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| ISBN-13: | 9780486147956 |
|---|---|
| Publisher: | Dover Publications |
| Publication date: | 09/19/2012 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 256 |
| File size: | 17 MB |
| Note: | This product may take a few minutes to download. |
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The Age of Reptiles
By EDWIN H. COLBERT
Dover Publications, Inc.
Copyright © 1997 Edwin H. ColbertAll rights reserved.
ISBN: 978-0-486-14795-6
CHAPTER 1
Time, Tetrapods and Fossils
THE PAST is mysterious, ever more so the farther we look back from our vantage point in the twentieth-century world. As we follow the procession of years back through time the earth and its inhabitants seem to us less real and less substantial the more distantly they are removed from this age in which we live. How vague in our minds is the golden age of the Greek heroes, in spite of the Homeric legends that have come down to us through the generations, in spite of the tangible relics of that distant time, the ruined temples and statues, the shining black and white vases and amphorae, the exquisite gems and coins. How much more vague is the far more ancient age of the Magdalenian hunters in western Europe, in spite of the burials and the skeletons, the flint tools, and the incomparable paintings that these men left on the walls of their caves. And how very much more vague still is the age when reptiles ruled the earth, in spite of the fossil bones - relics of animals that lived on the continents and in the seas in a world of long ago.
Yet although there are many mysteries, there is much to be learned from the past. If we probe deeply and in detail it is possible to thrust aside some parts of the curtain of obscurity that comes between us and the vanished ages of history - human and prehuman - thus to bring a certain degree of life and reality to those years before there were men, even before there were mammals. Such is the intended purpose of this book; we are to explore the Age of Reptiles.
What was the Age of Reptiles? It was, as the name implies, that portion in the story of the earth when reptiles were supreme. Let us look at it in its proper frame of reference, against the backdrop of life history.
When we trace the evolution of life back through time, so far back that the days of the ancient Greeks or even the years of the Magdalenian hunters seem as but yesterday, we can see the first grand entrance of animals on the stage of earth history, as abundantly revealed by the fossil record, about six hundred million years ago. At that far distant day the record of animal life, as preserved by the fossils in the rocks, bursts upon our view in almost full panoply, and we see before us a rich array of shelled animals that lived in ancient seas. Evidently there had been a vast preceding time span during a considerable part of the four or five thousand million years of earth history in which animals and plants were simple organisms, unprotected by outer shells, unsupported by woody tissues, and therefore seldom fossilized. Then, when animals had evolved to such a degree of complexity that many of them were protected by shells of various sorts, the Age of Invertebrates, as we know it, had begun.
Millions of years passed, and the first, primitive, jawless fishes made their appearance in the stony record of life. This event, followed by a long period during which the fishes multiplied in great diversity, marked the Age of Fishes.
While the fishes were evolving along lines that were to determine the course of their later success, the plants, heretofore simple marine forms, had migrated on to the land, to clothe the hills and valleys with a mantle of green, and accompanying the plants as early explorers of the land went various animals without backbones, particularly the ancestors of the insects. About three hundred million years ago some of the fishes, which had strong, leg-like fins and lungs to breathe with, ventured on to dry ground from the streams and lakes in which they lived. These were the progenitors of the amphibians, and the Age of Amphibians had begun.
The rule of the amphibians on the land, at the beginning quite unopposed, was relatively brief. These animals throughout their history have retained vestiges of their fish heritage, especially in their method of reproduction. In short, the modern amphibians (the frogs, toads and salamanders, and a group of tropical amphibians known as coecilians) generally lay unprotected eggs in the water, hatch as little fish-like tadpoles, and after an interval of a somewhat fishlike existence, these tadpoles change or metamorphose to become adult, four-legged animals. There is every reason to think that this method of reproduction was as characteristic of ancient amphibians as it is of those we know today. Animals that perpetuate their own kind in this way are and always have been tied closely to the water; it is and was in past ages necessary for them to return to the water or to moist places to lay their eggs. This has been a limiting factor in their lives.
So it was that not long after the amphibians had become established on the land that some of them trended away from this method of reproduction to become completely independent of the water. Thus arose the first reptiles, very much like their immediate amphibian forebears in many respects. But one very important thing distinguished the first reptiles from the amphibians, and this was the amniote egg. In these early reptiles, and in reptiles ever since, it was not and has not been necessary to return to the water to reproduce. These animals were freed from their ancestral habitat and could wander far and wide over the world, because they produced an egg that was protected from the environment. The reptile egg, in brief, is enclosed by a shell within which the developing embryo is bathed in fluids. One can say that the embryo lives in a sort of little pond of its own, in the dark world within the shelled egg. Then when the baby reptile breaks out of the crypt where life began, it appears as a miniature of its parents, ready to make a living in a strange and hostile world.
The rise of reptiles from their amphibian ancestors did not immediately usher in an Age of Reptiles, because for some time there was keen competition between amphibians and reptiles for living space on the earth. When the first reptiles arose the amphibians were well established and many of them were rather large and to a degree aggressive, so that the early reptiles had to fight their way upward. But in time the advantages of the shelled egg, together with improvements in the individual for life on dry land, did tell, and the reptiles came into their own.
The Age of Reptiles began something more than two hundred million years ago, and it continued until about seventy million years ago. It was a long and successful reign; in many respects the most spectacular phase of life history. It was the age when, through much of its extent, the dinosaurs ruled the land. And it came to an end only when the great dinosaurs and other large reptiles that lived with them became extinct. Since that event the world has been dominated by the mammals, and within the past few hundred thousand years one mammal, man, has established himself as the master of the globe. The Age of Mammals has recently passed, and we are living in the Age of Man. Being men we can do something that no animals in the past have been able to do: we can look forward and we can look back. We can look back to the Age of Reptiles.
Such a backward glance leads us through great expanses of geologic time. But how can one look back through the corridors of the years to long vanished ages, back to the Age of Reptiles and beyond?
Early in the last century one of the pioneers of geology, William Smith, an English surveyor, established the principle that each particular stratum of sedimentary rocks — of sandstones, shales or limestones - has enclosed within it (if it is fossiliferous) fossils that are distinctive for that layer. The sediments of the earth are piled up, one on top of another, in a great succession that records their deposition on the bottoms of shallow seas and ocean basins, in lakes, ponds, streams, and rivers, in the dunes of deserts or as the debris of glaciers.
It is obvious that the oldest sediments are at the bottom of this grand sequence, and that successively younger sediments follow in order. It is also obvious that the fossils in these sediments show a progression from oldest to youngest, and hence a record of the progression of life through the ages.
Of course it isn't nearly as simple as these few sentences might indicate. The story of earth history as recorded by rocks and fossils is incredibly complex, with parts missing at various places over the globe, with other parts confused by the processes of earth movements, by the upwelling of volcanoes and by other earth forces - in short with countless interruptions, modifications and the like that have taken place during millions of years of time. But as a result of long explorations and studies in field and laboratory, students of earth history have been able to put together many facts to make a reasonably orderly story. They have been able to draw up a table of geologic time, in which a sequence of periods has been established, each distinguished by its fossils, as well as by indications in the rocks of the physical events that took place while these rocks were being formed.
The table of geologic time is simply presented in Table 1. A few explanations are necessary.
The three eras in the Table are named on the basis of the general aspects of life through time: Paleozoic, ancient life; Mesozoic, middle life; Cenozoic, recent life. Geologic history before the fossil record has also been subdivided into several eras, but it is common practice to call this long early phase Precambrian time, and for the purposes of our book such usage is quite sufficient.
The periods within each era are named after regions where they were first studied, or according to attributes of their sediments. Cambria is the ancient name for Wales, where rocks of this age were first defined. The Ordovices and Silures were prehistoric tribes of southern Britain, inhabiting the regions where rocks of this age were originally studied. Devonian is named after Devonshire. Carboniferous, the term used by most European geologists, refers to the great carbon or coal deposits that typify these rocks. Mississippian and Pennsylvanian, used by North American geologists, refer to the regions where these rocks are most completely exposed. Permian is from the district of Perm in northern Russia. Triassic refers to the threefold division of rocks of this age first recognized in central Europe. Jurassic comes from the Jura mountains in the Alps. Cretaceous is from the Latin creta, meaning chalk, a reference to the cliffs of southern England. Tertiary is based on an old classification of rocks, of which these are the third part. Quaternary similarly refers to an original fourth division of geologic history. These last two terms are retained for convenience, whereas the correlative terms originally used for earlier divisions of geologic time, the Primary and Secondary, have long since been abandoned. All of which goes to show that man tries, but never quite succeeds, to be completely logical.
The duration of the various periods as shown in the Table are based upon careful studies made within the past few decades on radioactive elements. It is known, for example, that certain elements, such as uranium, break down through time, the end product of this process being lead. The rate of decay is known. By comparing in selected rocks, fortunately discovered at various levels through the geologic column, the ratios of radioactive elements and the end products, it is possible to reach figures that express with what is thought to be a considerable degree of accuracy the age of the rocks in years. On these studies our modern ideas about the duration of geologic time have been established and generally agreed upon.
As for the Ages shown in the chart, these are rather informal designations to indicate the types of animals that were generally dominant during the geologic periods to which the names apply. They are not to be taken too seriously, but they are useful.
For the purposes of this book the Age of Reptiles will be regarded as consisting of the Permian period, at the close of the Paleozoic era, and the Triassic, Jurassic and Cretaceous periods, making up the Mesozoic era. This is somewhat at variance with common geologic practice, which assigns only the three Mesozoic periods to the Age of Reptiles, thus making a neat coincidence of ages with the major geologic time divisions. Certainly such an orderly arrangement is very convenient to those interested in the large dimensions of earth history; but if we are to be concerned with the time when reptiles were truly dominant on the earth, we must include the Permian period within our Age of Reptiles.
This Age, so far as it concerns the evolutionary history of reptiles and amphibians, is readily divisible into two definite stages. The first of these, consisting of the Permian and Triassic periods, was the time when lands were ruled by large amphibians belonging to a group known as the labyrinthodonts, by primitive groups of reptiles, and by the mammal-like reptiles, which appeared very early in reptilian history. At the close of this Permian-Triassic stage of life history the labyrinthodonts, the primitive reptiles and the mammal-like reptiles became extinct. So it was that the second stage of the Age of Reptiles, composed of the Jurassic and Cretaceous periods, began, and new groups, most of them originating during the latter part of the Triassic period, became dominant on land and sea. These were the frogs, the various archosaurs, consisting of thecodonts, dinosaurs, crocodilians and flying reptiles, the turtles, lizards and snakes, the marine ichthyosaurs and plesiosaurs, and the primitive birds and mammals. Thus, instead of looking at the Age of Reptiles as a straight succession of four periods, Permian—Triassic—Jurassic—Cretaceous, we may look at it as a divided succession: Permian-Triassic/Jurassic-Cretaceous. If so regarded, the history of the tetrapods living during these several periods of earth history will take on a new significance.
Such is the setting of time within which our story takes place. Now let us become acquainted with the principal characters of the story, the reptiles, and the ancient amphibians, birds and mammals who were their contemporaries. What are reptiles? And what are these other groups of animals? In the first place, they are all tetrapods, a word which means 'four feet'. These are the backboned animals with a basic common inheritance of four legs (although in some of them the legs may be variously transformed into wings, or paddles, or they may even be lost) and lungs. They are the animals primarily adapted for life on the land, in other words the backboned animals other than fishes.
One way to compare modern tetrapods is on the basis of their body temperatures. Among the amphibians and reptiles there are no internal physiological mechanisms to regulate temperatures, so that the body heat in these animals varies more or less directly as does the temperature of the environment. Consequently the amphibians and the reptiles are often thought of as being ectothermic or 'cold-blooded' tetrapods, capable of continued activity only in warm or mild climates. In contrast the birds and the mammals, which are often designated as being endothermic or 'warm-blooded' tetrapods, do have internal physiological controls of their body temperatures, so that these temperatures remain more or less constant, however environmental temperatures may fluctuate. This distinction between the amphibians and the reptiles on the one hand, and the birds and mammals on the other, reflects something of their origins and past histories, for the amphibians and reptiles arose during Paleozoic time and retained their primitive lack of bodily temperature controls, whereas the birds and mammals arose from certain reptiles during Mesozoic times, and in the course of their evolutionary origins became endothermic animals.
If one compares reptiles downwardly with amphibians a crucial difference between them, namely their different modes of reproduction, is at once apparent. As has been shown, the amphibians lay an unprotected egg and go through a tadpole stage in the water, whereas the reptiles, having lost this ancient tie with the ultimate fish ancestors of all land-living tetrapods, lay an egg protected by a shell, thus freeing them completely from dependence on water for continuation of the species. The distinction between the two groups on this basis is quite clear, but when we look at the diagnostic features of amphibians and reptiles as delineated by anatomical structure, which is necessary when studying the fossils, it is not easy to draw sharp boundaries between primitive reptiles and their amphibian ancestors. The emergence of the reptiles from their amphibian forebears was a gradual process, and as is so often the case when we have a rather good fossil record, the lines of demarcation between the groups tend to be rather fuzzy and ill-defined. The modern wanderer in field and woodland has no difficulty in distinguishing a frog from a snake, but the student of fossils does not always find it easy to separate early reptiles from their amphibian cousins.
(Continues...)
Excerpted from The Age of Reptiles by EDWIN H. COLBERT. Copyright © 1997 Edwin H. Colbert. Excerpted by permission of Dover Publications, Inc..
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Table of Contents
Preface to the Dover EditionPreface to the First Edition
1 "TIME, TETRAPODS AND FOSSILS"
2 BEGINNING OF THE AGE OF REPTILES
3 EARLY REPTILIAN RULES OF THE LAND
4 THE TRANSITION
5 SUPREMACY OF THE REPTILES
6 THE FIRST WAVE OF EXTINCTION
7 NEW RULING REPTILES
8 DOMINANCE OF THE DINOSAURS
9 A NEW WORLD FORESHADOWED
10 ZENITH OF THE DINOSAURS
11 THE GREAT EXTINCTION
Bibliography
Addendum
Index