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1610s, "any thing dug up;" 1650s (adj.) "obtained by digging," from French fossile (16c.), from Latin fossilis "dug up," from fossus, past participle of fodere "to dig," from PIE root *bhedh- "to dig, pierce."
Restricted noun sense of "geological remains of a plant or animal" is from 1736; slang meaning "old person" first recorded 1859. Fossil fuel (1835) preserves the earlier, broader sense.
The remains or imprint of an organism from a previous geologic time. A fossil can consist of the preserved tissues of an organism, as when encased in amber, ice, or pitch, or more commonly of the hardened relic of such tissues, as when organic matter is replaced by dissolved minerals. Hardened fossils are often found in layers of sedimentary rock and along the beds of rivers that flow through them. See also index fossil, microfossil, trace fossil.
The evidence in rock of the presence of a plant or an animal from an earlier geological period. Fossils are formed when minerals in groundwater replace materials in bones and tissue, creating a replica in stone of the original organism or of their tracks. The study of fossils is the domain of paleontology. The oldest fossils (of bacteria) are 3.8 billion years old.
Note: The term is used figuratively to refer to a person with very old-fashioned or outmoded viewpoints: “That old fossil thinks that men should wear suits at the theater!”