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A living fossil is a living species (or clade) of organism that appears to be similar to a species otherwise known only from fossils, typically with no close living relatives. Normally the similarity is only by an imagined physical resemblance, between two different species, one extinct, the other extant. It is an informal non-scientific term, mostly used in the lay media. These species have survived major extinction events, and generally retain low taxonomic diversities. A species that successfully radiates (forming many new species after a possible genetic bottleneck) has become too successful to be considered a ''living fossil''. ==Overview== A subtle difference is sometimes made between a "living fossil" and a "Lazarus taxon". A Lazarus taxon is a taxon (either one species or a group of species) that suddenly reappears, either in the fossil record or in nature (i.e., as if the fossil had "come to life again"), while a living fossil is a species that (seemingly) has not changed during its very long lifetime (i.e., as if the fossil species has always lived). The average species turnover time (the time a species lasts before it is replaced) varies widely among the phyla, but averages about 2–3 million years. So, a living species that was thought to be extinct (e.g., the coelacanth, ''Latimeria chalumnae'') could be called a Lazarus taxon instead of a living fossil. Coelacanths disappeared from the fossil record some 80 million years ago (upper Cretaceous). Of course, species do not just appear out of nothing, so all living Lazarus species (excluding disappearing and reappearing red list species) are nonetheless considered living fossils, if it can be shown they are not Elvis taxa. Lastly, the term "living fossil" misleadingly appears to suggest that the organism has somehow "stopped evolving". Tadpole shrimp (''Triops'') are often presented as "living fossils", though a genetic study released in 2013 demonstrated the radiating diversity of notostracans in the genera ''Triops'' and ''Lepidurus'': "Our work shows that organisms with conservative body plans are constantly radiating, and presumably, adapting to novel conditions... I would favor retiring the term ‘living fossil’ altogether, as it is generally misleading," the author observed to the popular scientific press.〔("Diversification in Ancient Tadpole Shrimps Challenges the Term 'Living Fossil'" ''Science Daily'' ) accessed 2 April 2013; "The Falsity of 'Living Fossils'", ''The Scientist'' accessed 2 April 2013.〕 It should be noted that several recent studies pointed out that the morphological conservatism of coelacanths is a myth that is not supported by actual paleontological data. In addition, it was shown recently that studies concluding that a slow rate of molecular evolution is linked to morphological conservatism in coelacanths are biased by the ''a priori'' hypothesis that these species are ‘living fossils’. Accordingly, the genome stasis hypothesis is challenged by the recent finding that the genome of the two extant coelacanth species L. chalumnae and L. menadoensis contain multiple species-specific insertions, indicating transposable element recent activity and contribution to post-speciation genome divergence. Some living fossils are taxa that were known from fossils before living representatives were discovered. The most famous examples of this are the coelacanthiform fishes ''Latimeria chalumnae'' and ''Latimeria menadoensis'' and the dawn redwood, ''Metasequoia'', discovered in a remote Chinese valley. Others include glypheoid lobsters, mymarommatid wasps, eomeropid scorpionflies, and jurodid beetles, all of which were first described from fossils, but later found alive (2 species, 10 species, one species, and one species respectively). Others are a single living species that has no close living relatives, but is the survivor of a large and widespread group in the fossil record. Perhaps the best-known example is ''Ginkgo biloba'', though there are others, such as ''Syntexis libocedrii'' (the cedar wood wasp).The dinoflagellate ''Dapsilidinium pastielsii'' is also such an example, which was first described as a fossil dinocyst from the Eocene. Note that just because a living fossil is a surviving representative of an archaic lineage does not necessarily require that it retains ''all'' of the "primitive" features (plesiomorphies) of the lineage it is descended from; that is, they may possess one to many derived features (autapomorphies), that have evolved since the time of their lineage's divergence. All that is required is that they can be unambiguously assigned to an otherwise extinct lineage (rarely are they ''identical'' to the fossil forms). See for example the uniquely and highly autapomorphic oxpeckers, which are not "true" living fossils (as no fossils are known yet) but nonetheless appear to be the only survivors of an ancient lineage related to starlings and mockingbirds. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「living fossil」の詳細全文を読む スポンサード リンク
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