Extinction event

An extinction event (also known as: mass extinction; extinction-level event, ELE) is a sharp decrease in the number of species in a relatively short period of time. Mass extinctions affect most major taxonomic groups present at the time — birds, mammals, reptiles, amphibians, fish, invertebrates and other simpler life forms. They may be caused by one or both of:

• extinction of an unusually large number of species in a short period.
• a sharp drop in the rate of speciation.

Over 99% of species that ever lived are now extinct, but extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine invertebrates and vertebrates every million years.

Marine fossils are mostly used to measure extinction rates because they are more plentiful and cover a longer time span than fossils of land organisms.

Since life began on earth, several major mass extinctions have significantly exceeded the background extinction rate. The most recent, the Cretaceous–Tertiary extinction event, occurred 65 million years ago, and has attracted more attention than all others because it killed the dinosaurs. In the past 540 million years there have been five major events when over 50% of animal species died. There probably were mass extinctions in the Archean and Proterozoic Eons, but before the Phanerozoic there were no animals with hard body parts to leave a significant fossil record.

Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.

Major extinction events

The classical "Big Five" mass extinctions identified by Jack Sepkoski and David M. Raup in their 1982 paper are widely agreed upon as some of the most significant: End Ordovician, Late Devonian, End Permian, End Triassic, and End Cretaceous. In addition, there is mounting evidence for two other mass extinctions of a similar scale: the current Holocene extinction event and the end-Ediacaran extinction at the start of the Phanerozoic eon.

These and a selection of other extinction events are outlined below. The articles about individual mass extinctions describe their effects in more detail and discuss theories about their causes.

1. Present day — the Holocene extinction event. 70% of biologists view the present era as part of a mass extinction event, possibly one of the fastest ever, according to a 1998 survey by the American Museum of Natural History. Some, such as E. O. Wilson of Harvard University, predict that humanity's destruction of the biosphere could cause the extinction of one-half of all species in the next 100 years. Research and conservation efforts, such as the IUCN's annual "Red List" of threatened species, all point to an ongoing period of enhanced extinction, though some offer much lower rates and hence longer time scales before the onset of catastrophic damage. The extinction of many megafauna near the end of the most recent ice age is also sometimes considered part of the Holocene extinction event. Some paleontologists, however, question whether the available data support a comparison with mass extinctions in the past.
2. 65 million years ago (Ma) — at the Cretaceous-Paleogene transition (the K/T or Cretaceous–Tertiary extinction event) about 17% of all families and 50% of all genera went extinct. (75% species). It ended the reign of dinosaurs and opened the way for mammals and birds to become the dominant land vertebrates. In the seas it reduced the percentage of sessile animals to about 33%. The K/T extinction was rather uneven — some groups of organisms became extinct, some suffered heavy losses and some appear to have been only minimally affected.
3. 205 Ma — at the Triassic-Jurassic transition (the Triassic-Jurassic extinction event) about 20% of all marine families (55% genera) as well as most non-dinosaurian archosaurs, most therapsids, and the last of the large amphibians were eliminated. 23% of all families and 48% of all genera went extinct.
4. 251 Ma — at the Permian-Triassic transition, Earth's largest extinction (the P/Tr or Permian-Triassic extinction event) killed 53% of marine families, 84% of marine genera, about 96% of all marine species and an estimated 70% of land species (including plants, insects, and vertebrate animals). 57% of all families and 83% of all genera went extinct. The "Great Dying" had enormous evolutionary significance: on land it ended the dominance of mammal-like reptiles and created the opportunity for archosaurs and then dinosaurs to become the dominant land vertebrates; in the seas the percentage of animals that were sessile dropped from 67% to 50%. The whole late Permian was a difficult time for at least marine life — even before the "Great Dying".
5. 360-375 Ma — Late Devonian extinction. Near the Devonian-Carboniferous transition at the end of the Frasnian Age in the later part(s) of the Devonian Period. A prolonged series of extinctions eliminated about 70% of all species. This extinction event lasted perhaps as long as 20 MY, and there is evidence for a series of extinction pulses within this period. 19% of all families of life and 50% of all genera went extinct.
6. 440-450 Ma — at the Ordovician-Silurian transition two Ordovician-Silurian extinction events occurred, and together are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that went extinct. 27% of all families and 57% of all genera went extinct.
7. 488 Ma — a series of mass extinctions at the Cambrian-Ordovician transition (the Cambrian-Ordovician extinction events) eliminated many brachiopods and conodonts and severely reduced the number of trilobite species.

The older the fossil record gets the more difficult it is to read it. This is because:

• Older fossils are harder to find because they are usually buried at a considerable depth in the rock.
• Dating fossils is difficult.
• Productive fossil beds are researched more than unproductive ones, therefore leaving certain periods unresearched.
• Prehistoric environmental disturbances can disturb the deposition process.
• The preservation of fossils varies on land, but marine fossils tend to be better preserved than their sought after land-based cousins.

Minor events

Minor extinction events include:

Precambrian

• End-Ediacaran extinction - circa 542 Ma

Cambrian Period

• End Botomian - circa 517 Ma
• Dresbachian

Silurian Period

• Ireviken event
• Mulde event
• Lau event
• End Silurian

Carboniferous Period

• Middle Carboniferous

Permian Period

• End Middle Permian

Jurassic Period

• Toarcian turnover circa 183 Ma
• End Jurassic

Cretaceous Period

• Aptian extinction circa 117 Ma

Paleogene Period

• Eocene-Oligocene extinction event

Neogene Period

• Cat gap
• Middle Miocene disruption circa 14.5 Ma

Quaternary Period (disputed)

• Quaternary extinction event