Fossils: a Story of the Rocks and Their Record of Prehistoric Life

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.

This exhibit was taken from the famous fossil quarry at Agate, Nebraska. The fossilized remains are still partially imbedded in the sandstone which preserved them for millions of years.]

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.

Part 2

(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.

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