Mass Extinctions
Mass extinction.
The Causes of Mass Extinctions
Fossils and the Geologic Time Scale

It is commonly said that the final fate of all the species is Extinction, that is, animal and plant species appear and eventually disappear they do not longer exist on Earth. The cause of the extinction; however, is variable. For example, in the process known as pseudoextinctionthe species evolves into a new species that differs so much from its ancestral group that the parent species can be considered as extinct although in the strict sense no extinction event occurred. In other cases the species dies out (disappear) without giving origin to anything else, this process is know as  anextinction. Furthermore, there are times or episodes in the geological past in which the extinction rates are greatly accelerated resulting in the marked decrease in the diversity of organism. Such extinction that involves the disappearance a number of groups are known as mass extinction.
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During geologic history, there have been times when large groups of organisms became extinct within a short period (a few  million  years)- same of these  mass extinction have been used to define period breaks. 

Main episodes of mass extinction in the history of  the biosphere include:

1) Cambrian/Ordovician, 505 my ago: Many trilobites, sponges, and gastropods. About 52 percent of   biota involved. 

Ordovician-Silurian extinction, about 439 million years ago, caused by a drop in sea levels as glaciers formed, then by rising sea levels as glaciers melted. The toll: 25 percent of marine families and 60 percent of marine genera.

2) Devonian/Mississippian, 360 my ago: groups of corals, trilobites, bryozoan and fish. About 30 percent of the biota involved. 
Late Devonian extinction, about 364 million years ago, cause unknown. It killed 22 percent of marine families and 57 percent of marine genera. Erwin said little is known about land organisms at the time.

3) Permian/Triassic, 251  my ago: Rugose corals, trilobites, blastoids, brachiopods, and foraminifera became extinct. About 50 percent of the biota involved. Many scientists suspect a comet or asteroid impact, although direct evidence has not been found. Others believe the cause was flood volcanism from the Siberian Traps and related loss of oxygen in the seas. Still others believe the impact triggered the volcanism and also may have done so during the Cretaceous-Tertiary extinction. The Permian-Triassic catastrophe was Earth’s worst mass extinction, killing 95 percent of all species, 53 percent of marine families, 84 percent of marine genera and an estimated 70 percent of land species such as plants, insects and vertebrate animals.

The End Triassic extinction, roughly 199 million to 214 million years ago, most likely caused by massive floods of lava erupting from the central Atlantic magmatic province -- an event that triggered the opening of the Atlantic Ocean. The volcanism may have led to deadly global warming. Rocks from the eruptions now are found in the eastern United States, eastern Brazil, North Africa and Spain. The death toll: 22 percent of marine families, 52 percent of marine genera. Vertebrate deaths are unclear. 
4) Jurassic/Cretaceous, 144  my ago: Ammonites, brachiopods, and fish groups. About 35 percent of biota involved. 

5) Cretaceous/Tertiary, 65 my ago: Ammonites, dinosaurs, marines reptiles become extinct, many types of marine invertebrate were affected. 26 percent of biota involved. Cretaceous-Tertiary extinction, about 65 million years ago, probably caused or accelerated by impact of a several-mile-wide asteroid that created the Chicxulub crater now hidden on the Yucatan Peninsula and beneath the Gulf of Mexico. Some argue for other causes, including gradual climate change or flood-like volcanic eruptions of basalt lava from India’s Deccan Traps. The extinction killed 16 percent of marine families, 47 percent of marine genera (the classification above species) and 18 percent of land vertebrate families, including the dinosaurs.

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The Causes of Mass Extinctions
Plate Tectonics: the joining of all the continents into one supercontinent, Pangaea, destroyed much of the suitable shallow-water, marine, continental shelf area.  During times of active plate tectonics  continents join, and faunal diversity appears to decrease. When continents fragment, faunal diversity increases. Plate tectonics is invoked as the main cause of Paleozoic extinctions. 

Extraterrestrial Impacts: A controversial hypothesis for mass extinctions has been postulated, particularly for the extinction of the dinosaurs at the end of the Cretaceous--extraterrestrial meteor or asteroid impacts. While meteors and/or asteroids have certainly struck the earth In the past, the causal relationship between impacts and extinctions is still debated. 

Humans and human activities should be listed as a cause of extinction of species. As we know there is a long list of endangered biota (animals and plants) due to human intervention on the environment or by direct abuse of species. 
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The fossil record shows many evolutionary (new appearance) and extinction (disappearance) events, From an examination of the timing of these events, geologists have been able to divide geologic time into major units based on the history of life. Phanerozoic time (from about 700 million years ago to the present) is divided into three eras-Paleozoic, Mesozoic, and Cenozoic the basis of the overall nature or predominant character of fossils found in the strata. The -subdivision of eras into periods is clone primarily on the basis of extinction events, In general, the Paleozoic fossil record is water marine invertebrates characterized by shallow-water marine invertebrates. Thus, the Paleozoic is sometimes called the Age of Invertebrates. The Mesozoic rock record contains a relatively large percentage of continental deposits with fossils of reptiles. hence, the Mesozoic has been termed the Age of Reptiles. The Cenozoic fossil record contains a large number of mammals and flowering plants that evolved during this time. The Cenozoic is informally named the Age of Mammals.
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 Some species have escaped mass extinction through geologic time:
Other species did not cope with the presence of humans
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