Mass Extinction Events: The Causes And Effects
Kieran A. F. Burley
A mass extinction is defined as an event when there is a massive decrease in the abundance and diversity of all species on the planet, this will occur over a relatively short period of time, for any one species an extinction is catastrophic. Extinction events occur continually this results in regular change of all species on the planet and is known as background extinctions, sometimes however extinction rates rise suddenly for a relatively short time and this is a mass extinction.
Mass extinctions that have occurred through time.
There have been a number of examples the most famous being the Cretaceous-Tertiary mass extinction or K-T extinction, this is because the dinosaurs became extinct during this particular mass extinction. This is also the most recent large scale mass extinction and has been relatively well documented in the rock strata. (Jablonski D., 2001) A number of mass extinction events have occurred throughout geological time. There have been five major mass extinctions, they are the following...
the Ordovician-Silurian boundary
The Permo-Triassic boundary
The Triassic-Jurassic boundary
And the Cretaceous-Tertiary boundary
All of the extinctions mentioned above have in some way affected well-established genera. Extinction event | Time | Organisms most affected | Cretaceous-Tertiary | 65 Ma | Bivalves, Belemnites, dinosaurs, Ammonites, pterosaurs, plesiosaurs, mososaurs, numerous families of fish and many others all became extinct at this time. | Triassic-Jurassic | 200 Ma | Sea and land animals. Many Ammonoids. About 35% of all animal families die out. Most of the dinosaur families become extinction. | Permo-Triassic | 251 Ma | 95% of all marine species. In particular Brachiopods, Corals, Ammonoids, Echinoids and Trilobites. 50% of all animal families. Many tree and plant organisms also died out. | Late Devonian | 375 Ma | Shallow marine ecosystems, in particular the corals, Brachiopods and Trilobites. | Ordovician-Silurian | 443 Ma | Planktons and bottom dwellers. Brachiopods and trilobites. 100 families extinct. | Figure 1. Table showing the big 5 extinction events and how they effected organisms. (Hallam A. et al., 1997) While a mass extinction kills of many species, the niches and ecosystems that they leave behind allow other species and genera to adapt into new ecological niches, this allows life to diversify.
The Cretaceous-Tertiary or K-T mass extinction.
The Cretaceous-Tertiary extinction event occurred around 65 Ma and 75% of species became extinct. The event itself was gradual and shows a decline in species over several million years. (Hallam A. et al., 1997) Marine casualties included large reptiles, Brachiopods, foraminifera, belemnites, fish and bivalves. Loss on land included dinosaurs, pterosaurs and plants. While many species were virtually wiped out by this extinction event there were many groups that remained relatively unaffected. Animals such as mammals, crocodiles, lizards and birds managed to survive without taking to many casualties overall (Norris R. D. et al., 1999).
Possible theory to explain the Cretaceous-Tertiary mass extinction. Causes.
The most widely excepted theory commonly used to explain this mass extinction is an impact of an asteroid or meteorite, there is overwhelming evidence supporting an impact of a large object around 65 Ma. Evidence supporting this theory comes in the form of a layer of iridium, a transition element that is rare on Earth, however is found in meteorites, this can be found as a thin layer concentrated in clays at the K-T boundary. Another piece of evidence is shocked grains of quartz found in sediments forming a thin layer at the boundary, (Hallam A. et al., 1997) shocked quartz are grains of quartz that have been deformed in such a way that would...
References: Berner R. A., 2002. Examination of hypotheses for the Permo–Triassic boundary extinction by carbon cycle modelling.Hallam A. & Wignal P. B., 1997. Mass Extinctions and their aftermath.
Jablonski D., 2001. Lessons from the past: Evolutionary impacts of mass extinctions.
McGee Kenneth A.,Doukas Michael P., Kessler Richard, and Terrence M. Gerlach
1997. Impacts of Volcanic Gases on Climate, the Environment, and People. U.S. Geological Survey Open-File Report 97-262
Wignall P.B., 2000. Large igneous provinces and mass extinctions. Earth-Science Reviews. 1177.
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