Gaining Ground, Second Edition

The Origin and Evolution of Tetrapods

Jennifer A. Clack

• Publisher: Indiana University Press
• Year: 2012
• ISBN:
  9780253005373
• Edition: 2
• Topic: Biological Sciences
• Subtopic: Evolution
• Index: Biological Sciences

Around 370 million years ago, a distant relative of a modern lungfish began a most extraordinary adventure—emerging from the water and laying claim to the land. Over the next 70 million years, this tentative beachhead had developed into a worldwide colonization by ever-increasing varieties of four-limbed creatures known as tetrapods, the ancestors of all vertebrate life on land. This new edition of Jennifer A. Clack's groundbreaking book tells the complex story of their emergence and evolution. Beginning with their closest relatives, the lobe-fin fishes such as lungfishes and coelacanths, Clack defines what a tetrapod is, describes their anatomy, and explains how they are related to other vertebrates. She looks at the Devonian environment in which they evolved, describes the known and newly discovered species, and explores the order and timing of anatomical changes that occurred during the fish-to-tetrapod transition.

1.1. Family tree of the living tetrapod groups.

1 Introduction

The Origin and Evolution of Tetrapods

Approximately 380 million years ago, something strange and significant happened on Earth. That time is part of an interval of Earth’s history called the Devonian period by scientists such as geologists and paleontologists, but in more popular imagination, it is known as the Age of Fishes. The reason for this is that after about 200 million years of earlier evolution, the vertebrates—animals with backbones—had produced an explosion of fishlike animals that lived in the nearshore lagoons, river estuaries, and lakes of the time. The strange thing that happened from the middle to the later parts of the Devonian period is that some of these fishlike animals evolved limbs with digits—fingers and toes. Over the ensuing 350 million years, these tetrapods gradually evolved from their aquatic ancestry into walking terrestrial vertebrates. These have dominated the land ever since this initial explosive radiation allowed them to colonize and exploit the land and its opportunities. The tetrapods, with their limbs and fingers and toes, include ourselves as humans, so that this distant Devonian event is profoundly significant for humans as well as for the planet.

Today, the modern descendants of these early pioneers are divided into two major groups (Fig. 1.1). The modern amphibians include frogs (anurans—jumping, tailless amphibians), salamanders (urodeles—tailed amphibians), and caecilians (apodans—elongate, limbless amphibians). The modern amniotes include mammals (usually animals with fur and that produce milk, humans among them), turtles, crocodiles and their relatives the birds, and lizards and their relatives the snakes. (Note that the term “reptile” can be used to include all amniotes except mammals, provided birds are included in this group.) In practice, tetrapods include any animal with four legs or whose ancestor had four legs. (The situation is a bit more difficult with fossils, as will be shown.) These two major radiations of vertebrates also had many relatives that are now extinct, notably the dinosaurs, whose closest living relatives are the birds.

Because becoming terrestrial was apparently a slow process, I have called this book Gaining Ground, to suggest that it was achieved with some difficulty. At the same time, this evolutionary development has led to enormous innovations in evolutionary terms, and I hope to suggest not only something of the breadth of possibilities that it opened up, but also the contingent nature of the transition. In other words, much of this change occurred because of happenstance—being at the right place at the right time. It was not a directed process.

The origin, early evolution, and relationships of tetrapods form the focus for the interaction of several disciplines. Paleontology (the study of fossils) and the related studies of paleoecology, taphonomy (how the creatures died and became fossilized), and paleobiogeography (where the creatures lived and how they were distributed in time and space), as well as modern zoology, anatomy, physiology, and developmental and molecular genetics, all contribute various aspects to the understanding of past life. Most people are aware that at some stage creatures crawled out of the water and came onto land, and so can relate to the contents of this book. I hope this book will show how much more can be said than that—and how much more there is to know.

The study of the origin of tetrapods has gone through many phases in its history, but none has been more exciting than that of the present day. Over the last few years, more fossil material from this crucial period has been unearthed than at any time in the past (this is even more the case than when the first edition of this book was being prepared 10 years ago), and these discoveries have helped to reshape ideas about when, where, how, and even possibly why the transition occurred at all. In the not-too-distant past, there was almost no fossil material, and ideas were based largely on informed guesswork. Speculation was intense, and as is often the case, this speculation was in inverse proportion to the amount of data. To be truthful, there is still not a large amount of real data, so speculation is still active, and whatever is concluded today may be overturned by discovery of a new fossil tomorrow. In some ways, this is to be hoped for, because only in that way can guesses be falsified and tested as scientific hypotheses. Indeed, much of this has gone on since the first edition of this book, as subsequent chapters will testify.

This book tells the story of the evolution of tetrapods from their fish ancestry and puts the sequence of events into its ecological context. The story is founded on an understanding of the evolutionary relationships between tetrapods and their fishy relatives—their phylogeny—and traces the family tree of tetrapods from its roots to the point at which the major groups of modern tetrapods branch off from its original trunk. The tetrapod family tree is in fact more like a bush, with several main branches, some of which have died out during the course of evolution and some of which have become large and important from small beginnings.

This book looks at the changes that occurred in the transition from creatures with fins and scales to those with limbs and digits in an attempt to understand how and when the changes occurred, and to do this, it is necessary to understand something of the anatomy of the animals involved. Chapters 2 and 3 are devoted to these parts of the story. Chapters 4, 5, and 6 set out what is currently known of the earliest tetrapods and their lifestyles. By careful analysis of what is known of them from fossils, and by comparison with modern animals that live at the transition between water and land, it may be possible to understand a little of how the early tetrapods worked as animals. After the tetrapods had become established, they radiated into a range of forms requiring modifications to the original tetrapod pattern. Chapters 7, 8, and 9 carry the story forward from the origin of tetrapods to their ultimate conquest of terrestrial living. The final chapter draws together some of the threads that have been taken up in the preceding chapters and shows how they impact the study and understanding of tetrapods today.

A cautionary note should be added here. Many of the skull reconstructions, derived from the literature, have freely duplicated left or right sides to present a complete, but artificially symmetrical, picture. These reconstructions are therefore not suitable for use in morphometric studies. Also, in many cases, the dermal ornament on the skull roofs has been omitted.

I hope that this book brings the excitement of this field of study to a wider public, shows something of how paleontology progresses and what it can and cannot do, and, of course, most importantly, shows people a little more of how they fit into the broader picture of evolution.

The Geological Framework

Tools of the Trade: The Geological Framework, Fossilization, and Family Trees

To put the evolution of terrestrial tetrapods in its context, it is necessary to have an understanding of Earth’s history in general outline. This is not the place to discuss dating methods or techniques of stratigraphic correlation. These can be found in readily available geological textbooks such as that by Briggs and Crowther (2001) or Raup and Stanley (1979). However, it is necessary to explain the approximate dates and approximate lengths of time over which the story takes place, so that it can be put in the context of other major events in the story of evolution.

The geological column is the name that scientists give to the succession of times, dates, and names into which Earth’s history is divided. There are several ways of expressing this. It can be expressed in a way that accords each time interval a space proportional to its length, usually as a vertical column, always with the oldest at the bottom. Or it might be depicted as a sort of clock face. The problem with this method is that only a small proportion of Earth’s known history is represented by an abundant fossil record. The planet is estimated to be about 4,500 million years old, and the first signs of life (fossil bacteria) are dated at about 3,500 million years. Complex multicellular animals first appear commonly in the fossil record only about 550 million years ago, so that to use the clock face method has practical problems, in that most of it would effectively be empty. Another way is simply to set out the list of dates and names in their relative order, again with the oldest at the bottom, as shown in Figure 1.2. The numbers show the dates of the boundaries between the divisions and the lengths of time they have lasted.

The idea that the Earth is as old as this is a relatively recent one, dating back only to the early 19th century, and its appreciation has changed the perspective from which we view our place in its history. The concept has been called “deep time.” As an example, one of the important factors that study of deep time reveals is the complexity of climate change through Earth’s history, culminating in the appreciation of the possibility of human-induced global warming. This would not be possible without study of the Earth’s climate over the past few million years. Talking of perspectives, when considering the period in Earth’s history covered by this book, climate changes far more radical than recent ones are obvious. If global warming continues as predicted, eventually the climate may become something like it was about 15 million years ago in the Miocene period, but it will still be a long way from that which current study suggests prevailed when early tetrapods were alive.

1.2. Timescale showing the time of origin of major groups, and other events in Earth’s history. The shaded area indicates the period covered in this book. Dates from Gradstein et al. (2004).

The interval for which there are abundant fossils in the rocks is called the Phanerozoic, meaning “visible life,” and it represents a time of about 600 million years. The Phanerozoic is divided into three eras, originally named according to what proportion of its biota resembled that of the modern world. These divisions are named the Paleozoic (“ancient life”), Mesozoic (“middle life”), and Cenozoic (“recent life”). The ages are divided into periods, and the periods into stages. To a large extent the boundaries of the divisions are based on the fossils of animals and plants that lived at that time, although the names they receive do not necessarily reflect this. Names of the stages, for example, are often based on places where the representative strata were first found or where they are most clearly seen. These large time periods, rock sequences and their names, and the basic faunal complement of each were worked out during the 19th century, and they have not changed very much since then. The subsequent decades have seen a process of refinement, increasing resolution of time intervals, and precision of dating and correlation between sequences in different parts of the world.

Geologists and paleontologists usually date their finds by reference to an independently produced and well-established timescale. One of the most recent in use at the present time is that by Gradstein et al., published in 2004, and has used many different and complementary techniques in its construction. It is enormously detailed for the whole Phanerozoic.

The story of the origin and early evolution of tetrapods and the time frame of this book takes place almost entirely within the later part of the Paleozoic, during the Devonian, Carboniferous, and Permian periods (Fig. 1.2). More details of the stages into which these periods are divided are given in the relevant chapters. The story as we have come to understand it really begins about 380 million years ago, although some recent work suggests that an earlier date is likely. Some chapters set the scene by describing the history of plants and animals that were already present as the tetrapods started their evolutionary journey. It takes the story through about 132 million years to a time about 248 million years ago as the Permian period comes to a close. For comparison, the first dinosaur is dated at around 225 million years, while the last died out 65 million years ago, a comparable period of time. The earliest tetrapods that feature in this story are nearly twice as old as the oldest dinosaur. Humans can trace their lineage back to a split from the common ancestor of apes and humans about 5 million years ago, while Homo sapiens as a species is currently reckoned to be about 100,000 years old.

Fossils, Fossilization, and Techniques of Study

The only means of finding out about animals and plants that lived so long ago is from fossils. These are the preserved remains or traces of these ancient organisms, and to understand the story more fully, it is necessary to look at how and under what circumstances these remains are preserved and discovered.

Types of fossils can be categorized in a variety of ways according to what is preserved, or how it is preserved. What is preserved is usually the harder parts of an animal or plant, so for example bones and shells make good fossils. This type of fossil is often called a body fossil, to distinguish it from another category, that of trace fossil. Trace fossils are preserved impressions of features that an animal (usually an animal, although occasionally plants leave trace fossils too) has made—for example, footprints or burrows. Body fossils show the anatomy of the plant or animal it preserves, and from this it is possible to work out something of its evolutionary relationships and functional morphology. Trace fossils can sometimes be even more telling in that they can provide information about the behavior of the animal and clues to its lifestyle that body fossils cannot give. Both kinds of fossil are known in the story of the origin of tetrapods.

Body fossils are preserved in many different ways. Usually the process involves water to a large extent, and fossils of aquatic animals are much more common than those of terrestrial animals. Fossils of terrestrial animals are usually found only when the creatures’ bodies have been accidentally washed into bodies of water. Generally, the animal sinks to the bottom of the lake or sea and the soft, fleshy parts usually decay quite rapidly. The remains may be scavenged by other animals and disintegrate so that bones become isolated, but gradually they are covered with sediment that over the millennia hardens to preserve the bones. Most favorable to preservation are deep, still waters where the sediment particle size is small. Sediments can then take up small details of the bones or shells by filling in even the smallest crevices, and then not being disturbed again. If decay happens more slowly—for example in water low in oxygen, where predators are few and bacterial action slow—the carcasses may be preserved in a more complete form.

The remains may be subsequently preserved in a variety of ways. They may be more or less unaltered. Shells made of calcium carbonate, for example, may retain the same chemical structure they had in life if they are preserved in limestone. Bones, which are the main concern here, are formed of calcium phosphate, which may also be unchanged chemically in many instances. However, bones are not solid but have pores or spaces in them, for blood vessels, nerves and fluid or even air, to allow them to grow and to make them lighter and stronger than solid bone would be. During fossilization, these spaces are often suffused with solutions of chemicals that later precipitate out and harden, so that the fine internal structure of the bone is preserved. This is why fossil bone is often much heavier than recently dead bone. Figure 1.3 shows sections through part of the skull of Acanthostega, a Devonian tetrapod, to show how well the internal structure of the bone can sometimes ...