5/10/2016 Mesozoic ­ AccessScience from McGraw­Hill Education (http://www.accessscience.com/) Mesozoic Article by: Dubiel, Russell F. U.S. Geological Survey, U.S. Department of the Interior, Denver, Colorado. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097­8542.417200 (http://dx.doi.org/10.1036/1097­8542.417200) Content Triassic Jurassic Cretaceous Bibliography Additional Readings The middle era of the three major divisions of the Phanerozoic Eon (Paleozoic, Mesozoic, and Cenozoic eras) of geologic time, encompassing an interval from 251 to 65 million years ago (Ma) based on various isotopic­age dates. The Mesozoic Era is known also as the Age of the Dinosaurs and the interval of middle life. The Mesozoic Erathem (the largest recognized time­stratigraphic unit) encompasses all sedimentary rocks, body and trace fossils of organisms preserved, metamorphic rocks, and intrusive and extrusive igneous rocks formed during the Mesozoic Era. http://www.accessscience.com/content/mesozoic/417200 1/11 5/10/2016 Mesozoic ­ AccessScience from McGraw­Hill Education The Mesozoic Era was originally named for one of three principal divisions of the fossil record, or history of life, that was bounded before and after by significant mass extinctions that dramatically changed the biotic composition of the world. In England during the early 1840s, geologist John Phillips introduced the terms Mesozoic Era and Cenozoic Era, in conjunction with geologist Adam Sedwick's term 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 continents were amassed into one large supercontinent, Pangaea. Both the marine and continental biotas were impoverished from the mass extinction that marked the boundary between the Permian and Triassic periods, and the end of the Paleozoic Era. This mass extinction was responsible for the loss of over 90% of the species 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 evolution of modern ocean basins by continental drift, the rise of the dinosaurs, the ascension of the angiosperms (flowering plants), the diversification of the insects and crustaceans, and the appearance of the mammals and birds. The end of the Mesozoic Era is marked by a major mass extinction at the Cretaceous­Tertiary boundary that records several meteorite 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 tectonics (/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 epoch designated yet). The packages of rock themselves are subdivided into series 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 unconformities, 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: Unconformity (/content/unconformity/720400) http://www.accessscience.com/content/mesozoic/417200 3/11 5/10/2016 Mesozoic ­ AccessScience from McGraw­Hill Education 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 foraminifera and nannofossils (shelled protozoa), ammonites (cephalopods), and inoceramids (mollusks); continental plant spores and pollen (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); Mollusca (/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: Paleontology (/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 Europe and North America, with other important outcrops in India, China, Argentina, and South Africa. The Triassic Period was named originally the Trias in Germany in 1834 by Friedrich August von Alberti for its unique fauna 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 continent, and the monsoonal climatic setting, life changed substantially in both marine and continental ecosystems. The marine ecosystems witnessed the addition of large reptiles and the modern reef­building corals, the reemergence and diversification of the mollusks, and the emergence of pelagic life in the form of planktonic organisms. Ray­finned and bony fishes and sharks dominated the seas. Placodonts and nothosaurs were aquatic marine reptiles that fed on mollusks and other marine invertebrates. Ichthyosaurs appeared in the oceans for the first time. Fresh­water and terrestrial ecosystems were marked by the emergence and diversification of the dinosaurs, flying reptiles, frogs, turtles, terrestrial crocodiles, and birds; the appearance of the mammals, though quite small in size; the emergence of freshwater and terrestrial crayfish; and the emergence of new insects, such as the Isoptera (termites), Diptera (flies), and the Hymenoptera (bees, wasps, and possibly ants), appearing earlier in the Mesozoic than previously thought. Trace fossil 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 ferns and seed ferns were abundant, but gymnosperm floras continued to dominate the landscape. Therapsids 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 Equator, a worldwide monsoonal climate 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 seasons in many regions. Areas landward of the coasts experienced increased continentality of the climate and produced more pronounced wet and dry seasons. See also: Paleoclimatology (/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 Atlantic Ocean is the presence of rift 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