The idea of individual species becoming extinct is quite familiar; indeed it is a rather sad indictment of our stewardship of the planet that we are all too familiar with extinction. But, in fact, extinction is a rather complex phenomenon. At one end of the continuum we have the notion of a population of organisms evolving into something else. Here, the disappearance of the original phenotype might be accomplished by nothing more than natural turn-over of the generations (anagenesis).
At the opposite end of the spectrum, we have the mass extinctions, where huge proportions of the earth biota disappear more or less simultaneously, within an interval that is, in some sense, short. At least some of the more sensational explanations for these phenomena require the wholesale killing of individual organisms.
Between these two extremes we have a range of possibilities, further complicated by the vagaries of the fossil record and our imperfect interpretation thereof. And always, even in the case of the KT event which cannot be "explained away" in its entirety by meteorite impact, there is the enigma of underlying cause.
Working Definitions
For our purposes, extinction of a single taxon ?whether a species or higher level taxon ?is accomplished when the last representative of that taxon dies. Of course we could also distinguish the point at which the organism was no longer able to reproduce (e.g. when the population density of a dioecious species drops below its reproductive threshold) but any such subtlety is pointless: The fossil record is not a good witness to the fate of individuals, so our notions of extinction are necessarily approximate. In practice, to the paleontologist, extinction is the last (most recent) occurrence of an identifiable fossil.
We have more difficulty with the concept of mass extinctions. Ward 2000 (pp. 6-7) offers the definition that mass extinction events are geologically short intervals of intense species extinction. However, this definition admits events such as the decimation of the South American marsupial fauna following the establishment of a land bridge with North America in the late Pliocene, which is almost certainly not his intention. Of course such events are also interesting extinction phenomena. But to properly capture the idea of Mass Extinction, it also seems necessary that a mass extinction should be global in extent and involve participants?from widely diverse taxonomic groups.
http://www.peripatus.gen.nz/paleontology/extinction.html
At the opposite end of the spectrum, we have the mass extinctions, where huge proportions of the earth biota disappear more or less simultaneously, within an interval that is, in some sense, short. At least some of the more sensational explanations for these phenomena require the wholesale killing of individual organisms.
Between these two extremes we have a range of possibilities, further complicated by the vagaries of the fossil record and our imperfect interpretation thereof. And always, even in the case of the KT event which cannot be "explained away" in its entirety by meteorite impact, there is the enigma of underlying cause.
Working Definitions
For our purposes, extinction of a single taxon ?whether a species or higher level taxon ?is accomplished when the last representative of that taxon dies. Of course we could also distinguish the point at which the organism was no longer able to reproduce (e.g. when the population density of a dioecious species drops below its reproductive threshold) but any such subtlety is pointless: The fossil record is not a good witness to the fate of individuals, so our notions of extinction are necessarily approximate. In practice, to the paleontologist, extinction is the last (most recent) occurrence of an identifiable fossil.
We have more difficulty with the concept of mass extinctions. Ward 2000 (pp. 6-7) offers the definition that mass extinction events are geologically short intervals of intense species extinction. However, this definition admits events such as the decimation of the South American marsupial fauna following the establishment of a land bridge with North America in the late Pliocene, which is almost certainly not his intention. Of course such events are also interesting extinction phenomena. But to properly capture the idea of Mass Extinction, it also seems necessary that a mass extinction should be global in extent and involve participants?from widely diverse taxonomic groups.
http://www.peripatus.gen.nz/paleontology/extinction.html
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