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Spotlight on Mass Extinctions

Spotlight on Mass Extinctions

Mass extinctions are episodes in the history of life on Earth during which unusually large numbers of species die off. They stand in contrast to the background rate of extinction, which occurs even when the diversity of life is increasing.

Scientists recognize five major mass extinctions in the Earth’s history. The extinctions are measured in terms of large groups of related species, called families.

The five mass extinction episodes occurred because of major changes in the prevailing ecological conditions brought about by climate change, cataclysmic volcanic eruptions, or collisions with giant meteors.

The sixth mass extinction appears to be in progress now, and the main cause is environmental change brought about by human activities.

Period: Late Ordovician

Years ago: 440 Million

Families Died Out: 20 – 50 %

Tropical areas are hit hardest. No terrestrial vertebrates have evolved yet.

Period: Late Devonian

Years ago: 360 Million

Families Died Out: 20 – 30 %

Many fish and marine invertebrates die out.

Period: End of Permian

Years ago: 250 Million

Families Died Out: 50 %

Greatest mass extinction ever.

Period: Late Triassic

Years ago: 213 Million

Families Died Out: 20 – 35 %

Dinosaurs and mammals had just recently evolved; both live through this extinction.

Period: Late Cretaceous

Years ago: 66 Million

Families Died Out: 15 %

Dinosaurs become extinct, possibly caused by a meteorite colliding with the Earth. Mammals and fishes begin to proliferate.

Period: Holocene

Years ago: Now

Families Died Out: ??? %

anthropogenic causes

Related Topics:

Case Study on Pleistocene Megafauna

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