Mesozoic Era, second ofEarth’s three major geologic eras ofPhanerozoic time. Its name isderived from the Greek term for “middle life.” The Mesozoic Era began 252.2 million years ago, following the conclusion of thePaleozoic Era, and ended 66 million years ago, at the dawn of theCenozoic Era. (See thegeologic time scale.) The major divisions of the Mesozoic Era are, from oldest to youngest, theTriassic Period, theJurassic Period, and theCretaceous Period. The ancestors of major plant andanimal groups that exist today first appeared during the Mesozoic, but this era is best known as the time of thedinosaurs.

Earth’s climate during the Mesozoic Era was generally warm, and there was less difference in temperature between equatorial and polar latitudes than there is today. The Mesozoic was a time of geologic and biological transition. During this era the continents began to move into their present-day configurations. A distinct modernization of life-forms occurred, partly because of thedemise of many earlier types of organisms. Three of the five largestmass extinctions in Earth history are associated with the Mesozoic: a mass extinction occurred at the boundary between the Mesozoic and the preceding Paleozoic; another occurred within the Mesozoic at the end of the Triassic Period; and a third occurred at the boundary between the Mesozoic and subsequent Cenozoic, resulting in the demise of the dinosaurs.

Mesozoic geology

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At the outset of the Mesozoic, all of Earth’scontinents were joined together into thesupercontinent ofPangea (see themap of the Early Triassic). By the close of the era, Pangea had fragmented into multiple landmasses. The fragmentation began with continentalrifting during the Late Triassic. This separated Pangea into the continents ofLaurasia andGondwana. By the Middle Jurassic these landmasses had begun further fragmentation. At that time much of Pangea lay between 60° N and 60° S, and at theEquator the wideningTethys Sea cut between Gondwana and Laurasia. When rifting had sufficiently progressed, oceanic spreading centres formed between the landmasses. During the Middle Jurassic, North America began pulling apart fromEurasia and Gondwana. By the Late Jurassic, Africa had started to split off from South America, andAustralia andAntarctica had separated from India (see themap of the Late Jurassic). Near the close of the Cretaceous, Madagascar separated fromAfrica, andSouth America drifted northwestward (see themap of the Late Cretaceous).

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As the continents rifted and ruptured, thick sequences ofmarine sedimentsaccumulated in large linear troughs along their margins. Ocean basin deposits of Jurassic age are found today in the circum-Pacific region, along the coasts of eastern North America and theGulf of Mexico, and on the margins of Eurasia andGondwana (that is, along the northern and southern boundaries of the Tethys Sea).

Majormountain building (orogeny) began on the western margins of both North andSouth America and between the separating fragments of Gondwana. For example, the northwesterly movement ofNorth America resulted in a collision of the western edge of the North American continental plate with a complex ofisland arcs during the Late Jurassic. So-called exotic terranes, geologic fragments that differ markedly instratigraphy,paleomagnetism, andpaleontology from adjoiningcontinental crust, were accreted to the margin of the North American plate. As thrusting occurred in an eastward direction, hugegraniticbatholiths formed in what is now theSierra Nevada range along the California-Nevada border. Other notable episodes of mountain building during the Mesozoic include theSevier andLaramide orogenies, which took place in western North America during Cretaceous time. These events created theRocky Mountains.

Mesozoic rocks are widely distributed, appearing in various parts of the world. A large percentage of these rocks aresedimentary. At various times during the Mesozoic, shallowseas invaded continental interiors and then drained away. During Middle Triassic time, a marine incursion—theMuschelkalk Sea—covered the continental interior of Europe. Seas again transgressed upon the continents between the Early and Late Jurassic and in the Early Cretaceous, leavingextensive beds of sandstone, ironstone,clays, and limestone (seeSolnhofen Limestone). A last major transgression of marine waters flooded large segments of all the continents later in the Cretaceous. These sharp rises insea level and resultant worldwide flooding are thought to have had two causes. The first was warm global temperatures, which prevented large volumes ofwater from being sequestered on land in the form of ice sheets. The second was related to accelerated seafloor spreading; the attendant enlargement ofocean ridges displaced enormous amounts of ocean water onto the landmasses. Marine transgression was so extensive that in North America, for example, a seaway spread all the way from theArctic to the Gulf of Mexico in the Cretaceous Period. Widespreaddeposition ofchalk, clay, black shales, andmarl occurred. In parts of North America, lake and river sediments rich indinosaur fossils were deposited alongside marine sediments. (SeeMorrison Formation.)

A substantial amount ofigneous rock also formed during the Mesozoic. The orogenies of the Jurassic and Cretaceous periods involved volcanism andplutonic intrusion such as occurred during the emplacement of granites andandesites in the Andes of South America during the Late Jurassic. Two of the largest volcanic events in Earth’s history occurred during the Mesozoic. The Central Atlantic Magmatic Province, a huge volume of basalt, was created at the end of the Triassic during the initial rifting of Pangea. The surface area of this igneous province originally covered more than 7 million square km (about 3 million square miles), and its rocks can be found today from Brazil to France. Despite such a massive volume of basaltic material extruded,volcanic activity was probably short-lived, spanning only a few million years. At the end of the Cretaceous, another igneous province, the flood basalts of theDeccan Traps, formed on what is now theIndian subcontinent. Some scientists have suggested that both of these large igneous events may have injected significant amounts ofcarbon dioxide and aerosols into the atmosphere, triggering a change in global climate. The timing of these volcanic events appears to overlap the Triassic-Jurassic and Cretaceous-Tertiary, or Cretaceous-Paleogene, mass extinctions, and they may have played a role in them.

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Mesozoic life

The fauna and flora of the Mesozoic were distinctly different from those of the Paleozoic, the largest mass extinction in Earth history having occurred at the boundary of the two eras, when some 90 percent of all marine invertebrate species and 70 percent of terrestrial vertebrate genera disappeared. At the start of the Mesozoic, the remaining biota began a prolonged recovery ofdiversity and total population numbers, and ecosystems began to resemble those of modern days.Vertebrates, less severely affected by the extinction thaninvertebrates, diversified progressively throughout the Triassic. The Triassic terrestrialenvironment was dominated by thetherapsids, sometimes referred to as “mammal-like reptiles,” and thethecodonts, ancestors of dinosaurs and crocodiles, both of which appeared during the Late Triassic. The first truemammals, which were small, shrewlike omnivores, also appeared in the Late Triassic, as did the lizards, turtles, and flyingpterosaurs. In the oceans, mollusks—includingammonites,bivalves, andgastropods—became a dominant group. Fishes, sharks, and marine reptiles such asplesiosaurs,nothosaurs, andichthyosaurs also swam the Mesozoic seas.

Another majorextinction event struck at the close of the Triassic, one that wiped out as much as 20 percent of marine families and manyterrestrial vertebrates, including therapsids. The cause of this mass extinction is not yet known but may be related to climatic and oceanographic changes. In all, 35 percent of the existing animal groups suffered extinction.

In the oceans the ammonites and brachiopods recovered from the Late Triassic crisis, thriving in the warm continental seas. Ammonites rapidly became very common invertebrates in the marine realm and are now importantindex fossils for worldwide correlation of Jurassic rock strata. Many other animal forms, including mollusks (notably the bivalves), sharks, and bony fishes, flourished during the Jurassic. During the Jurassic and Cretaceous, theecology of marine ecosystems began to change, as shown by a rapid increase in diversity of marine organisms. It is believed that increasing predation pressures caused many marine organisms to develop better defenses and burrow more deeply into the seafloor. In response, predators also evolved more-effective ways to catch their prey. These changes are so significant that they are called the “Mesozoic Marine Revolution.”

The dominant terrestrial vertebrates weredinosaurs, which exhibited great diversity during the Jurassic and Cretaceous.Birds are believed to have evolved from dinosaur ancestors during the Late Jurassic. Ancestors of living vertebrates, such as frogs, toads, and salamanders, appeared on land along with the two important modernmammal groups, theplacentals and themarsupials.Plant life also exhibited a gradual change toward more-modern forms during the course of the Mesozoic. Whereas seed ferns had predominated in the Triassic, forests of palmlikegymnosperms known ascycads andconifersproliferated under the tropical and temperate conditions that prevailed during the Jurassic. The first flowering plants, orangiosperms, had appeared by the Cretaceous. They radiated rapidly and supplanted many of the primitive plant groups to become the dominant form of vegetation by the end of the Mesozoic.

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The Mesozoic closed with an extinction event that devastated many forms of life. In the oceans all the ammonites, reef-building rudistbivalves, and marine reptiles died off, as did 90 percent of thecoccolithophores (single-celled plantlike plankton) andforaminifera (single-celled animal-like plankton). On land the dinosaurs and flying reptiles became extinct. The Late Cretaceous extinctions have been variouslyattributed to such phenomena as global tectonics, draining of the continental seas, northward migration of the continents into different and much cooler climatic zones, intensified volcanic activity, and a catastrophicmeteorite orasteroid impact. The Cretaceous extinction may very well have had multiple causes. As the landmasses were uplifted by plate tectonism and migrated poleward, the climate of the Late Cretaceous began to deteriorate. In fact, some of the extinctions were not sudden but rather spanned millions of years, suggesting that a gradual decline of some organisms had already begun before the end of the Cretaceous. However, strong evidence supports thecontention that a large-scale impact played a significant role in the mass extinctions at the end of the Mesozoic, including the sudden disappearance of many groups (such asammonite and microfossil species), the presence of geochemical and mineralogical signatures that most likely came from extraterrestrial sources, and the discovery of theChicxulub crater in theYucatán Peninsula. It is believed that an asteroid with a diameter of about 10 km (6 miles) hit the Earth and caused wildfires,acid rain, months of darkness (because of the large amount of ash injected into the atmosphere), and cold temperatures (caused by increased reflection ofsolar energy back into space byairborne particles). An intense warming may have followed, heat being trapped by atmospheric aerosols. Whatever the cause, this major mass extinction marks the end of the Mesozoic Era. The end of the dinosaurs (except birds) and many other forms of life allowed the development of modern biota in the Cenozoic Era.