TABLE OF CONTENTS
Introduction
Part I. Origins and Transformations
Evolution of the Vertebrate Dentition: Teeth Transform Jaws Into a Biting Force -Moya Meredith Smith and Zerina Johanson
DOI: 10.7208/chicago/9780226268392.003.0001
[Early vertebrates, jaw evolution, organized dentitions, tooth origins, tooth succession, skin denticles]
This chapter reviews recent changes in our understanding of how teeth and dentitions evolved, a great transformation that resulted in jawed vertebrates becoming the dominant life forms on the planet. Did jaws and organized dentitions evolve together as a functional unit for feeding? Jaws and more caudal gill (pharyngeal) arches are rostrocaudally repeated, serially homologous structures, but in evolution were the latter co-opted for jaws? As with the jaws, the question of what system was co-opted for regulated tooth patterns—serially homologous tooth units or single modules—is still an open one and much debated. Dentitions are characterized by exquisite structural patterns of tooth shape changes and tooth addition regulated in space and time along the jaws. In both Chondrichthyes and Osteichthyes, survival depends on the ability to make large numbers of successive teeth that replace previous generations of teeth, consolidated into the bite as functional upper and lower dentitions. Many amphibians and reptiles replace their teeth several times in a regulated pattern, and we are now beginning to understand the genetic mechanisms regulating this spatiotemporal pattern. A better understanding of these genetic mechanisms and new paleontological discoveries will help us to further unravel this important evolutionary transformation. (pages 9 - 30)
This chapter is available at:
University of Chicago Press
Flexible fins and fin rays as key transformations in ray-finned fishes -George V. Lauder
DOI: 10.7208/chicago/9780226268392.003.0002
[Fish, Locomotion, Fin, Hydrodynamics, Kinematics]
Ray-finned fishes (Actinopterygii) are a highly speciose, monophyletic clade of vertebrates that exhibits remarkable diversification in locomotor structure and function. One of the most obvious traits shared by ray-finned fishes is ironically one of the least studied: the structure and function of fins and fin rays. Ray-finned fishes possess four key fin-related features that are likely to have been important components of evolutionary diversification into a wide range of aquatic habitats. First, fins are flexible, and this flexibility allows directional control over locomotor forces. Second, (most) fins in ray-finned fishes are collapsible, which allows control over surface area and hence the magnitude of both drag forces exerted by water on the body, and thrust forces exerted by the fish on the fluid. Third, fins in actinopterygian fishes are supported by flexible, jointed fin rays with a unique bilaminar structure that allows active control of bending and resistance to fluid loading. Fourth, ray-finned fishes possess multiple sets of fins, arranged both medially and in paired configurations. This allows fishes to take advantage of hydrodynamic interactions among fins, to exert fine control over body position, and to execute complex swimming behaviors requiring multifin control such as moving through obstacles or moving backward. (pages 31 - 46)
This chapter is available at:
University of Chicago Press
Major Transformations in Vertebrate Breathing Mechanisms -Elizabeth L. Brainerd
DOI: 10.7208/chicago/9780226268392.003.0003
[swim bladder, lung, buccal pump, aspiration, lung ventilation, respiration, XROMM, homoplasy]
Vertebrate breathing mechanisms have undergone several major transformations, the most profound of which were the transformation from breathing water with gills to breathing air with lungs, and the transformation from ventilating lungs entirely with bones and muscles of the head region (buccal pumping), to ventilating the lungs entirely with the trunk region (aspiration breathing). The series of smaller changes that together produced these great transformations are the subject of this chapter, and the order and timing of them are sometimes surprising. Lungs evolved in aquatic environments long before vertebrates evolved a terrestrial lifestyle, and the swimbladders of teleost fishes evolved from lungs, counter to the assumption of early evolutionary biologists that the swim bladders of 'lower vertebrates' must have come before the lungs of 'higher vertebrates.' Soft tissues such as gills, lungs and swim bladders are not generally preserved in the fossil record, but we are fortunate that comparative analysis of these structures in extant species can reveal much of the transformation series. In the case of costal aspiration breathing and rib structures, a new technology, X-ray Reconstruction of Moving Morphology (XROMM) is poised to help interpret the functions of ribs and costovertebral articulations preserved in the fossil record. (pages 47 - 62)
This chapter is available at:
University of Chicago Press
Origin of the Tetrapod Neck and Shoulder -Neil H. Shubin, Edward B. Daeschler, and Farish A. Jenkins, Jr.
DOI: 10.7208/chicago/9780226268392.003.0004
[Tiktaalik, Acanthostega, Eusthenopteron, stem tetrapod, pectoral girdle, cleithrum, fin, limb, neck]
New material prepared from the pectoral region of Tiktaalik
roseae, with comparisons to finned sarcopterygians and stem tetrapods, enables a re-evaluation of the major pectoral transitions involved with the origin of digited vertebrates. The pectoral skeleton undergoes significant changes in proportion of endochondral and dermal elements, glenoid shape and orientation, and position in the body. These changes point to coordinated shifts in locomotion, breathing and head mobility during the evolution of stem tetrapods. The transformation series of the pectoral girdle suggests two functional trends during the evolution of limbed forms. Head mobility, as inferred by the freeing of the cranium from the shoulder, predated the origin of digits. Comparison of Tiktaalik to limbed taxa reveals that the loss of the connection to the head happened prior to the complete reduction of the supracleithral series. The later shift involved freeing the dorsal shoulder from the body wall, a transition underway in Acanthostega and apparently complete in more derived Devonian forms such as Ichthyostega, Tulerpeton and crown tetrapods. Tiktaalik, like its close relatives on the tetrapodomorph tree, evolved intermediate structures of the skeleton to thrive in ecological settings that are themselves functionally transitional, or intermediate, between fully aquatic and terrestrial habitats. (pages 63 - 76)
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Origin of the Turtle Body Plan -Ann Campbell Burke
DOI: 10.7208/chicago/9780226268392.003.0005
[Chelonia, Testudines, plastron, carapace, paleontology, phylogeny]
The turtle's shell is a unique variation on the tetrapod body plan. The association of the trunk vertebra and ribs with a specialized dermis to form the carapace results in a novel relationship between axial and appendicular systems. The plastron contains highly modified elements of the secondary pectoral girdle, as well as possibly neomorphic bones. A wide array of tetrapod taxa has been suggested as the ancestral root of turtles and as of today the root of this significant branch of amniotes remains controversial. The fossil record continues to produce exciting and sometimes surprising specimens of early turtles, but the first appearance of the carapace and plastron remains elusive. Studies of the development of these structures in modern turtles provide an increasingly detailed understanding of how turtle embryos diverge from other tetrapods, and a fuller sense of what developmental changes led to one of the more extreme variations on tetrapod anatomy. This chapter will briefly review the state of current molecular phylogenies, and focus on highlights of both paleontological and developmental studies that explore this remarkable transformation in vertebrate evolution. (pages 77 - 90)
This chapter is available at:
University of Chicago Press
Anatomical Transformations and Respiratory innovations of the Archosaur Trunk -Leon Claessens
DOI: 10.7208/chicago/9780226268392.003.0006
[Archosauria, flight, ribs, ribcage, aspiration, respiration, breathing]
The Archosauria, a group of tetrapods that dominated the terrestrial ecosystem for the majority of the Mesozoic Era, provide a unique window into anatomical transformations of the trunk. Birds and crocodylians are the only living representatives of the Archosauria, a once diverse clade that included dinosaurs, pterosaurs, and other extinct groups encompassing an enormous range of anatomical disparity and functional diversity. Active flapping flight evolved twice in archosaurs, in both the pterosaur and in the bird lineages. The clade Archosauria is also marked by diverse respiratory mechanisms, including the hepatic piston pump of modern crocodylians and the lung-air sac system of modern birds. A transition from ectothermic to endothermic metabolic physiology must have occurred in the lineage toward modern birds. Increased ossification and a reduction in the number of elements that make up the ribcage preceded the evolution of flight in both pterosaurs and birds, changes that would have increased the structural integrity of the trunk and likely aided in accommodating the increased stresses transmitted through the trunk during flight. In addition, these changes in trunk skeletal morphology placed constraints on the skeletal aspiration pump, and may have played an important role in the evolution of the respiratory system. (pages 91 - 106)
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University of Chicago Press
Evolution of Hind Limb Posture in Triassic Archosauriforms -Corwin Sullivan
DOI: 10.7208/chicago/9780226268392.003.0007
[Archosauria, Archosauriformes, erect posture, hindlimb, Triassic, homoplasy]
Birds and crocodilians are survivors of the reptilian clade Archosauriformes, which also includes pterosaurs, non-avian dinosaurs and other extinct groups. Archosauriforms were the dominant large terrestrial and aerial vertebrates from the Middle Triassic until the end of the Cretaceous, and some investigators have linked their success to the evolution of erect, fully adducted hindlimb posture in some taxa. Analysis postural indicators implies that basal archosauriforms walked with a sprawling hindlimb posture, like extant lizards. Derived archosauriforms, the archosaurs, typically achieved an erect hindlimb posture based on one of two distinct hip configurations, buttress erect and pillar erect. Interpreted in the context of archosauriform phylogeny and biostratigraphy, the osteological evidence indicates that a shift to more upright postures coincided closely with the origin of Archosauria and occurred during the Early Triassic. Details of the shift, and of the complex pattern of subsequent transitions among the semi-erect, buttress erect and pillar erect conditions, remain unclear. Erect hindlimb posture was a prerequisite for the evolution of bipedality in the dinosaurian lineage, and may have contributed to archosaurian ecological success. However, the hypothesis that erect posture offered archosaurs an immediate competitive advantage is not supported by patterns of changing diversity in the Triassic record. (pages 107 - 124)
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University of Chicago Press
Fossils, Trackways, and Transitions in Locomotion: A Case Study of Dimetrodon -James A. Hopson
DOI: 10.7208/chicago/9780226268392.003.0008
[Synapsida, pelycosaurs, Sphenacodontidae, step length, sprawling, lateral bending, trackway]
Basal synapsids ("pelycosaurs") had sprawling locomotion, with primary movement of the humerus being long axis rotation that yielded short step lengths. To test the hypothesis that trunk bending was important for increasing step length, a protocol was developed for determining relative contributions to step length of limb movement and axial bending. Skeletons yield reconstructed step lengths based on functional analysis, but trackways yield actual step lengths. A common measure of body length, glenoacetabular length (distance between shoulder and hip joints), is determinable from mounted skeletons and trackways. A skeleton scaled to the body size of a trackmaker determines how reconstructed step length varies from known trackway step length. Where skeleton-based steps fall short of trackway steps, the amount of lateral bending of the trunk required to make up the difference can be determined. Analyses of the sphenacodontid Dimetrodon and a trackway of the probable sphenacodontid Dimetropus demonstrate extensive axial bending. (pages 125 - 142)
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University of Chicago Press
Respiratory Turbinates and the Evolution of Endothermy in Mammals and Birds -Tomasz Owerkowicz, Catherine Musinsky, Kevin M. Middleton, and A.W. Crompton
DOI: 10.7208/chicago/9780226268392.003.0009
[Archosauria, Crocodilia, Mammalia, Aves, cynodont, trachea, nasal cavity, countercurrent]
For almost thirty years, respiratory turbinates have been hailed as a bona fide hallmark of endothermy of mammals and birds. Located in the nasal cavity, turbinates act as a temporal countercurrent exchanger to reduce heat and water loss. Surface area of respiratory turbinates scales to (body mass)0.73 in both mammals and birds, but is three times greater in mammals. Tracheal surface area also scales to (body mass)0.73, but is 3.5 times greater in birds. Surgical ablation of respiratory turbinates in emus offers experimental evidence that temporal countercurrent exchange operates in the avian trachea. We propose that craniocervical organization of cynodonts—specifically, short necks and massive heads—constrained their counter-current exchanger to the nasal cavity. In contrast, a trend of neck elongation among theropod dinosaurs allowed their trachea to become a major site of respiratory heat and water conservation. Presence of a prominent nasal preconcha in extant crocodilians suggests that respiratory turbinates and ectothermy are not mutually exclusive. Instead of heat conservation, the original selection pressure for respiratory turbinates may have been heat dissipation by evaporative heat loss. We conclude that respiratory turbinates may have been exapted for heat and water conservation from their original function in selective brain cooling. (pages 143 - 166)
This chapter is available at:
University of Chicago Press
Origin of the Mammalian Shoulder -Zhe-Xi Luo
DOI: 10.7208/chicago/9780226268392.003.0010
[Mammalia, shoulder girdle, scapula, clavicle, forelimb, glenohumeral joint, embryogenesis, homoplasy]
The shoulder girdle and the forelimb underwent major transformation in the evolution of mammals from the pre-mammalian cynodonts. Shoulder girdles of therian mammals differ from those of cynodonts and mammaliaforms in having a much greater mobility of the scapula and the clavicle relatively to the axial skeleton, in a re-orientation of the glenohumeral joint for more parasagittal posture, and in having neomorphic scapular muscles for stabilizing the shoulder joint, all with important implications for locomotor functions of the forelimb. The sternal structure, the scapulocoracoid, and the glenohumeral joint are now better documented with the recently discovered Mesozoic mammal fossils, which have shown prominent evolutionary homoplasies of the procoracoid, a major primitive element in the shoulder-sternal structure, and the septal part of the scapular spine that is a derived feature of the scapula, among Mesozoic mammaliaforms. These homoplasies are consistent with the labile embryogenesis, and gene patterning of living mammals. The labile development can be hypothesized as a plausible mechanism underlying the major transformation of the shoulder girdle in the early evolution of mammals, and is an example of how development can inform evolution. (pages 167 - 188)
This chapter is available at:
University of Chicago Press
Evolution of the Mammalian Nose -A.W. Crompton, Catherine Musinsky, and Tomasz Owerkowicz
DOI: 10.7208/chicago/9780226268392.003.0011
[Mammalia, endothermy, respiratory turbinal, maxilloturbinal, synapsid, cynodont]
High basal metabolic rate and rhythmic breathing characterize endothermy in mammals. When the larynx is in an intranarial position, airflow to and from the lungs passes through the nose, bypassing the oral cavity. Ossified maxilloturbinals act as temporal countercurrent exchange sites and save energy by reducing the loss of heat and water in the expired air. The earliest nonmammalian synapsids were ectothermic and appear to have possessed indeterminate growth. They possessed a cartilaginous nasal capsule that opened directly into the mouth and lacked respiratory turbinals. Successive clades of nonmammalian synapsids dating from the mid-Permian to the mid-Jurassic progressively evolved morphological and physiological features that supported sustained activity. The earliest indication of the ability to isolate airflow through the nose or mouth shows up in lower Triassic nonmammalian cynodonts with the addition of a hard palate and a putative soft palate extending posteriorly. These animals actively foraged. They had a large respiratory chamber housing cartilaginous maxilloturbinals that functioned during panting as sites for evaporative cooling. The final step in the acquisition of endothermy occurred some time in the Middle to Late Jurassic when the common ancestor of Mammalia acquired an intranarial larynx, ossified maxilloturbinals, suckling of the young and determinate growth. (pages 189 - 204)
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University of Chicago Press
Placental Evolution in Therian Mammals -Kathleen K. Smith
DOI: 10.7208/chicago/9780226268392.003.0012
[Mammalia, Eutheria, Monotremata, natural selection, placentation, viviparity, reproductive strategy, life history]
The origin and early radiation of mammals stands as one of the most complex of all the transformations in vertebrate evolution. Within Mammalia, the adaptations for reproduction are particularly intriguing, as the three major groups, monotremes, marsupials and eutherians, possess markedly different strategies. The causes and consequences of these different reproductive strategies have long intrigued vertebrate biologists. This chapter reviews recent work on the evolution and function of the placenta in marsupial and eutherian (i.e. therian) mammals. In both groups there is significant variation in placental form and function that argues against prior work suggesting that the marsupial reproductive strategy is most likely due to constraints arising from the inability to develop sophisticated placentation. Instead, the evolution of reproductive traits suggest that selection has molded the patterns we observe. Data on variation in eutherian placentas and theoretical work on maternal-fetal conflict and gene imprinting are also reviewed. This work together suggests that in eutherians, complex patterns of natural selection may be responsible for the observed variation. The differing reproductive strategies in marsupial and eutherian mammals may have arisen from minor differences in selection for life history traits and/or maternal resource conservation during the evolution of viviparity in the earliest therians. (pages 205 - 226)
This chapter is available at:
University of Chicago Press
Going from small to large: mechanical implications of body size diversity in terrestrial mammals -Andrew A. Biewener
DOI: 10.7208/chicago/9780226268392.003.0013
[effective mechanical advantage, EMA, limb posture, scaling, moment arm, locomotion, energetics, biomechanics]
Studies of tetrapod locomotor evolution demonstrate the fundamental importance of shifts in limb posture relative to locomotor performance. Changes in limb posture have repeatedly facilitated the diversification of size within land vertebrates, influencing the biomechanics of body support and the energetics of movement. Within terrestrial mammals, changes from more crouched postures in small mammals to more upright postures in larger mammals enable a reduction in musculoskeletal force demands relative to body weight, keeping peak bone and muscle stresses within safe limits. Postural effects on musculoskeletal force transmission are determined by the ratio of muscle to ground force moment arms (r/R), defined as muscle and limb effective mechanical advantage EMA, which determines the ratio of ground force to muscle impulse (?G/?F). This chapter reviews and synthesizes key trends of postural changes in limb mechanics, which permitted the repeated evolution of large size within mammals from the first late Triassic ancestral mammals that were nocturnal and small. With an increase in limb EMA, muscle forces are reduced relative to the ground reaction forces, facilitating an evolutionary increase in body size. (pages 227 - 238)
This chapter is available at:
University of Chicago Press
Evolution of Whales from Land to Sea -Philip D. Gingerich
DOI: 10.7208/chicago/9780226268392.003.0014
[Cetacea, Cete, Artiodactyla, Cenozoic, herbivore, carnivore, adaptive zone, terrestrial, marine]
Whales, Cete or Cetacea, are very different from other animals. Their classification as mammals was not established until 1758 with publication of the definitive 10th edition of Carolus Linnaeus' Systema Naturae. Following this there was still uncertainty and controversy about whether whales were most similar to and evolved from primitive mammals deep in Mesozoic time, whether whales evolved from herbivorous land mammals in the Cenozoic, or whether whales evolved from carnivorous land mammals in the Cenozoic. Cetaceans have a good fossil record, and discovery of Eocene archaic-whale skeletons having skulls with carnivorous teeth but the feet of herbivores resolved the issue and confirmed the close relationship of Cetacea to Artiodactyla advocated by biomolecular systematists. Herbivorous mammals that supplement their diet with meat today provide a clue to the long slow transition from herbivory on land, to omnivorous scavenging on a shoreline, to an eventual diet of fish, meat, and zooplankton in the sea. Whales started as terrestrial herbivores and evolved through time to be marine carnivores, illustrating step by step, cumulatively, a macroevolutionary change of adaptive zone. Evolutionary reversals, including whales returning to the sea, show the process to be opportunistic. (pages 239 - 256)
This chapter is available at:
University of Chicago Press
Major Transformations in the Evolution of Primate Locomotion -John G. Fleagle and Daniel E. Lieberman
DOI: 10.7208/chicago/9780226268392.003.0015
[Cenozoic, quadrupedal, bipedal, homoplasy, hominin, behaviour, paleontology, biomechanics]
Living primates exhibit great diversity in locomotor abilities, including arboreal and terrestrial quadrupedalism, several types of leaping, suspension, and bipedalism. Early attempts at classifying primate locomotor abilities saw the evolution of primate locomotion as a progressive trend through the Cenozoic from vertical clinging and leaping to quadrupedalism, quadrupedalism to suspensory behavior, and suspensory behavior to bipedalism. However, a detailed functional assessment of the fossil record demonstrates that there have been numerous independent evolutionary transitions among these different locomotor abilities in the history of primate evolution in different, often geographically isolated primate clades. Both the morphological and behavioral aspects of the transformations often show clade-specific features, and the fossil record documents a much greater diversity of locomotor morphologies than are present in extant species. Hominin bipedalism poses difficulties for comparative analyses because it is practiced by only a single extant taxon. However, the growing fossil record demonstrates that during the past six million years there have been different types of bipedal hominins. Future advances in our understanding of primate locomotor evolution will require an integrated research program that combines the results of paleontological and behavioral fieldwork, experimental studies of biomechanics, and detailed studies of morphology of living and fossil primates. (pages 257 - 280)
This chapter is available at:
University of Chicago Press
Part II. Perspectives and Approaches
Ontogenetic and evolutionary transformations:The ecological significance of rudimentary structures -Kenneth P. Dial, Ashley M. Heers, and Terry R. Dial
DOI: 10.7208/chicago/9780226268392.003.0016
[bird, ecology, behaviour, environment, habitat, juvenile, wing-assisted incline running, WAIR, branching]
Major evolutionary transformations in vertebrate history are routinely framed as dichotomous habitat shifts between different physical media. However, both present-day and historical media contain(ed) myriad transitional environments (riverbanks, mountain slopes, ephemeral ponds, etc.) that are/were routinely negotiated by organisms seeking resources or avoiding predation. Access to refugia in these transitional environments provides protection against predators and harsh abiotic conditions, and thus represents a major selective pressure for all age groups – particularly for developing young. This chapter reviews the locomotor strategies that juvenile birds use to negotiate transitional habitats and reach refugia, demonstrating how each environment corresponds with the form-function relationships of incipient, rudimentary locomotor structures. During the ontogenetic acquisition of powered flight, developing birds transition between terrestrial, aquatic and aerial media by using their wings and legs cooperatively. Such behaviors are relevant to adult birds with secondarily reduced and rudimentary wings, and may have been relevant to extinct theropods with rudimentary flight apparatuses during the origin of flight. Though this chapter focuses on birds and their dinosaur ancestors, juvenile locomotor development offers unique opportunities to explore form-function relationships and ecologically relevant behaviors of transitional stages in many groups, and thereby to enhance our understanding of evolutionary transformations. (pages 283 - 302)
This chapter is available at:
University of Chicago Press
Skeletons in motion: an animator's perspective on vertebrate evolution -Stephen M. Gatesy and David B. Baier
DOI: 10.7208/chicago/9780226268392.003.0017
[kinematics, XROMM, osteology, skeleton, bone, fossil, biomechanics, locomotion, flight, Archaeopteryx]
Many of the great transformations in vertebrate history entail profound changes in movement as well as morphology. Yet the skeletons of extinct animals have traditionally been interpreted as static objects. Specimens are gleaned for phylogenetically informative characters and measured for comparison by morphometric methods. Functional interpretations are likewise based on correlations between the static osteology and behavior (locomotion, diet, etc.) in living taxa. As a compliment, we advocate for a more active understanding of skeletal form through 3-D analyses of skeletal motion. Recent advances in animating bone models based on X-ray video allow the dynamic relationship among hard and soft tissues to be reconstructed at unprecedented spatial and temporal resolution. An animator's perspective offers a continuity of form through time, a vibrant context for physiological data, and a solid foundation from which to study evolutionary history through reciprocal illumination. (pages 303 - 316)
This chapter is available at:
University of Chicago Press
Developmental Mechanisms of Morphological Transitions: Examples from Archosaurian Evolution -Arhat Abzhanov
DOI: 10.7208/chicago/9780226268392.003.0018
[Aves, Crocodilia, bird, skull, beak, evolutionary developmental biology, evo-devo, body plan]
Archosaurs experienced a tremendous degree of morphological change during their evolution. Some members of this specious clade evolved into massive quadruped herbivores weighing many tons, others into agile carnivorous bipeds with a long snout equipped with conical teeth, and yet others became powerful fliers with wings supported by highly modified fingers. Many archosaurs evolved feathers, horns, bony plates or toothless beaks. Owing to many recent important paleontological discoveries, zoological and taxonomic studies, there is now a much better appreciation of the archosaur phylogenetic history and diversity. Over the last 10-15 years, advances in vertebrate developmental genetics allowed for important breakthroughs in uncovering the molecular mechanisms underlying animal morphological evolution and at widely different taxonomic scales. This chapter reviews several informative case studies that provide novel insights into the evolution of archosaurian bodyplan, with a particular attention to the emergence of the specialized avian characteristics. (pages 317 - 332)
This chapter is available at:
University of Chicago Press
Microevolution and the Genetic Basis of Vertebrate Diversity: Examples from Teleost Fishes -Sydney A. Stringham and Michael D. Shapiro
DOI: 10.7208/chicago/9780226268392.003.0019
[microevolution, macroevolution, stickleback, cichlid, cave tetra, development, adaptation, homoplasy]
Despite longstanding interest in how vertebrates acquire novel traits that characterize macroevolutionary transformations, remarkably little is known about the number, location, and types of mutations that control major differences among vertebrate lineages. As major transformations among vertebrates occurred in the distant past, traditional genetic approaches typically will not work to understand the genes that actually mattered in the evolution of key innovations. This is because extant taxa with disparate traits or body plans are usually too distantly related for experimental genetics. In a limited number of extant species, however, different populations have evolved anatomical, physiological, or behavioral changes of a magnitude that typically characterizes different species. Not many species meet this criterion, but the ones that do are emerging as key models in evolutionary genetics and developmental biology. Using these special cases, we can gain important insights about the genetic architecture of adaptive traits, the types of mutations that occur (and their developmental consequences), and whether similar kinds of mutations occur repeatedly when similar traits evolve independently. In this chapter, we address these themes in the context of three diverse lineages of teleost fishes: sticklebacks, Mexican cave tetras, and African cichlids. (pages 333 - 350)
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University of Chicago Press
The Age of Transformation: The Triassic Period and the Rise of Today's Land Vertebrate Fauna -Kevin Padian and Hans-Dieter Sues
DOI: 10.7208/chicago/9780226268392.003.0020
[terrestrial, tetrapod, turnover, adaptation, homoplasy, dinosaur, crocodylomorph, mammaliaform, lepidosaur]
The Triassic Period (about 251 to 200 million years ago) was the most diverse geological period in the history of terrestrial tetrapods. At least two taxonomic turnovers and two functional-ecological transitions revolutionized these Triassic communities. Various late Paleozoic "holdovers" were replaced first by "indigenous" Triassic tetrapod groups and finally by groups that came to dominate terrestrial ecosystems ever since - including dinosaurs (including birds), crocodylomorphs, mammaliaforms, lepidosaurs, turtles, and extant amphibians. In addition, the ancestral mammalian lineage (synapsids) and the ancestral bird-dinosaur lineage (ornithodiran archosaurs) both evolved erect posture, parasagittal gait, and high growth rates (and likely high metabolic levels). Other functional-ecological advances include the evolution of an "aquatic" carnivore morphology in at least seven reptilian lines, at least five independent iterations of the terrestrial macrocarnivore body plan, specialized herbivory in at least seven independent lineages of reptiles and at least two synapsid groups, and the independent return to life in the sea by at least four and possibly as many as six reptilian lineages. By the end of the Triassic, terrestrial tetrapod communities looked entirely different than at the beginning of this period, and they were far more complex ecologically than any other time interval except the Cenozoic. (pages 351 - 374)
This chapter is available at:
University of Chicago Press
How do Homoplasies Arise? Origin and Maintenance of Reproductive Modes in Amphibians -Marvalee H. Wake
DOI: 10.7208/chicago/9780226268392.003.0021
[viviparity, caecilian, salamander, frog, fetal, phylogeny, homoplasy, development, physiology]
Homoplasies (similar features that have evolved independently in different lineages) are often identified by mapping characters on a tree that presents a phylogenetic hypothesis of relationships of taxa. Because the goal is usually either the phylogeny or an assessment of homoplasy, few attempts have been made to assess the mechanistic basis for the evolution of homoplasious conditions. A major focus of my research is the comparative biology of the evolution of derived modes of reproduction, especially viviparity (live-bearing) in amphibians. The independent origins of viviparity in diverse lineages are frequently labeled an example of homoplasy. Live-bearing is amenable to the examination of its component features that are either common to, or different in, both within and across, the frog, salamander, and caecilian lineages that share the purported homoplasies. This facilitates the assessment of the mechanisms by which similar conditions—in this case, suites of characters and their underlying developmental and physiological substrates—arise and evolve. Evidence to date for amphibians indicates that multiple aspects of maternal and fetal biology must be investigated at multiple levels of the hierarchy of biological organization in order to understand the origin and evolution of live-bearing. (pages 375 - 394)
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University of Chicago Press
Rampant Homoplasy in Complex Characters: Repetitive Convergent Evolution of Amphibian Feeding Structures -David B. Wake , David C. Blackburn, and R. Eric Lombard
DOI: 10.7208/chicago/9780226268392.003.0022
[Plethodontidae, Bolitoglossini, salamander, tongue, adaptation, phylogeny, adaptive radiation, convergent evolution]
Evolutionary transitions can exhibit repeated patterns of change when different lineages evolve into similar ways of life. This homoplasy, the appearance of similar features in independently evolving lineages, has been prominent in amphibian evolution, especially with respect to adaptations to terrestrial life. High levels of homoplasy in the evolution of terrestrial feeding were reported earlier, and we now revisit those studies in the light of more recent phylogenetic hypotheses based on DNA sequence data. Convergent evolution in terrestrial salamanders, starting with an ancestral attached protrusible tongue, was postulated to have led to derived projectile tongues a minimum of six times within family Plethodontidae. Our re-examination corroborates these estimates but shows that homoplasy is much more extensive than previously thought. Freely projectile tongues evolved at least three times, with six derived character states in common; in the two most extreme examples of this tongue form, an additional ten derived tongue characters evolved independently. We postulate that limited pathways toward change exist, and that these are entered repeatedly. Important evolutionary transitions may be nearly inevitable outcomes of past evolution that has set the stage by establishing an integrated system involving biomechanics, development, and genetics that acts as a guide for evolutionary trajectories. (pages 395 - 406)
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University of Chicago Press
Contributors
Index
