Life of the Past

An Unconventional Evolutionary History of the Skeleton

Matthew F. Bonnan

• Publisher: Indiana University Press
• Year: 2016
• ISBN:
  9780253018410
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

What can we learn about the evolution of jaws from a pair of scissors? How does the flight of a tennis ball help explain how fish overcome drag? What do a spacesuit and a chicken egg have in common? Highlighting the fascinating twists and turns of evolution across more than 540 million years, paleobiologist Matthew Bonnan uses everyday objects to explain the emergence and adaptation of the vertebrate skeleton. What can camera lenses tell us about the eyes of marine reptiles? How does understanding what prevents a coffee mug from spilling help us understand the posture of dinosaurs? The answers to these and other intriguing questions illustrate how scientists have pieced together the history of vertebrates from their bare bones. With its engaging and informative text, plus more than 200 illustrative diagrams created by the author, The Bare Bones is an unconventional and reader-friendly introduction to the skeleton as an evolving machine.

Setting the Stage

1

The sands of time were eroded by the river of constant change.

GENESIS, Firth of Fifth

Introduction: How Vertebrates and Cars Are (and Are Not) Similar

1

I KNOW VERY LITTLE ABOUT CARS. I KNOW THAT I LIKE TO GET FROM point A to point B in a reliable vehicle. I know that I like my car to start when I turn the ignition. I know that whereas I am theoretically capable of changing a tire on the expressway, it is probably best for me and passing drivers that this hasn’t occurred too often in my life. And it is probably obvious that manual transmissions and I have not made a proper acquaintance.

My wife and father-in-law, however, know very much about cars. They speak a foreign language to one another about gears, models, makes, and tire treads. Both have racing experience. They follow Formula One races religiously. The two of them even have a special bond with an old Porsche named “Helmut” – both can make that car do things that require special powers I just don’t possess.

And if my father-in-law and I were dropped into a junkyard full of discarded automobile parts, you can bet that he would be in a better place than I to tell you the make, model, and year of the particular bits we stumbled across. Here a discarded spoiler, there a V-8 engine block. To the untrained eye, such as mine, these are pieces of junk. To the eyes of someone with knowledge of car mechanics, however, these pieces may be valuable salvage, or at least they very clearly show their functionality. The spoiler’s shape causes the car to suck air downward, holding the vehicle more firmly on the pavement. The arrangement of the V-8 engine allows for both more horsepower and less vibration in a smaller space.

The appearance and shapes of the automobile parts not only highlight their function but often indicate their pedigree. Pin striping, the angle and orientation of a particular corner or side, or the size and shape of tail fins can inform the expert of whether he or she has a Chevy, a Ford, a Toyota. Furthermore, if you are a car enthusiast, you have probably seen the complete versions of many of these dismembered vehicles, and even know in great detail how the components have been modified over time by different manufacturers. In some cases, these changes have been functional – improving engine performance, controlling emissions, or damping vibrations. In other cases, the changes have been largely superficial and stylistic to attract new or returning car buyers.

Although I am about the farthest thing from a car enthusiast that one can be, it has often struck me that the skeleton and its associated soft tissues are perhaps best understood in machine metaphors. In fact, certain aspects of automobiles work well as concepts for explaining the evolution and functionality of the vertebrate body. I especially like the concept of the chassis, the framework that supports a car. The chassis in my mind is akin to the skeleton of the vertebrate animals I study. In cars, chassis shape, strength, and size can tell you a lot about a particular vehicle even when much else is missing. Likewise, even with the loss of soft tissues, the skeletons of vertebrate animals can reveal much about both the functionality and the pedigree of the particular individual you are studying.

The concepts of gear and torque are also useful. The size and shape of the gears in a car determine how fast the axle will spin and how much rotational force (torque) it will transfer to the tires. The arrangements of many skeletal muscles in the vertebrate body have similar mechanical consequences: muscles and bones act as gears producing torque around particular joints. Even concepts such as front-wheel and rear-wheel drive can be applied to understanding movement in vertebrate animals: for example, many land vertebrates utilize their hind limbs for propulsion (rear-wheel drive) and “steer” with their forelimbs.

A junkyard and a fossil-bearing layer of rock offer an intriguing comparison. My field paleontologist mentor, Jim Kirkland, once remarked that a vertebrate paleontologist in the field is equivalent to a car expert dumped into a mess of disassembled cars. As I now understand, a vertebrate expert can often identify the functionality and pedigree of a particular animal by examining the shape and structure of the disarticulated bones. Just as the features on a discarded engine can tell the car enthusiast the make, model, and size of the car, as well as its probable top speed, so can a femur (thigh) bone tell the vertebrate expert aspects of its possessor’s common ancestry, pedigree, probable top speed, and overall body size.

Of course, cars and vertebrates are not the same. If a manufacturer wants to change the chassis or engine of a car, they can redesign it from scratch, improving its efficiency or style in the process, without being tied to problems or limitations of the previous car body. Vertebrates are living creatures – a heart or brain or skeleton cannot simply be replaced or modified wholesale. Unlike a car’s engine, which can be shut off during modifications, the living engine of a vertebrate must continue to work throughout its life. Therefore, changes in form and function must occur while the animal continues to live. Often this means that the organs of vertebrate animals all begin from the same basic blueprint and are modified – many old parts are used or tweaked in ways different from their original functionality. It is as if an engineer was forced to redesign a car’s engine while it was still running, using only the parts already making up the car. Some interesting and weird but perhaps elegant solutions to various problems would most likely result. Hence, vertebrate skeleton shape is often a bizarre mix of retooled but old features containing stamps of long-lost ancestors.

Perhaps the most important difference between cars and vertebrates is that we can’t directly ask a particular animal lineage how and under what circumstances particular backbones fused, or limbs developed, or jaw power increased. Cars are made and designed by people, so we can ask them how and why they did what they did. Our approach to deciphering the skeletal evolution of vertebrates has to be much more indirect, more akin to reconstructing a crime scene or piecing together a family tree from old photos or lost memoirs. Fortunately, because the skeleton is the living, moving framework of the body, its shape, scars, articulations, and openings provide vital clues about the muscles, nerves, and guts of living and extinct vertebrates. Thus, if we can understand the shape and form of the skeleton, we purchase a window into the evolutionary history of the vertebrates, even if we can’t directly ask our ancestors how the various backboned animals came to be.

It can be exceedingly difficult for the uninitiated to become acquainted with the skeleton, its evolution, and function from a purely anatomical approach. Whereas professionally there is no substitute for discussing vertebrate evolution anatomically, many beginning students and laypersons will quickly become lost when we speak of animals being dorsoventrally compressed, or having craniofacial prognathism, or possessing a manus with ulnar deviation. However, although vertebrates and cars are not the same, approaching the skeleton in a way similar to that of the car enthusiast approaching a vehicle provides us with a framework within which vertebrate evolution and function can be understood and in which certain key anatomical terms can be introduced.

If you want to know about the history and functional design of an automobile, you are generally interested in knowing the following basic information:

1. Manufacturer

2. Date of production

3. Specialties of the chassis

4. Intended use – e.g., racing, utility vehicle, etc.

In this book, I modify and apply this approach of investigation to vertebrate animals. In each chapter, we will explore the evolution and functionality of the vertebrate skeleton using the following framework:

1. Pedigree – relationship to other vertebrates

2. Date of first appearance in the fossil record

3. Specialties of the skeletal chassis

4. Econiche – e.g., herbivore, carnivore, etc.

I should emphasize that our consideration of vertebrate skeletal function will not be limited to automobile analogies. Although we will approach vertebrates as a car enthusiast might, the mechanical analogies will run the gamut from scissors to engines, from medieval armor to socket wrenches. Overall, I wish to convey the functional anatomy of the vertebrate skeleton and its historical changes using a simplified, mechanical perspective rather than the more typical approach of anatomical analysis rooted in complex evolutionary diagrams.

Most of the chapters in this book are arranged by pedigree and skeletal chassis. The exceptions to these criteria fall under chapters 2 and 3. Chapter 2 deals with subjects such as evolution, deep time, and phylogeny, subjects that help us reconstruct the pedigree and functional anatomy of vertebrate animals. Chapter 3 introduces the basic vertebrate chassis and predicts what major anatomical structures would be present in the earliest vertebrate animals. This chapter also introduces the best-known earliest vertebrate in light of our predictions of what structures should be present in our common ancestor. From chapter 4 onward, we cover the long and intriguing history of the vertebrate skeleton stretching back over 540 million years. My hope is that, by approaching this topic from a mechanical perspective, you will ultimately appreciate the fundamental role that the skeletal chassis has played in ensuring the survival and diversity of the vertebrate animals.

2.1. This is evolution. Note that a common ancestor is not one individual but rather a group of two or more, just like on your family tree.

Evolution to Deep Time, Pedigree to Anatomy

2

Evolution

EVOLUTION IS MISUNDERSTOOD BECAUSE SCIENCE ITSELF IS misunderstood. Science has been conflated with atheism, social Darwinism, and the cold, inhuman march of progress that ultimately leads to the subjugation of humans by machines in many a science fiction novel. Science is no more any of these things than a mechanical engineer is the person with the striped hat in the locomotive engine. Science is a discipline narrowly focused on posing answerable questions about the physical universe (Popper, 1959). Science is simply a useful tool for understanding the physical world.

The confusion and maligning of science, and especially the theory of biological evolution, is unfortunate. Part of this confusion stems from the acceptance by some of a false choice: either you accept science and reject spirituality, or you accept spirituality and reject science. However, this misses the point: science is simply a tool for understanding the natural world – it cannot provide evidence for or against what is beyond nature.

To borrow an analogy from Eugenie Scott and Ron Toth, a scientist is no more qualified to show or falsify the existence of God than an auto mechanic. A vertebrate animal is (kind of) like a car, and an auto mechanic’s job, like that of a scientist, is firmly planted in the physical. I don’t know about you, but when I take my car to the auto mechanic, I expect that he or she will diagnose and fix the problems I have with the car physically. I expect to get a bill with items such as: low brake fluid, ruptured radiator, pinhole leak in the air conditioning compressor, or worn brake pad. I expect to see how the mechanic physically has altered my car so that it is once again safe to drive. I do not expect my auto mechanic to wax philosophic on the nature of good and evil, or to tell me that something supernatural is to blame for my car’s troubles. I also don’t plan to ask my auto mechanic for spiritual guidance or emotional counseling, or even for oboe lessons, because all these things fall outside his or her job description (unless the mechanic moonlights in oboe lessons, of course).

Science works similarly. There may be an ultimate purpose to the universe, and there may be a spiritual realm, but science is not the right tool to address those issues. On the other hand, understanding the pattern and functional evolution of vertebrates does fall under the purview of science, because it has occurred in the physical world and can be tested under the explanatory umbrella of the theory of biological evolution.

But what is the theory of biological evolution? Biological evolution is not simply “change over time” as is often claimed. Your watch changes over time, but it certainly does not evolve. It is telling that when Charles Darwin drew his first conception of evolution during his travels aboard the HMS Beagle, he drew a family tree of finches. In science lingo, Darwin drew a phylogeny – a branching diagram of common ancestry and descent of the finches on the Galapagos Islands (Darwin, 1859; Jones, 1999; Zimmer, 2001; Kelly and Kelly, 2009).

Darwin, and Alfred Russel Wallace independently, successfully united two concepts into one that provided a simple but effective explanation for the diversity of life: (1) common ancestry and (2) the passing of heritable traits via natural selection (Darwin, 1859; Zimmer, 2001; Kelly and Kelly, 2009). Put simply, the biological theory of evolution can be stated as descent with modification from a single, common ancestor. All life on earth is related through a great family tree. Different branches of the family tree have inherited modified characteristics unique to their portion of the pedigree through a process of natural selection.

Biological evolution is a theory, but that is not a weakness. A theory in science gets a specific definition: it is an overarching generalization that explains and makes sense of a given set of phenomena. A theory can be tested, has potential to be falsified, and has predictive power (Popper, 1959). Data ultimately decide whether or not a theory is rejected or modified.

Biological evolution is supported by data to be an effective explanation for many biological phenomena. For example, the groups-within-groups arrangement known as the hierarchy of life is the predicted end result of descent of modification from a single, common ancestor – we would predict there to be general traits shared by all organisms, followed by more and more exclusive traits shared by groups with more recent common ancestry, producing a groups-within-groups hierarchy. As another example, descent with modification over a long time would produce a sequential fossil record, such as the one we find around the world. The basic sequence of vertebrate evolution is the same everywhere you look in the rock record – birds never appear before fish in the fossil record, for example.

At its base, all of us recognize the family tree concept and its application in our own lives (Fig. 2.1). For example, I share common ancestors with my siblings – my mother and father. Further back in my family tree, my siblings and I along with our cousins share other, more distant common ancestors – our grandparents. Moving out to my second cousins, I share even more distant common ancestors with them – our great-grandparents. Take the common ancestry back several more generations, and we quickly approach a common ancestor for all of humanity. Beyond this, the family tree’s scope enlarges to encompass more distant common ancestors, such as those shared with other animals, plants, and eventually all living organisms on earth. Every living thing on earth is related. All vertebrate animals, including us, have a common ancestor.

Natural selection is the motor behind evolution. It was the insight of both Darwin and Wallace, and it provides a mechanism for how descent with modification from a common ancestor may occur. Natural selection is a biological law that can be summarized as follows: (1) all populations vary; (2) all populations produce more offspring than can survive; and (3) individuals within populations with traits that allow them to successfully mate and reproduce viable offspring will be selected for (Darwin, 1859; Jones, 1999; Zimmer, 2001). Having viable offspring means having children that can themselves successfully reproduce. Natural selection is sometimes, unfortunately and mistakenly, referred to as “survival of the fittest” (Kelly and Kelly, 2009), a statement that ignores the chance involved in survival. Many of the great vertebrate extinction events have had less to do with “fitness” than with the effect of rapid environmental change on organisms that are well adapted to the previously stable environment.

Within a given population, different individuals possess different traits or characteristics. Those individuals with traits that allow them to survive to reproductive age in a given environment, and to therefore mate and pass on these inheritable traits, are “selected for.” In other words, natural selection is simply the differential survival of individuals with any combination of traits that allow them to reproduce viable offspring.

We now understand, as Darwin, Wallace, and their contemporaries did not, that descent with modification is due to the inherited mutations (modif...

Table of contents

• Cover
• Half title
• Title
• Copyright
• Dedication
• Contents
• Preface
• Acknowledgments
• Part One: Setting the Stage
• Part Two: The Origin and Early Evolution of the Vertebrate Chassis
• Part Three: The Evolution of the Jawed Vertebrate Chassis and Something Fishy
• Part Four: The Vertebrate Chassis Moves to Land
• Part Five: Deep Scaly I: Reptilian Chassis from Early Reptiles to Sea Monsters
• Part Six: Deep Scaly II: The Archosaur Chassis, Those Ruling Reptiles
• Appendix: The Cards of Time
• References
• Index