Species of plants and animals regularly go extinct, but as we have only recently recognized that species can die out, we do not know what the rate of "background extinctions" are. Mass extinctions are recognized when a large number of species of different types of animals die out in a relatively short time all over the world, thus distinguishing it from a background extinction or a geographically centralized die off. In fact, it is the extinction of species and the rise of new species that geologists use to establish the stratigraphic ranges for different geologic time periods.
The end of the Permian Period (280 to 230 million years ago) witnessed the extinction of a large number of species, this event is often called the Permo-Triassic Boundary. The Permian Extinction was one of the largest extinction events in geologic history, with approximately 50% of all animal families going extinct. It is estimated that 85 to 95% of all marine species (predominately invertebrates), 70% of all terrestrial animal species, and many tree species went extinct in less than one million years.
The Trilobites went extinct during the Permian Extinction event, as did the Brachiopods, Foraminiferans, and many Ammonite families died out at this time. After a come-back in the early Triassic Period (230 to 195 million years ago), the ammonites were again decimated in number at the end of the Triassic Period. Although they did not go completely extinct, they live on today in the Nautiloids, the Ammonites never really recovered from the losses they suffered during the Triassic.
In addition, other invertebrates such as the Acritarchs, Archaeocyathids, Molluscs, Echinoderms, Corals, Diatoms, Dinoflagellates, Gastropods, Crinoids, Bryozoans, Eurypterids, Ostracods, Echinoderms, and Stromatoporoids also suffered heavy losses in the Permian Extinction event.
Terrestrial species that were affected included many insects, amphibians, and reptiles. Therapsids (mammal-like reptiles) which included the pelycosaurs, such as Dimetrodon, also went extinct during the Permian Period. The terrestrial plants like the gymnosperms, including the conifers. Marine species were also affected with many species of bony fish and sharks becoming extinct.
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