living fossil

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An organism, such as the coelacanth or ginkgo, that is similar in appearance to its fossilized distant ancestors and usually has no extant close relatives.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any species discovered first as a fossil and believed extinct, but which is later found living.
• noun Any living species which very closely resembles fossil relatives in most anatomical details.