16: Extinction

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**Table \(\PageIndex{1}\)** Major Extinction Events
Era	Period	Epoch	Approximate Duration of Era, Period or Epoch (millions of years before present)	Major Extinction Evens
CENOZOIC	Quaternary	Holocene	present - 0.01	\(6^{th}\) major extinction ? \(5^{th}\) major extinction (end of Cretaceous; K-T boundary) \(4^{th}\) major extinction (end of Triassic) \(3^{th}\) major extinction (end of Permian) \(2^{nd}\) major extinction (Late Devonian) \(1^{st}\) major extinction (end of Ordovician)
	Quaternary	Pleistocene	0.01-1.6
	Tertiary	Pliocene	1.6-5.3
		Miocene	5.3-23
		Oligocene	24-37
		Eocene	37-58
		Paleocene	58-65
MESOZOIC	Cretaceous		65-144
	Jurassic		144-208
	Triassic		208-245
PALEOZOIC	Permian		245-286
	(Carboniferous) Pennsylvanian		286-325
	(Carboniferous) Mississippian		325-360
	Devonian		360-408
	Silurian		408-440
	Ordovician		440-505
	Cambrian		505-570
PRECAMBRIAN			570-4500

Each of the first five mass extinctions shown in Table \(\PageIndex{1}\) represents a significant loss of biodiversity - but recovery has been good on a geologic time scale. Mass extinctions are apparently followed by a sudden burst of evolutionary diversification on the part of the remaining species, presumably because the surviving species started using habitats and resources that were previously "occupied" by more competitively successful species that went extinct. However, this does not mean that the recoveries from mass extinction have been rapid; they have usually required some tens of millions of years (Jablonski, 1995).

It is hypothesized that we are currently on the brink of a "sixth mass extinction," but one that differs from previous events. The five other mass extinctions predated humans and were probably the ultimate products of some physical process (e.g. climate change through meteor impacts), rather than the direct consequence of the action of some other species. In contrast, the sixth mass extinction is the product of human activity over the last several hundred, or even several thousand years. These mass extinctions, and their historic and modern consequences are discussed in more detail in the modules on Historical perspectives on extinction and the current biodiversity crisis, and Ecological consequences of extinctions..

Glossary

Extinct: a species is assumed to be extinct when there is no reasonable doubt that the last individual has died (IUCN, 2002)

Extinction: the complete disappearance of a species from Earth

Mass extinction: a period when there is a sudden increase in the rate of extinction, such that the rate at least doubles, and the extinctions include representatives from many different taxonomic groups of plants and animals