Five major mass global extinction events have occurred during the past 542 million years as shown in the fossil record. There have been numerous extinction events, but these five have been identified as the most significant by Raup and Seposki (1982).
(Graph showing timeline of mass extinctions and number of families lost.)
1/ Ordovician Crisis (444 mya)
2/ Devonian Crisis (360 mya)
3/ Permian Crisis (251 mya)
4/ Triassic Crisis (200 mya)
5/ Cretaceous Crisis (65 mya)
The graphs also includes the Jurassic Crisis, which I will not be including as a mass extinction event.
The causes of these extinctions are numerous and diverse. Scientists often differ on what they believe caused different extinction events, something which I will discuss in a later post. The one factor which all past mass extinctions share is that the extinctions all had natural causes, something which the potential sixth mass extinction may differ from.
Within the last 2000 years a large number of species have become extinct in ways which can be linked to human dispersal or activity. This has led many scientists to believe that a sixth mass extinction is underway, defined as the Holocene extinction event. In 1998 a survey by the American Museum of Natural History found that 70% of biologists view the present era as part of a mass extinction event.
This blog will try to identify the causes of this extinction event, whether it is the first mass extinction with a biotic cause. Focusing upon hard and soft evidence for species extinctions over the past 2000 years. I will also look back at the causes and rates of past extinction events, and see if any comparisons can be made, and look forward using modelling predictions to see what the future holds.
My next blog post will give an introduction to the sixth mass extinction.
References: New World Encyclopedia, http://mac122.icu.ac.jp/biobk/biobookpaleo3.html, Raup and Seposki (1982) - Mass Extinctions in the Marine Fossil Record, Science 215: 1501-1503.
Hi hannah, the figure you show for the previous 5 mass extinctions seem to relate to marine organisms. Did terrestrial species suffer the same fate?
ReplyDeleteHi Anson,
ReplyDeleteI should have mentioned that the diagram shown focused solely upon marine organisms. The reason that I chose to show a diagram that focused solely on marine organisms is because the preservation of fossils varies on land. Whereas marine fossils tend to be much better preserved than their terrestrial counterparts. This is especially true considering that we are looking back hundreds of millions of years. This means that it is easier to portray an accurate pattern of change of number of marine species than terrestrial ones.
In answer to whether terrestrial species suffered the same fate:
During the Cretaceous crisis (65mya) around 75% of species went extinct. Marine species suffered badly with over 65% of species being lost. Terrestrial animals did not suffer as badly and mammals and birds emerged as the dominant land vertebrates.
During the Triassic crisis (200 mya)55% of marine genera went extinct. On the land the story was also pretty dire with most non-dinosaurian archosaurs, most therapsids, and most of the land amphibians eliminated leaving the dinosaurs with little competition.
During the Earths largest extinction the Permian crisis (251 mya)96% of marine species and 70% of land species were lost. This was a time known as the 'great dying' and had enormous evolutionary significant resulting in large numbers of vacant 'niches' to be filled.
The age of the final two extinction events (Devonian and Ordovisian crises) makes it difficult to assess the proportion of terrestrial species that were lost. However, it is estimated that 70% and 57% of all Earths species were lost respectively.
I hope that this answers your question, and why the variation in effect on marine and terrestrial species is something that I will discuss further in my blog.