Monday, 29 October 2012

Mass Extinctions (part 2)

Last week, we introduced mass extinctions by explaining what they are, some keys to surviving extinction, and then describing the first three mass extinctions in Earth's history: the end-Ordovician, the Late Devonian, and the end-Permian. This week, we will talk about the last 2 major mass extinctions, and some other events that were major, but not considered to be huge mass extinctions. 

The Big Five mass extinctions continued:
4. The Triassic-Jurassic extinction event - This extinction event occurred at the boundary between the Triassic and Jurassic periods, approximately 200 million years ago. It affected both life on land and in the sea, with twenty percent of all marine families being lost, and many on land as well. Animals affected on land include many therapsids, large reptiles (non-dinosaurian archosaurs), and large amphibians. The extinction of these large animals allowed for the dinosaurs to really evolve and take over the landscape, filling the ecological niches left open by the extinct animals. Like most other mass extinction events, the cause is not clear. Possible causes include climate change resulting in sea-level changes or ocean acidification, an asteroid impact, or large volcanic eruptions. Of all of the mass extinctions, the Triassic-Jurassic extinction event is probably ranked fifth in terms of extinction rate. 
Smilosuchus was a phytosaur, a group of reptiles that went extinct in the end-Triassic extinction. Image from Wikimedia Commons user ArthurWeasley.
5. The Cretaceous-Palaeogene mass extinction - Also known as the Cretaceous-Tertiary extinction, or K-T/K-Pg for short, this is the most famous mass extinction to ever occur, although it is not the largest. It marks the end of the Cretaceous period, approximately 65 million years ago. In addition to the extinction of non-avian dinosaurs and pterosaurs, large marine reptiles like mosasaurs and plesiosaurs also went extinct, as well as many plants and invertebrates like ammonites. It affected both animals on land and in the oceans, with a total of 65-70% of all species going extinct, making it the second (or third) largest extinction event in Earth's history. This mass extinction is the only one that the cause is most likely known. It was almost definitely caused by a meteor impact in the Gulf of Mexico, although there is some evidence that it was caused by increased volcanic eruptions in what is now India. To learn more about the Cretaceous-Palaeogene extinction, check out Mesozoic Mondays next week.
Artists rendering of the bolide impact that likely caused the Cretaceous-Palaeogene extinction
Other lesser extinction events have occurred throughout history, during the Precambrian, Cambrian, Silurian, Carboniferous, Permian, Jurassic, Cretaceous, and Neogene. The last extinction event occurred during the Quaternary, and resulted in the extinctions of the large megafauna that existed. Only large animals were affected, so it is not considered to be a true mass extinction, although animals all over the world went extinct. Several events occurred during the Quaternary extinctions, and cause has been attributed to over hunting by humans, climate change, disease, predation, or a swarm of comets. As you can see, it can be very difficult to determine the cause of these mass extinctions. 
Large megafauna like the mammoths went extinct during the Quaternary. Image from Plos Biology credit Mauricio Anton.
Now you have seen a summary of what a mass extinction is, the Big Five extinction events in Earth's history, and some other extinctions. Next week, we will talk about the Cretaceous-Tertiary extinction in detail!

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