Fossil Remains of Organisms

remains and traces of the life activities of organisms of past geological periods. Many forms of the preservation of fossil remains are known. Plants, especially large ones, are not as a rule buried whole in rocks; there remain single leaves, fragments of branches, splinters of trunks, cones, fruits, separate seeds, spores, pollens, and, very rarely, flowers. When the organic matter is destroyed by oxidation processes (which happens most often), only imprints remain in the rocks; a carbonaceous film (cuticle) is sometimes preserved on these imprints. After its chemical processing, the cellular structure of the epidermis and stomata is visible under the microscope. When the bulky remains of plants decay (parts of trunks, cones), the surrounding sediment fills in the cavity that has formed, creating a natural mold. More rarely, the plant remains are replaced by various minerals (calcite, limonite, amorphous silica, siderite). These petrifactions preserve the anatomical structures of the plants, which make them especially valuable for science. Some algae have a siliceous (Diatomeae) or calcareous (Siphonales, Florideae) armor, which remains after the decomposition of the organic matter.

Whole carcasses of animals or their parts, which provide an idea of the skeletal structure as well as of the soft tissue, are preserved only in Anthropogenic deposits. Finds of well-preserved animal remains are known in regions of permafrost (mammoths, fish) and in ozokerite—a natural asphalt-like substance that sometimes forms considerable aggregates. Mummified remains also provide information about an organism. The skeletons of animals and individual parts of them are preserved considerably more often and rather well (even including the pigmentation on shells). Instances are known in which, as a result of the dissolution of skeletal remains in the rock, a cavity formed, which was then filled with various kinds of mineral substances. The resulting mold, which conveys the shape of the object, is called the external nucleus. In those cases where the internal cavity of a skeletal remain, for example, a shell, was first filled and then the shell dissolved, an internal nucleus is formed.

A rather frequent case of petrifaction occurs when all the pores of an organic remain become filled with mineral substances precipitated from aqueous solutions. The skeletal matter itself is sometimes replaced by these sediments, and its structure is preserved (so-called pseudomorphosis). The process of mineralization of organic fossil remains is called fossilization. Delicate, carbonaceous seams in rocks remain from invertebrates with chitinous skeletons. Sometimes various traces of the life activities of organisms are preserved: traces of crawling, walking, or boring; remains of predators’ meals; excrements of fossil animals; and the like. The intactness of the remains depends on the structure of the organism and conditions of its burial (solid massive skeletons or porous ones that are quickly permeated with mineral salts are better preserved) as well as on the rapidity of its burial in sediment and the degree of isolation from various destructive agents. The preservation of fossil remains is considerably better in basins of water than on dry land, where organisms are generally preserved only in conditions of permafrost or by mummification in deserts or special preservative media (ozokerite, amber).

The branch of paleontology and historical geology that studies the processes of formation of sites of fossil organisms is called taphonomy.