Fossil

Fossil, a term, originally including mineral specimens, now restricted to "any body, or the traces of the existence of any body, animal or vegetable, buried in the earth by natural causes." Thus worm-burrows, the cast, external or internal, of a shell, footprints, or the amber from a tree, are as much fossils as the bones, stem, or leaves themselves. No question of age enters into the definition, fossils being the remains either of extinct organisms or of those now living, which are termed "recent." Nor is there now any necessary notion of mineral change implied in the term. The original substance may be wholly or partly preserved, as in the case of the mammoths (q.v.) found frozen entire in Siberia or the little altered chitinous coverings of Paleeozoic trilobites and scorpions; or only the external or internal form may be retained with no internal structure; or the original substance may have been molecularly replaced by true petrifaction, so that the minutest internal structure is preserved, as in silicified wood. The expression "natural causes" is introduced into the definition to exclude the intentional burial of remains by man and so to delimit the province of archaeology from that of geology. The study of fossils is termed palccontoloyy (q.v.). Organic remains are seldom preserved as fossils unless speedily protected from air. Birds, insects, and other animals, or plants, that perish on dry land stand little chance, therefore, of preservation; whilst peat-moss vegetation, and aquatic animals, together with those entombed in caves or by mineral springs, are far better represented in the "geological record." Again, it is, as a rule, only hard parts that are preserved, such as the woody stems, leaf-veins and harder fruits of plants, and the calcareous shells, chitinous integuments, bones and teeth of animals. The chief mineral substances that replace the organic matter in fossilisation are calcite, aragonite, silica, mostly as opal, marcasite, chalybite, and less commonly glanconite, vivianite, native copper, fluor, etc. Fossils are important from three points of view: (1) Biologically, as evidencing the former existence of organisms forming series of forms ancestral to those now existing; (2) geographically, as evidencing former differences in the distribution of land and water and climate; and (3) geologically, as affording the chief means for the identification of stratified rocks in distant localities and tracing their chronological succession. [Geology, Homotaxis.]