Gaining Ground: The Origin and Evolution of Tetrapods (2nd Edition)

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As most fishes also lack a tongue that might assist in manipulation of food, options for intra-oral transport are limited. Exploitation of new sources of food has been hypothesized as a driving force for the invasion of the terrestrial environment—which implies that modifications of the skull that enable terrestrial feeding should appear early in the evolution of tetrapods.

However, Anderson et al.

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How might early tetrapods have fed when on land? Again, comparison with extant systems provides crucial insight. Van Wassenbergh , this issue showed that fully-aquatic eel-catfish are able to capture prey terrestrially by direct prehension of the jaw; however, to swallow they must return to the water and use hydrodynamic means of intra-oral transport.


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Thus, extant amphibious fishes are tied to the water in a previously unrecognized way, and this limitation may have also restricted the geographic dispersal of early tetrapods. Although the ancestors of the earliest tetrapods were almost certainly air-breathing fishes, locomotion in the gravity-dominated realm of land likely placed unaccustomed metabolic demands on the earliest tetrapods, which required the evolution of new, or enhanced, mechanisms capable of facilitating acquisition and transport of aerial oxygen.

However, it is unlikely that tetrapodomorph fishes had substantial sustained aerobic capacity, and this may have limited their capability for activity on land. Jew et al. Thus, the vertebrate invasion of land may have been facilitated and even accelerated by the favorable atmospheric conditions of the late Paleozoic.

For the majority of extant amphibians, their single strongest remaining tie to the aquatic environment is reproduction. Like extant amphibians, early tetrapods must have produced delicate anamniotic eggs, which would have been prone to desiccation and predation. Because of this, the amniotic egg, with its relatively water-impermeable shell, is often regarded as a key terrestrial innovation Kardong However, Martin and Carter , this issue considered the reproductive biology of extant fishes and reached a different conclusion.

Many extant teleosts that are not otherwise prone to emergence will leave the water to deposit their eggs on land; these fishes have evolved a suite of unusual behavioral and life-history characteristics that accommodate and facilitate this reproductive strategy. Because there are such a large number of fishes from phylogenetically diverse groups that reproduce on land, it appears that the benefits of placing eggs on land must outweigh potential costs.

Indeed, the terrestrial landscape during the Paleozoic must have presented many fewer predators, relative to the diversity of aquatic forms in oceans and streams of that time. Accordingly, terrestrial reproduction may have been another force driving tetrapods out of the water. Indeed, some vertebrate lineages demonstrate terrestrial adaptation in only one functional system e.

It is also clear that functional changes need not occur simultaneously during the evolution of terrestriality; for example, locomotor and feeding innovations can be decoupled Anderson et al.

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Yet, terrestrial habitats are most effectively exploited when a suite of terrestrial adaptations appear—and this may be the key to adaptive radiation. However, these findings also raise new questions. What role do phylogenetically diverse intrinsic morphogenic and physiogenic processes, such as development Davis , this issue , play in generating divergent evolutionary trajectories? A second theme is that the original invasion of land by tetrapods occurred under environmental conditions that were significantly different from those we see in the present e.

To what extent do specific abiotic and biotic environmental conditions influence or constrain the success of terrestrial radiations? Additional work assessing the functional capacities of modern amphibious taxa in the context of past environmental conditions will provide new opportunities to examine this question. As is clear from the symposium, our understanding of these environmental transitions is greatly improved by reciprocal integration of paleontological and neontological perspectives e.

A third emergent theme of the symposium is that our ability to elucidate the evolutionary steps that underlie the invasion of land improves as we expand our model systems. Studying unusual or novel taxa e. Finally, we conclude by suggesting that the colonization of the land by vertebrates was not inevitable. Clearly, the vertebrates that first invaded the land possessed a series of pre-adaptations, such as air-breathing and limb-based locomotion, that allowed them to move about effectively on land; however, other behaviors such as reproduction and swallowing likely tied these vertebrates to the water.

In addition, the more we survey the ecology of extant osteichthyian fishes, the more we realize that aquatic vertebrates have invaded the nearshore terrestrial environment hundreds of times—and that they may continue to invade the terrestrial realm in the future. Thus, as-yet-to-be-identified approaches, new model species, and innovative research techniques will be required to elucidate the key evolutionary steps taken by vertebrates in past invasions, to outline the similarities and differences of past invasions to the ongoing incursions onto land by extant osteichthyian fishes, and to predict how amphibious vertebrate species might be affected by future global environmental change.

Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Sign In. Advanced Search. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents. Future directions. Oxford Academic. Google Scholar. Tonia Hsieh. Alice C. Richard W. Cite Citation. Permissions Icon Permissions. Abstract The transition from aquatic to terrestrial habitats was a seminal event in vertebrate evolution because it precipitated a sudden radiation of species as new land animals diversified in response to novel physical and biological conditions.

Late to the table: diversification of tetrapod mandibular biomechanics lagged behind the evolution of terrestriality. Search ADS. Kinematics of level terrestrial and underwater walking in the California newt, Taricha torosa. Functional diversity in extreme environments: effects of locomotor style and substrate texture on the waterfall-climbing performance of Hawaiian gobiid fishes. Google Preview.

The deep homology of the autopod: insights from Hox gene regulation. Thrash, flip, or jump: the behavioral and functional continuum of terrestrial locomotion in teleost fishes. Environmental effects on undulatory locomotion in the American eel Anguilla rostrata : kinematics in water and on land. How muscles accommodate movement in different physical environments: aquatic versus terrestrial locomotion in vertebrates. From swimming to walking with a salamander robot driven by a spinal cord model.

Atmospheric oxygen levels affect mudskipper terrestrial performance: implications for early tetrapods. Where are we in understanding salamander locomotion: biological and robotic perspectives on kinematics. Propulsive forces of mudskipper fins and salamander limbs during terrestrial locomotion: implications for the invasion of land.

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.


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  • 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.

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    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. 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 or Raup and Stanley 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, million years old, and the first signs of life fossil bacteria are dated at about 3, million years. Complex multicellular animals first appear commonly in the fossil record only about million years ago, so that to use the clock face method has practical problems, in that most of it would effectively be empty.

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    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. 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. 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.

    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 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.