Contrary to the images most of us hold of mass extinction events, they are in fact are rarely due to asteroid impacts. Abigail Rooney explores the surprising variety of factors behind Earth’s ‘Big Five’ mass extinction events.

Over the past 550 million years, since complex life forms evolved from the microscopic life that dominated the Earth over the preceding 3 billion years, five major mass extinction events have been recognised in the fossil record (each outlined below). As landmasses move, and environments change, ecosystems will evolve and adapt to the new conditions they face. This process can lead to the natural extinction of certain species unable to adapt, as well as new speciation and competition. What then defines a major mass extinction event? When the rate of extinction exceeds the rate at which new species evolve, the net adjustment is a reduction in the number of species on the planet. If this condition persists at a significant rate, a major mass extinction event will take place, and will be evident in the fossil record. The ‘Big Five’ extinction events saw a reduction of approximately 50 % of genera. Many causes have been suggested for these events, such as asteroid impacts, massive volcanism (flood basalt events), oceanic anoxia, sea level change, climate change, gamma ray bursts from nearby supernova and Earth’s changing location within our galaxy. Each extinction event was more than likely initiated by a combination of these triggers.

Fossils constitute only a small proportion of life that has existed on Earth, with soft bodied organisms often being forgotten through the sands of time, as they do not contain hard parts that fossilise easily. Geologists have used what evidence exists in the rock record, including geochemical tracers for life to decode the past, but the further back in time you go; the more difficult it is to decipher exactly what happened.

The ‘Big Five’ Mass Extinction Events:

Late Ordovician: 445 Mya, 57 % loss of genera, likely due to global cooling and sea level drop

Most living creatures lived in the sea at this time. It has been suggested that as the supercontinent Gondwana passed over the South Pole, large land based glaciers developed, causing a drop in sea level and a change in climate. This led to a reduction in accommodation space within the oceans, which may have led and/or contributed to a mass extinction event. The northern continents were also beginning to move towards each other at this time, which led to faunal mixing and an overall reduction in total diversity. Another hypothesis for this extinction is that a gamma ray burst from a ‘nearby’ (less than 6000 light years away!) supernova may have reduced the strength of the ozone layer, leaving the planet open to attack from harmful UV radiation coming from the sun.

Late Devonian: 370 Mya, 50 % loss of genera, likely due to oceanic anoxia and global cooling

At this time, the supercontinent of Gondwana was moving back towards the South Pole, initiating the onset of glaciation and lower sea levels once again. The development of expansive black shales at the end of the Devonian also suggests that very low oxygen conditions may have been present in large areas of the oceans. As these black shales develop, they trap large quantities of organic carbon, which can lower carbon dioxide (CO 2 ) levels in the atmosphere. The Earth’s first forests were also beginning to flourish at this time, possibly contributing to a further drawdown of CO 2 . As CO 2 is a greenhouse gas which keeps our planet warm, a reduction in CO 2 can decrease global temperatures, exacerbating the ongoing glaciation and sea level drop, which in turn may have led to a mass extinction event.

My research at Trinity College Dublin involves extracting fossil algae from Late Devonian shales from Kentucky, USA. The deep marine shales were deposited in an epi-continental sea that covered much of Central USA during the Late Devonian. The aim of the project is to use the changes in nitrogen isotopes trapped in the algae to trace the environmental changes that were taking place approximately 360 million years ago (the Devonian – Carboniferous boundary).

Late Permian: 250 Mya, 83 % loss of genera, likely due to volcanism, climate change and oceanic anoxia

This was the largest of all known mass extinction events. Many factors most likely contributed to this ‘Great Dying’, as it is sometimes referred to, such as massive volcanism, climate change, oceanic anoxia, sea level fluctuations and possibly, an asteroid impact. It is known that the Siberian Traps, a massive outpouring of basalt, erupted at this time. This event would have produced large quantities of dust and volcanic gases, blocking out sunlight and disrupting photosynthesis. Large quantities of CO 2 would also have been released, leading to global warming. A carbon isotope excursion has been recognised in the rock record and has led to speculation that the release of large quantities of methane, previously trapped on the continental margins, took place, possibly due to instability caused by the eruption of the Siberian Traps. This would have led to a further increase in global temperatures and climate disruption. The supercontinent of Pangaea had also formed at this time, and may have led to climate extremes and a change in water currents.

Late Triassic: 200 Mya, 48 % loss of genera, likely due to rifting and global warming

The end-Triassic mass extinction event coincided with the breakup of the supercontinent, Pangaea, and the eruption of the flood basalts of the Central Atlantic Magmatic Province as Pangaea rifted apart to form the Atlantic Ocean. This may have led to increased levels of CO 2, global warming and rising sea levels. Suggestions such as an asteroid impact or sluggish speciation have also been presented as possible causes of this extinction.

Late Cretaceous: 65 Mya, 50 % loss of genera, likely due to asteroid impact, volcanism and sea level drop

The extinction that occurred at the end of the Cretaceous is the most widely known, as it brought an end to the animals which have sparked our imaginations over the centuries, as their almost fictional monster-like fossils have been unearthed. It has been widely acknowledged that an asteroid impact is the likely cause of the demise of the dinosaurs and many other species at this time, however, the eruption of the Deccan Traps in India and a sharp fall in sea level have also been accepted as probable contributing factors to the extinction event.

Following each extinction event, new species found ways to capitalise on the demise of the fallen. Speculation has been made that due to human impact on our climate, we are now entering a sixth mass extinction event. How the Earth will react to these changes remains to be seen, but whatever happens, it is likely that life, through its evolutionary genius, will find a new niche and take advantage of it.

Abigail Rooney

Ph.D. student, Department of Geology, Trinity College Dublin

References, further reading and links:

David, Archibald; David Fastovsky (2004). “Dinosaur Extinction”. In Weishampel David B, Dodson Peter, Osmólska Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 672–684.

Erwin, D. H., S. A. Bowring, et al. (2002). “End-Permian mass extinctions: a review.” Special papers-Geological Society of America: 363-384.

Hallam, T. (2005). Catastrophes and lesser calamities: the causes of mass extinctions, Oxford University Press, USA.

Sandberg, C. A., J. R. Morrow, et al. (2002). “Late Devonian sea-level changes, catastrophic events, and mass extinctions.” Special papers-Geological Society of America: 473-488.

Southwood, Richard (2004). The story of life, Oxford University Press, USA.

http://dsc.discovery.com/earth/wide-angle/mass-extinctions-timeline.html