaurs disappeared. Most of these groups recovered in the Jurassic.

http://www.accessscience.com/content/mesozoic/417200 6/11 5/10/2016 Mesozoic ­ AccessScience from McGraw­Hill Education Extinction also claimed the large and mammallike reptiles from freshwater and terrestrial ecosystems. The cause of these mass extinctions is unknown, though some scientists hypothesize that either a meteorite impact or increasing global aridity caused many genera to go extinct. See also: Triassic (/content/triassic/708500)

Jurassic

The middle part of the Mesozoic Era is represented by the Jurassic Period, which constitutes about one­third of the total time of the era. Jurassic rocks are well exposed, especially in North America and Europe, and other important outcrops exist in and . In 1839, German geologist Leopold von Buch established the Jurassic as a for rocks in Switzerland, Germany, and England. The new system was based on descriptions of equivalent rocks made by the German geologist Alexandre von Humboldt (1795) and the English geologist William Smith (1797–1815). They described massive of the in Switzerland as the Jura­Kalkstein and the Lias­Oolite rock sequences in England and Wales, respectively. See also: (/content/limestone/383000); Oolite (/content/oolite/469600)

During the Jurassic Period the Pangaean landmass continued to separate into two large continental masses, with one in the Northern Hemisphere and the other in the Southern (Fig. 2b). The Northern Hemisphere landmass, , was composed of North America and , while the Southern Hemisphere landmass, Gondwanda, was composed of South America, Africa, India, , and . Continued plate spreading and more rapid sea­level fluctuations late in the Jurassic created the Tethyan Seaway, which extended between Laurasia and Gondwanda, allowing oceans to flow freely between the continents, and caused epicontinental seas to flood large areas of North America and Europe. The opening of the ocean basins and the resulting increased oceanic circulation created a zonal atmospheric climate pattern that ranged from tropical at the Equator to warm temperate near the Poles, with local zones of aridity due to orographic and latitudinal rain shadows. See also: Continents, evolution of (/content/continents­evolution­of/159300); Rain shadow (/content/rain­shadow/572100)

Oceanic and continental biotas shifted in composition during tectonic and climatic transformations of the Jurassic Period. Numerous reef communities of modern reef­building corals flourished in shallow tropical oceans along with bivalves, ammonites, belemnoids, sea urchins, and fishes. Planktonic life began to prosper in the warm, shallow seas with the appearance of nannoplankton. Marine reptiles included plesiosaurs and ichthyosaurs, and the invasion of the oceans by crocodiles. Terrestrial and freshwater ecosystems were dominated by plants such as , cycadeoids, , , and to a lesser extent ferns. The Jurassic is also known as the age of the cycads. The dinosaurs' greatest rise to dominance occurred with the radiation of large such as the sauropods , , , the plated stegosaurs, and the heavily armored ankylosaurs. The herbivores were pursued by predatory dinosaurs such as and . Many flying reptiles speckled the skies, including and the feathered reptilelike , . Mammals were still small in size but began to increase in diversity. See also: Dinosauria (/content/dinosauria/196800); (/content/phytoplankton/515000); Reptilia (/content/reptilia/581800); Zooplankton (/content/zooplankton/756950)

The end of the Jurassic Period was marked by moderate extinctions of biota in both marine and continental ecosystems. In the marine realm, diversity declined steadily, other varied in diversity, and marine reptiles, such as the , became nearly extinct. On the continents, the major

http://www.accessscience.com/content/mesozoic/417200 7/11 5/10/2016 Mesozoic ­ AccessScience from McGraw­Hill Education extinctions eliminated the last of the therapsids and affected the large herbivorous sauropods, stegosaurs, and ankylosaurs, as well as their predators. See also: Jurassic (/content/jurassic/361100)

Cretaceous

The last part of the Mesozoic is represented by the Cretaceous Period, which constitutes a little less than one­half the total time of the Mesozoic Era. The Cretaceous is represented well by rocks in North America, South America, Europe, Asia, Africa, and Australia. The Cretaceous Period was named in 1822–1823 by French geologist J. J. d'Omalius d'Halloy for exposures at the , which are composed of marine and can be traced throughout Europe and North Africa. These widespread units are composed predominantly of microscopic plates of calcareous nannoplankton, and had once been an ancient sea floor. See also: Chalk (/content/chalk/124200); Micropaleontology (/content/micropaleontology/423200)

During the Cretaceous Period the face of the Earth began to take on an appearance more similar to the continental and oceanic configuration (Fig. 2c). Early in the Cretaceous, both Laurasian and Gondwanan continental masses separated into the continents still recognizable today. The separation of in the marked the onset to the formation of the South . Increased sea­floor spreading rates, which opened the oceanic gaps between the continents, resulted in the expansion of shallow epicontinental seaways in North America, Africa, and Northern Europe, along with the flooding of most of southern Eurasia. These variations in sea­floor spreading rates caused the to fluctuate constantly throughout most of the Cretaceous. The Pacific and Atlantic Ocean basins began taking form, and the Tethyan Seaway, between what is now the western Mediterranean and southeastern Asia, persisted throughout most of the period. New, expanded oceanic realms coupled with the changed continental configurations and increased atmospheric transformed the early Cretaceous climate into a humid zonal climate, warmer than today. The climate began cooling down, beginning near the end of the Mesozoic.

The biotic composition during the Cretaceous Period contained a mixture of both intermediate and modern forms of life in both marine and continental ecosystems. In the marine realm, modern types of gastropods, bivalves, and modern fishes shared the oceans with marine reptiles such as and plesiosaurs, ammonoids, belemnoids, and other gigantic, coiled oysters and sedentary bivalves. Other marine invertebrates, such as planktonic and benthic foraminifera, flourished together with bryozoans, corals, reef­building rudist bivalves, , , and other crustaceans. In the continental Cretaceous realm, the greatest change in freshwater and terrestrial ecosystems occurred with the appearance and diversification of angiosperms or flowering plants that became more diverse over the early and middle Mesozoic gymnosperm floras near the end of the Cretaceous. At the same time, freshwater and terrestrial insects continued to diversity and exploit new niches and resources provided by the angiosperms. Many groups of , including , modern types of turtles, crocodiles, , and amphibians, diversified from ancient stocks during the Cretaceous to coexist with the dinosaurs that continued to rule the Earth. Mammals continued to evolve and diversify, but remained very small in size in comparison with their modern descendants. See also: Magnoliophyta (/content/magnoliophyta/401000); Mammalia (/content/mammalia/402500)

http://www.accessscience.com/content/mesozoic/417200 8/11 5/10/2016 Mesozoic ­ AccessScience from McGraw­Hill Education The dinosaurs diversified for the last time in the Cretaceous and formed ecological communities similar to faunas inhabiting the African plains today. Herbivores such as the great horned and large duck­billed dinosaurs such as Hadrosaurus traveled in herds, roamed the plains, and followed watercourses in seasonally migrating for food. The predators that followed these herds were the largest of all , and , together with other pack and ambush predators such as the and crocodiles, respectively. The few flying reptiles that remained were spectacular, one form attaining a wingspan of nearly 11 m (35 ft), and these creatures shared the skies with modern types of shorebirds and wading birds.

The end of the Cretaceous, and thus the end of the Mesozoic Era, is marked by a mass extinction known as the Cretaceous­Tertiary boundary (Tertiary is the earliest system of the Cenozoic Era, now divided into the and ). This mass extinction is widely known for the demise of 60% of the organisms on Earth, including the ammonoids, the rudist corals, marine reptiles, the dinosaurs, and the flying reptiles. There was a large reduction in the diversity of various marine plankton and continental faunas and floras, but they later recovered early in the Cenozoic. There have been many heated debates over the cause of the terminal Mesozoic extinctions, because some of them focus on extraterrestrial causes such as bolide and impacts onto the Earth's surface. Intriguing evidence in the form of anomalies comes from marine deposits and continental deposits within rocks spanning the Cretaceous and Tertiary boundary worldwide. Iridium is an element that is typically depleted in rocks derived from the Earth's curst but is enriched in extraterrestrial stony . Some scientists hypothesize that a large 10­km (6­mi) meteorite or comet struck the Earth, exploded on impact, and released a blast greater than all the nuclear weapons on Earth. Such an impact would have thrown enormous volumes of dust and smoke into the atmosphere, blocked a large fraction of sunlight, and thus caused severe hardships to all marine and continental biota by lowering worldwide temperatures and disrupting the . If the impact occurred in the ocean, huge volumes of water would have been vaporized instantly and caused gigantic tsunamis or tidal waves, which could have swept across most of the lowlands of continents. The devastating results of such an impact have been termed , because the effect would be similar to that produced by an all­out nuclear war. Other scientists hypothesize that extensive volcanic outgassing and cooler due to plate tectonic movements produced major changes in global climate and environmental disturbances that forced many organisms into extinction. Despite the similarities in global climate change and mass extinction interpreted by both of these hypotheses, other environmentally sensitive and presumably vulnerable groups of organisms were little affected by the mass extinction . Environmentally sensitive vertebrates, such as crocodiles, lizards, turtles, frogs, and , were paradoxically spared and made it through the mass extinctions with little loss of species. The birds also persist through the mass extinctions and in fact radiate in the Paleogene and Neogene. See also: Cretaceous (/content/cretaceous/167800); Extinction () (/content/extinction­biology/249000); (/content/geologic­time­scale/286500); Meteorite (/content/meteorite/420500); Tertiary (/content/tertiary/686000)

S. T. Hasiotis R. F. Dubiel

Bibliography

M. V. Caputo, J. A. Peterson, and K. J. Franczyk (eds.), Mesozoic Systems of the Rocky Mountain Region, USA, Rocky Mountain Section, Society for Sedimentary Geology, 1994

http://www.accessscience.com/content/mesozoic/417200 9/11 5/10/2016 Mesozoic ­ AccessScience from McGraw­Hill Education B. U. Haq and F. W. B. van Eysinga, Geologic Time Table Chart, 4th ed., 1994

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M. Moullade and A. E. M. Nairn (eds.), The Phanerozoic Geology of the World II: The Mesozoic, 1978

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Additional Readings

British Mesozoic Fossils, 6th ed., British Museum of Natural History, no. 872, 1983

K. Carpenter, D. J. Chure, and J. I. Kirkland, (eds.), The Upper Jurassic : An Interdisciplinary Study, Modern Geology Special Issue, vol. 23, no. 1–4, 1997

K. Carpenter, K. F. Hirsch, and J. R. Horner (eds.), Dinosaur Eggs and Babies, Cambridge University Press, 1994

P. J. Currie and K. Padian, Encyclopedia of Dinosaurs, Academic Press, San Diego, 1997

R. F. Dubiel et al., The Pangean megamonsoon—Evidence from the Upper Triassic , , PALAIOS, 6:347–370, 1991 DOI: 10.2307/3514963 (http://dx.doi.org/10.2307/3514963)

C. T. Gee (ed.), Plants in Mesozoic Time, Indiana University Press, Bloomington, IN, 2010

P.­C. Graciansky et al. (eds.), Mesozoic and Cenozoic of European Basins, SEPM Spec. Publ. no. 60, Tulsa, 1998

S. T. Hasiotis, Complex ichnofossils of solitary to social soil organisms: Understanding their evolution and roles in terrestrial paleoecosystems, Palaeogeog. Palaeoclimatol. Palaeoecol., 192:259–320, 2003 DOI: 10.1016/S0031­ 0182(02)00689­2 (http://dx.doi.org/10.1016/S0031­0182(02)00689­2)

S. T. Hasiotis, The invertebrate invasion and evolution of Mesozoic soil ecosystems: The ichnofossil record of ecological innovations, in R. Gastaldo and W. Dimichele (eds.), Phanerozoic Terrestrial Ecosystems, Paleontological Society Short Course, vol. 6, pp. 141–169, 2000

B. P. Kear and R. J. Hamilton­Bruce, Dinosaurs in Australia: Mesozoic Life from the Southern Continent, CSIRO Publishing, Collingwood, Australia, 2011

P. Kearey and F. J. Vine, Global Tectonic, 2d ed., Blackwell Science Limited, Oxford, England, 1996

J. A. Long, Dinosaurs of Australia and New Zealand and Other Animals of the Mesozoic Era, Harvard University Press, Cambridge, MA, 1998

S. G. Lucas (ed.), The Triassic Timescale, Geological Society of London, Bath, UK, 2010

P. D. Mannion et al., A temperate palaeodiversity peak in mesozoic dinosaurs and evidence for geographical partitioning, Global Ecol. Biogeogr., 21(9):898–908, 2012 DOI: 10.1111/j.1466­8238.2011.00735.x (http://dx.doi.org/10.1111/j.1466­8238.2011.00735.x)

C. R. Scotese and J. Glonka, Paleogeographic atlas: PALEOMAP Project, Department of Geology, University of at Arlington, 1992

A. G. Smith, D. G. Smith, and B. M. Funnell, Atlas of Mesozoic and Cenozoic Coastlines, Cambridge University Press, 1994

http://www.accessscience.com/content/mesozoic/417200 10/11 5/10/2016 Mesozoic ­ AccessScience from McGraw­Hill Education S. L. Wing and H.­D. Sues, Mesozoic and Early Cenozoic terrestrial ecosystems, in A. K. Behrensmeyer et al. (eds.), Terrestrial Ecosystems through Time: Evolutionary Paleoecology of Terrestrial Plants and Animals, University of Chicago Press, 1992

International Commission on Stratigraphy (http://www.stratigraphy.org/)

Introduction to the Mesozoic Era (http://www.ucmp.berkeley.edu/mesozoic/mesozoic.html)

Mesozoic Era of the Phanerozoic Eon (http://www.palaeos.com/Mesozoic/Mesozoic.htm)

Paleogeography of the Southwestern United States (http://jan.ucc.nau.edu/~rcb7/paleogeogwus.html)

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