living fossil

Noun

living fossil (plural living fossils)

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.

   • 2001, "Samurai Jack: The Premiere Movie — Part II: The Samurai Called Jack", season 1, episode 2 of Samurai Jack

     Angus: Impossible! If what you say is true, he'd have to be thousands of years old.

     Rothie: Astounding! The age-o-meter dates your particles all the way back to 25 B.A., twenty-five years before Aku enslaved the Earth. You, my friend, are a living fossil.

     Jack: So the question is not where I am but when I am. […] The spell Aku cast must have ripped me from my own time and flung me into the distant future.

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.