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Mesozoic
*** Shopping-Tip: Mesozoic
The '''Mesozoic''' Era is one of three
Geologic time scale geologic eras of the
Phanerozoic eon (geology) eon. The division of time into eras dates back to
Giovanni Arduino in the
18th century, although his original name for the era now called the Mesozoic was "Secondary" (making the modern era the
Tertiary). Lying between the
Paleozoic and the
Cenozoic, Mesozoic means "middle animals" in
Greek language Greek: ''meso'' for middle and ''zoo'' for
animals. It is often called the "Age of Medieval Life" or the "Age of the Dinosaurs", after the dominant fauna of the era.
The Mesoic was a time of
Plate tectonics tectonic,
climate climatic, and
evolutionary activity. The continents gradually shifted from a state of connectedness into their present configuration; this rifting provided for
speciation and other important evolutionary developments. The climate was exceptionally warm throughout the period and also played an important role in the evolution and diversification of new
animal species. By the end of the era, the basis of modern life was in place.
Geologic periods
Following the
Paleozoic, the Mesozoic extended roughly 180 million years: from 251 million years ago (
annum Ma) to when the
Cenozoic era began 65 Ma. This time frame is separated into three geologic periods. From oldest to youngest:
*
Triassic (248.2 Ma to 205.7 Ma)
*
Jurassic (205.7 Ma to 144.2 Ma)
*
Cretaceous (144.2 Ma to 65.5 Ma)
The lower (Triassic) boundary is set by the
Permian-Triassic extinction event Permian-Triassic extinction, during which approximately 90% of marine species and 70% of terrestrial vertebrates became
extinction extinct. It is also known as the "Great Dying" because it is considered the largest mass extinction in history. The upper (Cretaceous) boundary is set at the
Cretaceous-Tertiary extinction event Cretaceous-Tertiary (KT) extinction, which may have been caused by the meteor that created the
Chicxulub Crater on the
Yucatán Peninsula. Approximately 50% of all genera became extinct, including all of the non-
bird avian dinosaurs.
Tectonics
In general, the Mesozoic era was one of hightened tectonic activity. It began with all of the world's continents collected into a
supercontinent called
Pangea. Pangea gradually split into a northern continent,
Laurasia, and a southern continent,
Gondwana. By the end of the era, these continents had rifted further into their present form. Laurasia became
North America and
Eurasia, while Godwana split into
South America,
Africa,
Australia,
Antarctica, and the
South Asia Indian subcontinent, which collided with
Asia to form the
Himalayas.
Mesozoic climate
The Triassic was generally dry, a trend that began in the late
Carboniferous, and highly seasonal, especially in the interor of Pangea. Low sea levels may have also exacerbated temperature extremes.
Water acts as a temperature-stabilizing heatsink due to its high
specific heat capacity, and land areas near large bodies of water, especially the
oceans, experience less variation in temperature. Because much of the land that constituted Pangea was distant from the oceans, temperatures fluctated enormously, and the interior of Pangea probably included expansive areas of
desert. Abundant evidence of
red beds and evaporites such as
salt support these conclusions.
Sea levels began to rise during the Jurassic, probably due to an increase in
seafloor spreading. The formation of new crust beneath the surface displaced ocean waters by as much as 200m more than today, which flooded coastal areas. Furthermore, Pangea began to rift into smaller divisions, bringing more land area in contact with the ocean by forming the
Tethys Ocean Tethys Sea. Temperatures continued to increase and began to stabilize.
Humidity also increased with the proximity of water, and deserts retreated.
The climate of the Cretaceous is less certain and more widely disputed. Due in part to higher levels of
carbon dioxide in the
atmosphere, the
Temperature gradient world temperature gradient from North to South became almost flat: temperatures were about the same across the planet. Average temperatures were also higher than today, about 10°
Celsius C. In fact, by the middle Cretaceous, equatorial ocean waters, perhaps as warm as 20°C in the deep ocean, may have been too warm for sea life, and land areas near the equator may have been deserts despite their proximity to water. The circulation of
oxygen to the deep ocean may also have been disrupted. For this reason, large volumes of organic matter accumulated because they were unable to
Decomposition decompose and were eventually
deposition deposited as "
Oil shale black shale".
Not all of the data support these hypotheses, however. Even with the overall warmth, temperature fluctuations should have been sufficient for the presence of
polar ice caps and
glaciers, but there is no evidence of either. Quantitive models have also been unable to recreate the flatness of the Cretaceous temperature gradient.
Mesozoic life
The extinction of nearly all animal species at the end of the
Permian period allowed for the
adaptive radiation radiation of many new lifeforms. In particular, the extinction of the large
herbivore herbivorous and
carnivore carnivorous dinocephalia left those
ecological niches empty. Some were filled by the suriving
cynodonts and
dicynodonts, the latter of which subsequently became extinct.
Animal life during the Mesozoic was dominated, however, by large
Archosaur archosaurian reptiles that appeared a few million years after the Permian extinction:
dinosaurs,
pterosaurs, and aquatic reptiles such as
ichthyosaurs,
plesiosaurs, and
mosasaurs.
The climatic changes of the late Jurassic and Cretaceous provided for further adaptive radiation. The Jurassic was the height of archosaur diversity, and the first
birds and
Eutheria placental mammals also appeared.
Flowering plant Angiosperms radiated sometime in the early Cretaceous, first in the
tropics, but the even temperature gradient allowed them to spread toward the poles throughout the period. By the end of the Cretaceous, angiosperms dominated tree floras in many areas, although some evidence suggests that
biomass was still dominated by
cycad and
ferns until after the KT extinction.
Some have argued that
insects diversied with angiosperms because insect
anatomy, especially the
mouth parts, seems particularly well-suited for flowering plants. However, all major insect mouth parts preceded angiosperms, and insect diversification actually slowed when they arrived, so their anatomy originally must have been suited for some other purpose.
As the temperatures in the seas increased, the larger animals of the early Mesozoic gradually began to disappear while smaller animals of all kinds, including
lizards,
snakes, and perhaps the ancestor
mammals to
primates, evolved. The KT extinction exacerbated this trend. The large archosaurs became extinct, while birds and mammals thrived, as they do today.
References and further reading
*''British Mesozoic Fossils'', 1983, The Natural History Museum, London.
-
The Mesozoic from Palaeos
-
Lecture19: Mesozoic Terrestrial Ecosystems
-
Global Climate Change Student Guide: Mesozoic Climates
-
Paleozoic/Mesozoic Climate
{{Phanerozoic Footer}}
{{Mesozoic Footer}}
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ru:Мезозой
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| style="border-bottom: 3px solid; background: #efefef; width: 130px" | This period is part of the
Mesozoic era.
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Cretaceous
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Jurassic
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Triassic
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Category:Phanerozoic
*** Shopping-Tip: Mesozoic
