Tuesday, 23 October 2012

Mass Extinctions (Part 1)

Throughout geological time, several mass extinctions have taken place, significantly changing the makeup of plants and animals over time. This post will be talking about mass extinctions, what they are, what causes them, and some examples, followed by another post next week. 

The first question to ask is 'what exactly is a mass extinction'? A mass extinction (also known as an extinction event) is a time when levels of extinction are much higher than normal background levels for a large number of groups, and is not limited to one group or one environment. Mass extinctions are widespread: the have global affects, and they result in a large decrease in diversity and abundance in microscopic and macroscopic life. 

No organism is immune to extinction, but there are characters that can help one survive a mass extinction. Organisms that are widespread are more likely to survive, as they are often more flexible and able to live in different environments. For example, if an animal is found only in a specific environment and a certain area of the world, and that area has a massive fire, then that animal will most likely die. Another thing that helps is numbers. If there are many of the animal, they are more likely to survive. For more details on what helps an animal survive a mass extinction, check out UK palaeontologist Dave Hone's post "How to survive mass extinction.

In the past, there have been several mass extinctions. In palaeontology, we refer to the "Big Five" mass extinctions. This week, we will talk about the first 3: 
Graph showing number of families of animals over time, indicating the 5 major mass extinctions. Image from UMass.
1. The Ordovocian extinction event - occurred at the end of the Ordovician, about 443 million years ago. It was characterised by 2 peaks of extinctions separated by as much as a million years. As the Ordovician is quite early in the evolution of life on land, most life was still in the ocean, and therefore it was marine life that suffered, including a large number of brachiopods and bryozoans. More than 60% of all marine invertebrates went extinct, and as much as 85% of all life, making it the second (or possibly third) most deadly mass extinction of all time. Although the immediate cause is difficult to determine, it seems to have been caused by a decrease in temperature worldwide, resulting in glaciation and fall of sea level. 
Diorama of typical Ordovician life before the extinction. Image from Wikimedia Commons
2. The Late Devonian mass extinction - This likely occurred as a series of events (2 or more) and the event as a whole was approximately 360 million years ago at the end of the Devonian period. Again, primarily marine life was affected, with shallow seas being particularly deadly. Approximately 75% of all life died, including nearly all the corals, which had previously colonised much of the Earth's sea, producing large reef systems. Again, the cause of this extinction is not clear, with theories including a bolide (extraterrestrial) impact, sea level changes, and lack of oxygen in the oceans. This is the least deadly mass extinction.
Typical Devonian ocean life. Image credit: University of Michigan University of Paleontology
3. The Permian mass extinction - The Permian mass extinction occurred at the end of the Permian and the end of the Paleozoic, approximately 250 million years ago. This event is also known as the Permian-Triassic event, as it separates the Permian from the Triassic, the first period in which dinosaurs occur. This is by far the largest mass extinction event in history. It's estimated that as many as 96% of all marine species and 70% of terrestrial species went extinct at this time. This caused a major faunal turnover, with many new groups appearing in the Triassic to exploit the areas left empty by all of the animals that went extinct. Several causes have been proposed including a large release of methane gas, large fires, huge volcanic eruptions, bolide impacts, and sea level changes, although it is likely a result of several different events. Whatever the cause, this is definitely the most catastrophic event that has happened on Earth. Trilobites, foraminiferans, brachiopods, and ammonites suffered substantially, with trilobites going extinct completely. On land, many mammal-like therapsids as well as other reptiles and amphibians died out. 
Trilobite fossil from the Permian of Russia. Image from the Carnegie Institute for Science
Next week, we will continue talking about mass extinctions, including a description of the final two: the end Triassic extinction, and the end Cretaceous extinction, which is the end of the dinosaurs! 

For more information, check out the BBC Nature website on mass extinctions!

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