Living fossils

A living fossil is an extant taxon that closely resembles organisms otherwise known only from the fossil record. As a rule, to be considered a living fossil, the fossil species must be old relative to the time of origin of the extant clade. Living fossils commonly are species-poor lineages, but that is not a necessary condition. The term living fossil is not formally defined, but in scientific literature commonly appears to imply a bradytelic group, one tending to a slow rate of evolution.

In the popular literature "living fossil" commonly embodies radical misunderstandings such as that the organism somehow has undergone no significant evolution since fossil times, with practically neither morphological nor molecular evolution, but scientific investigations have repeatedly discredited any such claims.

Living fossils have three main characteristics. They:

Because such criteria are neither well-defined nor clearly quantifiable, the term "living fossil" necessarily is arbitrary in several respects and 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. No definitive criteria for what qualifies as a living fossil are possible, because for one thing, the term is an analogy rather than an unambiguous or fundamental biological concept, and accordingly it is often used by independent parties in mutually exclusive senses.

One example of a concept often 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.

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 2–3 million years. So 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.

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