Noun
(evolutionary theory) 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. [from 1859]
The coelacanth and the dawn redwood are living fossils.
(evolutionary theory) Any living species which very closely resembles fossil relatives in most anatomical details.
Crocodiles are living fossils that haven’t changed their appearance much in millions of years.
Thus it is seen that a polyp is an animal of very simple structure, a living fossil that has not changed significantly for about half a billion years (per generally accepted dating of Cambrian sedimentary rock ). Source: Internet
With the discovery of their evolution, 'living fossil' no longer seems like an appropriate term to describe these unique creatures. Source: Internet