a fossil — remains or impressions of ancient life preserved in rock and studied in paleontology/geology.
博物館で恐竜の化石を間近で見た。
I saw dinosaur fossils up close at the museum.
この化石は何百万年も前の生物のものだ。
This fossil belongs to a creature from millions of years ago.
海底の堆積層から化石が見つかった。
Fossils were found in sedimentary layers under the sea.
petrifaction; fossilization — the process or state of something becoming fixed, rigid, or outdated (often used as 化石化する).
慣習が化石化して変化を拒んでいる組織もある。
Some organizations have customs that have fossilized and resist change.
古い技術は市場から徐々に化石化していった。
Old technologies gradually became obsolete (fossilized) in the market.
化石になるまでには何百万年もかかる。
It takes millions of years for something to become a fossil.