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The idea that a species could become extinct was an unpopular belief until fairly recently in human history (mid to late 1700s). It was even considered blasphemous because it contradicted accepted notions of a benevolent God and a “Great Chain of Being”. Thomas Jefferson instructed Lewis and Clark to look for the animals belonging to fossil bones and wrote in 1781 “Such is the economy of nature, that no instance can be produced of her having permitted any one race of her animals to become extinct; of her having formed any link in her great work so weak as to be broken...” This was the prevailing view of the time. The modern view of extinction is obviously different. Of an estimated 5 to 50 billion species that have lived on this planet throughout history, fewer than 50 million are alive today. Thus, about 99.9% of all species that have ever lived are now extinct.
Why do species become extinct? There are many reasons that can contribute to individual extinctions such as competition and predation from new organisms. For example, many trilobites became extinct after cephalopods evolved and many amphibians became extinct after reptiles evolved.
There have been a number of periods in history in which large numbers of organisms have gone extinct at the same time. Five major mass extinctions are often recognized and referred to as the “Big Five”. The timing of these events is indicated on the following chart by solid lines.
1) Late Ordovician Extinctions
Towards the end of the Ordovician, it is estimated that 57% of marine genera became extinct. The diversity of trilobites was greatly reduced. Many types of echinoderms, brachiopods (more than half species), bryozoans (more than half), early corals, cephalopods, acritarchs, and conodonts became extinct. Global cooling might have caused this mass extinction since the animals of warmer water seem to have been affected most. Because it lasted 2 million years, no one single event (like an asteroid) is likely to have caused it.
The ‘Great Ordovician Biodiversification Event’ (GOBE) is associated with a change in reef organization, with metazoans composing the major organisms and a increased colonization of deeper water environments (Servais, 2010).
2) Late Devonian Extinctions
At the end of the Devonian, 50% of the marine genera and 75% species became extinct during a period of 4 million years. Warm-water invertebrates the most severely affected. Trilobites, tabulate corals, rugose corals, ammonoids, graptolites, stromatoporoid sponges, conodonts, acritarchs, and brachiopods virtually disappeared. Many groups of agnathans and placoderms became extinct. Global cooling seems to have been a primary cause and there may also have been some meteorite impacts during this time.
3) End Permian Extinctions
The mass extinction that occurred at the end of the Permian was the worst mass extinction in history. An estimated 96% of marine species became extinct. The dominant groups of the Paleozoic seafloor (crinoids, bryozoans, and brachiopods) were replaced by groups which continue to dominate today (bivalves, gastropods, and echinoderms). Bivalves and gastropods were less affected and lost perhaps 30% of their diversity. Some groups had been reduced by earlier mass extinctions and now were completely wiped out (trilobites, blastoids, tabulate corals, rugose corals, orthid brachiopods, and many mollusks). Fusulinid foraminiferans and productid brachiopods were thriving in the Permian but went extinct at its end. Only one group of crinoids and two groups of ammonoids survived the end of the Permian. The tetrapod species which became extinct during the end Permian extinction seem to have disappeared in at least two separate events which were not synchronized with the marine extinctions (Lucas, 2008). Plants suffered a mass extinction at the end of the Permian. In some areas, the coal beds and seams that were typical of the Permian are absent from the end of that period (Peng, 2009).
What could have caused this extinction which lasted 8 million years?
THE SIBERIAN TRAPS
THE SIBERIAN TRAPS
Large igneous provinces result from the release of immense quantities of magma. The Siberian Traps represent the largest continental igneous province formed by magma quantities estimated at a maximum of 500 million square kilometers (Reichow, 2009; Erwin, 1994).
Some feel that the cause of the Siberian traps was a superplume of material from the earth’s mantle which reached the core and vented gases through vast pipes into the atmosphere. A reversal of the earth’s magnetic field occurred during this time and may have had a role in causing this superplume (Isozaki, 2008).
THE COMPLETION OF PANGEA
One of the most significant events of the Permian Period was the joining of the continents. The various landmasses on earth came together in the middle Permian to form a single supercontinent known as Pangea. As the ocean which separated what would someday become the Americas from Europe and Africa disappeared, many shallow marine habitats were lost. The change in bodies of water would have affected climate. Terrestrial ecosystems seem to have collapsed after the events which devastated marine ecosystems, and caused both animal and plant extinctions. Climatic fluctuations may have been a major cause (Looy, 2001, Erwim 1996). Some have argued that the halogens which would have been released by the Siberian Traps at the end of the Permian may have destroyed enough ozone to contribute to an ecological crisis (Visscher, 2004).
The formation of a supercontinent would certainly affect local climates since rainfall patterns would have been altered. In addition, all of the organic material from former marine continental shelves which were exposed to air would have decomposed. This would have used tremendous amounts of oxygen, perhaps reducing the atmospheric oxygen content by half. It appears that the world’s single ocean (named Panthalassa) suffered a major anoxic event which lasted millions of years at the end of the Permian. As oxygen levels dropped, carbon dioxide levels in the ocean seem to have increased substantially. Not only would this have been toxic for marine life, it appears that the carbon dioxide entered the atmosphere causing a brief greenhouse effect which melted the polar ice caps which had formed earlier in the Permian (for the first time in tens of millions of years. Whatever initiated the end Permian extinction (probably either mass volcanic eruptions and/or a meteorite impact), the result seems to have been a greenhouse effect which raised the earth’s temperature by an estimated 6 degrees Celsius. Both terrestrial and marine ecosystems were decimated and required tens of millions of years to recover their previous biodiversity (Benton, 2003; Isozaki, 1997, Erwin, 1996; Erwin 1994; Waters, 1997).
A reduction in the oxygen of ocean water is associated with the end-Permian extinction. This anoxic event did not end until the mid-Triassic (Takahashi, 2009).
The Permian marks the end of the Paleozoic Era; the Mesozoic begins in the wake of these extinctions. In the period after the end-Permian extinctions, dinosaurs evolved. Two additional mass extinctions would occur in the Mesozoic Era: the end-Triassic extinction and the end-Cretaceous extinction. After the end-Triassic extinction, dinosaurs became the dominant land vertebrates but the end-Cretaceous extinction would cause their demise.
In the aftermath of the end- Permian extinction, the recovery of earth’s biodiversity was slow. In the Early Triassic, conifers and lycopsids were the dominant trees, therapsids such as Lystrosaurus were the dominant land vertebrates, and paper clams and inarticulate brachiopods were the most common marine fossils. While it may be that this slow recovery was a result of the magnitude of the biodiversity loss in the end-Permian, there is also evidence of multiple greenhouse gas crises in the aftermath of the end-Permian extinction which made the time period which followed it less hospitable (Retallack, 2010).