Amniote Origins

Completing the Transition to Land

Stuart Sumida, Karen L.M Martin

• Publisher: Academic Press
• Year: 1997
• Print ISBN: 9780126764604
• eBook ISBN: 9780080527093
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

Amniote Origins integrates modern systematic methods with studies of functional and physiological processes, and illustrates how studies of paleobiology can be illuminated by studies of neonatology. For this reason, comparative anatomists and physiologists, functional morphologists, zoologists, and paleontologists will all find this unique volume very useful. Inspired by the prospect of integrating fields that have long been isolated from one another, Amniote Origins provides a thorough and interdisciplinary synthesis of one of the classic transitions of evolutionary history. - Integrates modern systematic methods with studies of functional and physiological processes - Illustrates how studies of paleobiology can be illuminated by studies of neonatology - Provides a thorough and interdisciplinary synthesis of one of the classic transitions of evolutionary history

CHAPTER 1

AN INTEGRATED APPROACH TO THE ORIGIN OF AMNIOTES: COMPLETING THE TRANSITION TO LAND

Karen L.M. Martin and Stuart S. Sumida

The origin of amniotes was a critical step in vertebrate evolution. It is apparent that fundamental, macroevolutionary changes in the natural histories and physiological features of tetrapods occurred at or near this transition, which set the stage for the Age of Reptiles and, ultimately, for the radiation of mammals and birds. Despite its tremendous evolutionary importance, the origin of amniotes has been addressed directly by relatively few studies. Other biologically important transitions, for example the transition from water to air breathing or the evolution from ectothermy to endothermy, have been examined independently by many disciplines. However, the origin of amniotes has been, for the most part, studied by paleontologists and thus limited to osteological data. Our goal has been to bring together paleontologists and neontologists specializing on both extant and extinct organisms, to examine this evolutionary transition that is rich in importance, but historically poor in testable hypotheses.

Amniotes were the first vertebrates to complete the transition to land, and this release from life history stages requiring bodies of water provided a profound freedom for new evolutionary radiations. Their descendants accomplished innovations in terrestrial herbivory, behavioral thermoregulatory strategies, respiration in air, water balance and conservation, and new means of locomotion, including flight. In this volume we provide a holistic approach to understanding primitive tetrapods that should increase our understanding of this macroevolutionary change. Our aim is to provide a broad overview of the evolution of the vertebrate body with both paleontologists and neontologists addressing the origin of amniotes and making inferences from a variety of different perspectives. Vertebrate paleontologists have been slow to adopt experimental approaches, and reproducible studies addressing function by vertebrate paleontologists are still disappointingly few. Conversely, functional morphologists and physiologists rarely address the influences that their work on extant organisms might have on understanding the extinct animals that made the transition from anamniote to amniote. We hope that this volume will prove to be heuristic, generating dialogue and forging new connections to stimulate novel ways of thinking about the evolution of the tetrapods.

SCIENTIFIC AND HISTORICAL PERSPECTIVE

Early focus on the origin of amniotes inevitably turned to a search for progressively more primitive, and in some cases hypothetical, ancestors (Romer, 1966; Carroll, 1969a,b, 1970). Over the course of the past thirty years, the context in which primitive tetrapods have been examined has changed radically. New concepts in biogeography and the acceptance of plate tectonics forced a reexamination of the taxa presumed to be close to the origin of amniotes (Hotton, 1992). The advent of cladistic methods of phylogenetic analysis has suggested profoundly different hypotheses regarding the primitive radiations of amniotes (e.g., Heaton, 1980; Holmes, 1984; Heaton and Reisz, 1986; Gauthier et al., 1988a, Berman et al., 1992; Lee, 1993, 1995; Carroll, 1995; Laurin and Reisz, 1995). This volume has grown out of our desire to address new advances in our understanding of the phylogenetic relationships of early amniotes and to add the critically important experimental approaches of functional biology.

The authors gathered here address the integrated influences of morphology, physiology, evolutionary biology, behavior, developmental biology, histology, ecology, and phylogeny on the origin of amniotes. However, caveats are necessary. With fossil animals, only the morphology of hard tissues can typically be studied; the other aspects of their biology must almost always be inferred. Yet morphology may be a poor predictor of physiology; animals with similar morphologies may have very divergent physiologies (Burggren and Bemis, 1990). Moreover, changes in physiology or function may leave few morphological clues in the fossil record (Lauder, 1981). Clearly, many inferences are not testable by experimentation or observation when studying extinct organisms. Nevertheless, we believe that the present can be gainfully considered as a key to the past, provided caution is used (Garland and Carter, 1994). With appropriate care, we believe that useful analogies can be made between extant and extinct animals.

OVERVIEW OF TEXT

A major concern in approaches to the study of the origin of amniotes is the controversy surrounding nomenclature and phylogeny of the groups formerly known as the Amphibia and the Reptilia. In a rigorous use of cladistic terminology, these two groups are radically re-defined (Gauthier et al., 1988b). The word “amphibian” is not used in a taxonomic sense, although it is often used by authors included here as a colloquial means of communicating the concepts of anamniote tetrapods. Groups of animals previously placed under the term “Reptilia” are considered to be paraphyletic. Therefore, we have attempted to focus this work on the earliest amniotes and their immediate sister groups. In order to provide context, however, neontological examples are taken from extant animals that may of necessity be far removed phylogenetically from this transition. Where inferences are drawn, chapter authors have stated their assumptions explicitly. Laurin and Reisz examine current hypotheses about the phylogeny of tetrapods in “A New Perspective on Tetrapod Phylogeny,” as do Lee and Spencer in “Crown-Clades, Key Characters and Taxonomic Stability: When Is an Amniote Not an Amniote?” Significantly, the first chapter suggests that extant lissamphibians may be more closely related to early amniotes than was previously supposed, thus making them potentially much more useful surrogates for testing physiological hypotheses. In “Biogeography of Primitive Amniotes,” Berman, Sumida, and Lombard update the biogeographic context for the evolution of early amniotes and point out the importance of Pangea and plate migration in the spread of tetrapod groups.

The physical and physiological transition to land began long before the origin of amniotes. Some of the most primitive fishes may have had lungs for air breathing (Randall et al., 1981). Many primitive tetrapods were semi-terrestrial and may have spent part of their lives on land (Smithson, 1980), perhaps even breeding terrestrially (Carroll, 1988). Their eyes, ears, and lateral lines must have adjusted to the medium of air instead of water (Duellman and Trueb, 1986). These changes may be considered exaptations (Gould and Vrba, 1982). Insects and plants completed the transition to land before the vertebrates came fully ashore, providing habitat, cover, and food for the explorers of this brave new world, but a diet of high-fiber plants probably required changes in digestive physiology (Diamond and Buddington, 1987). Habitat and climate are discussed in the chapters “Biogeography of Primitive Amniotes” and “The Late Paleozoic Atmosphere and the Ecological and Evolutionary Physiology of Tetrapods” by Graham, Aguilar, Dudley and Gans. Lauder and Gillis explore ways that these animals exploited food resources in a chapter on “Origin of the Amniote Feeding Mechanism: Experimental Analysis of Outgroup Clades.” Hotton, Olson, and Beerbower take a parallel approach with extinct taxa in the chapter “Amniote Origins and the Discovery of Herbivory.” Movement on land required adjustments to the axial and appendicular skeleton (Sumida and Lombard, 1991), discussed in the chapter “Locomotor Features of Taxa Spanning the Origin of Amniotes” by Sumida.

The evolution of an amniotic egg was a key innovation (Thomson, 1992) that defines this clade and provided one of the means of invasion of a new habitat. Aspects of this process are included in the chapters by Packard and Seymour in “Evolution of the Amniote Egg” and by Stewart in “Morphology and Evolution of the Egg of Oviparous Amniotes.” However, the differences between the adults of the earliest amniote and its immediate anamniote “predecessor” may have been minor and apparently unremarkable at the time of the transition, and indeed, the exact identity of the earliest amniotes may still be in doubt (Romer, 1966; Carroll, 1969a,b; Lombard and Sumida, 1992; Smithson et al., 1994).

The differences between present-day lissamphibians and extant “reptiles” go far beyond the evolution of the amniotic egg. It has been suggested that the group Reptilia should not be defined solely on the basis of morphology but also on shared life history and physiological traits (Gans and Pough, 1982). Such traits may permit wide variation in structure. We suggest that this may also be true for the “Amphibia,” and that the origin of amniotes may have been a step that left few tangible morphological clues but engendered enormous physiological changes that profoundly altered the course of evolutionary history. These differences are examined as clues to past physiological evolution in the chapters on “The Role of the Skin in the Origin of Amniotes: Permeability Barrier, Protective Covering, and Mechanical Support” by Frolich and “Water Balance and the Physiology of the Amniote Transition” by Martin and Nagy. In the final chapter, “Reconstructing Ancestral Trait Values Using Squared-Change Parsimony: Plasma...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• ACKNOWLEDGMENTS AND DEDICATION
• CONTRIBUTORS
• Chapter 1: AN INTEGRATED APPROACH TO THE ORIGIN OF AMNIOTES: COMPLETING THE TRANSITION TO LAND
• PHYLOGENETIC PERSPECTIVE
• BIOGEOGRAPHIC PERSPECTIVE
• FEEDING
• THE AMNIOTE EGG
• MORPHOLOGICAL CHANGES
• INTEGRATIVE BIOLOGY AND THE ORIGIN OF AMNIOTES
• SUBJECT INDEX