Wiktionary
n. 1 Any species discovered first as a fossil and believed extinct, but which is later found living; an organism that has remained unchanged over geological periods 2 Any living species which very closely resembles fossil relatives in most anatomical details.
Wikipedia
A living fossil is a living species (or clade) of organism that closely resembles species otherwise known only from the fossil record. Normally, to be considered a living fossil, the fossil species must be old, relative to the age of the (living) clade. In several cases, living fossils are species-poor lineages, but this is not always the case. The term is not formally defined, but is commonly used in the scientific literature to mean a group characterized by an exceptionally low rate of evolution. The more technical term for such groups is "bradytelic". The term living fossil is sometimes used in the popular literature, as if to imply a lack of evolution, and has occasionally also been taken to imply very low rates of molecular evolution, but scientific investigations have repeatedly discredited claims that these species do not change at all, as well as other misconceptions about living fossils.
"Living Fossil" is a science fiction story on the concepts of human extinction and future evolution by L. Sprague de Camp. It was first published in the magazine Astounding Science-Fiction for February, 1939. It first appeared in book form in the anthology A Treasury of Science Fiction (Crown Publishers, 1948); it later appeared in the anthologies Gates to Tomorrow (Atheneum, 1973), and The SFWA Grand Masters, Volume 1 ( Tor Books, 1999). The story has been translated into Danish, Swedish and Italian.
It is perhaps the earliest work of fiction dealing with the afterwards popular theme of humanity being replaced by other intelligent primates in the future, later epitomized by Pierre Boulle's Planet of the Apes.
Usage examples of "living fossil".
Someday he, too, is going to be a lobster, swimming around and waving his pincers in a cyberspace so confusingly elaborate that his uploaded identity is cryptozoic: a living fossil from the depths of geological time, when mass was dumb and space was unstructured.
But the assumption that Australia has a 'living fossil' fauna is hard to justify.
Forever its petrified prisoners whispered their crazed memories of the greed or fierce desire or yearning which had led them into that cave, and which had spurred the living fossil to open its stone door.
Pre-cellular life, of course, more primitive than the living fossil blue-green algae back home.
She had viewed him as a living fossil, an annoying survival from an earlier, less evolved era.
Even if the brags were true, which Alex gravely doubted, a great seducer would be nothing but a living fossil on the Moon, under the conditions de Tohil Vaca had specified.