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Early analyses of evolutionary rates emphasized the persistence of a taxon rather than rates of evolutionary change.Contemporary studies instead analyze rates and modes of phenotypic evolution, but most have focused on clades that are thought to be adaptive radiations rather than on those thought to be living fossils.I would favor retiring the term ‘living fossil’ altogether, as it is generally misleading." 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’.
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").
These anomalous forms may almost be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition.
Queensland lungfish (Neoceratodus fosteri) is an example of an organism that meets this criterion, fossils identical to modern Queensland lungfish have been dated at over 100 million years making this species one of the oldest if not actually the oldest extant vertebrate species.
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).
Such studies, however, challenge only a genome stasis hypothesis, not the hypothesis of exceptionally low rates of phenotypic evolution.
The term was coined by Charles Darwin in his On the Origin of Species from 1859, when discussing Ornithorhynchus (the platypus) and Lepidosiren (the South American lungfish): ...
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.
The more technical term for such groups is "bradytelic".