An extinction occurs when the last member of a species dies. Background extinction refers to the fact that species are constantly going extinct but at a slow rate. Most of the creatures living on Earth today were not around a few hundred million years ago, just as the creatures that were around then haven't all made it to the present day. But there have been events in Earth's history when lots of species have all gone extinct at once, in a rapid (well at least rapid from a geological viewpoint) amount of time. These events are known as mass extinction events and it is thought by some people that we could be heading towards the next one very soon.
There have been five recognised mass extinction events in Earth's history, where over 75% of lineages have been truncated and extinction has occurred to a global extent. The big five events likely all have different causes and they have been studied to varying extents. The cause of these events are complicated, not very well understood and often due to a variety of factors interacting. To fully go into the causes and consequences of each would be way beyond the scope of this article so take this as a basic introduction.
Iguanodon skeleton in the Oxford University Museum of Natural History: https://www.oumnh.ox.ac.uk/
Ordovician-Silurian
When? About 444 million years ago at the boundary between the Ordovician and Silurian periods.
What? Wiped out about an estimated 85% of species that were around at the time. Second largest extinction in Earth's history. Only had major effects on life in the oceans.
Why? It is thought to have taken place in two stages. The first due to global cooling was due to either volcanic activity or the evolution of terrestrial land plants (this is still debated). The second stage has been attributed to glaciation causing sea levels to drop and the ocean to become more anoxic.
Late Devonian
When? About 374 million years ago, towards the end of the Devonian period
What? Mostly affected organisms that lived in shallow seas such as corals and reef building sponges known as stromatoporoids. Also animals such as brachiopods and trilobites.
Why? The causes of the Late Devonian mass extinction are still not entirely known. There is evidence that global cooling may have been involved. The first large trees and forests formed in the late Devonian which may have impacted atmospheric carbon dioxide levels. Other hypotheses such as sea level change and ocean anoxia have been proposed. The likelihood is it was a range of factors all interacting to cause conditions leading to large scale extinction.
End Permian aka Permo-Triassic (PT)
When? About 252 million years ago at the boundary between the Permian and Triassic periods which is also the boundary between the Paleozoic and Mesozoic eras.
What? The largest mass extinction in Earth's history. Wiped out up to 96% of marine species and 75% of terrestrial species. Heavily effected marine animals with calcium carbonate skeletons due to ocean acidification causing an increase in carbonate dissolution.
Why? One of the most common theories is the effects of the Siberian Traps volcanism lead to widespread extinction. The Siberian Traps is a large igneous province (LIP) that erupted up to 10,000,000 cubic kilometres of lava over 100,00 years causing various gases to be emitted in large volumes such as carbon dioxide and sulphides. Kill mechanisms could have included gas poisoning, acid rain and global cooling (volcanic winter) followed very quickly by global warming (due to greenhouse gases).
Triassic-Jurassic
When? About 202 million years ago, boundary between the Triassic and Jurassic periods.
What? Up to 80% of land and marine species.
Why? Global warming perhaps driven by another LIP called the Central Atlantic Magmatic Province caused a quadrupling in atmospheric carbon dioxide levels and ocean acidification.
Cretaceous-Paleogene (KPg)
When? About 66 million years ago at the boundary between the Cretaceous and Paleogene periods as well as the Mesozoic and Cenozoic eras.
What? Perhaps the most famous mass extinction as it lead to the end of the dinosaurs as well as wiping out around 76% of the planet's species. Small mammals were one of the few groups to survive the KPg extinction, leading us into the 'age of mammals' which we still exist in today.
Why? The only mass extinction to be attributed to an asteroid impact. The asteroid, who's crater can be found in the Yucatan Peninsula of Mexico, would have flung up very large volumes of dust that could trigger global cooling as well as starting wild fires, and a tsunami. There is also evidence that the Deccan Traps (another LIP) erupted at the same time (perhaps triggered by the impact but this has not been confirmed) which would then have lead to global warming.
More information on the KPg and how its cause was discovered are in a great book called T.Rex and the Crater of Doom by Walter Alvarez, see 10 Geoscience(ish) Book Recommendations for more!
Anthropocene (now)
When? Now, it is suggested by some scientists that we are living in a new epoch known as the Anthropocene (others argue that we are still in the Holocene) and are currently creating what could turn in Earth's 6th mass extinction.
What? If all species that are currently threatened continue to go extinct at current rates we could find ourselves in a mass extinction within the next 200-500 years. Even if we slow down the rate so only critically endangered species go extinct we could enter the next mass extinction in the next 200 years.
Why? Human activities such as deforestation, fossil fuel burning, overfishing, spread of disease and many others are leading to a heightening of background extinction rates. Anthropogenic climate change poses a long term threat and our release of carbon dioxide into the atmosphere has the potential to get to the level of mimicking a LIP eruption. Eruptions such as the Siberian Traps which triggered the PT extinctions emitted much higher (up to 1,400 times more) volumes of carbon dioxide than humans are but at a rate very similar to our current rate of emissions.
Sources:
National Geographic Website https://www.nationalgeographic.com/science/article/mass-extinction
My notes on a lecture series by Frankie Dunn (Oxford University)
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