because no one knows exactly what defines a species. Species extinction rates, even of background extinction, is not stable over time.
MASS EXTINCTION Mass extinctions are also referred to as extinction events or biotic crises. It results in a major loss of biodiversity on earth over a short period of time. No one knows the exact number of extinction events there have been, mostly because it is hard to define what is major and what is meant by a rapid change in biodiversity. Extinction events are usually defined based on their effect on multicellular organisms because it isn t known what the effect has been on microbes of the earth. Marine animals are most used to assess these events because their fossil records are the most accurate and stratification is easier to determine in water-based environments. There have been five major mass extinction events and many more minor events throughout geologic history. We will talk about these events and their possible causes. The first extinction event occurred at the transition between the Ordovician and Silurian eras about 450 million years ago. There were two events that killed off about 60 to 70 percent of all the earth s species. The earth was warm prior to the events, which were a major fall in sea levels affecting coastal areas and glaciation, which cooled the earth, killing off many marine organisms. The Late Devonian extinction happened at the transition between the Devonian and Carboniferous eras. There were multiple events that killed off 70 percent of species and that lasted 20 million years. No one knows how many events contributed to this but it primarily affected marine animals. No one knows what events likely caused this extinction. The largest extinction on earth killed off up to 96 percent of all species. It was called the Permian-Triassic extinction event, killing off insects, trilobites, and many plant species. Mammal-like reptiles died off and it took 30 million years for vertebrates to recover from this great dying event. No one knows if it was due to one or several events but it may have been caused by volcanic eruptions, meteor impact, or climate change causing the release of methane from the oceans. The Triassic-Jurassic extinction event occurred 200 million years ago, killing off about 75 percent of all species. Archosaurs, large amphibians, and therapsids were largely made extinct. It allowed the dinosaurs to ascend without competition. No one knows what caused it but it caused much of the earth to become much drier.
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