Mesozoic Era

The Mesozoic era was a major episode in Earth’s history during which primitive flora and fauna and the physical environment changed and became progressively more familiar. During this era, continents and ocean basins nearly achieved their present configuration, and dinosaurs and other large reptiles flourished.

Breakup of Pangaea

The Mesozoic era and system are, respectively, a major subdivision of geologic time and the rocks of that age. The Mesozoic began about 251 million years before the present and ended about 65 million years before the present. It was preceded by the Paleozoic era and followed by the Cenozoic era and is divided into three periods: Triassic (about 251 to 201 million years before the present), Jurassic (about 201 to 145 million years before the present), and Cretaceous (about 145 to 65 million years before the present).

All the continents were gathered in a single large landmass, the supercontinent Pangaea, at the beginning of the Triassic period. South America, Africa, Antarctica, Australia, New Zealand, Arabia, and Peninsular India previously formed the supercontinent Gondwanaland during the Carboniferous (about 359 to 299 million years before the present), while North America, Greenland, Eurasia (less Peninsular India), and Borneo formed Laurasia. Laurasia and Gondwanaland merged during the Permian (about 299 to 251 million years before the present) by welding northwest Africa and South America to the south and east margin of North America. At this time, the Atlantic Ocean and Gulf of Mexico did not exist, but a large wedge-shaped seaway, Tethys, separated the former Laurasia and Gondwanaland blocks between the Mediterranean and opened to the east. The equator transected Mexico, the Sahara Desert, and northern India, continuing eastward to divide the Tethys seaway. The South Pole was slightly offshore of the western base of the Palmer Peninsula, and the North Pole was in eastern Siberia.

During the Late Triassic, Pangaea began to break up as North America moved away from Europe and northwest Africa. The resultant narrow North Atlantic Ocean and Caribbean Gulf of Mexico widened in the Jurassic and Cretaceous, reaching something near the present size and shape by the end of the period. Starting in the Jurassic, North America pulled away from South America, leaving a Caribbean Gulf of Mexico seaway connecting the Atlantic and Pacific. The South Atlantic opened later in the Cretaceous, resulting in a relatively narrow strait between Africa and South America by the end of the Mesozoic. The Late Triassic also was when India, Australia, and Antarctica began moving away from Africa as a single block. India separated from the Australian-Antarctica block as the movement continued, and, in the Late Jurassic, Madagascar split from Africa. Cenozoic movement then divided Antarctica and Australia and fused Peninsular India to Asia, eventually resulting in the present configuration of lands surrounding the Indian Ocean.

By the end of the Cretaceous, the equator passed just north of South America, crossed Africa at the southern margin of the Sahara, clipped the southwest corner of Arabia, traversed India along a north-south line through western India, and crossed Sumatra and Java. The North Pole was in the Arctic Ocean off western Siberia, and the South Pole lay within Antarctica.

The movement of the continents and opening of ocean basins caused compressional deformation of Earth's crust accompanied by massive igneous intrusion. The Andes and the mountains of western North America resulted from North and South America moving westward over oceanic crust in the Pacific Basin. Thrust faulting, folding, and batholithic (igneous) intrusion began in the Late Jurassic from California and Nevada northward to British Columbia and Alaska. This process continued during the Cretaceous, spreading to South America, the Palmer Peninsula, and northwest Antarctica. Similar deformation associated with subduction also took place along the northern margin of the Tethys seaway from the Mediterranean through the Balkans, Caucasus, and into Indonesia. Rifting along the East Coast of North America from Nova Scotia to Florida caused fault-block mountains and major basaltic flows as the Atlantic Ocean began opening in the Triassic and early Jurassic. Similar rifting with mountain-making and basaltic flows occurred in South Africa, India, and Australia as the eastern part of Gondwanaland fragmented during the Late Triassic through the Cretaceous.

Sedimentary Deposition

Triassic sedimentary deposits are characterized by terrestrial red beds laid down on the supercontinent, Pangaea, under arid climatic conditions. These conditions occur between paleolatitudes (lines of latitude shown on the present pattern of land and sea where those lines existed in the past) from 10 to 30 degrees north and south. Thus, red beds were deposited north of the equator, from the north coast of Africa through Spain and Germany to the Ural Mountains and northwestward into Britain and Scandinavia. Similar rocks in East Greenland and southwestern North America are part of the same belt, as well as red beds in the rift valley system between Nova Scotia and Florida. Red beds and evaporites (salt and associated minerals precipitated from evaporating water) also accumulated in a parallel southern belt in Brazil, South Africa, Madagascar, eastern India, Australia, and Antarctica. Triassic marine rocks are sparse because Pangaea stood well above sea level during most of the Triassic. However, fairly complete sequences do occur between the Alps and southern China along the northern margin of the Tethys seaway. Similarly, marine Triassic sediments occur in the Alps and western Cordillera of North America, which then were on the continental side of a subduction zone. Epicontinental seas encroached on both the Americas and Eurasia from these bordering areas, leaving marine rocks interbedded with the great red bed accumulations on their western and southern margins.

The growth and expansion of spreading centers as Pangaea began breaking up probably caused the advance of epicontinental seas in the Jurassic and through the Middle Cretaceous. Resultant marine sedimentary rocks are widespread in North America from the Rocky Mountains westward and in western Europe. In addition, the growth of mountain ranges along continental margins, due to either subduction or collision, led to extensive alluvial (river or stream) deposition. Also, relatively deep sea deposition continued in subduction zones bordering the Pacific and the northern shore of Tethys. Epicontinental seas also encroached on the western Americas prior to the Late Jurassic and were responsible for extensive deposits in western Europe. As the North Atlantic opened, Late Jurassic deposits first filled the Nova Scotia to Florida rift zone and then overlapped the continent in the Middle Jurassic and into the Cretaceous. Also, a great evaporite basin in the Gulf of Mexico area accumulated Jurassic salt deposits. Rising mountains from California to western Canada shed extensive alluvial deposits eastward as far as central Kansas during the latest Jurassic. Widespread alluvial deposits associated with basalt flows in India, Australia, Antarctica, Madagascar, and New Zealand. Terrestrial, coal-bearing rocks also border the northern and western shores of marginal seas from Iran to Siberia.

Processes responsible for Jurassic depositional events accelerated in the Cretaceous, with maximum expansion of epicontinental seas in the middle of the period. Drying and filling of most of these seas followed due to the expansion of the mountain-making toward the end of the Cretaceous. Thus, deep-water, continental margin deposition continued around the Pacific perimeter and on the southern margin of Eurasia until growing mountain chains engulfed these basins. Epicontinental seas expanded over Europe west of the Urals, western and northern Africa, central Australia, western and northern South America, and on the Atlantic coast of North America. In addition, a new seaway opened between the Arctic Ocean and the Gulf of Mexico, separating the rising mountains of the western Cordillera from the eroded stumps of the Appalachians. Widespread chalk deposits characterize these seas in areas remote from actively rising mountains, including the notable chalk cliffs of Dover and Normandy. The rising western North and South American mountains, however, shed abundant detritus to their east in the form of alluvial plains, deltas, and muddy marine deposits, which very nearly filled the North American midcontinent seaway by the end of the Mesozoic. Most, but not all, of the other Cretaceous epicontinental seas either dried up or were greatly constricted at the end of the period or early in the Tertiary.

Plant Life and Dinosaurs

Early Mesozoic plants resembled those of the preceding Paleozoic. Primitive Carboniferous coal swamp plants persisted in restricted moist environments in the Northern Hemisphere but died out at the end of the Triassic. Cycadeoids, gingkoes, conifers, and ferns dominated the Triassic through Early Cretaceous floras, with the cycads, primitive conifers—including the auracariaceans of the Petrified Forest—and gingkos first expanding in the Triassic. This plant life was joined by more modern conifers and early relatives of cypress in the warmer, moister Jurassic and Early Cretaceous. Abruptly, in the middle of the Cretaceous, modern broad-leaved trees appeared and quickly dominated the forests. Thus, Late Cretaceous forests closely resembled those of the present.

Large amphibians and an expanding contingent of reptiles constituted the terrestrial vertebrate fauna at the beginning of the Triassic period. Phytosaurs (large crocodile-like reptiles) were the dominant reptiles. Still, the first dinosaurs appeared in the Late Triassic, along with the first ichthyosaurs (porpoise-like reptiles), nothosaurs (ancestors of plesiosaurs), and the first turtle. The first frogs also were Triassic. Mammal-like reptiles (reptiles ancestral to mammals), living mostly in the Southern Hemisphere, declined during the Triassic but probably gave rise to the first mammals late in the period.

Rapid dinosaur diversification in the Jurassic led to bipedal (two-footed) and quadrupedal (four-footed) forms and herbivores and carnivores. Brachiosaurus, a quadrupedal herbivore, was probably the largest land animal of all time. Other, bizarre, nondinosaurian reptiles also appeared in the Jurassic. Ichthyosaurs and the first plesiosaurs (large animals with long necks and tails and four paddles for limbs) became abundant in the seas, and the first batlike pterosaurs invaded the skies. The only mammals were small, inconspicuous, and generalized. Archaeopteryx, from the Upper Jurassic of Germany, is generally considered the first bird.

Dinosaurs continued to dominate terrestrial Cretaceous faunas but began a slow decline, culminating in extinction at the end of the period. Large sauropods (quadrupedal, herbivorous saurischians, saurischians being “reptile-hipped” dinosaurs) declined throughout and disappeared before the end of the period. Theropods (bipedal saurischians with short forelimbs), including Tyrannosaurus, the largest known terrestrial carnivore, also declined, but a few persisted to the end of the period. Ornithischians (“bird-hipped” dinosaurs) included several quadrupedal herbivores. One of these, the Triceratops, with its distinctive three horns and fringed neck, persisted to the very end of the Cretaceous. Bipedal ornithischians included the duckbilled dinosaurs. Pterosaurs also declined to extinction at the end of the period, but one of them, Quetzalcoatlus, was a giant, having a wingspan of 10 meters. Ichthyosaurs declined, but plesiosaurs and giant lizard-like mosasaurs in the marine fauna were abundant. All, however, became extinct at or before the end of the Cretaceous. The Cretaceous also saw the rise of turtles, snakes, lizards, and crocodilians—all currently prominent reptiles. Mammals of the time included both marsupials (mammals retaining the newborn in a pouch on the mother's abdomen) and placentals (mammals in which the young are not nurtured in a pouch). All of them were small, resembling modern shrews and insectivores, but included the ancestors of all modern mammals. Cretaceous birds were rare, but a diversity of toothed birds is known, including the flightless, marine, diving bird Hesperornis. More modern birds appeared in the very latest Cretaceous.

In the twenty-first century, scientists made discoveries about dinosaur life in the Mesozoic era. In 2020, paleontologists discovered what is believed to be the smallest known Mesozoic dinosaur when they found a small, bird-like skull in Burmese amber in northern Myanmar named Oculudentavis khaungraae. New dinosaur fossils believed to belong to dinosaurs that developed during the Mesozoic era have also been located in Spain and China. Scientists have also found fossils that lead them to believe that flowering plants propagated from seeds that had also developed during the Mesozoic era. 

Marine and Insect Life

Chondrostian fish (ray-finned, but with largely cartilaginous skeletons) diversified greatly in the Early Triassic as sharks declined and more primitive fish characteristic of the Paleozoic persisted in low numbers and variety. Cartilaginous fish, including sharks, continued to dominate the Jurassic, but holostean fish gave rise to the first teleosts (bony fish) in the Late Jurassic. This group continued to expand and diversify in the Cretaceous and persists to the present.

Primitive insects occurred sporadically in the Triassic through Middle Cretaceous fossil faunas. Still, they were reduced to a minor role by an explosive appearance of very modern-appearing insects accompanying the appearance of broad-leaved forest trees in the Middle Cretaceous.

Marine invertebrates suffered extensive extinction in the Permo-Triassic transition. The Paleozoic corals, trilobites, and graptolites completely disappeared, and other groups, such as brachiopods, bryozoans, and crinoids, were decimated. By the Late Triassic, however, new organisms had occupied the environments inhabited by the extinguished animals. Modern corals, not at all closely related to Paleozoic corals, already were constructing reefs. The old bryozoans were replaced by new bryozoan types, and the reduction in brachiopods was matched by expansion among the pelecypods (clams, oysters, and scallops). Ammonites descended from a single group of shelled cephalopods surviving into the Early Triassic, quickly evolved into many species, so much so that the Mesozoic is known as the age of ammonites as well as the age of dinosaurs. Other characteristic organisms appeared during the Jurassic and Cretaceous, including redistid clams (giant pelecypods that built reefs), a wide variety of echinoids (sea urchins and sand dollars), globigerinids and other planktonic foraminifera (single-celled animals secreting shells), and coccoliths (single-celled, planktonic algae secreting minute carbonate plates), along with crabs, shrimp, and lobsters. Globigerinids and coccoliths became sufficiently abundant in the early part of the Late Cretaceous to cause worldwide deposition of chalk and chalky limestone.

By the end of the Mesozoic, many organisms, including dinosaurs and ammonites, had become extinct. Though not as extensive as those at the end of the Paleozoic, these extinctions are the reason for recognizing separate Mesozoic and Cenozoic eras. However, not all the characteristic Mesozoic animals that failed to survive into the Cenozoic became extinct at the very end of the period. In addition, very little change occurred in the flora, as the significant change occurred in the middle of the Cretaceous.

Close examination of the rocks at the Mesozoic-Cenozoic boundary, as defined by occurrences of planktonic foraminifera, coccoliths, ammonites, and dinosaurs, has uncovered the widespread occurrence of a thin layer containing anomalous concentrations of iridium and, more questionably, sooty material and minerals of the sort associated with meteoritic impacts. This evidence has been used to advance a theory that the end-of-Cretaceous extinctions were caused by meteoritic impact, perhaps through drastic cooling caused by vast quantities of dust and smoke thrown into the atmosphere. However, physical evidence of an actual impact has not convinced all researchers, and many paleontologists do not believe that the gradual disappearance of the Mesozoic fauna can readily be explained by an instantaneous event, even though it might be synchronous with the final extinctions.

Mesozoic rocks are rich in mineral fuels. Coal is sparse in the Triassic, reflecting the aridity of the time. Much coal, however, occurs in the Jurassic of Asia, deposited on alluvial plains north of the Tethyan orogenic belt, and even more extensive deposits in the Cretaceous of North America and northern South America are in a similar position relating to the growing western American mountains. Jurassic and Cretaceous oil and gas are very important, especially around the Gulf of Mexico and in the Persian Gulf region. Triassic ore deposits are rare because of little mountain building and igneous intrusion, but Jurassic and Cretaceous metallic ores are very extensive. The gold of the California Mother Lode is in Jurassic rocks, and many of the precious and base metal reserves of the Rocky Mountains and the Andes are Cretaceous. Similarly, the mountains formed elsewhere on the Pacific Rim and on the northern margin of the Tethys are rich in metallic ores. Finally, the diamond pipes of southern Africa are Cretaceous.

Principal Terms

epicontinental sea: a sea covering part of a continental block; such seas generally are less than 200 meters deep and are the depositional site of most exposed sedimentary rocks

Gondwanaland: an ancient, large continent in the Southern Hemisphere that included Africa, South America, India, Australia, and Antarctica

Laurasia: an ancient, large continent in the Northern Hemisphere that included North America and Eurasia

Pangaea: the supercontinent containing all continental crust that existed at the beginning of the Mesozoic

period: a unit of geological time forming part of an era and subdivided, in decreasing order, into epochs and ages

rifting: a process of faulting and basaltic intrusion occurring where crustal plates separate during continental drift; may cause mountains

subduction: a process by which one crustal plate rides over another, which descends and melts, generating molten rock that then intrudes the deformed plate above; it causes mountains

system: the rocks deposited during a period, which is defined by the age of the rocks making up its system

Tethys: a seaway embayed into Pangaea between the southeast corner of Asia, the western end of the Mediterranean, and the southeast end of Pangaea

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