What is a mass extinction? What usually occurs after a mass extinction? What is the greatest mass extinction of our planet’s history?
A mass extinction event occurs when there is a sharp decrease in the diversity and abundance of life on earth. This usually happens when undue stress is placed on living things because of a changing environment. The “official zoological definition” is the decrease of two to five taxa such as Classes or Orders. Generally, this means that at least ten percent of all families, forty percent of all genera (genus), and seventy percent of all species die out at one time.
Major Mass Extinction Events
There were five major extinction events in the earth’s history. The most severe was the Permian-Triassic Event. Known as “The Great Dying”, 57 percent of all families, 83 percent of all general, and at least 90 percent of all species died out. In addition, this extinction event ended the primacy of mammal-like reptiles and ushered in the Age of Dinosaurs.
The second worst extinction event was the Ordovician-Silurian Event, when 27 percent of all families and 57 percent of all genera died out. The only mass extinction of insects occurred at this time. After this event came the diversification of land species and new ecosystems.
The other extinction events were the Late Devonian Event (19 percent families, 50 percent genera, and 70 percent species) when the trilobites disappeared. The Triassic-Jurassic Event (23 percent families and 48 percent genera) saw the end of large amphibians and many mammal-like reptiles. The well-known Cretaceous-Paleogene (Tertiary) Event (17 percent families, 50 percent genera, and 75 percent species) wiped out the dinosaurs and gave rise to the Age of Mammals.
Since the cause of mass extinction varies from glaciations to volcanism to meteorite strikes, the organisms that emerge in the recovery after the event are different from those before. However, first “disaster taxa” take over and proliferate. After the Permian-Triassic Event, ninety percent of land vertebrates consisted of one species – Lystrosaurus, a pig-like reptile. Then, the other organisms organize themselves and take advantage of the new ecological niches. Afterwards, the “disaster taxa” recede and new dominant species emerge.
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