The New International Encyclopædia/Fossil
FOSSIL (Fr. fossile, from Lat. fossilis, dug up, fossil, from fodere, to dig; connected with Corn. bedh, Welsh bedd, grave, OChurch Slav. bosti, Lith. badyti, to pierce). Any remains or trace of the form of animals or plants found buried by natural causes in deposits or rocks before the present era. The term was formerly applied to anything dug up out of the ground, and included minerals, prehistoric implements, etc. At the present day the word is used as an adjective in this latter sense, and also to designate anything pertaining to prehistoric times. Thus, we read of fossil salt, fossil rain-drops and mud-cracks, and fossil lakes, deserts, sea-beaches, and shores. The word petrifaction is often incorrectly employed as a synonym for fossil, although it properly designates only such organic remains as have been turned to stone, as described below. Fossils are the relics of the animals and plants that have lived upon the earth and in the waters of the earth during the long periods of its geological history, and study of their organization, occurrence, and relations to each other and to modern organisms constitutes the science of paleontology (q.v.). Fossils are naturally absent from all rocks of igneous and volcanic origin, and, on the other hand, they are present originally in nearly all rocks of sedimentary origin. From large masses of these latter they have been obliterated by chemical and physical changes, so that they are now seldom or sparingly found in metamorphic rocks. The processes by which organic remains have been preserved are grouped under the term fossilization. This includes entombment and the subsequent changes that have ensued. The place of entombment may be on land, in fresh water, or in the salt water of bays, seas, or oceans.
The degree of preservation of fossils varies greatly. In some few cases the flesh of animals has been preserved as if in an ice-box. Mammoth carcasses embedded in the frozen mud cliffs of Siberia for thousands of years had meat so fresh that it was eaten by the dogs of the exploring party. The most perfectly preserved fossils are undoubtedly those insects found in the Tertiary amber of the Baltic Provinces, where the form, structure, and colors are retained intact. Then we find shells preserved in the rocks with their original organic matter replaced by some mineral, usually silica, or perhaps barytes, pyrites, or even zinc blende. Such replacements rightly receive the name of ‘petrifactions.’ In other cases we find cavities in rocks, the sides of these retaining impressions of the outer and inner surfaces of shells which have been dissolved and destroyed by percolating waters. These ‘molds’ are sometimes filled with calcite, or quartz, or other mineral matter, and then we have ‘casts’ of the original organic forms. The study of these molds is puzzling to the beginner, because of the multiplication of forms so caused. A single shell like a limpet, if preserved in the rocks, may present four different aspects as a fossil—the outer and inner surfaces of the shell itself, and the molds of each of these. The mold of the outer surface may pull away such delicate spines as may ornament the shell, and for this reason molds should always be carefully collected and treated with acid, after which the impression of the original shell surface is often shown with the utmost fidelity to detail. Another class of fossils consists of the impressions or trails made by animals crawling over the bottom of the water or over the beach, and also of burrows or casts of burrows that served as dwelling-places or passageways for worms, crustaceans, etc. The study of the footprints of reptiles and supposed birds, which are so abundant on the surfaces of the Jurassic sandstones of Massachusetts and Connecticut, was named ichnology by E. Hitchcock, who described and figured a host of such impressions. Similar footprints are found in rocks of shallow-water origin of Mesozoic and Tertiary age all over the world.
The parts of animals likely to be preserved are always those that resist longest the destructive agencies that may attack them both before and after their entombment. The soft parts are seldom preserved, and often also the hard parts are destroyed. Because of this certain groups of animals are represented by insignificant parts of their anatomy, which, though of great importance to the paleontologist, are usually laid aside by the zoölogist as of trivial interest. Thus, the presence of sponges in certain formations is demonstrated by their isolated spicules; holothurians are recognized by their minute calcareous plates and anchors; worms by their teeth and dwelling-tubes; dibranchiate cephalopods by their internal shells; and many fish by their teeth, ear-bones (otoliths), spines, and dermal scales.
The manner of entombment of fossils varies greatly. In many cases the shells of mollusks have been dead a long time, and have become incrusted with polyzoans and corals before they were entombed. In other cases they were washed along the shore and broken and worn by the waves so that now in fragmentary condition they form ‘shell limestones.’ Among the crustacean fossils we find those that were killed suddenly, perhaps by some change in the temperature of the water, in which case their remains are usually well preserved. In some rocks of fresh-water or estuarine origin certain layers are covered with the remains of fish. These evidently lived in shallow pools that were either dried up suddenly or became so heated by the sun that the fish were killed, soon to be covered by sediment. Such conditions are frequent in the Catskill, Old Red Sandstone, and Jurassic formations. Myriads of insects of Tertiary time became entangled in the soft gum of coniferous trees, and are now preserved in the amber of the Baltic and the fossil resins of Africa and New Zealand.
The old ideas regarding fossils were curious and often fantastic. A few of the Greek and Roman philosophers had well-defined ideas of their true nature as entombed animals and plants that had once lived in the sea and upon the earth; but the majority of early writers attached to them some fanciful or supernatural origin. Thus they were explained as due to the vis plastica, or creative force that formed living things out of inorganic materials; as sports of nature; as due to some peculiar fermentative process in the earth, or as originating in some unknown influence of the stars. Another hypothesis, maintained for centuries, and even now persisting in uneducated communities, explains fossils as the remains of animals and plants washed up on the land, and there stranded by the waters of the Noachian deluge. These erroneous ideas persisted in the face of true explanations by some observers, until the beginning of 1800, when slowly the true nature of fossils and their relations to the rocks in which they are entombed began to be more universally understood, and at last during 1800 to 1840 there were laid the foundations of the science of paleontology.
For further information on the early ideas regarding fossils, consult: Lyell, Principles of Geology, vol. i. (New York, 1872); Von Zittel, History of Geology and Palæontology, translated by Ogilvie-Gordon (London and New York, 1901). For modes of fossilization and the relations between fossils and the rocks containing them, consult: Geikie, Text-Book of Geology (London and New York, 1893); White, “The Relations of Biology to Geological Investigation,” in Smithsonian Institution Report of the United States National Museum for 1892 (Washington, 1894); Marr, Principles of Stratigraphical Geology (Cambridge, 1898); Schuchert, “Directions for Collecting and Preparing Fossils,” in Smithsonian Institution United States National Museum, Bulletin No. 39 (Washington, 1895). See also Paleontology; Paleobotany; Geology.