In order for this blog to make sense, it needs to be placed into its historical context. This post is intended to provide some background to future discussion, hence enabling deeper analysis of mass extinction-related issues.
Recognisable life on earth was initiated around 700mya (million years ago) when single-celled organisms combined to create multicellular beings. 600mya witnessed the development of the skeleton, and between 500 and 400ma, the seas began to contain egg-laying fish. Mammalian life is thought to have begun around 260ma, at the end of the Palaeozoic era.
Fossil evidence suggests that there have been 5 mass extinction events to date, though these were likely to have been accompanied by smaller extinction events.
1) End-Ordovician (Ashgillian) 434 mya. This event took place over a period of several million years, and at a time of high global temperatures caused by greenhouse gases. Causes: sea level fluctuations, polar glaciations, changes in ocean temperatures, circulation and chemistry, also possibly due to extreme levels of CO2. It is thought that during this time, 90% of earth’s species vanished, and that the remaining 10% of species were severely affected by the ecological imbalance caused, so up to 99% of Palaeozoic species could have died out (Courtillot 2002).
2) Late Devonian (Frasnian-Framennian) 360 mya. Possible causes include bolide (meteor) collision, a fall in CO2 levels through increased uptake of plants, fluctuations in global sea level, and ocean anoxia. With regard to the exact causes of this event, McGhee (1988) notes that the most important question to answer is ‘what is the inhibiting factor that caused the cessation of new species originations?’
3) End-Permian 251 mya, also known as the ‘Great Dying’. It has been suggested by White (2002) and others that this was the worst loss of life the earth has ever witnessed. Perhaps up to 96% of marine species became extinct, and many land plant, reptiles, amphibians and insect species also vanished. Fossil evidence suggests that this event was incited by environmental disturbances. Oceans became stagnant and anoxic, with high levels of hydrogen sulphide, and large-scale methane released contributed to global warming. There is still much debate as to whether these instabilities came about due to changes within the earth system or because of a catastrophic event.
4) End-Triassic (Novian) 205mya. This event occurred between the Triassic and Jurassic Periods. 50% of genera were lost. It has been noted that this extinction occurred at the same time as the increase in volcanic activity caused by continental movements within the Pangaea earth mass (Deenen et al 2010). Though others argue that meteorite impact may have been responsible (Courtillot and Renne 2003). Extreme atmospheric CO2 levels, short-term sea level fluctuations, changes in ocean chemistry.
5) End-cretaceous (end-Maastrichtian) 65mya. This event was tends to be remembered because it marked the end of the dinosaur era, but also wiped out most other large land animals and plants. Other taxa, however, including freshwater fish, amphibians, turtles, crocodiles, snakes and lizards, and placental mammals were unaffected. On average, temperatures were between 6 and 14 degrees higher than at present, and up to 40 degrees higher at the poles. This extinction is thought to have been caused by the after effects of a bolide collision, evidence for which is visible in the Yucatan Peninsula, Southeast Mexico. This collision triggered tsunamis and volcanic eruptions, which released clouds of stratospheric volcanic dust and cooled the earth, creating a ‘nuclear winter’. Acid rain, methane release from continental slopes and intense greenhouse warming are also thought to have arisen. Over a period of hundreds of thousands of years, the combination of these effects led to large-scale species extinction.
The causes of mass extinction will be discussed in greater detail in later posts.
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