Fossils are remains of prehistoric organisms. Preserved by burial under countless layers of sedimentary material, they are a record of the history of life beginning approximately 3.5 billion years ago, the study of which is called paleontology.
Some fossils are abundant in the strata of the Earth's crust. The chalk cliffs of Dover and the Niobrara Chalk of Kansas are composed of complex platelets of algae, so small that millions fill a cubic millimeter. Shells of invertebrate marine animals, such as brachiopods, bryozoans, clams, snails, corals, and echinoderms, are preserved in many beds of limestone, and the bones and teeth of vertebrates are sometimes so numerous that they form deposits called "bone beds." In other sedimentary rock, such as the majority of the world's red beds, shells and bones are rarely found, although tracks and burrows may be abundant. Fossils are rarely found in sediments of precambrian time, although some rocks of the latter part of that era have yielded a moderately diverse assemblage known as the ediacaran fauna.
KINDS OF FOSSILS
Entire or partial bodies of organisms are called body fossils. In contrast, marks left in rock by the activities of organisms are called trace fossils. These include artifacts, burrows, feces, tracks, and trails. Fossils so small as to be measured in micrometers or millimeters are called microfossils, and those in the centimeter and meter range are called megafossils. Microfossils are studied in the field of micropaleontology. A related study of microscopic spores, pollen grains, and cysts extracted from sediment by hydrofluoric-acid treatment is called palynology (paleobotany). Chemical paleontologists study the fossil record of organic macromolecules, the presence of which in rocks may reveal the existence of certain groups of organisms in the distant past.
HOW AND WHERE FOSSILS FORM
The Earth is teeming with life, all of it searching for food--organic compounds of hydrogen and carbon. As a result, very little digestible organic matter escapes destruction, and indigestible skeletal material, such as shells, bones, and teeth, has a much better chance of burial and preservation. Shell material is typically composed of calcium carbonate, as are mollusk shells; teeth and bones are composed of calcium phosphate; sponge spicules, diatom frustules, and radiolarian tests are composed of opaline silica. The highly indigestible organic jackets of spores and pollen grains also commonly escape destruction. Such materials form most of the body fossils common in layered rocks.
Much rarer are accumulations of sediment in settings from which scavengers are excluded and in which the bodies of plants and animals, carried in from outside, may retain their general form. Although few such occurrences are preserved and discovered, they are of greatest value to the paleontologist because they give the most comprehensive view of past life. Certain rock formations are well known for this reason. One is the Precambrian Gunflint Chert on the north shore of Lake Superior. It contains well-preserved bacteria and blue-green algae approximately 2 billion years old. Another, the Burgess Shale of British Columbia, contains carbonaceous films of soft-bodied marine worms and crustaceans of the Cambrian Period. At the Carboniferous Mazon Creek locality in Illinois, both land plants and marine invertebrates are preserved. The Holzmaden oil shale of southern Germany, of Early Jurassic age, is well known for its many fish and crinoids, as well as for its ichthyosaurs and other marine reptiles that have been found there and displayed in museums throughout the world. Even better known is the Late Jurassic Solnhofen limestone of southern Germany, where the quarries, worked for lithographic stone, have yielded not only large numbers of well-preserved jellyfish and horseshoe crabs, but also flying reptiles (pterodactyls) and the world's earliest known bird, archaeopteryx.
Some of the most spectacular fossils of the Cretaceous Period, including fish, marine reptiles, pterodactyls, and birds, have come from the Smoky Hill Chalk, near Hays, Kans. A remarkable Eocene lake fauna has been uncovered in the Green River Formation, near the town of Fossil, Wyo. Along the shore of the Baltic Sea, insects, spiders, and the like are found preserved in amber, fossilized resin exuded from trees.
The most information on Pleistocene faunas, from vultures to bisons, wolves to saber-toothed cats to beetles, has come from tar seeps, such as the La Brea Tar Pit in Los Angeles, Calif., where prehistoric animals got mired and embalmed in the tar, as they do in modern tar pits. Most extraordinary are the few finds, in Alaska and Siberia, of prehistoric but presumably post-Pleistocene mammoths (see mammoth), frozen into the arctic permafrost and preserved by natural refrigeration.
Important as these unusual localities are, most of the fossil record is composed of skeletons that can be recovered from ordinary sediment, in a cliff or quarry or roadcut, or from wells drilled deep into the ground. These skeletons and skeletal fragments may be preserved as the original material, as hollows (molds) formed by dissolving the original matter to leave only an imprint, or as replacement material, with a mineral such as quartz or pyrite having replaced the original bone or shell.
FOSSILS AND EARTH HISTORY
Toward the end of the 17th century, the naturalist Robert Hooke turned his attention to spectacular marine fossils found in his native England. Determining that these must be the remains of once-living animals, he noted that they did not resemble any living species then known, causing him to believe that life might have changed at some time in the past and that fossils might be a chronological guide to geologic history. Hooke also noted that these fossils looked more like tropical shells than species then living on British shores and wondered whether Britain's geographic latitude had also changed since the time these animals lived. The first suggestion was verified a century later, and the second three centuries later with the discovery of continental drift.
The Earth's sedimentary strata are initially layers of muds and sands, each covering an older stratum and being covered, in turn, by a younger one (stratigraphy). They form, in this manner, a historical sequence, and the fossils that they contain can be arranged in time, by what has come to be known as the law of superposition. Early in the 1800s, William Smith, in England, noted that fossils were distinctive of individual beds or groups of beds in such sequences of strata, and that distinctive assemblages of fossils could be traced cross-country. Geologists soon discovered that the sequences of fossils in England could be matched with similar sequences elsewhere in the world.
Any given area contains a stratigraphic record of only some part of Earth history. By combining information from many different areas, geologists can determine global Earth history. Nearly two centuries of such efforts, including the description of fossils in monographs and journals, have resulted in ever more detailed classification of the more fossiliferous part of Earth history--the last 600 million years.
The smallest units of this classification, generally characterized by certain species or combinations of species, are called zones. These may be recognizable only locally, but many have been traced worldwide. Combinations of zones are the chief criteria for the recognition of worldwide units of geologic time, called stages, generally having a duration of approximately 10 million years. These in turn include the larger time periods, called systems and eras.
In terms of time, the fossil record yields a relative chronology rather than an absolute one (geologic time), but the development of radiometric age-dating methods, using the decay of radioactive isotopes, has enabled scientists to calibrate Earth history in actual years.
THE PROGRESSION OF PREHISTORIC LIFE
The earliest megafossils, some 1 to 3.5 billion years old, are stromatolites, calcareous masses formed in shallow water by blue-green algae. Microfossils from cherts this old include a variety of blue-green algal bacteria forms. Approximately 1 billion years ago, a wider variety of microscopic cells had appeared, including some that may have had nuclei. In deposits approximately 600 million years old, imprints of soft-bodied invertebrates are found. In sediments 570 million years old (the beginning of the Cambrian Period), the first skeletal invertebrates appeared--mollusks, brachiopods, and trilobites. The appearance of these invertebrates, in rocks deposited at the beginning of the Paleozoic Era, coincided with the first widespread signs of burrowing. In rocks of the Ordovician Period (500-425 million years ago), researchers have found fossil animal burrows that are evidence of the earliest known land animals; these trace fossils indicate that terrestrial ecosystems may have evolved sooner that was once thought. Most of the modern classes of invertebrates as well as the ostracoderms (the fishlike organisms), were represented by this time; marine faunas had become much more diverse.
In the Silurian Period (425-400 million years ago) landmasses were colonized by rapidly evolving higher plants, whose supporting structures and water-conducting vessels now made life on dry land possible. In the Devonian Period (400-345 million years ago) the main groups of FISH--coelacanths, lungfish, sharks, bony fish, and the extinct arthrodires --were differentiated. Forests and the first primitive insects appeared on land, as did amphibians. The Carboniferous Period (345-280 million years ago), known for its great coal deposits, witnessed the development of reptiles, the first animals having an amniote egg, which enables the embryo to develop on dry land.
The Mesozoic Era (225-65 million years ago), which includes the Triassic, Jurassic, and Cretaceous periods, is known particularly for the evolution of gigantic reptiles, both in the sea (Ichthyosaurus, Plesiosaur, and Mosasaur) and on land (Dinosaur). Flying reptiles (pterodactyl) and birds, as well as mammals, appeared during the Jurassic Period (190-135 million years ago). On land, forests of conifers and cycads had largely replaced the lycopod- and seed-fern-dominated forests of the Paleozoic Era. In the sea tiny calcite-armored, photosynthetic unicells called coccolithophorids appeared, and massive calcium carbonate (chalk) deposition began in the deeper oceans (ooze). The Cretaceous Period (135-65 million years ago) is the time of origin of two great groups of plants--the flowering plants (Angiosperm) on land and the diatom in water. Flowering plants changed the face of the Earth in many ways and triggered a great wave of evolution among the insects. At the end of Cretaceous time, the extinction of dinosaurs resulted in the spectacular evolution of terrestrial mammals, and giant sharks and marine mammals replaced large reptiles in the sea.
The group of mammals known as the primates--now represented by lemurs, monkeys, apes, and humans--dates back to the beginning of the Cenozoic Era (65 million years ago to the present), but humanlike creatures (prehistoric humans) are known only from the last few million years, the Pliocene and Pleistocene epochs. Homo sapiens, modern humans, appeared in the Old World during the Pleistocene but did not reach the Americas until its latter part.
Fossils record the progressive evolutionary diversification of living things, the progressive colonization of habitats, and the development of increasingly complex organic communities. The development of new species and such larger groups of species as genera and families has gone on throughout time, but so also has the loss of species by extinction. The rate of extinction at some times in Earth history greatly exceeded the rate of speciation, and the faunas and floras of the world became reduced. Notable biotic crises occurred in Cambrian time, near the end of the Devonian time, at the close (permian period) of the Paleozoic Era, and at the end of the Cretaceous Period. Various theories have been advanced to explain the cause of these extinctions.
FOSSILS AND EVOLUTION
The fossil record corresponds to the general theory of organic evolution, and any group of plants or animals can be seen to change through the record of strata. The smallest general unit of classification is the species, recognized in the fossil record by great similarity of form. Most species are seen to have an existence that is short in terms of Earth history--usually one to several million years, more rarely tens of million years, and very rarely hundreds of million years. Most genera (groups of related species) can be traced through time spans of tens of millions of years; larger units of this classification system, or taxonomy, tend to persist through longer time spans. The majority of the classes and phyla of invertebrate animals, for example, have records beginning in the Early Paleozoic Era.
All of the larger groups of animals are seen to have evolved a wide range of life forms. Reptiles, for example, having evolved in late Paleozoic time out of small or medium-sized predatory amphibians, developed into a great diversity of animals that included herbivores and carnivores, dwarves and giants, creepers, runners, climbers, burrowers, swimmers, and flyers, all in competition with other groups of animals living in the same ways. They have continued to occupy some of these functions (niches, in ecology) throughout their history and have vacated others. Reptiles are still among the more effective small creeping and running predators on land (lizards, the tuatara), replaced by birds as aerial dwellers and by mammals as large terrestrial herbivores and predators. A succession of animals have been marine "superpredators." In the Triassic and early Jurassic periods, this was the domain of ichthyosaurs; in late Jurassic and early Cretaceous time, these were replaced by plesiosaurs; in the Cretaceous, by mosasaurs; in the Eocene, by the zeuglodont whales (mammals); and since the Miocene, by the great carcharodont sharks. The fossil record suggests that evolution has resulted mainly from the tendency of all species to experiment with new ways of living, to thereby exploit new opportunities as they arose in an ever-changing world, gaining a new foothold in one place and losing one in another.
With the theory of evolution by Charles Darwin in the mid-1800s came the expectation that the fossil record would provide unbroken evolutionary sequences, in which species after species would be seen to emerge gradually from their ancestors and pass, equally gradually, into their descendants. Most species, however, are seen to appear abruptly, to maintain their typical form for most of their history, and to vanish as suddenly as they appeared. This failure to trace coherent lineages of ancestors and descendants does not prevent recognition of changes in larger groups of animals: horses are seen to have developed through Cenozoic time from small to large, from five-toed to one-toed, and from short- to long-toothed, but complete records of the transition from one species to another have not been found.
This caused some paleontologists to doubt Darwin's belief that evolution proceeds by the gradual accumulation of small changes. It has now been determined that evolution does not proceed steadily, in response to some mysterious internal force, but in response to new opportunities. In a stable, unchanging environment, a well-adapted species is not likely to change, whereas in a changing one it may find better opportunities by changing its way of life. In addition, evolution does not normally occur throughout a species, but in interbreeding populations, occupying some small part of the geographic range of the species as a whole. It is such populations and adjacent populations, linked by exchange of genes, that deviate from the ancestors and from the species as a whole, to form races or subspecies, and eventually, species. At any one time, a species is a combination of such groups, diverging episodically from each other. As geography and habitat change, these groups shift about, either blending when they meet, abruptly displacing each other, or coexisting side by side in different ways of life.
One of the most difficult problems in evolutionary paleontology has been the almost abrupt appearance of the major animal groups--classes and phyla--in full-fledged form, in the Cambrian and Ordovician periods. This must reflect a sudden acquisition of skeletons by the various groups, in itself a problem. Paleontologists are not certain whether the soft-bodied forms of the Precambrian Ediacarian fauna are in fact ancestral to modern groups; in any case, the lack of well-documented animal remains in older rocks indicates that differentiation of the major groups occurred more rapidly than did their subsequent evolution.
FOSSILS AND ANCIENT LANDSCAPES
The fossil record contains a history of the evolution of life on Earth and provides geologists with a chronology far more detailed and widely applicable than that of geochemistry. It also contains much information about the geographical and ecological changes that have occurred in the course of geologic time. This interpretation of the fossil record predates the other, in that some of the early Greek philosophers and Renaissance naturalists recognized certain strata as marine and as evidence of former higher sea levels, on the basis of the enclosed fossils, long before the evolutionary nature of fossils was known.
The best example of this is the recognition of ancient seas and land masses. The deposit of loess containing grass seeds and land-snail shells can be quite easily recognized as the windblown accumulation of dust in an ancient grassland or prairie. The accumulation of PEAT or coal, containing abundant woody material along with spores or pollen, and possibly skeletons of land animals, is evidence of an ancient peat bog or swamp. A bed of limestone containing a wide variety of clams and snails belonging to marine families, as well as the remains of sea urchins or other echinoderms - a phylum that seems always to have been restricted to the sea--is evidently of marine derivation.
The fossil record can be used to reconstruct ancient environments. For example, strata of Tertiary age in the oil-bearing "transverse basins" of California, such as the Los Angeles and Ventura basins, contain many microfossils of the protozoan group called foraminifera. These were studied because they provided a means of tracing strata from one oil well to another, in accumulations of sediment thousands of meters thick. This sedimentary sequence began with stream deposits in the Oligocene epoch, passed through a long marine phase in the Miocene and Pliocene, and reverted to mammal-bearing alluvial deposits in the Pleistocene Epoch (see also quaternary period). In order to learn something about the conditions under which the oil-bearing marine portion was deposited, paleontologists compared the fossil assemblages with the depth range of the same species or the most closely related species living today off the California coast. Interpreted in this manner, environmental change from continental, through shallow near-shore, into deep-water (more than 1,500 m/5,000 ft), back through the shallow water, and into alluvial was revealed. This history has been confirmed by the study of fossil fish scales. Although sardine scales are found throughout the marine parts, angler fish and other species of the bathyal zone are found only in association with deep-water foraminifera.
This is one example of the field of study that is called paleoecology. Palynologists studying pollen grain assemblages from lake beds and peat bogs have determined the shifting forests and grasslands that occurred in the later stages of the Pleistocene ice age and in the interval since then and are establishing a much-needed history of climates, which can be related directly to the historical record (pollen stratigraphy). Study of the foraminifera in marine cores enables scientists to chart the distribution of ocean currents and water masses during the height of the last glaciation, 18,000 years ago, in an effort to understand the Ice Age and its cause.
Fossil chemistry is another area of research for paleoecologists. Many elements occur in two or more atomic forms that differ from each other in weight--isotopes. Thus; oxygen occurs as O(16) and O(18), O(16) being by far the most abundant. When an organism such as foraminifera builds a skeleton of calcium carbonate, it incorporates O(16) and O(18) in a proportion that depends on the ratio in which they occur in the ambient water, and on the temperature of the water. The higher the temperature, the less O(18) is put into the skeleton. If the ratio of the isotopes has remained constant in the oceans, then the ratios of such isotopes in a foraminiferal shell are a direct indication of temperature. The isotopic composition of the seas, however, has not remained constant, and the ratios in the shells can be disturbed by chemical changes after burial, but paleoecologists have demonstrated, for example, that the thermal structure of the oceans in Cretaceous times was much different from what it is today, with much warmer mid-water masses.
FOSSILS AND PALEOGEOGRAPHY
The distribution of plant and animal species in the world today reflects the interplay between the physical environment, which, for example, restricts polar bears to the high latitudes, and geographical barriers to migration, which, for example, have kept polar bears from invading the Antarctic and keep the marine snakes of the Pacific side of Panama from invading the Caribbean Sea. Such barriers divide the world into biogeographic provinces. The fossil record enables paleontologists to reconstruct such provinces for the past and to study their history. South America, for example, was joined to Africa during most of the Mesozoic Era, as its Triassic and Jurassic reptile faunas demonstrate. When it broke away, in mid-Cretaceous time, with the opening of the South Atlantic Ocean, the dinosaurs of South America succumbed to the Cretaceous crisis and the mammals took their place as the dominant land animals, forming a very different kind of mammalian fauna that evolved independently of the Eurasian-American fauna. Australia underwent similar changes except that it remained isolated, and its indigenous mammalian fauna (a marsupial one) has evolved to its modern state. South America became linked with North America during the Pliocene Epoch, and the two very different mammalian faunas invaded each other's territories, pitting species against species for existence. Eventually, North America assimilated some South American mammals (ground sloths, armadillos, opossums, and porcupines), but the majority of South American species became extinct.
The fossil record also shows that the Caribbean and eastern Pacific marine faunas were essentially identical in Miocene times, diverging only since the Panamanian land bridge closed in Pliocene time. The question of whether to build a Panamanian sea-level canal has focused attention on what would happen to present faunas if communications were to be reestablished. Would the Pacific sea snakes, for example, invade the Caribbean to the detriment of fisheries there?
The fossil record spans three-quarters of Earth history but forms a coherent whole only since the time that animals with skeletons appeared at the beginning of the Cambrian period. Both the origin of life and the origin of the major groups of animals remain unknown. The seafloor became heavily populated with animals in Cambrian time, and the lands were colonized in the Silurian Period. Organic communities have become more complex through geologic time, but not in a linear fashion. Extinction of species is normally counterbalanced by speciation, but at times it has prevailed, and during the great biotic crises some communities were reduced to low levels throughout the world, and major groups of organisms, such as the dinosaur and the ammonite, were lost. That part of the record is a reminder that organisms exist at the tolerance of the environment, and that this environment, throughout geologic time, has exhibited variations of a nature and magnitude outside the realm of human experience.
Alfred G. Fischer