The Top 5 Extinctions

  The Earth goes through periods of radiation of the organisms on the planet and extinction periods. These extinctions, small or large, make way for new species to diversify and fill spaces that extinct animals once did.

  Did you know that 98% of documented species are now extinct? That’s a lot, especially considering how many different organisms are alive today. Imagine if none of these creatures died out; the world would be crammed, over populated and we humans probably wouldn’t even exist. So, horrible as it sounds, extinctions are very important to the Earth’s growth and evolution.

  The extinction of invertebrates and vertebrates normally happen every million years or so, and mostly in the oceans. These marine fossils are used to measure extinction rates because they have a better fossil record and stratigraphic range compared to land-dwellers.

  Sepkoski and Raup in 1982 identified five mass extinctions, which I will discuss here, starting with the oldest first.

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The Ordovician-Silurian

  The first of our top five is the Ordovician-Silurian extinction. This happened 450-440 million years ago, creating the boundary between the Ordovician period and the Silurian. This is known as the second greatest major extinction ever- the worst is yet to come!

  This extinction killed off 27% of all families, 57% of all genera and 60-70% of all species. At this point in time, all life was constrained to seas and oceans. 60% of all marine invertebrates died, including two thirds of all brachiopod and bryozoa families. Echinoderms and corals were also greatly affected.

  So why did this happen? It is suggested that it is due to the movement of Gondwana, the supercontinent. This caused global cooling, leading to glaciations and an eventual sea level fall.

 

The Late Devonian

  Number two on our list is the Late Devonian extinction 375-360 million years ago. In this mass extinction, 19% of all families, 50% of all genera and 70% of all species disappeared. This was a very lengthy and dragged out extinction, which lasted for 20 million years! This was prolonged by ‘extinction pulses’- where lots of little things build up into a big extinction.

  One event that helped with extinction is the Hangenberg Event. Black shale is found in rock at this point due to an environment lacking oxygen. Plants were moving onto land, causing an increase in nutrient supply in rivers. This stimulated the growth of algal blooms, making it difficult for any other life. There was also rapid sea level fall and possible global cooling and oceanic volcanism and maybe even comets.

  However, as dreadful as this sounds, it only affected marine life such as brachiopods, trilobites (unfortunately the end of our favourite prehistoric creepy crawlies) and reef building organisms. Corals didn’t make their great comeback after this until the Mesozoic!

  Thankfully, our ancestors, the jawed vertebrates were unaffected.

 

The Permo-Triassic

  Next up, what can only be described as the ‘Great Dying’. This catastrophic event happened 251 years ago, at the Permian-Triassic boundary and, you guessed it, is the worst and largest mass extinction ever. 57% of all families, 83% of all genera and 90-96% of all species were wiped out! These were mostly marine but it is the only known mass extinction where insects were greatly affected. This extinction also ended the primacy of mammal-like reptiles and gave rise to the great Archosaurs (crocodiles, dinosaurs and their relatives), but this took them a good 30 million years to do. New plant taxa also became dominant after this ‘Great Dying’.

  This extinction also had phases, first starting with a gradual environmental change. These environmental changes include sea level change, anoxia, increasing acidity in the water and a shift in ocean circulation. The next phase was a catastrophic event. Not just any catastrophic event, but every bad thing you could imagine; meteors, comets, volcanoes, coal and gas fires, explosions from and area called the Siberian Traps and a sudden release of methane clathrate from the sea floor. The world was a horrible place to live at this point, and very few things made it through. This extinction was a terrifying giant in comparison to the others.  

 

The Triassic-Jurassic

  Now, a less worrying mass extinction that happened 201.3 million years ago; the Triassic-Jurassic extinction. In this extinction, conodonts and 20% of all marine families disappeared. All non-dinosaurian archosaurs, apart from crocodiles were wiped out, along with large amphibians and some theraspids. These deaths on land allowed dinosaurs to fill niches in the Jurassic that were not available in the Triassic. This was a quick extinction, happening in less than 10000 years and occurred just before Pangaea started to break up.

  Marine losses at this time suggest that the decrease in diversity was caused more by a decrease in speciation, rather than an increase in extinctions.

  This extinction could be due to many things, like gradual climate change; sea level fluctuations; pulse of ocean acidification, but none of these explain the suddenness of extinction in the marine realm.

  Asteroid impact has also been suggested, but there hasn’t been any evidence of a big enough at this boundary to cause this. One was found; the Rochechouart crater in France, which was about 201 million years old (fitting this boundary) but it was too small.

  There might have also been massive volcanic eruptions, shown by flood basalts of the Central Atlantic Magmatic Province. This either caused a release in carbon dioxide, causing global warming, or sulphur dioxide and aerosols, causing global cooling. Carbon isotopes show that this volcanic theory is a possibility.

 

The Cretaceous-Tertiary

  And finally, the mass extinction we all know and love, the Cretaceous-Palaeogene extinction some 66 million years ago. Three quarters of plant and animal species on Earth died, including non-avian dinosaurs, pterosaurs, mosasaurs, plesiosaurs, ammonites and birds with teeth. Some mammals, lizards, insects, plants, fish, sharks and plankton were also affected but oddly enough crocodiles and turtles were barely harmed.

  It is marked on the geological record by a thin layer of sediment called the K-Pg boundary found in marine and terrestrial rocks containing clay showing high levels of metal iridium. This is rare in the Earth’s crust but very abundant in asteroids.

  A massive asteroid, the Chicxulub crater in the Gulf of Mexico, 180km wide hit Earth, causing global warming and then a lingering impact winter. This made it impossible for plants and plankton to carry out photosynthesis.

  After this, mammals diversified and birds and fish radiated. We would have never come to be if this extinction had not happened.

 

  So there you have it, the Big 5 mass extinctions. These extinctions were the Earth’s way of making way for some of the big players, such as dinosaurs to dominate, and then fall to pave the way for us humans. One can only imagine when the next big one will be and how it will affect us…

 

References:

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 Smith, Roff (2011-11-16). “Dark days of the Triassic: Lost world”. Nature 47 (7373): 287–289.

T.M. Quan, B. van de Schootbrugge, M.P. Field, “Nitrogen isotope and trace metal analyses from the Mingolsheim core (Germany): Evidence for redox variations across the Triassic-Jurassic boundary”, Global Biogeochemical Cycles, 22 2008: “a series of events resulting in a long period of stratification, deep-water hypoxia, and denitrification in this region of the Tethys Ocean basin”; M. Hautmann, M.J. Benton, A. Toma, “Catastrophic ocean acidification at the Triassic-Jurassic boundary”, Neues Jahrbuch für Geologie und Paläontologie 249.1, July 2008:119-127.

Sole, R. V., and Newman, M. Patterns of extinction and biodiversity in the fossil recordJin YG, Wang Y, Wang W, Shang QH, Cao CQ, Erwin DH (2000). “Pattern of Marine Mass Extinction Near the Permian–Triassic Boundary in South China”. Science 289 (5478): 432–436.

Image from earthmagazine.org

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About danniteboul

Palaeobiologist at the University of Portsmouth- Undergrad 20 years old Follow: @danniteboul
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3 Responses to The Top 5 Extinctions

  1. Pingback: End-Triassic mass extinction link to CAMP: It’s official? |

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  3. Pingback: Deep-sea animals in the age of dinosaurs | Dear Kitty. Some blog

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