Any organism whose closest relatives are extinct and that was once itself thought to be extinct. An example is the coelacanth, a primitive fish that was common in the Devonian era, the first recent living specimen of which was discovered in 1938.
Geology and Earth Sciences
An organism that has persisted, essentially unchanged, since its first appearance. For example, Lingula (a brachiopod) has remained much the same since Ordovician times and Sphenodon (tuatara) since the early Mesozoic.