Megafauna: Giant Beasts of Pleistocene South America

By Richard A. Fariña, Sergio F. Vizcaíno and Gerry De Iuliis

“An enjoyable read that provides a substantial amount of detail on the biology, ecology, and distribution of these fantastic animals . . . Highly recommended.” —Choice

More than 10,000 years ago spectacularly large mammals roamed the pampas and jungles of South America. This book tells the story of these great beasts during and just after the Pleistocene, the geological epoch marked by the great ice ages. Megafauna describes the history and way of life of these animals, their comings and goings, and what befell them at the beginning of the modern era and the arrival of humans. It places these giants within the context of the other mammals then alive, describing their paleobiology—how they walked; how much they weighed; their diets, behavior, biomechanics; and the interactions among them and with their environment. It also tells the stories of the scientists who contributed to our discovery and knowledge of these transcendent creatures and the environment they inhabited. The episode known as the Great American Biotic Interchange, perhaps the most important of all natural history “experiments,” is also an important theme of the book, tracing the biotic events of both North and South America that led to the fauna and the ecosystems discussed in this book.

“Collectively, this book brings attention to the discovery and natural history of ancient beasts in South America while providing a broader temporal and geographic background that allows readers to understand their evolution and potential immigration to South America.” —Quarterly Review of Biology

“An excellent volume . . . This book is likely to facilitate progress in the understanding of fossil mammals from the Americas.” —Priscum

• Language: English
• Publisher: Open Road Integrated Media
• Release date: May 22, 2013
• ISBN: 9780253007193

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© 2013 by Richard A. Fariña, Sergio F. Vizcaíno, and Gerardo De Iuliis

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Library of Congress Cataloging-in-Publication Data

Megafauna : giant beasts of Pleistocene South America / Richard A. Fariña, Sergio F. Vizcaíno, and Gerry De Iuliis.

p. cm. — (Life of the past)

Includes bibliographical references and index.

ISBN 978-0-253-00230-3 (cloth : alk. paper) — ISBN 978-0-253-00719-3 (eb) 1. Mammals, Fossil—South America. 2. Paleobiology—South America. 3. Geology, Stratigraphic—Pleistocene. I. Fariña, Richard A. II. Vizcaíno, Sergio F. III. De Iuliis, Gerardo, [date]

QE881.M475 2012

569.098—dc23

2012017801

1 2 3 4 5 17 16 15 14 13

To past and current researchers of South American fossil mammals.

Those of the past are an endless source of inspiration, those still current of intellectual motivation.

To the memory of Mirta Tosar, my mother, who taught me to be bold and love animals as a person, and to Neill Alexander, who encouraged me in the same way as a scientist.

R.A.F.

To the memory of my parents, Eric and Negra, who instilled in me the value of hard work and honesty.

To Susi, Rulo, Leo, Tano, Guille, and Nestor, the people with whom I share every day the joy of doing this job.

S.F.V.

To my family and the memory of my father and father-in-law, whose sacrifices allowed me the luxury of doing what I love.

To Charles Rufus Churcher, who instilled in me the intellectual discipline to carry it out.

G.D.I.

The number of the remains embedded in the grand estuary deposit which forms the Pampas and covers the granitic rocks of Banda Oriental, must be extraordinarily great. I believe a straight line drawn in any direction through the Pampas would cut through some skeleton or bones. Besides those which I found during my short excursions, I heard of many others, and the origin of such names as the stream of the animal, the hill of the giant, is obvious. At other times I heard of the marvellous property of certain rivers, which had the power of changing small bones into large; or, as some maintained, the bones themselves grew. As far as I am aware, not one of these animals perished, as was formerly supposed, in the marshes or muddy river-beds of the present land, but their bones have been exposed by the streams intersecting the subaqueous deposit in which they were originally embedded. We may conclude that the whole area of the Pampas is one wide sepulchre of these extinct gigantic quadrupeds.

Charles R. Darwin, November 26, 1833

C

Contents

Preface & Acknowledgments

1 Paleontology and Science: What Is Science?

2 Distinguished Paleomammalogists

3 Geological and Ecological History of South America during the Cenozoic Era

4 North American Late Cenozoic Faunas

5 The Great American Biotic Interchange and Pleistocene Habitats in South America

6 Bestiary

7 Physics of the Giants

8 General Paleoecology

9 Extinction

Epilogue. Lessons from the Deep Past

Appendix 1. A Primer on Skeletal Anatomy

Appendix 2. Skeletal Anatomy of Xenarthrans

Appendix 3. Equations Used to Estimate Body Masses Based on Dental and Skeletal Measurements and Their Respective Sources

Appendix 4. Calculations

References

Index

P

Preface & Acknowledgments

The first reports, during the late 1700s and early 1800s, of the fossil remains of South America’s magnificent Pleistocene beasts, so fantastically bizarre, immediately caused a stir among the general public and, in particular, the European scientific community. The first notices of their discovery described them as monsters, firing the imagination and interest of several eminent scientists and politicians, and leading some of them to believe that these great beasts still wandered among the unknown (for Europeans, at any rate) reaches of the New World. The fossils helped usher in a new episode among the fledgling nations of both South and North America, striving then for recognition and validation in the eyes of the established European powers: finally they had something of their own that rivaled the great treasures of the Old World. Eventually, the fossils contributed significantly to the establishment of new scientific institutions and traditions as the New World countries took hold of their destinies and exploration of their territories.

The fossil mammals of both North and South America began to reveal an unimagined chapter in the history of mammals, based as it then was mainly on knowledge unearthed from European deposits, but it was those from South America that were most strikingly different and garnered much of the early attention. Perhaps because of this distinctness, largely as a result of the long, past isolation of South America from other continental landmasses, they played crucial roles in the development of modern biological thought. We may note as examples of their scientific achievements that a South American fossil mammal (Megatherium americanum, a giant fossil sloth) was the first fossil to be formally described and named scientifically, and its skeleton was the first to be mounted in a lifelike pose. The sharp mind of Georges Cuvier, the great French comparative anatomist, forged the concept of extinction (in the modern sense of this word) based on this fossil sloth (as well as on North and South American remains of fossil elephant relatives). Perhaps most significantly, it was the giant sloths, the giant armadillo-like glyptodonts, and the majestic and ponderous toxodonts (among other South American fossil remains) that struck most fervently upon the fertile mind of the young Charles Darwin, both during and after his famous voyage aboard the HMS Beagle, as he worked out his ideas on evolutionary theory.

Despite the relative isolation of the new South American countries, these ideas greatly affected scientists and intellectuals on both sides of the Río de la Plata, several of whom (such as the Ameghinos) took on the void created by Darwin’s return to England and restarted the study of the South American fossil mammals with renewed enthusiasm. Such was their influence that it even affected the excited atmosphere of the newly born city of La Plata, which inspired an illustration that adorns one of the entrances of the División Paleontología de Vertebrados of the Museo de La Plata. As explained in Chapter 8, it was conceived at the end of the nineteenth century, just after the city’s founding, as an artist’s rendition of a plan to embellish the gardens between the museum and the neighboring zoo, two of the proud city’s new jewels. In these gardens, visitors could stroll between the institutions that housed the living and the long dead and have a sense of those extinct beasts, brought back to life in the form of life-sized sculptures.

Despite such an early and auspicious beginning, the study and our understanding of South America’s extinct mammals has generally lagged behind those from most other continents. In part this is certainly due to their distinctness, generally leading scientists to either consider them too odd to expend much energy on, or regard them as somewhat inferior variants of more typical mammals, as was once done for dinosaurs. In accepting such views, the past was condemned to be more or less like the present and the magnificent mammals of South America relegated to being antiquated curiosities of better and more modern mammalian designs.

In this book we endeavor to reveal that such views are erroneous. Despite their differences and the fact that no living analogues exist for many native South American mammals, thus making comparisons difficult, we show that these mammals and the environments in which they lived and evolved were likely not merely slight variations of those that exist today. Their study is thus entirely worth the effort. By combining a wide variety of techniques from several biological disciplines, researchers over the past decade or so have begun bringing these beasts back to life. The picture that has begun to emerge is that of a marvelous biota that resists being pigeonholed, one that at once enlightens our concept of mammalness and enhances our knowledge of the past.

We begin our story in Chapter 1 with an introduction to the splendor of the South American megafauna, the assemblage of large-sized and gigantic beasts (or megamammals) that are the main subject of this book. In addition, we provide an analysis of how paleontology (and science in general) works, discussing its place among the evolutionary sciences, and consider other relevant background topics such as the role of fossilization, geology, and biological classification. This sets the stage for the more detailed discussions of the mammals and their environments provided in later chapters. We continue in Chapter 2 by outlining the historical contributions to the study of South America’s fossil mammals: a pantheon of researchers whose efforts illuminate our present knowledge, for these are the giants on whose shoulders modern paleontologists stand. Chapter 3 provides more detailed discussions, begun in Chapter 1, of the geological and ecological contexts in which this fauna evolved. These historical contexts are indispensible, for they allow us to understand the natures of and differences between the Pleistocene and modern South American fossil assemblages. This theme is continued in Chapter 4, which focuses particularly on the North American fauna and its relationship with that of South America—although we are getting ahead of ourselves; much of the Pleistocene faunas, and to an even greater degree modern South American faunas, are ultimately of North American origin, and thus the northern stock plays an integral role in our story.

In Chapter 5 we bring together the diverse threads of the previous chapters as we discuss the history of the native South American mammals, the geological processes leading to the emergence of the Isthmus of Panama, finally ending South America’s long isolation, and the episode that followed: the extensive intermingling of the North and South American faunas known as the Great American Biotic Interchange and its consequences. As well, we consider the habitats present in the Pleistocene, which served as the essential backdrop to the events occurring among the mammals. Chapter 6 rejoices in the wonders of this splendid assemblage by focusing on many of the main large-bodied species, noting their relationships, main characteristics, and roles as members of the fauna. Chapters 7 and 8 outline our more recent attempts at bringing these mammals back to life—or at least in viewing them as having once been real flesh-and-blood beasts rather than the dry and scattered bones that we have left of them. We provide explanations of some of the methods used to study the paleobiology and paleoecology of these animals, particularly of the giants, and present the results of such research into their habits, ecology, metabolism, appearance, lifestyles, and diets.

As is generally known, the magnificent megafauna of South America (and several other continents) is no more. Chapter 9 presents our attempts to explain the demise of these beasts. The extinction of the megafauna is a touchy subject: whereas we are generally content to explain away other extinction events (of the nonavian dinosaurs, for example) as due to any, all, or some combination of environmental, tectonic, catastrophic, and climatic factors, the disappearance, only some 10 thousand years ago, of the South American Pleistocene megafauna occurred in the presence of humans and debate rages on the role, if any, that humans played. Our treatment of the subject is set within the framework of the competing hypotheses with all their scientific and ethical implications. Last, the Appendices provide guidance on the basic anatomy required to interpret our discussions and descriptions of the mammals, and more in-depth explanations of some of the methods described in this book’s chapters.

The publication of this book is due to the efforts and resources of many people and institutions, and so we have many to thank. Foremost among these, we acknowledge the generosity and graciousness of Edmundo Canalda. In addition to his much-appreciated encouragement, Edmundo (and Fin de Siglo) kindly allowed us to borrow liberally from Hace Sólo Diez Mil Años (Fariña and Vizcaíno, 1995). Though briefer in scope and less sophisticated and detailed in treatment of the themes presented in the current book, the similarities between them are unmistakable. In addition, we thank Ángeles Beri for help with and information on Cenozoic plants, Sebastián Tambusso and Néstor Toledo for many of the illustrations, Eva Fariña for recreating some of the medieval bestiary figures in Chapter 6, Signe Haakonsson for reviewing the spelling in some of the references in Danish, Guillermo Cassini for help with Fig. 3.14, Mauro Muyano for many illustrations, Jonathan Perry for reading parts of this book and making many good suggestions, Dino Pulerà for kind permission to reproduce several anatomical illustrations, Celestino De Iuliis for reading and commenting on the proposal and initial drafts of the earlier chapters, Peter Reali for help with photography and image reproduction, Silvia Ametrano, director of the Museo de La Plata, for permission to reproduce images of publications, specimens, and art of this institution, Susi Bargo for collaborating with the three of us on many aspects of this book and much of the original research, Eduardo Mizraji and Andrés Pomi for their help in finding Vilardebó’s image, Antonio Carlos Sequeira Fernandes and Deise Henriques for their help with the photograph of Paulo Couto, Jhoann Canto and the Eberhardt family for their help with the photograph of Captain Eberhardt, Juan Carlos Fernicola for helpful discussions on glyptodonts and the image of H. G. Burmeister, the American Museum of Natural History (New York) and the Field Museum of Natural History (Chicago) for permission to reproduce images and the staff who facilitated our requests, the many other colleagues who provided original images and whom we acknowledge individually in the figure captions, colleagues too numerous to list who provided original information, shared their views, and collaborate with us in the ongoing process of learning about South American fossil mammals, our graduate students who have developed their research under our supervision (and the innumerable other tasks that they inevitably take on), Marco Tosi for a long-ago evening in Rome that helped launch G.D.I.’s long and fruitful relationship with South American researchers. S.F.V. acknowledges the contributions of conicet (Consejo nacional de Investigaciones Científicas y Ténicas), Universidad Nacional de La Plata and the Agencia Nacional de Promoción Científica y Tecnológica, for these agencies’ continued support for much of his research activities. Last, a huge thanks to our spouses, Ángeles, Miriam, and Gina, and children, Eva, Josefina, Julieta, Theodore, and Jacob, for their encouragement and tolerance throughout the duration of this project.

1

Paleontology and Science: What Is Science?

Introduction

South America, the southern half of the pole-to-pole landmass named, according to the usual attribution, after the Italian merchant and cartographer Amerigo Vespucci—or, as convincingly argued by Lloyd and Mitchinson (2008), after the wealthy Bristol merchant Richard Ameryk, a main investor in Giovanni Caboto’s second transatlantic voyage—remains a territory full of interest, intrigue, and biological treasures. Artificially severed from the northern half by the Panama Canal, it extends from the tropics, where the marvelous Amazonian rain forest offers its biological diversity and chemical riches of trapped carbon, to the elevations and endless steppes of Patagonia, its tapered south that points at and nearly touches frozen Antarctica. From west to east, the assortment of landscapes includes the soaring Andes, followed in places by the Altiplano that so aroused past greed for silver and gold, and then descends into the low-lying eastern plains, where the fossils discussed in this book have mainly been found.

Despite this variety in habitats, latitudes, and altitudes, the great naturalist Georges-Louis Leclerc, better known as Comte de Buffon, claimed that South America lacked enough vital energy to yield true giants among its fauna. Indeed, the present-day fauna boasts no true megamammals, i.e., those for which body mass is given in tonnes (or megagrams, hence the term). Impressive as it is for a rodent, the capybara, at 60 kg, can hardly lay claim for membership in the same category as elephants and rhinos. Also of similar size are two xenarthrans (a group of mammals particularly characteristic of South America), the giant armadillo Priodontes maximus and the giant anteater Myrmecophaga tridactyla (both, sadly, of dwindling populations), for which the adjective applies only in comparison with their close kin, but not if placed side by side with hippos and giraffes. The largest South American mammal is the tapir, whose bulk (approximately 300 kg) is less than striking compared with bison. Among carnivores, the jaguar is the largest, but it cannot match other top predators, such as lions and tigers, respectively two and three times its size.

It was Charles Darwin himself who corrected Buffon: rather than absence, the reality for huge South American mammals is recent demise. Ten thousand years ago, an instant in terms of geologic time, South America was inhabited by a mammalian fauna so large, diverse, and rare that today’s African national parks would pale in comparison. Bears, sabertooth cats, and elephant-like gomphotheres lived alongside much-larger-than-extant 150-kg capybaras and oversized llamas. Horses roamed these lands and went extinct thousands of years before Spanish explorers reintroduced the domestic species. In addition to these relatively familiar mammals, there were also bizarre creatures only distantly related to modern forms, such as terrestrial sloths several meters in height when standing bipedally; completely armored glyptodonts, hippo-sized animals related to armadillos; the camellike macrauchenids; and the rhinolike toxodonts, weird yet strangely familiar in echoing the ungulate, or hoof-bearing, types common in North America, Africa, and Asia.

Thanks to the efforts of many people, both collectors who found and prepared the material, often in decidedly uncomfortable and in some cases outright dangerous conditions, and scholars who interpreted those remains, we have now resurrected them for our contemplation and awe.

Traveling today through the sparsely populated regions of South America, it boggles the mind to think that the plains and low hills were once filled by a fauna so grand that it was rivaled only by the dinosaurs in magnificence. But unlike these long-extinct creatures of the Mesozoic, the South American fauna is much closer to us in time and in the phylogenetic (or genealogical) relationships to living mammals, as well as being more abundantly preserved. It is therefore easier to understand the biology and infer the ways of life of these magnificent mammals; further, the extraction of genetic material from the more recent fossils is already being accomplished, in contrast to the currently impossible science fiction world of Jurassic Park. Perhaps even more intriguing is that some of these mammals coinhabited these lands with early Americans, who may, regrettably, have had a hand in their demise.

In this book, we deal with the life and times of these remarkable beasts, the adaptations they evolved, their origin and journeys, the ecology of those (not so) long ago times, and the possible reasons for their extinction. All these subjects are dear to paleontology, a discipline that claims its own place within science. Most people think of paleontologists as adventurous individuals dashing through remote areas of the world to find and dig out dinosaur bones. Although it is true that paleontologists need fossils to understand life of the past, their scope of knowledge and skills is broader than that required to unearth old dead things. Paleontologists usually receive broad training in both biology and geology, and they are generally equally comfortable in the field, deciphering the geology of the area under investigation, and in the lab, researching and describing the material they have collected. The latter in particular requires a wide knowledge of living organisms—how else could we hope to understand the way extinct animals appeared and behaved from the meager scraps we have left of them? This requires, among other things, an extensive knowledge of anatomy—it is no coincidence that many paleontologists also teach comparative anatomy in universities and human anatomy in medical schools.

Paleontology and science

Paleontology falls under the aegis of evolutionary biology, the scientific field that deals with the remarkable diversity of the organic world, past and present. The theory of evolution, the cornerstone of modern biology, includes the set of ideas explaining how and why evolution occurs. It is one of science’s most robust theories, supported by an impressive array of evidence. Surprisingly, it also happens to be among those scientific theories that the general public is least confident in. A recent poll among Britons, for example, indicated that fewer than half of respondents accept the theory of evolution as the best explanation for the development of life, with nearly 40% opting instead for some form of creationism—the set of ideas that rely on some sort of supernatural creator or designer as an explanation for the origin and diversity of life.

The situation in the United States is pretty much the same as in the United Kingdom. It seems incredible that one of science’s strongest theories should be viewed with such skepticism in such a scientifically and technologically advanced nation. Then again, some have cast doubt on the theory of evolution and voiced support for the teaching of intelligent design theory in U.S. public schools. This position was soundly routed in U.S. district judge John E. Jones’s 2005 ruling against Pennsylvania’s Dover area school board’s decision to insert intelligent design into the public school science curriculum. In one of the most stinging criticisms of creationism to date, Judge Jones noted the overwhelming evidence that establishes intelligent design as a religious view, a mere relabeling of creationism, rather than a scientific theory, and he condemned the breathtaking inanity of the Dover board’s policy, accusing some of the board members of lying to conceal their true motive of promoting religion.

Much of the public’s suspicion of evolution is the result of a consistent battle waged by a small, but politically active and well-connected fundamentalist religious faction that has tried to disparage evolution while trying to impose its own viewpoint on U.S. public schools; that is, on students during what is probably their most intellectually vulnerable stage. Originally a concern mainly in the United States, creationism has taken root in other Western nations over the last two decades. Its onslaughts on science have both contributed to and played upon, in synergistic fashion, the general public’s misperception of what science is. It is one thing when people who have not received a higher level of education are swayed by creationists, especially as many people start out their school careers already inculcated with some worldview that includes elements of creationism. It is quite another when the more highly educated members of society (U.S. polls indicate about 40% of college graduates do not believe that humans evolved from some other mammals) and indeed a former leader of the nation—a Yale University graduate with any number of highly qualified advisors supposedly at his disposal—misunderstand science. It is even worse when many professional academics venture into a field not their own and, with an embarrassingly limited understanding of the subject (remedied by even a first-year university-level course), decide they are qualified to hold forth on the perceived inadequacies of evolutionary theory. To place this sort of dabbling into perspective, image an evolutionary biologist trying to set a nuclear physicist straight on real or imagined inconsistencies in the theory of quantum mechanics due to interpretations over the value of particle accelerators. It is so preposterous an idea that we wouldn’t even take its proposition seriously. Yet we must wonder why so few eyebrows are raised when (rather than if) the reverse happens.

There seem to be two main reasons why so many people have an aversion to evolution. One is that evolution happens to touch on aspects of humanity dear to many of us: our place in this universe, and the meaning and purpose of our lives. The centrality of these facets leads many of us to assert that our personal convictions are as equally valid explanatory tools as evolutionary theory in dealing with the diversity, development, and history of life on this planet. However, this is simply not true. Evolutionary theory analyses the physical and behavioral changes in the forms of life over time and provides explanations for how those changes occurred. That such changes have occurred is beyond any reasonable doubt. Opinions on such questions are not equally valid, any more so than is any particular person’s opinion on whether matter is really composed of atoms or the earth revolves around the sun. There may still be people somewhere in the world that believe in a flat earth. Their opinion, bluntly put, simply does not count. Evolutionary theory does not deal with, nor does it purport to deal with, questions of spirituality, which, for reasons explained below, are outside the realm of science. Many scientists, including a good number of evolutionary biologists, such as Francisco J. Ayala (Fig. 1.1), have reconciled religion and science, maintaining their spiritual faith while recognizing the fact of evolution.

Another reason why people mistrust evolution has to do with how they view science. This calls into question the effectiveness and role of science education in much of Western society. Science education has made great strides in the last half century or so, particularly in conveying scientific knowledge. Yet we have to wonder whether our emphasis on presenting the results and successes of scientific investigation has overshadowed the more basic principles of how we arrive at that knowledge, on what actually qualifies as science. If we have done our job properly, then by the time a student graduates from high school—given that we start science education in the elementary grades—the student would have a perfectly good grasp of what science is and what it is meant to do, and there would be little if any reason for debate on whether we can accept evolution as scientific or whether it is true or not because it is only a theory, after all. When almost half of college graduates—in the United States, at least—are confused, it might be a sign that we are not doing all we can as educators. Given the current level of misunderstanding about science in general, and to place the information of this book in context, it is worth spending a few pages to consider what science is.

1.1. Francisco J. Ayala in 2008, former Dominican priest and famed geneticist and evolutionist. Although he does not discuss his personal opinions, his views represent an appropriate way of dealing with science and religion.

Image courtesy of F. J. Ayala, University of California, Irvine, USA.

Science is so important to modern society that rarely does a day go by without a media report on some new discovery. It is perhaps because science is so ubiquitous that we have begun to take it for granted and fail to reflect on just what it is that’s behind the headlines. This is certainly one reason why science is so misunderstood, but the fact that so many people complete their formal education without understanding the fundamentals of science means that we may not be instructing students properly—or at least not emphasizing the essentials.

What is it about science that makes it so special? Most people view scientists as diligently working away to fill in some piece of a giant puzzle, a great edifice of knowledge that, once completed, would represent the truth about the way the nature works. This picture is misleading. Scientists are not out to seek some universal truth—at least, not in the way most people understand truth. At best, we can say that scientists are out to describe the way that nature works, and that the best we can usually do is to arrive at some approximation of the truth.

Science provides us with tentative answers. Any scientist worth his or her salt realizes that any answer is provisional and can expect to be corrected or revised (Tattersall, 2002). Eventually a larger understanding of nature’s workings is achieved, but this knowledge is always based on our current abilities to perceive nature. If new techniques or conceptual frameworks provide us with novel ways of analyzing phenomena, then we can expect our conceptualization of nature to change. This is precisely what happened when we switched our picture of matter and motion from a Newtonian to a quantum perspective.

1.2. Science is a human activity that is both a body of knowledge (scientific knowledge) and a way (scientific method) of investigating natural phenomena. The method is peculiar to science and results in the knowledge we consider scientific. As explained in the text, there isn’t a single scientific method, although most people are unaware of this. The important aspect of scientific methodology is that it provides us with an objective or unbiased way to investigate natural phenomena.

This means that there is no giant puzzle to fill in. A puzzle is static, whereas science is a dynamic process, constantly rearranging its view of how nature works (Tattersall, 2002), with scientists accepting new ideas and rejecting old ones, as the need arises. The fact that there is change does not mean that the methods used to discover the knowledge are invalid or somehow inferior. Indeed, we have made enormous strides over the past 300 hundred years in understanding the natural world using these methods, which have been far more successful than any other method humankind has devised. The success of science lies in its methodology. Science, as summarized in Fig. 1.2, is factual knowledge of the natural world gained through an unbiased procedure that is generally labeled as the scientific method. That is, we can think of science as consisting of a body of information or knowledge (facts, in most people’s estimation)—for example, an atom of hydrogen has a single proton in its nucleus. As well, science is a human endeavor; it also includes the way (by using scientific methodology) we obtain that knowledge.

In considering how scientists carry out their activities, we can begin to explore the essence of what makes science so successful at revealing the workings of the natural world. Through prior knowledge and observations, scientists propose ideas about some particular facet of nature. Why do they do this? In part, it is because, like most people, scientists are inquisitive beings. Further, scientists are generally driven to understand their particular corner of the universe; it is simply part of who they are. However, many people have ideas, so simply coming up with ideas isn’t what sets science apart. Once a scientist formulates an idea, it is evaluated by putting it through some sort of test, then published so that it is available to the scientific community (though usually only a small number of scientists are interested in or qualified to evaluate it). There, in the scientific arena, it is open to further scrutiny and testing. Other scientists can check this idea, evaluating whether it might indeed be a valid description of nature. How do scientists do this? By banding together and trying to prove it, as many people might believe? No, it is precisely the opposite: scientists try to knock it down; they try to show that it is not correct. In short, they try to falsify it. Scientists are not striving to prove something, because they realize that certainty is unattainable.

So how does science advance our knowledge? Before we get to this, let’s consider why other scientists would want to test another’s ideas. There can be many reasons. Some scientists may disagree, on the basis of their own observations and research. They may have competing ideas that they either have already proposed or are in the process of developing. They may simply doubt a scientist’s results based on their understanding of the way nature works. Being human, scientists also sometimes dislike one another, they may be envious of each other, or they may be competing for a limited pool of research funds.

Whatever the reasons (and we hope that they are more often than not of the nobler variety), the point is that ideas are tested. When scientists formulate ideas, they are careful to state them in such a way that they are open to testing or to being falsified. If a proposal is not open to falsification, then it is not considered scientific. This is why creationism (whether labeled as scientific creationism or intelligent design theory) is not and can never be scientific. These attempts begin by asserting that creationism and its tenets are true based on faith. If there is evidence that refutes any part of creationism (say, that the earth is only a few thousand years old), then it is the evidence that is wrong, because creationism must be true. Faith, by definition, is not open to testing, otherwise it wouldn’t be faith. Science accepts only natural explanations because supernatural explanations are not testable. The instant we resort to a supernatural explanation, we have stopped doing science altogether.

Science is grounded in doubt, rather than faith (Tattersall, 2002), and so ideas must be testable. When a testable idea, technically termed a hypothesis, resists falsification, scientists’ confidence that it is an accurate description of nature is strengthened. We can reformulate the sequence of events given above into the more formal (and stereotypical) flowchart (Fig. 1.3) that most students are exposed to during the primary and secondary school years.

This is the classic scientific method that so many of us have grown up with. This stereotype, however, is yet another hurdle to overcome, for students come away with the idea that it is the scientific method. Educators rarely go beyond it to explain that this method, the experimental method, is not the only one. It is critically important in many scientific fields, particularly those amenable to investigating phenomena in artificial environments, such as a laboratory, where the researcher has control over which and how many variables (or factors) are manipulated. Examples are much of physics and chemistry. However, it is unsuitable for investigating many scientific questions, such as those posed by much (but clearly not all) of cosmology, biology, geology, and meteorology—and paleontology.

These disciplines, which rely on other equally valid methods, are usually considered historical sciences because they investigate phenomena resulting from past events that cannot be recreated in a laboratory setting. Although aspects of these sciences are important components of school curricula, it is the experimental method (which is inappropriate for these sciences) that is typically presented to students. If students fail to make the link between the material they are learning and the method they are told is used to investigate scientific questions, we should not be surprised—the correlation does not exist! A volcanic eruption or the movement of the earth’s crust cannot be recreated in a laboratory; certainly, they may be modeled, but that is not the same thing, as explained in Chapter 9. Neither can the beginning of the universe, nor the interactions among the factors involved in ecological settings. For example, in many cases there is no way to experiment with an ecological system that does not also alter the system. If we wanted to know what would happen to a system if one element—say, a top carnivore, were eliminated—we could not do this experimentally. It would be neither ethical nor desirable to exterminate the species in question to determine the outcome of its absence. In such realms of study, scientists are presented with outcomes that are the result of historical and therefore nonrepeatable events. The experiment, so to speak, has already been performed, in this case by nature, and it is the task of the scientist to unravel the conditions and factors responsible for producing the results. Scientists involved in such questions use a methodology called the comparative method.

1.3. A flow diagram outlining the steps in the experimental method of scientific investigation. This represents the stereotypical view of how science is carried out, but in reality this is the general pattern followed in the experimental method, where a hypothesis is amenable to being tested by experiment. However, not all scientific hypotheses can be tested by conducting laboratory experiments, in which variables can be strictly controlled. Such testing is not easily applicable to scientific investigations of historical phenomena because the conditions that produced the phenomenon under investigation cannot be artificially duplicated. The appropriate methodology applied to the investigation of historical phenomena, where the experiments have been already conducted by nature, is the comparative method.

The comparative method, then, is used to investigate situations—or natural experiments—where variables cannot be manipulated because they occurred in the past, because they are impossible to manipulate, or because it is unethical to do so. It is termed comparative because the researcher investigates by comparison of situations that differ in the variable of interest. It is often necessary to set up classification systems to accomplish this. If we are interested, for example, in comparing habitats, then categories such as top predators and physical factors must be defined and described so that similarities and differences may be recognized in the first place. The researcher proceeds by observing the outcomes of the natural experiment based on the varying components of the different systems under study—in other words, by reconstructing what must have been the conditions that produced the outcomes. Let’s consider a simplified scenario to illustrate how the comparative method may be used to test hypotheses. In our example, we are investigating an ecosystem that contains a top predator such as a tiger and a potential prey species such as an antelope. We have noticed a dramatic decline in the numbers of antelope and hypothesize that the tigers must be responsible. How can we test this hypothesis? One way is to seek another ecosystem where tigers are scarce or absent. If the antelope are in decline there as well, it suggests our hypothesis is wrong. If the antelope population is in good shape, however, it suggests (but does not prove) that our hypothesis might be correct.

Although many aspects of biology and other sciences rely on the comparative method, this does not mean that all activities in these fields do. For many biological disciplines, such as physiology, anatomy, and molecular biology, the experimental method is clearly applicable. Others, such as ecology, systematics, and paleontology, normally rely on the comparative method. Many of the results discussed in this book, therefore, were obtained by means of the comparative method. In some cases, though, a combination of comparative and experimental methods were used, such as in the analysis of functional anatomy.

Before we leave the subject of the nature of science, we must address yet one more aspect that causes confusion, and this is the difference between fact and theory, and what scientists mean by theory. Many people equate facts with certainty but are less confident about theories. Most believe that there is a gradation between fact, an incontrovertible truth, and theory, as though a theory were not quite as good as fact, but something that is still awaiting proof (Gould, 1983).

A fact is an observable (keeping in mind that observation includes more than just sight) attribute of nature. Let’s consider gravity as a familiar example. If an object were dropped from a height, it is possible that it would fall up, but we recognize that the innumerable observations made about what happens to dropped objects makes the probability of such an event infinitesimally small, and we recognize that gravity exists; it is a fact (Gould, 1983). The theory of gravity is an attempt to explain why it exists and how it functions, and is decidedly not a discourse on whether gravity exists. Gravity would exist regardless of why we think it exists. Similarly, that evolution has occurred and continues to do so is indicated by numerous observations from many areas of knowledge. The theory of evolution is an explanation of why and how evolution occurs. It may be that some aspects of our ideas on how evolution occurs may be wrong, which is something that only scientific methodology could reveal and correct. More to the point here, however, is that our being wrong would have absolutely no effect on evolution’s factuality, which is supported by overwhelming evidence. As another example, consider that when, some four centuries ago, the religious establishment debated leading philosophers on whether to accept or reject the heliocentric explanation for the motion of celestial bodies in our solar system, the planets took no mind and continued to do what they had always done; they did not care in the least what humans thought.

A theory in science has a precise meaning that is distinct from its colloquial use. In everyday language, a theory is something akin to an opinion, as when, in a murder-mystery TV show, one detective asks another for his theory on what happened. In science, a theory is an explanation or a set of explanations for some attribute of the natural world that scientists have come to accept as being the best explanation at a particular time. Scientists do not accept theories because they have been proven to be correct—recall that scientists do not strive to provide proof (although scientists, being human, sometimes slip up in everyday talk and refer to the proof of one thing or another)—but because they have resisted repeated attempts to disprove them. Generally, theories start out as hypotheses that are tested over and over again. When scientists are sufficiently confident of their explanatory power, they come to be considered theories. A further caveat about scientists is necessary. They often speak about their own ideas as theories, when in fact they should refer to them as hypotheses—unless of course, the ideas have been accepted by other scientists as the best possible explanation. To start applying these concepts to the disciplines that study the life of the past, the raw materials of paleontology, the fossil remains of once-living organisms, are discussed in the following section.

Fossils and taphonomy

Who has not marveled at the discovery of something ancient, or come across dusty family albums in the attic, the staid expressions of our great-grandparents staring back at us, as though wondering what planet they were being observed from? Who has not stood in wonder at discovering an ancient object casually found in a field and reflected on the hand that gave it form? Human beings, like many other mammals, are curious creatures, and questions on origins, on what once was and now is no longer, often stand uppermost in our thoughts.

This is probably the main reason why there are professional students of the past, men and women who earn their living scrutinizing the past through the years, centuries, millennia, or eras, according to their own personal interests. In particular, the historian of ancient life is designated by a wonderful word of Hellenic elegance: paleontologist. To decipher its meaning, let’s examine its component parts: palaios (παλαιóς in Greek) means old, ontos ( ) refers to existing things and thus is a roundabout way of referring to life, and logos (λoγóς) means a treatment or discussion, and by extension the body of knowledge associated with it—or in other words, the corresponding branch of science. Thus, paleontology is the scientific field that is concerned with ancient life.

The paleontologist lives and breathes fossils. The etymology of the word fossil is ambiguous. It is certainly derived from Latin, but its precise meaning is not easy to translate. Probably the most appropriate translation is dug up, referring to the fact that fossils are usually buried and that a pick is the favored, or at least stereotypical, tool of the trade. Actually, in centuries past, the term fossil was used to describe a wide array of things that were dug up, such as gems and interesting concretions, as well as objects that we now recognize as fossils (Rudwick, 1985). As we shall see, one South American mammal in particular was important in helping establish what fossils, in the modern usage of this term, represent.

1.4. Fossil pollen: reproductive structure of a 250-million-year-old gymnosperm (cone-bearing plant). Width approximately 40 μ.

Image courtesy of Ángeles Beri.

The erstwhile pick is the tool of choice for most of the cases we discuss in this book, but not all fossils are the same. Most people think of fossils as the lithified remains of bones, teeth, and shells. A good number are, but fossils include much more than animal remains that have been turned to stone. A fossil is defined as any trace, impression, or remains of a once-living organism. Thus, a set of footprints preserved along an ancient mudflat is a fossil, as are tracks, trails, and burrows made by crawling invertebrates, or even more esoterically, boreholes made by predatory organisms in the shell of clams—two fossils in one! Some fossils may be microscopic (Fig. 1.4). For example, the deposition of plant pollen, as with dead plankton in general that sinks to the bottom of a body of water, is almost imperceptible.

The fragmentary remains of small mammals and birds are surrounded by resistant matrix. The use of a pick to recover such fossils, given the good fortune that they are preserved at all, is akin to using a shotgun to scatter flies. Instead, the remains are removed along with a good deal of the surrounding rock (Fig. 1.5)—a rock saw often comes in handy—and then packaged in a protective jacket, usually made of plaster, for safe transfer back to a laboratory. But even when digging out the bones that are normally thought of as fossils, we face a common dilemma: the bones, large or small, are often fragile. Their extraction requires great patience and the delicacy of a brush and needle, rather than brute force; the surrounding matrix must be removed, often grain by grain (in the laboratory, various techniques may be used), and the fossil itself must be coated with frequent applications of a dilute cohesive that penetrates its substance (Fig. 1.6). The old saying that paleontology could not exist without glue is an exaggeration, but it is not too far off the mark.

However, before the wondrous moment when the remains of an animal or plant from an age long past are extracted, a seemingly miraculous series of events must occur. The scientific field that studies these events is called taphonomy, which refers, according to the paleontologist I.  A. Efremov (Fig. 1.7; also a great novelist), who coined the term, to the laws of burial or, more expansively, the study of the transition of remains from the biosphere (the realm of living organisms) to the lithosphere (rocks or the inanimate world). If something is to be transformed into a fossil, then the natural recycling of material must be interrupted at some point. Usually, nature recycles all the materials previously incorporated into once-living organisms (Fig. 1.8) because the earth’s resources are limited and we cannot count on meteors and comets for perpetual deliveries of extraterrestrial supplies. Even though these resources exist in great quantities, they would be exhausted with the inexorable passing of time. This does not occur, though, because the lithosphere receives continuous transfer of material from the biosphere.

1.5. Recovery of a fossil. (A) Fossil remains being extracted along with the surrounding rock from the early Miocene Santa Cruz Formation along the Atlantic coast of Argentine Patagonia. (B) The block (including the fossil and surrounding rock) is then packaged in a protective jacket for safe transport back to a laboratory, where further preparation is carried out.

Fossilization may appear to be as nothing less than miraculous, the outcome of an improbable series of events. It occurs, however, because of the uncountable number of opportunities that are available without pause during the extremely long periods of time that nature takes in going about its business. For an organism to become fossilized, it must first die in such a way that it avoids being completely devoured by another animal (not such an easy trick in the real world), and then further hope that its more resistant parts are not feasted on by putrefying bacteria. One common way for this to happen is to be buried quickly, but even in such cases, not all bones that survive postmortem events reach a sedimentary environment conducive for preservation—in other words, it depends on where they are finally laid to rest. Fluvial sediments are often a good bet for a bone bent on becoming a fossil. On the other hand, for example, we have a relatively poor record of forest-dwelling animals because the acidic soils of forests lead to disintegration of buried bones.

1.6. In the laboratory, preparator Leonel Acosta of the Museo de La Plata (Argentina) patiently picks away the matrix surrounding the fossil remains. Note the bottle of glue in the right foreground of the image.

Courtesy of the Museo de La Plata, Argentina.

The different parts of an organism have different potentials for fossilization. Fortunately for us, we share a common interest with most organisms. They happen to require protective and supportive structures, which, as a result of the nature of their functions, are quite resistant. We, on the other hand, can discern a great deal from such structures, when and if we can find them. Protective and skeletal structures are formed by a few primary materials common in the biological world. The shells of mollusks, for example, are made of calcium carbonate, a material with great potential for fossilization.

It is not mere coincidence that the bones of vertebrates are those most commonly associated with the word fossil. The calcium phosphate that forms them is stupendously well preserved. Similarly, this occurs with teeth, which are the hardest elements of the vertebrate skeleton. The most common examples of fossils are often formed by petrification, which results in stony or lithified remains, and may occur by two processes. Permineralization occurs when soft structures decay while water containing dissolved minerals works its way into every nook and cranny of the hard structures. Minerals are then deposited in these cavities and pores, resulting in a fossil that still contains a good deal of its original hard parts. Replacement occurs when water dissolves the original material forming the hard structures and replaces them with minerals. This may happen so slowly that the new minerals duplicate almost exactly the shape and structure of the original organic material (Ausich and Lane, 1999).

1.7. Ivan Antonovich Efremov (1907–1972), pioneer in the field of taphonomy, during an expedition to Mongolia in 1949.

Image from the Paleontological Institute of the Russian Academy of Sciences, courtesy of Sergey Rozhnov and Valeriy Golubev.

Other readily fossilized substances include silicon, which forms the coverings of diatoms and other microscopic organisms; chitin, present primarily in the covering of insects and other arthropods, but also in lichens, earthworms, and other invertebrates; and cellulose, which provides rigidity to the cell walls of plants. Once in a while, the impossible happens: soft tissues, the daily bread of bacteria and predators, escape decomposition and present us with precious information that is almost always missing in the fossil record, as can be seen in the skin and feces of a ground sloth in Fig. 1.9 (see Chapters 5 and 7). Even more remarkably, once in a long while, an organism may be so completely preserved that its last meal is too, as occurs in deposits from near Messel, Germany.

The process of decomposition may be interrupted or altered in the absence of oxygen. This is the case in natural mummification, when the environment is too dry and hot for bacteria to carry on their activities. One example is the famous mammoths of Siberia, some so well preserved in ice that a banquet, with a main course of mammoth trunk, was served at an academic gathering. At least that’s the way this story is usually told. However, it may be apocryphal and based on the assumption that the mammoths were frozen extremely rapidly in some sort of extraordinary climatic catastrophe. The famous paleontologist Björn Kurtén suggested that what probably happened is that one member of the mammoth-hunting expedition made a heroic attempt to consume a bit of mammoth meat, but could not quite get it down—it seems that the carcasses had been partly decomposed before freezing (Farrand, 1961). Still, the flesh was so well preserved that someone might actually have tried.

Soft tissues may also be preserved by substitution of the organic matrix by minerals (Fig. 1.10). At times, this happens in so perfect a manner that the nature of the original structures may be studied microscopically. Wonderful examples of this sort of preservation occur in lower Cretaceous rocks (dated at about 125 Mya) of the Yixian Formation of Liaoning Province, China. These deposits preserve not only such delicate structures as skin and insect wings, but also feathered dinosaurs, findings that have revolutionized our idea of how some dinosaurs looked and lived.

Fossil tree trunks, which are not overly rare, may also be subject to such fine preservation. In Patagonia, there is a petrified forest, declared a national monument, that preserves the trunks, branches, and even fruit in exquisite detail. There is also a petrified forest in Arizona, and the conifer trunks in Yellowstone Park are petrified.

Further, fecal material may be fossilized under the right conditions. These fossils, termed coprolites, are invaluable because they can reveal dietary information, such as pollen, fiber, bones, and shells. The coprolites of several fossil sloth species have been recovered (Fig. 1.9).

Paleontologists’ traditional ideas of how fossilization occurs involve the replacement of the original tissue, including soft tissues, by minerals to produce a hard or lithified representation of the original. Several remarkable recent discoveries hint at other methods of preservation that suggest that fossilized remains need not be restricted to lithification of structures. Remains of what seem to be Mesozoic soft tissues of dinosaurs have been reported, essentially like those of modern species (Asara et al., 2007; Schweitzer et al., 2005, 2007). In addition to preservation of tissue, scientists have uncovered preserved protein sequences—these are real molecules present in the organism during the time it was alive! Fossilization of such materials was once thought to be impossible, as scientists could not conceive of a way that such fossilization could occur. Although the fossilization method is still unclear, it appears, at least in the case of tissue preservation, to have something to do with postmortem formation of polymers, long-chained molecules that become inert and thus resistant to further chemical changes.

1.8. (A) The recently dead remains of a guanaco from the steppes of Argentine Patagonia. The flesh and most of the skin have already been recycled by nature, but the skeleton is almost intact. Many people believe paleontologists routinely find such complete and well-preserved skeletons as fossils. However, such finds are exceedingly rare. (B) Much more commonly, the bones of animals are disarticulated, scattered about, eaten, or otherwise destroyed, as is shown in this image of a fossil sloth from the Pleistocene of Neuquén, Argentina.

1.9. A portion of the hide, including some fur, and dung of Mylodon darwinii from Última Esperanza cave, Chile, exhibited at the Museo de La Plata, Argentina.

Courtesy of the Museo de La Plata, Argentina.

But it does not end here. Fossilization requires still further luck. Once buried, structures still face the possibility of destruction from the pressure of overlying sediments. Finally, there is erosion, the paradoxical ally of the paleontologist. Erosion exposes a fossil originally buried deep within sediments, but there is only a small window of opportunity, for if the remains are not recovered, the same forces will destroy them.

A fragment of an organism is exposed to and must survive all of these risks before it may finally rest in relative peace in the drawers of a collection. Then it must be studied, but here the adventure becomes, to use Einstein’s definition of science, one of thought. These adventures are referred to below and in Chapter 2, exemplified through the activities of the people who have dealt with the fossil animals that are the subject of this book.

1.10. A portion of lithified pebblelike skin of an embryonic sauropod dinosaur from Auca Mahuida, Neuquén, Argentina.

Image courtesy of Rodolfo Coria, Museo Carmen Funes, Plaza Huincul, Neuquén, Argentina.

Linnaeus and classification

Taxonomy is the branch of the biological sciences that establishes the rules for and carries out the classification of living organisms. Classifying things, be they animate or inanimate, is an essential practice that helps humans keep track and make sense of the numerous objects and concepts that we must deal with daily. We can classify most inanimate objects fairly easily on the basis of their unchanging features. Nearly anyone living in a modern industrialized society would be able to pick out a van from an SUV, pickup, jeep, convertible, sedan, hatchback, station wagon, and so on. The reason is that we all agree on the features that distinguish these different subcategories of the category motor vehicles. If we were to switch languages, say from English to Spanish, then we would simply have to learn a new vocabulary (well, and grammar too . . . ).

Living things also need to be named. For many purposes, their names are not terribly important—it doesn’t much matter if people refer to their local species of chickadee as the black-capped chickadee, the boreal chickadee, or simply call it a chickadee, especially if it’s the only one in the area. For formal scientific purposes, however, the naming of organisms is critically important and not as straightforward. Further, as we shall see, we cannot use characters to define groups of organisms.

Scientists face two problems. One is naming all the different units or kinds of life, technically known as species, and making sure that no two closely similar species have the same name. When scientists from different parts of the world communicate about a species, they need to be sure that they are talking about the same thing. It may sound trivial, but the vast diversity of life forms can lead to considerable confusion. In some cases, a single species may have many informal local or common names (not due solely to the fact