Mesozoic

The Mesozoic Era is a period from about 250 million years ago to about 67 million years ago. It is called the Age of Dinosaurs because most dinosaurs developed, and went extinct, during that time. The Chicxulub impact and other events ended the era when a majority of species on earth went extinct.

It is one of three geologic eras of the Phanerozoic 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 everything after, including the modern era, the "Tertiary"; the current term Quaternary was later proposed for the modern era, following the same numbering principle). Lying between the Paleozoic and the Cenozoic, "Mesozoic" means "middle animals", deriving from the Greek prefix meso-/μεσο- for "between" and zoon/ζωον meaning "animal" or "living being". It is often called the "Age of the Reptiles", after the dominant fauna of the era.

The Mesozoic was a time of tectonic, climatic and evolutionary activity. The continents gradually shifted from a state of connectedness into their present configuration; the drifting provided for speciation and other important evolutionary developments. The climate was exceptionally warm throughout the period, also playing 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 (Ma) to when the Cenozoic era began 65 Ma. This time frame is separated into three geologic periods. From oldest to youngest:

• Triassic (251.0 Ma to 199.6 Ma)
• Jurassic (199.6 Ma to 145.5 Ma)
• Cretaceous (145.5 Ma to 65.5 Ma)

The lower (Triassic) boundary is set by the Permian-Triassic extinction event, during which approximately 90% to 96% of marine species and 70% of terrestrial vertebrates became extinct. It is also known as the "Great Dying" because it is considered the largest mass extinction in the Earth's history. The upper (Cretaceous) boundary is set at the Cretaceous-Tertiary (KT) extinction event (now more accurately called the Cretaceous–Paleogene (or K–Pg) extinction event^[1]), which may have been caused by the impactor that created Chicxulub Crater on the Yucatán Peninsula. Approximately 50% of all genera became extinct, including all of the non-avian dinosaurs.

Paleogeography and tectonics

Compared to the vigorous convergent plate mountain-building of the late Paleozoic, Mesozoic tectonic deformation was comparatively mild. Nevertheless, the era featured the dramatic rifting of the supercontinent Pangaea. Pangaea gradually split into a northern continent, Laurasia, and a southern continent, Gondwana. This created the passive continental margin that characterizes most of the Atlantic coastline (such as along the U.S. East Coast) today. ^[2]

By the end of the era, the continents had rifted into nearly their present form. Laurasia became North America and Eurasia, while Gondwana split into South America, Africa, Australia, Antarctica and the Indian subcontinent, which collided with the Asian plate during the Cenozoic, the impact giving rise to the Himalayas.

Africa

The African prosauropod Massospondylus.

At the beginning of the Mesozoic Era, Africa was joined with Earth's other continents in Pangaea.^[3] Africa shared the supercontinent's relatively uniform fauna which was dominated by theropods, prosauropods and primitive ornithischians by the close of the Triassic period.^[3] Late Triassic fossils are found throughout Africa, but are more common in the south than north.^[3] The boundary separating the Triassic and Jurassic marks the advent of an extinction event with global impact, although African strata from this time period have not been thoroughly studied.^[3]

Early Jurassic strata are distributed in a similar fashion to Late Triassic beds, with more common outcrops in the south and less common fossil beds which are predominated by tracks to the north.^[3] As the Jurassic proceeded, larger and more iconic groups of dinosaurs like sauropods and ornithopods proliferated in Africa.^[3] Middle Jurassic strata are neither well represented nor well studied in Africa.^[3] Late Jurassic strata are also poorly represented apart from the spectacular Tendeguru fauna in Tanzania.^[3] The Late Jurassic life of Tendeguru is very similar to that found in western North America's Morrison Formation.^[3]

Midway through the Mesozoic, about 150-160 million years ago, Madagascar separated from Africa, although it remained connected to India and the rest of the Gondwanan landmasses.^[3] Fossils from Madagascar include abelisaurs and titanosaurs.^[3]

The African theropod Spinosaurus, the largest known predatory land animal of all time.

Later into the Early Cretaceous epoch, the India-Madagascar landmass separated from the rest of Gondwana.^[3] By the Late Cretaceous, Madagascar and India had permanently split ways and continued until later reaching their modern configurations.^[3]

By contrast to Madagascar, mainland Africa was relatively stable in position through-out the Mesozoic.^[3] Despite the stable position, major changes occurred to its relation to other landmasses as the remains of Pangaea continued to break apart.^[3] By the beginning of the Late Cretaceous epoch South America had split off from Africa, completing the southern half of the Atlantic Ocean.^[3] This event had a profound effect on global climate by altering ocean currents.^[3]

During the Cretaceous, Africa was populated by allosauroids and spinosaurids, including the largest known carnivorous dinosaurs.^[3] Titanosaurs were significant herbivores in its ancient ecosystems.^[3] Cretaceous sites are more common than Jurassic ones, but are often unable to be dated radiometrically making it difficult to know their exact ages.^[3] Paleontologist Louis Jacobs, who spent time doing field work in Malawi, says that African beds are "in need of more field work" and will prove to be a "fertile ground...for discovery."^[3]

Climate

The Triassic was generally dry, a trend that began in the late Carboniferous, and highly seasonal, especially in the interior of Pangaea. Low sea levels may have also exacerbated temperature extremes. With its high specific heat capacity, water acts as a temperature-stabilizing heat reservoir, and land areas near large bodies of water—especially the oceans—experience less variation in temperature. Because much of the land that constituted Pangaea was distant from the oceans, temperatures fluctuated greatly, and the interior of Pangaea 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, which was probably caused by an increase in seafloor spreading. The formation of new crust beneath the surface displaced ocean waters by as much as 200 m (656 ft) more than today, which flooded coastal areas. Furthermore, Pangaea began to rift into smaller divisions, bringing more land area in contact with the ocean by forming the 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. Higher levels of carbon dioxide in the atmosphere are thought to have caused the world temperature gradient from north to south to become almost flat: temperatures were about the same across the planet. Average temperatures were also higher than today by about 10°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 that was unable to decompose accumulated, eventually being deposited as "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. Quantitative models have also been unable to recreate the flatness of the Cretaceous temperature gradient.

Oxygen levels in the Mesozoic atmosphere were probably lower (12 to 15%) than today's level (20 to 21%). Some researchers have postulated levels of 12% because that was assumed to be the lowest concentration at which natural combustion could occur. However, a 2008 study concludes that at least 15 % is necessary.^[4]

Life

The extinction of nearly all animal species at the end of the Permian period allowed for the radiation of many new lifeforms. In particular, the extinction of the large herbivorous and carnivorous dinocephalia left those ecological niches empty. Some were filled by the surviving cynodonts and dicynodonts, the latter of which subsequently became extinct. Recent research indicates that the specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to reestablish, recovery did not begin until the start of the mid-Triassic, 4M to 6M years after the extinction^[5] and was not complete until 30M years after the P-Tr extinction^[6]. Animal life was then dominated, by large archosaurian reptiles: 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 placental mammals also appeared. 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 diversified 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

• British Mesozoic Fossils, 1983, The Natural History Museum, London.

1. ↑ Gradstein F, Ogg J, Smith A. A Geologic Time Scale 2004. http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=0521781426. 
2. ↑ Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6
3. ↑ ^{3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20} Jacobs, Louis, L. (1997). "African Dinosaurs." Encyclopedia of Dinosaurs. Edited by Phillip J. Currie and Kevin Padian. Academic Press. p. 2-4.
4. ↑ Chemical & Engineering News, Vol. 86 No. 35, 1 Sept. 2008, "O2 Requirement for Burning Rises", p. 12
5. ↑ Lehrmann, D.J., Ramezan, J., Bowring, S.A., et al. (December 2006). "Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from south China". Geology 34 (12): 1053–1056. doi:10.1130/G22827A.1. http://geology.geoscienceworld.org/cgi/content/abstract/34/12/1053. 
6. ↑ Sahney, S. and Benton, M.J. (2008). "Recovery from the most profound mass extinction of all time" (PDF). Proceedings of the Royal Society: Biological 275 (1636): 759. doi:10.1098/rspb.2007.1370. PMID 18198148. PMC 2596898. http://journals.royalsociety.org/content/qq5un1810k7605h5/fulltext.pdf. 

External links

• The Mesozoic from Palaeos
• The Mesozoic, age of cycads and dinosaurs
• Mesozoic terrestrial ecosystems
• Mesozoic climates
• Paleozoic/Mesozoic climate