Living fossil

A living fossil is an extant organism or lineage that resembles ancient fossil relatives and retains many of their traits. The label describes evolutionary persistence, not an absence of change.

Evolutionary stasis: Long-term persistence of similar traits within a lineage despite evolutionary change elsewhere. Living fossils are commonly identified by their apparent morphological stasis across geological time.
Coelacanth: A lineage of lobe-finned fishes represented today by deep-sea species in the genus Latimeria. Its fossil relatives were known long before living coelacanths were discovered in 1938.
Stabilizing selection: Natural selection that favors intermediate phenotypes and reduces variation around an existing trait value. It can preserve established anatomy when intermediate forms perform best in a stable environment.
Living fossil (paleontology): A fossil organism described as resembling an older fossil form while occurring in a younger geological interval. The phrase also has a paleontological use distinct from its usual application to extant organisms.
Morphological conservatism: The persistence of a relatively stable body form within a lineage over evolutionary time. The label usually rests on recognizable body-form resemblance, not genetic or ecological unchangedness.
Ginkgo biloba: The sole living species of a seed-plant lineage with a fossil record extending to the Mesozoic. Its distinctive leaves resemble fossil ginkgo leaves, although the lineage has continued to evolve.
Purifying selection: Natural selection that removes harmful genetic variants from a population. It can conserve functional genes even while neutral mutations accumulate elsewhere.
Molecular clock: A method that estimates evolutionary divergence times from accumulated molecular differences. Molecular dates can reveal deep divergence even when living forms look much like ancient fossils.
Molecular evolution: Evolutionary change in DNA, RNA, and protein sequences within populations and lineages. Some living fossils show substantial molecular change despite conserved visible anatomy.
Tuatara: A New Zealand reptile, the sole living representative of the order Rhynchocephalia. Its order was more diverse in the Mesozoic, making the tuatara a surviving lineage remnant.

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