Living fossil

Living fossils have two main characteristics, although some have a third: • Living organisms that are members of a taxon that has remained recognizable in the fossil record over an unusually long time span. • They show little morphological divergence, whether from early members of the lineage, or among extant species. • They tend to have little taxonomic diversity. The first two are required for recognition as a living fossil; some authors also require the third, others merely note it as a frequent trait. Such criteria are neither well-defined nor clearly quantifiable, but modern methods for analyzing evolutionary dynamics can document the distinctive tempo of stasis. Lineages that exhibit stasis over very short time scales are not considered living fossils; what is poorly defined is the time scale over which the morphology must persist for that lineage to be recognized as a living fossil. The term living fossil is much misunderstood in popular media in particular, in which it often is used meaninglessly. In professional literature the expression seldom appears and must be used with far more caution, although it has been used inconsistently. One example of a concept that could be confused with "living fossil" is that of a "Lazarus taxon", but the two are not equivalent; a Lazarus taxon (whether a single species or a group of related species) is one that suddenly reappears, either in the fossil record or in nature, as if the fossil had "come to life again". In contrast to "Lazarus taxa", a living fossil in most senses is a species or lineage that has undergone exceptionally little change throughout a long fossil record, giving the impression that the extant taxon had remained identical through the entire fossil and modern period. Because of the mathematical inevitability of genetic drift, though, the DNA of the modern species is necessarily different from that of its distant, similar-looking ancestor. They almost certainly would not be able to cross-reproduce, and are not the same species. The average species turnover time, meaning the time between when a species first is established and when it finally disappears, varies widely among phyla, but averages about 2–3million years. A living taxon that had long been thought to be extinct could be called a Lazarus taxon once it was discovered to be still extant. A dramatic example was the order Coelacanthiformes, of which the genus Latimeria was found to be extant in 1938. About that there is little debate – however, whether Latimeria resembles early members of its lineage sufficiently closely to be considered a living fossil as well as a Lazarus taxon has been denied by some authors in recent years. Some living fossils are taxa that were known from palaeontological fossils before living representatives were discovered. The most famous examples of this are: • Coelacanthiform fishes (2 species) • Metasequoia, the dawn redwood discovered in a remote Chinese valley (1 species) • Glypheoid lobsters (2 species) • Mymarommatid wasps (10 species) • Eomeropid scorpionflies (1 species) • Jurodid beetles (1 species) • Soft sea urchins (59 species) All the above include taxa that originally were described as fossils but now are known to include still-extant species. Other examples of living fossils are single living species that have no close living relatives, but are survivors of large and widespread groups in the fossil record. For example: • Wollemia nobilis • Ginkgo biloba • Syntexis libocedrii, the cedar wood wasp • Dinoflagellates (typified on coccoid dinocysts: occasionally calcareous cell remnants) All of these were described from fossils before later being found alive. The fact that a living fossil is a surviving representative of an archaic lineage does not imply that it must retain all the "primitive" features (plesiomorphies) of its ancestral lineage. Although it is common to say that living fossils exhibit "morphological stasis", stasis, in the scientific literature, does not mean that any species is strictly identical to its ancestor, much less remote ancestors. Some living fossils are relicts of formerly diverse and morphologically varied lineages, but not all survivors of ancient lineages necessarily are regarded as living fossils. See for example the uniquely and highly autapomorphic oxpeckers, which appear to be the only survivors of an ancient lineage related to starlings and mockingbirds. == Evolution and living fossils ==