Page:The New International Encyclopædia 1st ed. v. 15.djvu/281

PALEONTOLOGY. Faunas. T1u> fossils contained in a rock for- mation are in general indicative of the animal and vegetable life that lived during the period of time in which that rock was deposited. Suc- cessive faunas and lloras of variable expression have succeeded each other on the earth and have left their remains in the rocks that were forming during the periods of their existence. The large divisions of time are distinguislied by the pre- dominant tyi)es of life, as the Paleozoic era by invertebrates, the ilesozoio era by reptiles, etc., and the smaller ])eriods of time, represented by the .stages and substages, are characterized by particular genera and species. Thus the Helder- bergian .series is identiliable by the brachiopod genus Gypidula, and a division of that series, the C'oeymans limestone, is recognizable by the ])res- ence of (li/jiiihilii (/(ilralu a well-marked s])ecies that serves tor the identification of that hoiizon in many parts of America. Such a characteristic genus or species, which can l)e relied upon for the recognition of geological horizons or zcmcs, is called an index fossil, and its use is the result primarily of careful observation in the field. This empirical usage of index fossils is that adopted by geologists who have not been trained in the methods of paleontological research. There is a second and more im])ortant usage of index fossils based ujion a knowledge of the phylogeny of races of animals and plants. By recognition of the phyletic ])osition of a fossil the ex])ert t:v- ontologist can determine within close limits the relative age of the rock from which the fossil was obtained. IxnicENK AND Exotic Faunas. In a single Iiasin it is sometimes found that a series of similar faunas, evidently evolved each from its predecessor in this basin, is eventually succeeded at a higher horizon by a new fauna totally dif- ferent ironi those below. The lower faunas are the indig<'ne faunas, developed or evolved in this province through a long period of time and con- sisting of members nicely adapted to their en- vironment and to each other. The new fauna, generally associated with a change of sedimenta- tion, is called an alien or exotic fauna, and it has invaded this region in consequence of phys- iographic or climatic changes. It may find itself in congenial surroundings, and its species will then multiply and evolve, and if left undis- turlied it will in turn become the indigene fauna of the region. But if its new environment hap- pens to become uncongenial it may sufl'er partial extinction of its members or suppression of de- velopment of its individuals, and it will remain in the region for a short time only, to be suc- ceeded by another alien fauna or by a returning party of the original indigene fauna. The migra- tions of faunas are largely due to changes in the 'facies' conse(|uent upon physiographic and cli- matic changes. Facies Diovelopment and Bionomy. Facies is the combination of physical and biological char- acteristics exhibited by a geological formation at a particular point. These are determined at the present time by climate, depth, tides, nature of medium, distance from shore, etc. (see Dis- tribution OF Animals; Ecology; Fauna; Flora), and just as different types of facies are being developed in the ocean and on the land at the present time, so they have been developed during all past periods of geological history. It follows then that the deposits formed during any one period may be represented in dillcrent regions by littoral, sublittoral, abyssal, corallic, estua- rine, lacustrine, or terrestrial facies, and each of the.se facies will have its own distinctive fauna. Study of the life habits of modern organisms en- ables us to restore the habits of extinct forms of life, and we are able to portray with a considerable degree of correctness the conditions under which the ancient faunas lived, and consequently also to picture the physiography of past times. The large majority of fossiliferous rocks are of ma- rine origin, and hence a study of modern marine organisms is essential to a proper understanding of the bionomic conditions of the past. Fresh- water and terrestrial deposits are also claiming more attention than they formerly received; they with their jjcculiar faunas and floras occur chief- ly in the Jlesozoic and Cenozoic formations. Bioxoiiv. Marine organisms are broadly di- vided into ]ielagic, or those that inhabit the open sea, and littoral, those that live in the vicinity of the coasts. According to their modes of life they are divided into plankton, nekton, benthos. Plankton includes the majority of pelagic organisms that are more or less passively drifted about by the waves and currents of the ocean. Many organisms are ])lanktonic only during their larval stages and when adult become attached and adopt a benthonic life (nieroplankton) . Such are s|)onges, medusa', annelids, eehinoderms. brachiopods, bryozoans, most mollusks. and ( 'riis- taeea. Others are planktonie througlidut their lives, as Foraminifera. Kadiolaria. Siphonophora, Ctenopliora, Ch;rtognalh:i, Pteropoda. Hetero- poda, and some crustaceans. Planktonie organisms are generally widely distributed and their fossil remains occur principall.y in the sublittoral and abyssal facies, though they may be found in the littoral facies. Nekto.n includes the organisms that are ac- tive swimmers independent of storms, currents, and tides. Here are included fishes, most nia- lacostraean crustaceans, the dibranchiate cephalo- pods. and also the marine reptiles, including the extinct ichthyosaurs, plesiosaurs, Pythonomor- plia, and the cetaceans. Be.xtiios includes all forms which live at the bottom, and of which there are recognized two sub-groups: the sessile and the vagile benthos. The benthos includes by far the larger proportion of marine organisms that are found in a fossil state. Among them are the marine alga-, some foraminifers, sponges, hydroids, corals, ecliino- dernis, « onns, brachiopods, bryozoans, gastropods, lamellibranchs, cirripedes, tunicates, and many cephalojiods, such as the orthoceratites and belemnifes. Radial symmetry is most highly de- veloped in the sessile benthos (corals and eehino- derms). Littoral Facies includes deposits formed on the beach and in the shallow sea in the vicinity of the coast. They consist mostly of terrigenous materials ranging in size from fine mud, through sand to coarse conglomerate, and some kinds of organic deposits. The coarse beds contain few fossils, but those of finer grain abound in them. Fossils of the littoral facies comprise remains of the organisms that inhabited the shallow water and the region between tides: also those of tha beach, and others that have drifted in to shore from the open sea. The majority of the fauna is made up of benthonic forms and all the ani- mals and many of the plants have strong cal-