The Big Five Mass Extinctions

Mounted Brachiosaurus skeleton at the
Naturkunde Museum in Berlin, Germany. The dinosaurs
were the major casualties of the K/T Extinction. Image
credit: Bill Sellers

As I said in my Devonian post yesterday, I am going to cover the Five biggest mass extinctions in the Earth's history. We will look at these from the one with the least impact on global species to the one with the most impact. You will notice that when reading the statistics for what went extinct that the number of species will be higher than the number of families or genera that were lost, this is because you can have vast numbers of species die out without losing a genera if some species are to survive. For instance, the Sarcopterygii that was discussed in the Devonian post largely have died out but the whole class was not lost as it survives with the lungfish and Coelacanth. I am also using marine life families as some of the extinctions took place when there was little or no life on land.

#5: The Cretaceous Tertiary Extinction 
Also known as the K/T Extinction or K/Pg Extinction, Pg being the Palaeogene period that followed the Cretaceous, is famed for being the extinction that wiped out the dinosaurs and closed the Age of Reptiles. Extensive studies, including the drilling of boreholes, of the Chicxulub crater off the coast of the Yucatan Peninsula in Mexico have given a clearer picture of what happened 66 Ma. An asteroid nearly six miles wide smashed into the earth, this caused a chain reaction of events that doomed the dinosaurs. After the impact, mass volcanism, particularly around the Deccan Traps area in India, polluted the atmosphere and caused the Earth to heat up. Sea levels also fell by 150 metres which could be attributed to the loss of the marine life as many would have depended on shallower seas to survive. 
Marine Families Lost: 16%
Genera Lost: 67%
Species Lost: Approximately 76% including non avian dinosaurs, marine reptiles, pterosaurs and ammonites.

Artist's impression of the Chicxulub Crater. Image credit:
Detlev van Ravenswaay

#4: End Triassic Extinction
Little is known about the causes of this extinction that took place 201 Ma, but we do see evidence of falling sea levels which is potentially the reason behind a larger loss of ocean species, including some marine reptiles. There is also sedimentary evidence for volcanic rifting that took place as Pangaea broke apart. Taking place over where North America, Europe and Africa would have been on the supercontinent, the air would have been toxic to nearby ecosystems, another possible reason for an extinction event. 
Marine Families Lost: 22%
Genera Lost: 53%
Species Lost: Approximately 80% including most mammal-like reptiles and large amphibians, surprisingly plants made it through largely unscathed. 

#3: Devonian Period's Two Extinction Events
There were two separate events of extinction in the Devonian; the first being the Kellwasser, in the late middle Devonian, which pushed corals and jawless fish to extinction as well as reducing trilobite species. The second was the Hangeberg, which took place on the Devonian-Carboniferous boundary, was responsible for the extinction of Placoderms and many early ammonite species. The Hangeberg extinction event is believed to have been the result of global cooling due to increased volcanic activity. There is also evidence of eutrophication in the shallow seas. This is where there is an excess of nutrients, usually caused by the run off water from the land, this causes algal blooms. These blooms are disastrous for marine life as it prevents sunlight from penetrating the water, thus there is no replenishment of the oxygen that the algae removes from the water, which in turn kills the marine life that depend on the oxygen rich waters. The result of this is the coral reefs not making a return for another 100 million years.
Marine Families Lost: 22%
Genera Lost: 57%
Species Lost: Approximately 83% 

#2: End Ordovician Extinction
Taking place between 445 and 440 Ma, it is believed that an intense global ice age was the trigger for the event. As the supercontinent of Gondwana moved further South to take it's place over the South Pole, vast glaciers spawned gradually lowering global temperatures and therefore causing a fall in sea levels. It is believed that the sea levels fell by between 70 and 100 metres. 
Marine Families Lost: 26%
Genera Lost: 60%
Species Lost: 85%

#1: End Permian Extinction
The largest extinction event in Earth's history took place at the end of the Permian period and the end of the Palaeozoic Era around 252 Ma. Also known as the "Great Dying", the causes are unknown for certain, however, we already know that Pangaea was incredibly hot and dry which would push a great many of species to the brink. But we also see a spike in greenhouse gases such as Carbon Dioxide and Methane that would have been released from frozen stores in the oceans. Combined with one of the biggest volcanic eruptions ever and we can start to see why so many species were lost at the close of the Permian. 97% of all species that were present on Earth in the Permian were wiped out 252 Ma, all life that followed, the majesty of the dinosaurs, the radiation of mammals and eventually the success of Homo sapiens can all be traced back to the 3% of species that clung to survival at the end of the Palaeozoic Era.
Marine Families Lost: 51%
Genera Lost: 82%
Species Lost: 97%

I hope you find this helpful and interesting, let me know what you think in the comments.