Part 1

FOSSILS A Story of the Rocks and Their Record of Prehistoric Life

By Harvey C. Markman Curator of Geology and Paleontology

Cover Design and Murals by Mary Chilton Gray

DENVER MUSEUM OF NATURAL HISTORY

Popular Series No. 3 Alfred M. Bailey, Editor

Third Edition, Reprinted October 1, 1954

CONTENTS

Page Introduction 5 The Prehistoric Record 5 Varieties of Fossils 8 Fossilization 9 Floras and Faunas 13 Formations 16 Geological Time 18 Explanation of the Time Chart 23 The Geological Section 25 Before the Age of Reptiles 31 The Pre-Cambrian Complex 31 Cambrian Life 33 The Ordovician Record 35 Silurian Events 36 Devonian Progress 37 Carboniferous Forests 40 Permian Hardships 43 The Age of Reptiles 47 Dinosaurs 48 Plant Life and Climate 56 Coal and Fossil Footprints 57 Mesozoic Invertebrates 58 Extinct Birds 60 Ancestors of the Mammals 61 The Age of Mammals 64 Prehistoric Horses 70 Mastodons and Mammoths 80 The Rancho La Brea Fossil Pits 88 The Age of Man 93 Supplementary Reading 95

LIST OF ILLUSTRATIONS

Page At Work on a Dinosaur Skeleton 2 Fossil Bones in Rock Formation 7 Insect Fossils 10 Restoration of Rhinoceros 12 Dinosaur Tracks 17 Time Chart 22 Geological Section Showing Positions of Formations 26 Marine Beds of the Benton Formation 28 Plesiosaur Bones in Place 28 Invertebrate Fossils 34 Modernized Fishes 38 Prehistoric Plants 41 Marine Reptiles 46 Diplodocus 49 Trachodon 51 Stegosaur 52 Sea Turtle 54 Murals, Hall of Mammals 63 Uintatheres and Contemporary Life 65 Moropus 67 Titanotheres 69 Oligocene Mammals 71 Pleistocene Horse 73 Structure of Molar Teeth 75 Grazing Type of Molar Teeth 77 American Mastodon 79 Long-Jawed Mastodont 81 Molar Tooth of Mammoth 83 Nebraska Mammoth 85 Rancho La Brea Fossils 87 Folsom Bison 90 Man and Mammoth 92

FOSSILS

INTRODUCTION

In the recent growth of knowledge there has been rapid progress in two directions. The commercial exploitation of natural resources, being fundamental to modern civilization, attracts a liberal share of the talents and energies of workers trained for the industrial professions. A second trend has specialized in the further development of the sciences which are characteristic of our time. Such activities, in the natural history field, deal largely with the refinements of exact definition, nomenclature and classification, all of which means little or nothing to the layman who is otherwise engaged.

For the latter, however, there is a quality of interest which may be described as a wholesome curiosity about what has happened, how it happened, how we know it happened, and what it may signify to one who is neither industrialist nor scientist. This booklet is intended for the many who feel that there is more to be obtained from a natural history museum than an occasional glimpse of a bewildering “marvel.” In addition to being a guide to fossil exhibits it supplies parts of a great story which specimens alone can not relate.

THE PREHISTORIC RECORD

All that is known of the extinct plants and animals which inhabited the earth before man began the practice of recording his observations has been obtained from a study of the rocks. The few possible exceptions to this rule, in which animal and plant remains have been preserved by freezing or drying, are so unusual as to be hardly worth mentioning.

Explanation of this is that plant and animal tissues quickly decay under ordinary conditions when life ceases. Unless protected from destructive agencies which are especially active at the surface of the ground, even the heavier bones of animals and the large trunks of fallen trees will soon crumble into shapeless masses. The usual method employed by nature to prepare a fossil specimen is so closely related to the natural process of rock making that a little knowledge of that subject will be necessary in order to know what fossils are and how they are preserved for so long a time.

It should be understood first that a fossil is some record, commonly preserved in rock, of a kind of plant or animal which no longer exists as a living type. This, at least, is the ordinary sense of the word and more elaborate definitions are of small service to anyone. It may be necessary to add, however, that all things which have lived at any time are regarded as either plants or animals.

Nature’s way of producing rocks and fossils remains a mystery to many of us because we are so wrapped up with the importance of finding names for things and materials that we frequently neglect the consideration of sources and histories. Everyone knows a rock when he sees it in a large mass, but when he looks at sand, mud, dust, or soil, he seldom thinks of it as related in any way to rocks. Although the difference is almost entirely a matter of size, our use of words makes it seem unreasonable to speak of the finer particles as rock.

There can be no reality or meaning in the natural record for an individual who has failed to observe a few simple facts which involve changes going on in all parts of the world at the present time. With regard to rocks, it is supposed that what happens in our day also occurred under like circumstances ages ago. Anyone wishing to do so may see for himself that rock masses break down wherever they are exposed to the elements, that the larger pieces are reduced to smaller fragments, and that the final product is sand or dust.

He may also note that this finely ground material is being moved and sorted, by rain, wind, and streams, transported to lower levels and accumulated in great quantities wherever it finds a resting place. Along with it go sticks and leaves, bugs, shells, bones and carcasses of animals, some of which in time may become fossils. In large lakes and seas there is a steady distribution of such materials over broad areas, yesterday’s accumulation of sediments being buried by the contributions of today, the most recent of the settlings always resting upon older ones until something happens to disturb that arrangement.

Not so readily observed are other parts of the process, such as the consolidation of sands and muds into the firm sandstones and shales which we again recognize as rocks. Much of this requires more than the few score years of a human lifetime for its accomplishment, but many of us have seen muds become so solidified, by merely drying, that they could hardly be distinguished from prehistoric shales. It is to be noted also that some ancient fossils come from sandstones which are scarcely more rock-like than the loose sands of an ocean beach. Thus we learn that firm consolidation of rock-making materials is not always a sign of great antiquity, and that hardness of rock is not always essential to the preservation of imbedded plants and animals.

The rocks themselves must explain the many things which have happened during the course of millions of years, and this they do remarkably well when carefully studied, for many of the factors involved in their histories leave characteristic marks. Changing climates, the draining of seas, the uplifting of mountain ranges, all have ways of registering their occurrence which are as convincing and reliable as anything ever written by man. Piece by piece the story has been patched together through the efforts of thousands of investigators. Parts of the narrative remain buried at inaccessible depths, and whole chapters, no doubt, have been destroyed by the same forces that composed this tremendous record of prehistoric times.

VARIETIES OF FOSSILS

It would be a serious mistake to regard nature as divided into a number of distinct and independent schools of fossil making, each refusing to use the methods and devices of another. There are, however, certain features which stand out so prominently that a little classification becomes helpful. While this procedure brings out differences it should be understood that processes actually work together, several of them usually being involved in the production of any individual specimen.

(1) Impressions of animals and plants, or parts of these, are frequently left in soft sand or mud which later becomes converted into more durable rock. This type of fossils is represented by animal foot-prints and the imprints of leaves, flowers, insects, and like objects which may be mingled with the finely ground materials of the common sedimentary rocks.

(2) Parts of plants and animals may be gradually replaced by mineral matter with little or no change from original form and texture. Fossils of this class are said to be petrified or turned to stone. They are also known as replacements. The fleshy parts of animals do not petrify.

(3) Many animals among the invertebrates use mineral substances for protective or supporting structures. Small plants of various kinds follow a similar practice. These structures, being produced in stony materials, are readily converted into fossils. The shells of mollusks are the best known illustrations in this field, and all that is required for a shell to become a fossil is the extinction of the species of animal that produced it. Fossils of this type are extremely abundant.

(4) Preservative substances other than those which produce common rocks may be mentioned among fossil-making possibilities. Bones are known to have been preserved in asphalt, and insects in resins, but such cases are few in comparison with the products of other methods.

(5) In rare instances there has been preservation of extinct creatures by the process of drying or by refrigeration. Occasional mummies are found with shriveled flesh and skin still in place, but better preservation of all tissues occurs when the temperature is quickly reduced below freezing point and held there without interruption. This can happen only in the colder parts of the earth and is always subject to climatic change. The effect of drying also may be undone at any time by a slight increase in the amount of moisture.

(6) Coal beds often produce fossils of an unusual sort. In the formation of coal, plant material gradually loses some of its more perishable substances but retains carbon which has better lasting qualities and slowly accumulates to produce the seams and beds that are mined. In the early stages of the process the original vegetation undergoes little change in appearance but eventually practically all of its character is lost. Many fossil leaves are found as thin layers of carbon, bedded in the clays which are commonly associated with coal deposits.

(7) Concretions, which are hardened lumps of mineral substances occurring commonly in sandstones and shales, are often mistaken for fossils because of their peculiar shapes. However, there are localities in which the mineral solutions have been concentrated and deposited around shells, leaves, seeds, or similar objects, thus producing an abundance of fossils which may be obtained by opening the concretions. Fossils of this type are well known from Mazon Creek and other districts in Illinois, Kansas, Colorado, and elsewhere.

FOSSILIZATION

Footprints need little explanation other than a consideration of the factors which make it possible for them to be preserved. The sand or mud must be neither too soft nor too hard to take the form of the foot and retain its shape when the foot is withdrawn. Then in some manner the impression must be protected while the rock-making process goes on. When such protection is obtained it is usually in the form of more mud and sand, deposited over the surface which received the impression. At a later time the covering may be separated from the lower part of the deposit, which serves as a mold, and if the separation be accomplished successfully a natural cast of the foot will be obtained as well as the mold in which it was produced. Since conditions for perfect work are not always present in a laboratory of this kind, it is not surprising that fossil footprints are very rare considering the number and variety of tracks left by wandering animals.

Impressions of leaves are explained in much the same way except that the leaf remains under its protective covering until it decays. Similar impressions may be obtained from the bodies of delicate invertebrate animals but they are seldom preserved because of the softness of the tissues. The smaller fishes provide much better material for the production of fossils according to this method. While the fish is being flattened by the weight of surrounding sediments, scales, fins, and soft bones retain their positions and provide the necessary resistance to leave an impression of the body form when the flesh is gone.

The larger and more spectacular fossils, such as skeletons, skulls, and detached bones are nearly always of the replacement type. Replacement of plant and animal substances by mineral matter is a slow process and in younger fossils the change is rarely completed, some of the original material being present in a partially altered condition or not modified at all. Since air does not often carry the necessary materials and provide other essential conditions, replacement may be regarded as something which happens underground or in water. It is perhaps best explained in connection with limestones, because calcite or “lime” is frequently the replacing substance although other minerals, especially quartz, may serve the purpose.

Besides converting bony or woody objects into rock substance, mineral replacements may assist in the production and preservation of fossils in another manner. It often results in the filing of cavities with some rock-making substance which retards destruction through crushing or other injury. In many cases, so-called fossil shells are not shells at all; instead, they are merely a stony filling which was once surrounded by shell substance. In other instances the original shell remains as it was during the life of its former occupant, preservation of the shell being due largely to the substitution of a mineral filler for the soft animal tissues once present.

Limestone comes into existence through a more elaborate process than that which produces sandstone and shales. It is one of the three types of common rocks, known collectively as the sedimentaries, in which fossils are found. It differs from sandstones and shales, however, in that much of its substance has been dissolved in water instead of being transported in the form of finely ground rock particles. Lime occurs in many varieties of rock which are exposed to the wear and tear of the elements throughout the world. Slowly but more or less continuously it is taken from this source by ground and surface waters coming in contact with it. Particularly active is carbonated water, moving underground through pores and crevices.

This underground circulation of mineral matter in a dissolved condition explains the occurrence of fossils in land areas which have not necessarily been submerged during any great length of time, for it is well known that plant and animal remains are not invariably washed into lakes or seas, and that all sedimentary deposits have not been built up in large bodies of water. Here we are dealing with what is known as the continental type of sedimentation and such fossils as dinosaurs, mastodons, three-toed horses, and other former inhabitants of land areas.

In order to become properly fossilized, certain conditions are absolutely necessary, and only a small percentage of the once-living multitude secures the required treatment. There must be present, soon after death, some protection from the activities of the carnivorous birds and beasts that would separate and scatter the parts of a carcass, also from the smaller gnawing animals that would continue the destruction, and finally from wind, sun, rain, frost, and bacterial and chemical activities which in the course of only a few years would remove everything but possibly a few scraps of tooth enamel, which is the hardest of animal tissues.

A slight covering of earth substance in any form serves to check the disintegration, and this may be acquired in several ways. Animals that perish in bogs or quicksands are soon covered over; in many localities wind-blown dust and sand do the work; and flooded river valleys provide an abundance of mud for the necessary burial of others. Even underground, the decay of soft tissues is too rapid to permit of replacement by mineral substance in a manner that would reproduce form and texture. Skin and flesh are almost invariably lost, although in a few instances the thick scaly hides of dinosaurs are known to have produced natural molds and casts by the method explained in connection with footprints and other impressions.

With regard to the more durable tissues found in the teeth, bones, and shells of animals, or the woody parts of plants, the case is different. These parts become firmly imbedded in the ground, but moisture still has access, and it begins to work immediately; for all water moving underground finds soluble substances which it picks up and carries with it wherever it goes, and much of the load consists of mineral matter which may be unloaded again when the necessary conditions are found.

Mineral-laden waters will drop one kind of substance to take up another which dissolves more readily, and this happens sooner or later when a buried bone or log is encountered. Complications of various sorts enter into the process, but the final outcome frequently is a complete change from one chemical composition to another which is more enduring, the transformation being brought about so gradually and thoroughly that in many fossils the inner structure of the original tissue is as accurately reproduced as the fine detail of surface features.

Converted into stone, however, the result is still far from permanent. While yet underground the fossil is subjected to distortion and breakage due to earth movements which bend and dislocate the rock deposits. What causes these upheavals and depressions of the earth’s surface remains the subject of much discussion, but that they have occurred on a large scale and continue to occur is clearly evident. At higher altitudes the surface rocks and fossils are exposed to a larger variety of destructive activities than at lower levels where protective coverings are more likely to be provided and retained. Once stripped of that protection there is little chance for a fossil to survive. Beyond a doubt there are many thousands of tons of prehistoric remains damaged or destroyed each year, by weather and stream erosion.

FLORAS AND FAUNAS

As the various types of sediments continue to accumulate on land and in water they produce deposits of sandstones, claystones, and limestones which in time may acquire great thickness and cover wide areas of sea floor, or continental surface. Usually there is more or less mixing of sediments resulting in sandy limestones, limy clays, and other combinations. Quite commonly, however, the types remain fairly pure but become arranged in layers which alternate from one kind of material to another. At all times the character of the deposit will depend upon the nature of the rocks which supply the materials, and any fossils that may be produced will consist of such plants and animals as live and die during the time the rock is in the making.