Lazarus taxon

A Lazarus taxon is a taxonomic group that disappears from the known record and later reappears. In palaeontology, the phrase commonly describes a gap in fossil occurrences: fossils of a group are found in older rocks, are missing from a later interval, and are found again in younger rocks. It can also refer to groups whose living representatives are discovered after scientists had considered them extinct. These usages are related but not identical. A gap in fossils does not by itself demonstrate that the organisms vanished from the living world; nor does the modern discovery of a living member mean that an ancient species has returned unchanged.

Fossilization is uneven. Organisms must die in settings where remains can be buried, preserved, exposed by erosion and eventually found. A group's habitat may leave few accessible fossils, or its range may be small and poorly sampled. The University of California Museum of Paleontology uses coelacanths to illustrate these effects: living coelacanths are associated with deep-water environments around volcanic islands, while sediments from those settings can be difficult to preserve and expose on land. In the fossil record, a missing interval therefore may mark a gap in evidence rather than the true duration of a group's existence. In some cases, especially after mass extinctions, genuine population declines and rarity also contribute to intervals without fossil occurrences.

Coelacanth fossils are known from ancient rocks, but no younger fossils had been identified after the Cretaceous before a living specimen was found in South Africa in December 1938. Museum curator Marjorie Courtenay-Latimer recognized the unusual fish and contacted ichthyologist J. L. B. Smith, who identified it as a coelacanth. This discovery established that coelacanths survived beyond the youngest previously known fossils; it did not mean that the same ancient species had been presumed extinct and then re-evolved. Fossil ages and estimates of the gap differ among accounts, and the 1938 catch is best understood as a discovery by science, not the first knowledge of the fish: local fishers in the Comoros already knew coelacanths by the name gombessa.

The term is also used for fossil groups that disappear from geological sections around a mass extinction and appear again in younger layers. A 1999 study of the end-Permian and late-Triassic crises examined whether such patterns could be explained by the distribution of fossil-bearing sites or refuges. Its authors found that neither explanation accounted for the cases they assessed, and suggested that extreme rarity after the crises was a likely factor. The phrase thus describes an observed pattern, not a single cause: preservation gaps, sampling, migration and real changes in abundance must be evaluated for each example.

A later fossil or living population changes the evidence available to researchers and can extend a group's known history. It does not automatically settle how closely the later representatives are related to earlier forms, whether every intervening population persisted locally, or why the record was discontinuous. Researchers compare anatomy, stratigraphy, geographic distributions and, where available, genetic data. Lazarus taxa are a reminder that the fossil record is an incomplete sample of past life, while also providing a framework for testing extinction and survival hypotheses.