Amniote Paleobiology: Perspectives on the Evolution of Mammals, Birds, and Reptiles - A Volume Honoring James Allen Hopson

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Living amniotes—including all mammals, birds, crocodilians, snakes, and turtles—comprise an extraordinarily varied array of more than 21,000 species. Found in every major habitat on earth, they possess a truly remarkable range of morphological, ecological, and behavioral adaptations. The fossil record of amniotes extends back three hundred million years and reveals much about modern biological diversity of form and function.

A collaborative effort of twenty-four researchers, Amniote Paleobiology presents thirteen new and important scientific perspectives on the evolution and biology of this familiar group. It includes new discoveries of dinosaurs and primitive relatives of mammals; studies of mammalian chewing and locomotion; and examinations of the evolutionary process in plesiosaurs, mammals, and dinosaurs. Emphasizing the rich variety of analytical techniques available to vertebrate paleontologists—from traditional description to multivariate morphometrics and complex three-dimensional kinematics—Amniote Paleobiology seeks to understand how species are related to each other and what these relationships reveal about changes in anatomy and function over time. A timely synthesis of modern contributions to the field of evolutionary studies, Amniote Paleobiology furthers our understanding of this diverse group.

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Product Details

  • ISBN-13: 9780226094786
  • Publisher: University of Chicago Press
  • Publication date: 8/1/2006
  • Pages: 448
  • Product dimensions: 6.00 (w) x 9.00 (h) x 1.20 (d)

Meet the Author

Matthew Carrano is curator of dinosaurs in the Department of Paleobiology at the Smithsonian Institution. Timothy Gaudin is associate professor in the Department of Biological and Environmental Sciences at the University of Tennessee at Chattanooga. Richard Blob is assistant professor in the Department of Biological Sciences at Clemson University. John Wible is curator of mammals at the Carnegie Museum of Natural History.

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Perspectives on the Evolution of Mammals, Birds, and Reptiles

The University of Chicago Press

Copyright © 2006 The University of Chicago
All right reserved.

ISBN: 978-0-226-09478-6

Chapter One


Timothy J. Gaudin Matthew T. Carrano Richard W. Blob, and John R. Wible

Jim Hopson and Amniote Paleobiology

Living amniotes-including all extant mammals, birds, crocodilians, lizards, snakes, and turtles-together comprise an extraordinarily varied array of more than 21,000 species (Pough et al., 1999). These familiar animals are found in every major habitat on earth and are characterized by a dazzling range of morphological, ecological, and behavioral adaptations. The fossil record of the Amniota extends back in time over 300 million years (Carroll, 1988; Benton, 1993; Gauthier, 1994) and encompasses additional thousands of extinct genera (Carroll, 1988). In many respects, this impressive fossil assemblage is even more diverse than are the living amniote taxa. Given that more than ninety-nine percent of all species that have ever existed are extinct (Novacek & Wheeler, 1992), it is not surprising that extant amniote species provide only a glimpse of the total diversity of form and function that have existed historically. As a result, paleobiological analyses of the fossil record are crucial to understanding the origins of modern biological diversity and the processes that shapethe patterns we observe today.

Two critical questions are at the core of many paleobiological studies: (1) how are species related to one another and (2) what do species relationships reveal about changes in anatomy and function through time? These two perspectives-phylogenetic and functional-lie at the heart of this volume, which highlights the modern contributions of amniote paleobiology to evolutionary studies. At the same time, this volume serves as tribute to Dr. James A. Hopson, in honor of his retirement after more than thirty-five years of teaching and research at the University of Chicago. Throughout his career, Jim has been a strong advocate for both phylogenetic and functional approaches to studies of vertebrate evolution. Most importantly, Jim's work stands as an example of the successful integration of these two perspectives in amniote paleobiology.

The contributions in this volume focus primarily on phylogenetic and functional themes, but they also reflect the broad range of approaches used by modern paleobiologists. Therefore, the chapters are divided into four sections; the first two (entitled "New Fossils and Phylogenies" and "Large-Scale Evolutionary Patterns") are primarily phylogenetic in emphasis, and the second two (entitled "Functional Morphology" and "Ontogeny and Evolution") focus on other biological issues. However, the reader may be struck by the fact that many of these papers do not fit neatly into the categories that we, the editors, have erected. Indeed, to a certain degree such distinctions are difficult to make in any modern paleobiological compendium, given the importance of bringing varied perspectives to the task of elucidating any group's evolutionary history. This difficulty in discretely categorizing the various contributions also occurs in part by design, as the contributors were encouraged to integrate phylogenetic and functional perspectives into their work. Thus, it also testifies to the degree of integration increasingly seen in all aspects of modern paleobiology.

New Fossils and Phylogenies

Our knowledge of the amniote fossil record has increased rapidly over the past several decades. For example, Sereno (1997) noted that the diversity of dinosaur taxa alone has doubled in the past twenty years. The discovery, excavation, preparation, and description of new fossils is the most fundamental component of the science of paleontology. Because these actions represent the collection and integration of primary data, they underlie all other aspects of paleontological study. As long as new fossils remain to be discovered, paleontology will maintain this emphasis on descriptive work.

Early descriptions of fossil discoveries in the nineteenth century (and before) are typically used as benchmarks for the slow birth of paleontology as a field of study. Yet even the earliest paleontologists recognized that morphology does not exist in isolation but serves to illuminate the varied and complex connections between organisms. For example, Baron Georges Cuvier's comments on the first known European dinosaurs included prescient insights into their paleoecological relationships (Taquet, 1994).

With the advent of evolutionary theory and its profound impact on paleontology came an appreciation for the roles fossils could play in understanding the interrelationships of organisms. Description increasingly fed more directly into analyses of phylogenetic relationships for the simple reason that morphology formed the primary basis for such hypotheses until the advent of molecular studies in very recent decades. New fossils were seen as important markers of transitions between previously known forms and ultimately between distantly related modern groups. Indeed, few new fossils are described even today without accompanying statements or analyses of their phylogenetic placement, and these descriptions typically are written with phylogenetic informativeness in mind. Thus, description and phylogeny have become intertwined.

As is true of many workers in vertebrate paleontology, Jim Hopson's first scientific papers were descriptive (Hopson, 1964a, 1964b), and he continued to offer such studies throughout his later career. It is fitting, therefore, that several of the contributors to this volume describe new fossil taxa or significant new material from previously described taxa. In addition to their particular scientific content, they provide an excellent illustration of the modern state of the oldest of paleontological methods.

Lombard and Bolt (chapter 2) describe the mandibular morphology in Whatcheeria deltae, one of the oldest well-preserved tetrapods in North America. The authors then analyze the systematic implications of this morphology for basal tetrapods. As the authors explain, Whatcheeria was thought to represent a critical link in our understanding of basal tetrapod relationships, given its proposed phylogenetic position at the base of the anthracosaur radiation (Lombard & Bolt, 1995), which ultimately gave rise to all living amniotes. However, more recent analyses have thrown doubt on its anthracosaur affinities (see references in chapter 2). Nevertheless, Lombard and Bolt's analysis suggests that Whatcheeria provides novel insights into mandibular evolution in early tetrapods, and has improved our understanding of this important character complex.

In chapter 3, Munter and Clark describe new remains of small theropod (predatory) dinosaurs from the latest Early Jurassic of Mexico. A formal description is followed by a series of phylogenetic analyses that demonstrate the coelophysoid affinities of these fragmentary remains. The new coelophysoids represent the youngest known fossils of this clade. The group is common throughout Late Triassic and Early Jurassic terrestrial faunas but had disappeared by the Late Jurassic. The poorly known Middle Jurassic terrestrial record has hampered our understanding of their demise, but new fossils such as these ultimately may shed light on this process.

Sidor and Rubidge (chapter 4) discuss the morphology and phylogenetic affinities of a new biarmosuchian, which they named in honor of Jim Hopson. This new taxon is based on a nearly complete skull and broadens our understanding of this relatively poorly known group. The biarmosuchians are particularly important to studies of synapsid evolution, as they represent the oldest and most basal clade of therapsids. The authors use modern numerical cladistic techniques to elucidate the systematic relationships of the new taxon, ultimately supporting the monophyly of Biarmosuchia.

Sues and Jenkins (chapter 5) present a detailed description of the postcranial skeleton of the tritylodontid therapsid Kayentatherium. This taxon contributes greatly to our knowledge of the morphology of these cynodonts (specifically by suggesting that tritylodonts are not nearly as mammal-like as previously had been supposed) and simultaneously offers new clues on their relationships. The authors mine this wealth of new data for important phylogenetic implications, some of which bear directly on the origin of mammals themselves. As they point out, the phylogenetic placement of Kayentatherium makes it a particularly appropriate subject for a volume honoring Jim Hopson.

It is noteworthy that all these contributions include data matrices and/or some form of systematic analysis. The ability to place fossils into their appropriate systematic context has long been a central focus of paleontology. However, the role of systematics has changed dramatically since Jim Hopson began his career in the 1960s. At that time, investigations of higher-level vertebrate phylogeny were almost exclusively the domain of paleontologists (Ross, 1974; Patterson, 1981). Groups that lacked a significant fossil record typically were portrayed as having unknown (and by implication unknowable) phylogenetic histories (Patterson, 1981). Patterson (1981; p. 195) described the logical sequence whereby paleontology was granted its pivotal role in the reconstruction of phylogenetic or evolutionary relationships: "evolution is a theory about the history of life; evolutionary relationships are historical relationships; fossils are the only concrete historical evidence of life; therefore fossils must be the arbiters of evolutionary relationships."

The ensuing years have witnessed a dramatic revolution both in systematic practice and in the role of paleontology in systematics. Beginning in the late 1960s and 1970s, two important objections were raised to traditional systematic practices. The first objection concerned subjectivity in systematics. The practices of traditional systematists came to be viewed as overly dependent on intuition and authority, with groupings often established on the basis of subjective or cryptic selection and weighting of characteristics (e.g., see criticisms by Patterson, 1980; Wiley et al., 1991). Cladistics or phylogenetic systematics rose to prominence in response to such concerns and enjoyed particular success among systematic paleontologists over the past few decades. Indeed, in even a brief scan of important systematic paleontology journals such as the Journal of Paleontology or Journal of Vertebrate Paleontology, one would be hard pressed to find more than a handful of papers that used traditional ("evolutionary" sensu Mayr [1980]) systematic methodologies over the past decade. In recent years, the incorporation of computer-driven analyses into cladistic methodology has allowed ever more sophisticated investigations of ever larger systematic data sets. This transition in the vertebrate paleontological community from traditional "evolutionary" systematics to cladistic methodology to computer-driven analyses was embraced by Jim Hopson and his students and is mirrored in their systematic publications (e.g., see Hopson & Crompton, 1969; Hopson & Barghusen, 1986; Sidor & Hopson 1998).

The contribution by Gaudin and Wible (chapter 6) is another example of the use of such computer-driven cladistic analysis to analyze large data sets. They examine the phylogenetic relationships among living and extinct armadillos based on a data set of 163 craniodental characters sampled across nineteen taxa. The armadillos are the most diverse of the living members of the placental mammalian order Xenarthra. Along with their extinct kin, armadillos have been placed in the clade Cingulata, a group whose systematic relationships remain poorly investigated (Gaudin, 2003). This contribution overturns many of the traditional ideas about armadillo interrelationships and is notable in that the phylogenetic results for living taxa are changed substantially by adding fossil taxa to the analysis.

The results reported in chapter 6 are particularly pertinent in relation to the second primary objection to traditional systematics that began to surface, particularly in the 1970s and 1980s. This second objection concerned the role of paleontology in systematics. Influential papers published by Patterson (1981), Rosen et al. (1981), and Gardiner (1982) pointed out that, because of gaps in the fossil record, phylogeny could rarely if ever be read from the rocks in any straightforward manner. Cladistic methodology did not depend on stratigraphic information, meaning that living taxa could play a role in cladistic analyses equivalent to that of fossil taxa. Furthermore, because fossils typically do not preserve information on soft tissue structure or molecular composition, they contain only a fraction of the potentially useful systematic information provided by living forms. Patterson (1981, 218) claimed that "instances of fossils overturning theories of relationship based on Recent organisms are very rare, and may be nonexistent." Patterson (1981), Rosen et al. (1981) and Gardiner (1982) suggested that fossils may play only a minor role in phylogenetic reconstruction. A number of recent papers addressed their challenge of the relevance of paleontology to systematic analysis (e.g., Gauthier et al., 1988; Smith & Littlewood, 1994; O'Leary, 1999), and readers are referred to these works for more detailed arguments advocating the continued importance of fossils in investigations of phylogenetic relationships. We believe the papers throughout this volume provide further corroboration for the continued relevance of vertebrate fossils in advancing our understanding of amniote phylogeny.

Large-Scale Evolutionary Patterns

One of the benefits of cladistics' emphasis on the importance of living taxa in phylogenetic analysis has been a more widespread realization of the centrality of phylogeny to comparative biology and evolutionary biology. Many authors have discussed the importance of using phylogenetic information to better understand the evolution of myriad aspects of organismal biology, from functional morphology and physiology (e.g., see Lauder et al., 1995, and references therein) to behavior and ecology (Brooks & McLennan, 1991). Morphological, functional, ecological, and behavioral characters can be incorporated into cladograms as a means of elucidating evolutionary patterns and thereby enhancing our understanding of evolutionary processes. The importance of using cladograms to further our understanding of the biology of extinct vertebrates has also been recognized by the vertebrate paleontological community (e.g., see Polly & Sidor, 1999).

Such systematics-driven analyses of evolutionary patterns have long figured importantly in the work of Jim Hopson, who used such analyses to improve our understanding of a variety of topics from the evolution of phalangeal formulas in nonmammalian therapsids (Hopson, 1995) to the evolution of the mammalian ear (Allin & Hopson, 1992).

Modern workers have also developed an array of statistical techniques to address macroevolutionary questions regardless of whether they are based explicitly on a particular phylogeny. Trend analyses (e.g., Stanley, 1973; McShea, 1998; Alroy, 2000) complement cladogram-based analyses by investigating the distributional, variational, and probabilistic aspects of large-scale evolutionary trends. A number of our contributions use both systematic and trend-based techniques as a means of understanding evolutionary patterns of morphological change.

Parrish's work in chapter 7 draws paleoecological inferences from specific morphological observations. On the basis of previously published observations of fossil taxa, Parrish quantifies ranges of motion for cervical vertebrae in sauropodomorph dinosaurs (often using computer modeling as a guide). These ranges allow him to hypothesize potential feeding regimes for primitive members of this group and subsequently to place them into a larger phylogenetic and faunal context. This type of analysis is especially meaningful for taxa like most dinosaurs that have no obvious functional or behavioral analogues among the modern fauna.

In chapter 8, Carrano takes an even broader look at dinosaur history, quantifying and analyzing patterns of body size evolution in this group. Using several different analytical techniques, he tracks specimen-based size measures through the entire phylogeny of Dinosauria. This allows size changes to be quantified, revealing that most dinosaur lineages seem to follow Cope's Rule, showing marked size increases over time. A few exceptions are also identified, among both theropods and sauropods, that may prove worthy of future paleobiological study.


Excerpted from AMNIOTE PALEOBIOLOGY Copyright © 2006 by The University of Chicago. Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Table of Contents

1. Introduction

Part One: New Fossils and Phylogenies

2. The Mandible of Whatcheeria deltae, an Early Tetrapod from the Late Mississippian of Iowa
3. Theropod Dinosaurs from the Early Jurassic of Huizachal Canyon, Mexico
4. Herpetoskylax hopsoni, a New Biarmosuchian (Therapsida: Biarmosuchia) from the Beaufort Group of South Africa
5. The Postcranial Skeleton of Kayentatherium wellsi from the Lower Jurassic Kayenta formation of Arizona and the Phylogenetic Significance of Postcranial Features in Tritylodontid Cynodonts
6. The Phylogeny of Living and Extinct Armadillos (Mammalia, Xenartha, Cingulata): A Craniodental Analysis

Part Two: Large Scale Evolutionary Patterns

7. The Origins of High Browsing and the Effects of Phylogeny and Scaling on Neck Length in Sauropodomorphs
8. Body-Size Evolution in the Dinosauria
9. Major Changes in the Ear Region and Basicranium of Early Mammals

Part Three: Functional Morphology

10. Shoulder Girdle and Forelimb Multituberculates: Evolution of Parasagittal Forelimb Posture in Mammals
11. Tooth Orientation during Occlusion and the Functional Significance of Condylar Translation in Primates and Herbivores

Part Four: Ontogeny and Evolution

12. Neoteny and the Plesiomorphic Condition of Plesiosaur Basicranium
13. Scaling of the Hind Limb skeleton in Cynognathian Cynodonts: Implications for Ontogeny and the Evolution of Mammalian Endothermy
14. Cranial Variability, Ontogeny, and Taxonomy of Lystrosaurus from the Karoo Basin of South Africa

Part Five: Reflections on James Allen Hopson

15. James Allen Hopson: A Biography

Appendix. James Allen Hopson: A Bibliography (1964-2005)

Subject Index
Taxonomic Index

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