The Age of Reptiles

Copyright

Copyright © 1965, 1997 by Edwin H. Colbert.

All rights reserved under Pan American and International Copyright Conventions.

Published in Canada by General Publishing Company, Ltd., 30 Lesmill Road, Don Mills, Toronto, Ontario.

Published in the United Kingdom by Constable and Company, Ltd., 3 The Lanchesters, 162–164 Fulham Palace Road, London W6 9ER.

Bibliographical Note

This Dover edition, first published in 1997, is an enlarged and corrected republication of the work originally published by W. W. Norton & Company, Inc., New York, in 1965. A preface and an addendum have been specially prepared for this edition by the author.

Library of Congress Cataloging-in-Publication Data

Colbert, Edwin Harris, 1905–

The age of reptiles / Edwin H. Colbert.

p. cm.

An enlarged and corrected republication of the work originally published by W. W. Norton & Company, Inc., New York, in 1965. A preface and an addendum have been specially prepared for this edition—T.p. verso.

Includes bibliographical references (p.–) and index.

9780486147956

1. Reptiles, Fossil. I. Title.

QE861.C72 1997

$67.9—dc20

96-31472

CIP

Manufactured in the United States of America

Dover Publications, Inc., 31 East 2nd Street, Mineola, N.Y. 11501

Table of Contents

Title Page

Copyright Page

Acknowledgments

Plates

Table of Figures

Preface to the Dover Edition

Preface to the First Edition

Chapter 1 - Time, Tetrapods and Fossils

Chapter 2 - Beginning of the Age of Reptiles

Chapter 3 - Early Reptilian Rulers of the Land

Chapter 4 - The Transition

Chapter 5 - Supremacy of the Reptiles

Chapter 6 - The First Wave of Extinction

Chapter 7 - New Ruling Reptiles

Chapter 8 - Dominance of the Dinosaurs

Chapter 9 - A New World Foreshadowed

Chapter 10 - Zenith of the Dinosaurs

Chapter 11 - The Great Extinction

Bibliography

Addendum

Index

DOVER BOOKS ON ANIMALS

Acknowledgments

With the exception of figures 2 to 19 inclusive, all of the figures were drawn especially for this book. The restorations, figures 22, 24–27, 29–32, 35-43, 46-48, 50-52, 54–58 and 61–65, were made by Margaret Matthew Colbert under the supervision of the author. The four maps, figures 23, 34, 49 and 59 were drawn by George Matthew Colbert under the supervision of the author. The charts, figures 1, 20, 21, 28, 33, 44, 45, 53, 60, 66 and 67 were drawn by Michael Insinna under the supervision of the author.

Acknowledgments for figures not listed above are as follows.

Figures 2, 4 and are from Gregory, W. K. (1951) Evolution Emerging, New York, Macmillan, vol. 2, p. 371, fig. 11.31 ; p. 387, fig. 12.9; p. 417, fig. 12.34. Figure 3 is from Olson, E. C. (1951) Fieldiana: Geology, Vol. 11, No. 2, pl. 4. Chicago Natural History Museum. Figure 9 is from Olson, E. C. and Robert Broom (1937) Journal of Paleontology, Vol. 11, No. 7, p. 615, fig. 3. Figures 6 and 8 are from Andrews, C. W. (1910) A Descriptive Catalogue of the Marine Reptiles of the Oxford Clay. Part 1, p. 62, fig. 42; p. 118, fig. 66. Figure 7 is from Gregory, J. T. (1945) University of Texas Publ. 4401, Pl. 33. Figure 10 is from Walker, A. D. (1961) Philosophical Transactions of the Royal Society of London, B, Vol. 244, p. 161, fig. 22. Figure 11 is from Osborn, H. F. (1917) Bulletin of the American Museum of Natural History, Vol. 35, Pl. 26. Figure 12 is from Brown, Barnum (1917) Bulletin of the American Museum of Natural History, Vol. 37, PI. 13. Figure 13 is from Reese, A. M. (1915) The Alligator and its Allies. New York, Putnam, p. 50, fig. 16. Figure 14 is from Williston, S. W. (1925) Osteology of the Reptiles. Cambridge, Harvard University Press, p. 297, fig. 188. Figure 15 is from Romer, A. S. and L. I. Price (1940) Geological Society of America, Special Paper No. 28, p. 324, fig. 59. Figure 16 is from Colbert, E. H. (1948) Bulletin of the American Museum of Natural History, Vol. 89, p. 393, fig. 22. Figure 17 is from Young, C. C. (1947) Proceedings Zoological Society, London, Vol. 117, p. 543, fig. 2. Figure 18 is from Heilmann, Gerhard (1927) The Origin of Birds. New York, Appleton, p. 33, fig. 23. Figure 19 is from Simpson, G. G. (1928) American Museum Novitates, No. 329, p. 6, fig. 3.

The four double-page plates, showing reconstructions of Permian (Plate 3), Triassic (Plate 8), Jurassic (Plate 13) and Cretaceous (Plate 18) scenes, were drawn by Margaret Matthew Colbert under the supervision of the author. Plates 1, 2, 4–7, 9–11 and 15 are from photographs taken by the author. Plates 12, 14, 16, 17, 19 and 20 are from the photographic files of the American Museum of Natural History in New York.

Plates

[Between pages 148 and 149]

The Lower Permian Wichita beds of northern Texas

The Upper Permian Cistecephalus zone in the Lower Beaufort beds of the Karroo Series of South Africa

Life of Early Permian time in Northern Texas

The Upper Triassic Red Beds and Cave Sandstone of the Stormberg Series of South Africa

A gully or sanga exposing the bright red sediments of the Upper Triassic Santa Maria formation in the state of Rio Grande do Sul, Brazil

The Upper Triassic Chinle formation, as exposed in Arizona

The Mesozoic sequence at Ghost Ranch, northwest of Abiquiu, New Mexico

Life of Late Triassic time in Western North America

The Lower Jurassic or Liassic cliffs of the Channel Coast near Charmouth, Dorset, England

The Upper Jurassic Oxford clays, as exposed in a quarry in Dorset

The Upper Jurassic Morrison formation, at Dinosaur National Monument, Utah

Excavating Jurassic dinosaurs at the turn of the century in the Bone Cabin Quarry, at Como Bluff, Wyoming

Life of Late Jurassic time in western North America

The Cretaceous Djadochta formation at Shabarakh Usu, Mongolia

The white Upper Cretaceous chalk cliffs of southern England

Hauling dinosaur bones out of the valley of the Red Deer River, Alberta, from exposures of the Cretaceous Belly River beds

Exposures of the Upper Cretaceous Edmonton formation, Red Deer River, Alberta

Life of Late Cretaceous Belly River time in western Canada

Excavating the skeleton of a duck-billed dinosaur

The Upper Cretaceous Hell Creek beds of Montana

Table of Figures

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Figure 20

Figure 21

Figure 22

Figure 23

Figure 24

Figure 25

Figure 26

Figure 27

Figure 28

Figure 29

Figure 30

Figure 31

Figure 32

Figure 33

Figure 34

Figure 35

Figure 36

Figure 37

Figure 38

Figure 39

Figure 40

Figure 41

Figure 42

Figure 43

Figure 44

Figure 45

Figure 46

Figure 47

Figure 48

Figure 49

Figure 50

Figure 51

Figure 52

Figure 53

Figure 54

Figure 55

Figure 56

Figure 57

Figure 58

Figure 59

Figure 60

Figure 61

Figure 62

Figure 63

Figure 64

Figure 65

Figure 66

Figure 67

Preface to the Dover Edition

THIRTY YEARS and more have elapsed since The Age of Reptiles was published simultaneously in 1965 by Weidenfeld and Nicolson of London and by W. W. Norton & Company of New York (Norton also published a paperback edition in 1966).

During the past three decades a monumental body of knowledge has appeared pertaining to late Paleozoic and Mesozoic tetrapods (the air-breathing, four-footed backboned animals), much of it based upon new paleontological discoveries made throughout the world. At the same time our concept of past intercontinental relationships has been profoundly revolutionized by the facts of Plate Tectonics. Since many details of fact and interpretation have been added to or have superseded the information on which the original text of this book was based, some recognition and discussion of our new knowledge are needed, if a reprinting of The Age of Reptiles is to be valid and useful to the present generation of readers. These desiderata are attempted in an Addendum to the original text, kindly authorized by Dover Publications.

It is to be hoped that this Addendum will be satisfactory to the reader and will provide a summary of sorts, outlining our modern understanding of the evolution of tetrapods, and of the continents and the oceans on which and in which they lived during some 200 million years of earth history.

Flagstaff, Arizona

EDWIN H. COLBERT

1996

Preface to the First Edition

IN 1831 Gideon Algernon Mantell, an English physician living in Lewes, published a three-column communication in a local paper, the Sussex Advertiser, entitled ‘The Age of Reptiles’. Mantell, one of the small, dedicated group of men who did so much in their so-called spare time to help found our modern sciences, had come to realize that during a long span of geologic history the earth was ruled by great reptiles. This interesting phase in the history of the earth and the evolution of life, so much of it marked by the dominance of the dinosaurs, has since Mantell’s day been the subject of extensive study in field and laboratory by men of many nations on all of the continents. It is a subject that has attracted much popular attention.

The purpose of the present book is to attempt a review, and it is to be hoped a concise one, of the Age of Reptiles, with particular emphasis on the tetrapods, the four-legged vertebrates, that lived during the years of reptilian supremacy. It is intended as a story of the tetrapod life of an ancient age, of the interrelationships between amphibians and reptiles, between birds and mammals, and between all of these animals and their environments, at successive times through some two hundred million years of earth history. It is a story of faunas that followed one after the other through time, and changed as they developed — as they mirrored evolution through the years, as they recorded the origin and the extinction of species. The story is not concerned in its primary aspects with descriptions of the tetrapods which are the chief actors, but these are necessarily briefly described, in order that the reader who is not familiar with them from other sources may have some idea of their principal features and their relationships. It is hoped that the illustrations will be of much assistance on this score.

The story is a large one, and in a book such as this a great deal must be omitted. Even so, it may seem to some readers that parts of the account are filled with details. The author can only ask for the indulgence of such readers, with the assurance that no more details have been included than are necessary to tell the story of the Age of Reptiles in a meaningful way.

I am indebted to many sources and to many people for the background and the help that has made possible the writing of this book. First I wish to acknowledge my debt to paleontologists and geologists, who through their publications, and in many cases through their letters and conversations, have provided the base on which this present work has been erected. Specifically I wish to extend particular acknowledgments to Margaret Matthew Colbert for the restorations of animals and scenes illustrating this book, to George M. Colbert for the maps and to Michael Insinna for the diagrams and other illustrations. Richard Carrington read the original manuscript and offered many helpful suggestions. The manuscript in its several phases was typed by Mrs Catharine Minerly.

1964

EDWIN H. COLBERT

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 fish-like 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.

Table 1. Geologic time *

(To be read from the bottom up)

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.