Fossil Animals

animals of past geological periods whose skeletons (fossil remains) are preserved in various rocks of sedimentary origin and sometimes even form these rocks (for example, shell limestone). The science that studies fossil organisms is called paleontology; the study of fossil animals is paleozoology. With rare exceptions, only a few isolated skeletal parts of fossil animals are preserved; for this reason a more or less complete picture of animals that existed at some time in the past may be obtained only by comparing them with related existing forms, that is, by the comparative anatomical method.

The oldest remains of fossil animals from metamorphosed Precambrian deposits show that most of the presently existing animal phyla probably already existed at that time, except for the chordates, whose remains are known only from the early Ordovi-cian period. Protozoans (one-celled animals) are represented in the fossil state by Foraminifera, which secreted a calcareous or agglutinated shell, and by Radiolaria, which had skeletons made of silica. Their skeletons, often barely visible with the naked eye, are first encountered in Cambrian deposits, sometimes forming thick strata of sedimentary formations (Fusulina limestone, radi-olarites, and others). Lower many-celled animals—sponges— appeared in the Proterozoic era; the closely related Archaeocya-tha flourished only in the early Cambrian period. Coelenterates are known from the early Paleozoic, in which they were represented by extinct forms: Stromatoporoidae (which outwardly resembled modern hydroid polyps), Tabulata, Tetracoralla (Rugosa), and others. Remains of fossil worms are very rare. Only traces of their life activity (pathways, calcareous tubes) are usually found, on the basis of which it is difficult to establish the structure of the worms themselves; only the isolated finds of forms closely related to modern ones in the Cambrian deposits of North America show the considerable antiquity of this group and their diversity as early as that time. Mollusks are known from the Lower Paleozoic, but developed especially in the Meso-zoic and Cenozoic eras. The most highly organized of these—the cephalopods—were represented in the Paleozoic by the Nautiloi-dae and closely related groups and also by the primitive Am-monoidae, which gave rise to the true ammonites, which abounded in Mesozoic seas. A large role during the Mesozoic was also played by the belemnites, which arose as early as the Paleozoic from the Bactritoidea. The modern cuttlefish and squids belong to this same branch of mollusks. Bryozoa and brachiopods are well-known from the Paleozoic; brachiopods, especially numerous during the Paleozoic, usually inhabited the littoral zone of the seas, sometimes forming shell shoals and great strata of limestone. Arthropods were represented in the ancient Paleozoic by groups of trilobites and eurypterids; other classes of arthropods appeared in the Devonian—insects, arachnids, and others—whose remains are generally rare but are found in some places in considerable numbers. The remains of echinoderms are represented in Lower Paleozoic deposits by primitive groups (cystoids, Carpoidae). It is probably from the cystoids or forms closely related to them that other classes of echinoderms arose, of which the crinoids (sea lilies) and blastoids were especially developed in the Paleozoic and the sea urchins and starfish in younger deposits of the Mesozoic and Cenozoic.

Vertebrate fossils are more rarely found. The oldest, early Paleozoic, representatives of vertebrates belong to the superclass Agnatha, to which the modern lampreys belong. True fish appeared somewhat later, represented in Devonian deposits by sharklike armored fish, dipnoans, crossopterygians, and lower actinopterygians. Bony fish arose only in the Mesozoic. The first remains of terrestrial quadrupeds, which still preserved some features that related them to the crossopterygians, are known from the Upper Devonian in Greenland. The remains of primitive stegocephalic amphibians (Stegocephalia) and reptiles (Cotylosauria, Theriodontia, and Deinocephalia) are widely distributed in Carboniferous and Permian deposits. Especially rich sites of such forms in the USSR are in the Permian deposits of the Severnaia Dvina River and the Trans-Volga Region. At the end of the Paleozoic and beginning of the Mesozoic, most of the stegocephalic amphibians became extinct. There was a great variety of reptiles during the Mesozoic: crocodiles, turtles, and especially dinosaurs. Aquatic reptiles (Ichthyosauria, Plesiosauria) were numerous, and there existed flying reptiles—pterosaurs (Rhamphorhynchus, Pteranodorn)—whose wing span ranged from 10 cm to more than 7 m. Simultaneously there already existed the first large-toothed, long-tailed birds and small, primitive mammals, which preserved in their organization the traces of their descent from reptiles. The vigorous development of birds and mammals began with the onset of the Cenozoic era. The mammals included such unique animals as the titano-theres, giant rhinoceroses (Indricotherium), mastodons, three-toed horses (hipparions), and saber-toothed tigers; some of these were the ancestors of modern animals. Many sites with the remains of these animals have been discovered in the USSR in Kazakhstan, the Ukraine, the Caucasus, the Crimea, and other places. Man appeared at the end of the Neocene.

The changes in the animal and plant worlds in time have made it possible to divide the history of the earth into geological periods. The study of the remains of animals that were once widely distributed and that changed relatively rapidly yields the key to determining the relative ages of sedimentary rocks, without which it is impossible to link the cross sections of different regions. The paleontological method is being extensively applied in geological prospecting, since it provides the basis for establishing the succession of strata and facilitates determination of the conditions of deposition of a given mineral. The confinement of animals to certain conditions of existence gives an idea of the conditions under which the deposition of sediments and the minerals associated with them occurred and makes it possible to reconstruct scenes of the earth’s past and its development. Finds of the remains of individual animals that deviated sharply from modern forms yield valuable material for elucidating the paths of development of large groups of modern animals (for example, the study of the Archaeopteryx from Jurassic deposits makes it possible to understand the origin and history of the development of the class Aves).