This introduction to the life of the past as revealed through fossils includes:
-Descriptions of the typical plants and animals of major geological eras
-Maps showing where fossils can be found
-The history of the development of life on earth
Full-color illustrations and concise information make this an invaluable and enjoyable guide to a fascinating subject.
About the Author
Golden Guides first appeared in 1949 and quickly established themselves as authorities on subjects from Natural History to Science. Relaunched in 2000, Golden Guides from St. Martin's Press feature modern, new covers as part of a multi-year, million-dollar program to revise, update, and expand the complete line of guides for a new generation of students.
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LIFE, PAST AND PRESENT
The earth teems with life. Mountains, prairies, deserts, beaches, lakes, rivers and seas — every part of land, sea and air is inhabited by living things. The number of different species of living things is enormous. More than 350,000 species of plants and 1,120,000 species of animals are known.
How did these many species originate? Has life always been the same as it is now? Men have asked these questions for thousands of years. To answer them we must turn to fossils and to a knowledge of living organisms and their structure. Only an understanding of living animals can put life in the fragments of bones and shells millions of years old.
The elephant is the largest living land animal. But the study of fossils shows not only that elephants are a recent group in the long history of living things but also that early elephants looked more like hogs. As geologists trace elephant fossils from older to younger rocks they piece together the history of elephant evolution. Fossil bones and teeth reveal the structure of early elephants, but by studying these fossils in the light of the anatomy of living elephants, complete reconstructions of extinct elephants can be made with reasonable accuracy. Some unusual occurrences of mastodon fossils with crude flint weapons prove that these elephants were hunted by our ancestors.
EVOLUTION OF ELEPHANTS
Living elephants are survivors of an ancient, more widespread and varied group, which evolved from pig-sized ancestors of the Upper Eocene. (See the geologic clock, here.) Only a few of the many extinct elephants and their kin are shown.
OVER 1,000,000 ANIMAL SPECIES
5. Worm-like phyla-38,000
6. Other invertebrates-21,000
ALL FORMS OF LIFE have evolved from early beginnings, some three billion years ago. From relatively few primitive forms, the major groups of plants and animals developed. Living things became more complicated and adapted to many different ways of living. The number of different species gradually increased until they reached the tremendous diversity of today. The study of fossils (paleontology) traces the various paths by which animals and plants evolved to their present forms. Some, like elephants and horses, have changed greatly through the ages. Others, like the horseshoe crab and cockroach, have not changed in hundreds of millions of years. Still other fossils show lines of development that came to a dead end. Giant Sloths, once plentiful, are known only as fossils.
GLYPTODONT, 9 ft., an armored mammal from the late Cenozoic, is a fossil that shows spectacular and obvious adaptation. This relative of the armadillos was protected against carnivores and other enemies by a thick, solid, domed armor, which reached 5 ft. in length in some forms. The head and tail were also armored, and in some species the tail terminated in a spiked, mace-like club. Yet despite, or because of, these unusual adaptions glyptodonts became extinct.
ABOUT 350,000 PLANT SPECIES
1. Flowering Plants — 250,000
2. Ferns, Conifers, etc. — 10,000
3. Mosses and Liverworts — 23,000
4. Algae, Fungi, etc. — 60,000
About 350,000 plant species
ADAPTATION Most plants and animals exist only because they are successfully adapted to their environments. Each distinct environment such as a desert, pond or mountain top supports a more or less distinct population of animals and plants. Those which, over long periods of time, have become fitted to cope with local conditions have survived. All the rest have become extinct. Many living things are uniquely adapted to particular environments. The streamlined shape of a fish and the structure and function of its fins and tail are adaptations to life in the water. The fleshy stems of a cactus are adaptations that conserve water in the desert. Such adaptations succeeded, but the fossil record is strewn with the remains of those that failed. The slogan of life may well be — adapt or perish.
Survival in animals depends on adaptations as varied and as intricate as the animals themselves. Virtually every structure of a plant or animal may be regarded as adaptive. Many animals have protective coloring and a few forms, such as the bottom-living flounder, are able to change their color to conform to their background. Such an intricate adaptation is rarely discernible in fossils. However, if the adaptation affects bone or shell, it may show up clearly in the fossil record.
EVOLUTION OF THE VERTEBRATES
Adapted from W. K. Gregory
DEVELOPMENT OF MODERN ANIMAL LIFE is difficult to trace because the fossil record is incomplete. Enough is known to suggest the general pattern of evolution and to reconstruct in some detail history of groups in those areas where fossils occur abundantly. The chart shows relationships among major groups of vertebrates. Those animals within a given color probably developed from common ancestors.
INTRODUCTION TO FOSSILS
Fossils are the remains of prehistoric life or some other direct evidence that such life existed. To become fossilized a plant or animal must usually have hard parts, such as bone, shell, or wood. It must be buried quickly to prevent decay and must be undisturbed throughout the long process. Because of all this very few plants or animals that die are preserved as fossils.
In rare cases whole animals may be preserved. In Siberia and Alaska fossil mammoths have been found in the frozen ground, completely refrigerated for some 25,000 years. In Galicia (Poland) an Ice Age Woolly Rhinoceros was well preserved in asphalt. In semi-arid South America, parts of mummified Ground Sloths have been preserved in caves. In each of these cases an unusual condition — cold, chemical action and dryness — was involved.
SOFT PARTS are rarely found intact but insects' exoskeletons and minute appendages have been preserved in amber, the hardened resin of ancient trees. Leaves and small, soft marine animals buried in mud which hardened into shale have sometimes left behind a thin film of carbon outlining their form and preserving delicate details of their structure. And, in western Canada, sandstone casts of dinosaur skins have been preserved.
Cast of dinosaur skin in sandstone
Insect preserved in amber
HARD PARTS are often preserved with little or no alteration. Teeth of sharks and mammals are examples, and small jaws of ancient sea worms have been found. Bones may be preserved but more often have been altered and replaced by dissolved mineral matter. Shells frequently remain intact and in a few places logs and stumps have been preserved in peat or coal.
ALTERATION of hard parts preserved as fossils is common. Circulating water dissolves chemicals from shells and bones and leaves them light and spongy. More often as chemicals are dissolved they are replaced by others. Silica, lime and iron compounds are commonly deposited in fossils. Sometimes this replacement preserves the original structure of the plant or animal completely. In some petrified wood, silica has replaced the original woody structure so perfectly that the cells and annual rings show clearly. In most petrified wood and most replacement fossils, the replacement is less perfect and shows only the general form.
1. Animal dies and shell is buried in the sand.
2. Sand hardens to rock. Shell interior unfilled.
3. Shell material dissolved. Cavity wall is mold of shell.
4. Dissolved chemicals fill mold to form cast.
5. Both mold and cast are fossils, replicas of the original surface.
MOLDS AND CASTS Not all fossils are bones, shells and other remains. Some are mere indications of prehistoric life. All the original plant or animal material may be dissolved away so that only a cavity remains — the walls of which are a natural mold of the fossil. Later, dissolved substances may fill the cavity, forming a natural cast of the original. Such casts are common fossil forms. Footprints or trails of animals may harden as a mold. Filled with fresh mud, casts are formed and both may be preserved, as in the red sandstone (Triassic) of the Connecticut Valley which contains tracks of dinosaurs.
OTHER TYPES OF FOSSILS include some curious forms, all of which are evidence of ancient life.
BORINGS of worms and mollusks indicate that these animals lived millions of years ago. Such fossils are common. Sometimes petrified wood shows borings also.
GASTROLITHS are smooth, rounded pebbles found in rib cages of dinosaurs. These stones probably aided the dinosaurs' digestion just as gravel in their gizzards helps chickens crush grain. Polished gastroliths are found only in "dinosaur country."
COPROLITES are fossil excreta and give a clue to the diet of ancient animals. These lumpy fossils are usually associated with land animals of the past 50 million years.
ARTIFACTS are stone tools or weapons made by ancient man. Found in many parts of the world, the oldest have been found with bones of animals now extinct. The first stone artifacts were crude and difficult to recognize. More recent ones were chipped and polished to make beautiful implements.
SEDIMENTARY ROCKS contain nearly all the fossils that are found. These rocks are formed of sediments — mud, sand, clay — deposited mechanically, chemically or by organisms, in seas, lakes, caves, deserts and river valleys.
STRATA or layers are a characteristic of sedimentary rocks. The bottom layers are naturally the oldest. But not all sediments are evenly or clearly bedded.
LIMESTONE, mainly calcium carbonate, common in warm, shallow seas, often has fossils.
SHALE is a fine-grained rock formed from silt and clays. It preserves fossils well.
SANDSTONE is widespread in desert deposits and in shallow water sediments.
RIPPLE MARKS AND MUD-CRACKS characterize many sedimentary rocks formed in shallow waters. Ripple marks are common in shale. Mudcracks may form as mud and clays dry. These imply the presence of sunlight, water and moderate temperatures — conditions related to the possibilities of life.
SEDIMENTARY ROCKS, often rich in fossils, occur over much of North America. But in many places the solid rocks are covered with soil or glacial deposits, or the fossil-bearing layers lie deep beneath other rocks. Hence fossil hunting is restricted to outcrops — places where the sedimentary rock is exposed at the surface, as in cliffs, river banks, roadcuts or quarries.
The fact that fossils are found in sedimentary rocks is no coincidence. Other rocks are subjected to forces or conditions which destroy fossils easily. The processes that wear down the earth's surface produce the sediments from which sedimentary rocks are formed. These wearing down processes (degradation) involve rainfall, evaporation, wind, running water and transportation among other things. And the fossil fish pictured below not only proves that fishes lived in the distant past, but that conditions in the lake in which it lived were not greatly different from those in many areas today. This and older fossils provide evidence that basic physical conditions making life possible today existed not only 50 million years ago when this fish became a fossil but probably go back about two and a half billion years. Every fossil, even the most common, tells a fascinating story of the changing surface of the earth and the development of life upon it.
PSEUDOFOSSILS are rock structures that resemble fossils. They may have any shape and often look like parts of plants or animals. A geologist will usually recognize a pseudofossil at once, but an amateur may be misled. Pseudofossils resemble fossils only in external form. They never have the detailed structure of true fossils. They may occur in improbable situations, as for instance a "footprint" in rock formed long before any creatures walked on land.
Pseudofossils are formed in many ways. Some are water-worn fragments of rock. Concretions which form in sedimentary rock may contain a fossil, though most do not. Concretions, harder than the rock in which they occur, are often found on the surface. Some minerals form dendrites or fernlike deposits on or in rocks. Moss agates are dendrites, not fossil moss.
FOSSILS FOR AMATEURS
Collecting and studying fossils can be an interesting hobby as well as an important science. Only during the past two centuries has paleontology, the study of fossils, moved to the professional level. Amateurs have collected and studied fossils much longer and today they enjoy field trips and collecting as much as ever. Major discoveries have been made by amateurs and many have won acclaim from professional geologists.
Unless the ground is covered with snow, collecting fossils is an all-year occupation. It takes you out-of-doors and off the beaten track. You learn to know your region intimately and enjoy the company of other local "rock-hounds." No other hobby can open such wide vistas of time and space. The study of fossils still has many unsolved problems which a serious amateur can tackle with some chance of personal success. Such a person will understand fossils better if he also keeps up a continued interest in living animals and plants.
THE RAREST FOSSILS are those of human beings. This jawbone unearthed in Africa in 1961 pushed the origin of humans or near-human, tool-using animals back to nearly 1,750,000 years ago.
FOSSILS RECONSTRUCT LIFE They enable scientists to picture accurately many kinds of long-extinct plants and animals.
FOSSILS PLOT GEOGRAPHY They indicate ancient land and water areas and show the changing continents.
WHY COLLECT FOSSILS? Most people collect for the simple fun of it — for the fun of tramping and exploring; for the excitement of a rare find; for the challenge of "working out" a perfect specimen. But in the course of doing all this, the layers of sedimentary rocks unfold like pages of a gigantic book, revealing the fascinating story of the earth's long and exciting past. Events 50, 100 or 500 million years ago become real because the fossils you have found provide a clear connection with bygone ages.
With the aid of fossils the reconstruction of prehistoric plants and animals was possible, and the story of the evolution of life became clear. Without the evidence of fossils, evolution would still be a theory, not a fact. Fossils help determine whether sediments were formed in shallow or deep seas, in rivers, in swamps or in deserts. Thus they give a clue to the geography and ecology of the past and show how the continents and seas have changed. Fossils prove that Alaska was once connected with Siberia and Australia with Malaya. The distribution of shallow-water mollusks aids in tracing ancient shorelines.
FOSSILS HELP CORRELATE STRATA Index fossils establish the time relationship between rocks of different areas.
FOSSILS ARE NATURAL RESOURCES Fossils are a source of coal, oil, lime, phosphate and building stones.
Fossils, in addition to being clues to ancient geography, are also clues to the climate of the past. Fossil corals show that warm, shallow seas once covered New York. And plant fossils show that the climates of Antarctica and Greenland were once mild.
Certain fossils of limited time distribution clearly mark certain beds or strata of rocks. These are index fossils and their occurrence in rocks located miles apart proves these rocks were formed at the same time. This use of fossils to correlate strata is important in mapping rock formations and in locating valuable mineral deposits.
Fossils themselves or rocks located by fossils provide natural resources valued at billions of dollars. Nearly all our fuels are fossil fuels. Coal and oil are the remains of ancient plants and animals. Fossil limestones make excellent building stones. Some are cut for ornamental use. Micro-fossils are used as filters, fillers, in polishes and for many other purposes. Some phosphate beds are associated with large deposits of fossil bones. Amber and jet are fossils used as jewelry.
STUDYING FOSSILS is something like making a rabbit stew — you first must get a rabbit. Considering the earth as a whole, fossils are rare. Many are buried beneath the sea. Forests, grasslands, swamps, deserts, soil and rock debris cover many more. Yet despite all this, fossils are often easy to find.
Excerpted from "Fossils"
Copyright © 1990 St. Martin's Press.
Excerpted by permission of St. Martin's Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
LIFE, PAST AND PRESENT,
The earth, its life and how living things have evolved.,
INTRODUCTION TO FOSSILS,
What fossils are. How they are formed. Different types of fossils.,
FOSSILS FOR AMATEURS,
Equipment needed. Where, when and how to collect. Museums and other exhibits.,
LIFE OF THE PAST,
A survey of life era by era.,
A systematic survey of typical groups.,
A look at some common groups.,
A brief survey of some common fossil plants, their evolution and their identification.,
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