Natural Extinction, Breeding Population, Extinction, Pseudoextinction, Mesozoic Ammonoid, Silurian Graptolite, Paleozoic

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	Natural Extinction - The species which are prone to extinction are characterized by small size of breeding population (thus promoting inbreeding) associated with high coefficient of variation. When the size of breeding population drops to a hundred or less, the likelihood of extinction is enhanced still further due to inbreeding depression.
The overall extinction rate estimated from fossil data is one species per 107 species per year.This rate refers to true extinction, which means extinction of a species and its descendents and differs from 'pseudoextinction", which is caused due to evolution of one species into another. The longevity of a species defined as above (species and its descendents) varies from 1-2 million years (e.g. Mesozoic ammonoid and silurian graptolite) to as much as 10 million years (e.g. paleozoic and mesozoic invertebrates). It has also been shown that in general, planktonic and sessile marine animals, including corals and brachiopods, have had higher extinction rates than mobile benthic animals, such as gastropods and bivalves.