A mass extinction is well-defined in geological terms – it is when seventy-five percent of species existing at that time go extinct over a period of under two million years. This has happened five times in history – at the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods.

A summary of the extinction events is as follows:

  • The late Ordivician (443 million years ago), when most complex multicellular organisms lived in the sea. The Ordivician Event wiped out 86% of life, and is argued to have been caused by increased glaciation due to global cooling, resulting in a rapid drop in sea levels. For the mostly marine ecosystem of the time, this was disastrous.

  • The late Devonian Event (359 MYA) occurred when plants had just developed roots, seeds and water transport systems (although grass had yet to evolve) and animals were beginning to develop leg-like structures. Believed to have been caused, again, by climate change as the changes in plants and increased use of carbon dioxide in photosynthesis led to global cooling and glaciation.

  • The Permian Event is the most catastrophic in history, wiping out 96% of life on Earth, and has a long list of potential causes such as asteroid impacts, one of the largest volcanic eruptions in history, and a deficiency of oxygen in the atmosphere and subsequent increase of poisonous hydrogen sulphide in the oceans.

  • The Triassic Event allowed dinosaurs (who established a foothold in the world after the Therapsids died out during the Permian Event) to really establish themselves on the global stage. The Triassic Event is speculated to have been caused by the opening of the Mid-Atlantic Ridge.

  • The K-Pg (Cretaceous-Paleogean) Event is the most recent Extinction Event, and is best known for the asteroid impact that is commonly thought to have wiped out the dinosaurs as a single, sweeping cataclysm.

Of course, it is the Cretaceous Extinction Event that wiped out the dinosaurs. It is commonly touted in popular tellings that the dinosaurs’ final end came in the form of the Chicxulub Meteor, named for a 112 mile-wide crater in the Gulf of Mexico dating from 65 million years ago and believed by scientists to be the site of the impact that caused the Cretaceous Extinction Event. But this is only part of the story.

The geological record shows dinosaurs had been dying off long before the fateful impact, as their species diversity in the fossil record falls for millions of years prior to the meteor impact. Furthermore, most of the evidence we have for the effects of the impact are from the northern hemisphere – we have little idea of the after-effects further South. The fossil record suggests that the asteroid strike was not a single cataclysm that wiped out all the dinosaurs in one fell swoop – dinosaur fossils were still being found that dated to forty thousand years after the asteroid strike. Similarly, the fossil record suggests there was a decline in dinosaur diversity over ten million years prior to the asteroid impact, while pterosaurs declined in competition with the newly-evolved birds. What this suggests is that dinosaurs died out over a much longer period than we might expect – and indeed, were dying out long before the Chicxulub Meteor struck the Gulf of Mexico. Potential causes suggested for this mass extinction include the asteroid strike itself, alongside increased volcanic activity and falling sea levels that caused massive changes to the dinosaurs’ environments. Changes that they could not survive.

A general answer to the question may be ‘climate change’, as with most of the previous mass extinctions. Large volcanic eruptions in India combined with falling sea levels led to the injection of massive amounts of carbon dioxide and methane into the atmosphere – falling sea levels led to the drying out of the land and the release of trapped gas, similar to the release of trapped methane today in Siberia as the ice melts. The asteroid impact itself would have thrown dust and molten rock high into the Earth’s atmosphere, with the resulting impact winter lasting an entire year and ensuring that many plants and animals died long after the initial blast and the forest fires and tsunamis that followed.

References and further reading:

What causes mass extinctions?

http://www.livescience.com/1752-greatest-mysteries-mass-extinctions.html

Summary of past mass extinctions:

https://www.youtube.com/watch?v=FlUes_NPa6M

The Alvarez Hypothesis (The Asteroid Strike):

https://en.wikipedia.org/wiki/Alvarez_hypothesis

The Chicxulub Crater:

https://upload.wikimedia.org/wikipedia/commons/9/9c/Chicxulub_radar_topography.jpg

Ordovician-Silurian extinction event:

https://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_extinction_events

Late Devonian event:

https://en.wikipedia.org/wiki/Late_Devonian_extinction

Permian-Triassic Extinction Event:

https://en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinction_event

Triassic-Jurassic Extinction Event:

https://en.wikipedia.org/wiki/Triassic%E2%80%93Jurassic_extinction_event

Cretaceous-Palaeogene Extinction Event:

https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event

The eruption of the Siberian Traps

http://news.sciencemag.org/2011/11/eruptions-swept-life-land-and-sea?ref=em&elq=8f42909034db42aab865178f232755e9

Were there two Extinction Events in the Permian era?

http://www.iflscience.com/plants-and-animals/permian-era-had-two-mass-extinctions

Did the appearance of animals trigger the Earth’s first mass extinction?

http://www.iflscience.com/plants-and-animals/animal-arrival-triggered-earths-first-mass-extinction

Did a gamma ray burst cause the Ordivician Extinction Event?

http://kusmos.phsx.ku.edu/~melott/87551.pdf