Through study of the fossil record, paleontologists can frequently identify when an ancient species or taxon (a grouping of related organisms) first emerged in evolutionary history. (Paleontologists often prefer the term taxon over species because it is impossible to test whether populations of extinct organisms were reproductively isolated as are living species.) When a taxon disappears from the fossil record, the usual presumption is that it has gone extinct. Occasionally, however, a taxon thought to be extinct is found to be extant again during a later era (it may then become truly extinct during this later period). In extreme cases, organisms believed to be extinct may even show up among today's living biota. In acknowledgment of the New Testament biblical story in which Jesus raised Lazarus from the dead, biologists coined the term "Lazarus effect" to refer to such rediscoveries, and "Lazarus taxon" to describe a species that has been seemingly resurrected from the fossil dead. See also: Animal evolution; Extinction; Extinction (paleontology); Extinction and the fossil record; Fossil; Living fossils; Macroevolution; Paleontology; Systematics
Most gaps in the fossil record that produce a Lazarus effect are a few thousand years in length, but notable cases have exhibited gaps of millions of years. For example, the coelacanth fish (genus Latimeria) was rediscovered in 1938 after a gap in the fossil record of 66 million years, whereas the dawn redwood (Metasequoia glyptostroboides) was presumed extinct for 2–5 million years until being identified as a living species in the 1940s. Even more startling, because this phenomenon has been rarely documented in mammals, was the case of a squirrel-like rodent called the Laotian rock rat (Laonastes aenigmamus). Investigators discovered this living species in 2005. In 2006, however, paleontologists pointed out that it appeared to be identical to a fossil animal that had vanished 11 million years ago. The reasons why Lazarus taxa disappear from the fossil record for such lengthy spans of time are being actively investigated. The lack of any fossil evidence or remnants in the intervening years is likely the result of a combination of factors, including the random anomalies routinely associated with the fossil record, and the ways in which the climates and environments that these organisms inhabited may not be conducive to fossil preservation. See also: Coelacanthiformes; Rodentia; Taphonomy
