Living fossil

A living fossil is a living species (or clade) of organism that closely resembles species otherwise known only from the fossil record. Normally, to be considered a living fossil, the fossil species must be old, relative to the age of the (living) clade. In several cases, living fossils are species-poor lineages, but this is not always the case. The term is not formally defined, but is commonly used in the scientific literature to mean a group characterized by an exceptionally low rate of evolution. The more technical term for such groups is "bradytelic". The term living fossil is sometimes used in the popular literature, as if to imply a lack of evolution, and has occasionally also been taken to imply very low rates of molecular evolution, but scientific investigations have repeatedly discredited claims that these species do not change at all, as well as other misconceptions about living fossils.

Living fossils have three main characteristics: (1) they exhibit notable longevity; (2) they demonstrate little morphological divergence from early members of the lineage as well as low morphological diversity within the group and (3) they often also exhibit little taxonomic diversity.

Living fossils are not equivalent to a "Lazarus taxon", which 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"). In contrast, a living fossil is a species or lineage that has undergone exceptionally little change over a long time (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. It would also be considered a living fossil if it closely resembles the earliest members of its lineage. Coelacanths disappeared from the fossil record some 80 million years ago (upper Cretaceous) and if it does exhibit exceptionally low evolutionary rates, then it would qualify as both a Lazarus taxon and as a living fossil.

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). Dinoflagellates include also such examples, which have been first described as fossil-taxa (being typified on coccoid, occasionally calcareous cell remnants: dinocysts), but stratigraphically range until today.

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