living fossil

noun2

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.

   • The coelacanth and the dawn redwood are living fossils.
2. •
   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.