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 Mass extinctions are ecological disasters. Yet they also create evolutionary opportunities by removing once-dominant groups. Some biologists conclude that humans owe our present dominance to a mass extinction -- the K/T event that saw the end of the dinosaurs and cleared the way for mammals to diversify into all the many ecological roles they now occupy. Also, it should be kept in mind that mass exinctions probably account for the disappearance of only five percent of extinct species, the remainder having disappeared through the constant winnowing of natural selection and other continuous processes. Mass extinctions affect most major taxonomic classes present at the time — birds, mammals, reptiles, amphibians, fish, invertebrates and other simpler life forms. They may be caused by one or both of: 1. Extinction of an unusually large number of species in a short period. 2. A sharp drop in the rate of speciation. 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. Since life began on Earth, a number of major mass extinctions have greatly exceeded the background extinction rate present at other times. Though there were undoubtedly mass extinctions in the Archean and Proterozoic, it is only during the Phanerozoic Eon that the emergence of bones and shells in the evolutionary tree has provided a sufficient fossil record from which to make a systematic study of extinction patterns. Through evolution, new species arise through the process of speciation — where new varieties of organisms arise and thrive when they are able to find and exploit an ecological niche — and species become extinct when they are no longer able to survive in changing conditions or against superior competition. A typical species becomes extinct within 10 million years of its first appearance, although some species, called living fossils, survive virtually unchanged for hundreds of millions of years. Only one in a thousand species that have existed remain today. Prior to the dispersion of humans across the earth, extinction generally occurred at a continuous low rate, mass extinctions being relatively rare events. Starting approximately 100,000 years ago, and coinciding with an increase in the numbers and range of humans, species extinctions have increased to a rate unprecedented since the Cretaceous-Tertiary extinction event. This is known as the Holocene extinction event and is at least the sixth such extinction event. Some experts have estimated that up to half of presently existing species may become extinct by 2100.
There are differing estimates of the number of major mass extinctions in the last 540 million years, ranging from as few as five to more than twenty discrete extinctions. These differences stem primarily from the threshold chosen for describing an extinction event as "major"; and, what set of data one chooses as the best measure of past diversity. The classical "Big Five" mass extinctions identified by Raup and Sepkoski (1982) are widely agreed upon as some of the most significant: End Ordovician, Late Devonian, End Permian, End Triassic, and End Cretaceous. These and a selection of other extinction events are outlined below: 
1. 488 million years ago — 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.
2. 444 million years ago — at the Ordovician-Silurian transition two Ordovician-Silurian extinction events occurred, and togther these 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. 3. 360 million years ago — near the Devonian-Carboniferous transition (the Late Devonian extinction) a prolonged series of extinctions led to the elimination of about 70% of all species. This was not a sudden event — the period of decline lasted perhaps as long as 20 million years, and there is evidence for a series of extinction pulses within this period. 4. 251 million years ago — at the Permian-Triassic transition Earth's worst mass 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). The "Great Dying" had enormous evolutionary significance: on land it ended the dominance of the 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 of the late Permian was a difficult time for at least marine life - even before the "Great Dying", the diagram shows a late-Permian level of extinction large enough to qualify for inclusion in the "Big Five". 5. 200 million years ago — at the Triassic-Jurassic transition (the Triassic-Jurassic extinction event) about 20% of all marine families as well as most non-dinosaurian archosaurs, most therapsids, and the last of the large amphibians were eliminated. 6. 65 million years ago — at the Cretaceous-Paleogene transition (the K/T or Cretaceous-Tertiary extinction event) about 50% of all species became extinct. It has great significance for humans because it ended the reign of the dinosaurs and opened the way for mammals to become the dominant land vertebrates; and in the seas it reduced the percentage of sessile animals again, 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 got off relatively lightly. 7. Present day — the Holocene extinction event. A 1998 survey by the American Museum of Natural History found that 70% of biologists view the present era as part of a mass extinction event, possibly one of the fastest ever. Some, such as E. O. Wilson of Harvard University, predict that man'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 a part of the Holocene extinction event.  Insect bite marks in ancient leaf fossils are shedding new light on how nature bounced back after an asteroid impact killed off the dinosaurs and much of life on Earth 65 million years ago. Plant and insect biodiversity is strongly linked today: Where there are many types of plants, there are many insects to eat them. But after the mass extinction, the devastated plant and insect populations might not have been so in sync, according to a new study. "The recovery from a mass extinction was more interesting and chaotic than we thought," said study leader Peter Wilf, a paleontologist at Pennsylvania State University. Clearing the Slate The demise of the dinosaurs after this event, known as the K-T extinction, later brought about a restructuring of the animal world and the rise of mammals. But it initially marked the end of the biologically rich Cretaceous period and the beginning of the more anemic Paleocene epoch. Most Paleocene fossil sites show both low numbers of plants and insects. But scientists have discovered two sites in the western United States that show remarkable biodiversity—one in plants, and one in insects. Paleontologists looked at leaf fossils for signs of the bite marks left by different species of insects.
Wilf and his colleagues suspect that the plants were able to flourish because the ancient climate at the site was warm and wet. But the leaves unexpectedly showed few signs of insect predators. Wilf thinks the leaves were too hardy for insects to gnaw on. "It probably wasn't a good place for them to get started again," he said. A New Leaf... But what really surprised paleontologists were the fossils at another site in Mexican Hat, Montana that showed just the opposite relationship. The leaf fossils found there were more typical of those discovered at other Paleocene sites. But the insect bites left on them indicated a teeming insect population. Wilf and his team haven't found any other sites that show evidence of such a robust insect population. "We don't know where [the insects] went or where they came from," he said. Wilf believes these unusual fossil records show biodiversity recovery is more interesting than previously thought. "This may be a general pattern in the fossil record; it's certainly an interesting pattern in the fossil record," he said. "It's something that people can look for."
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