"A luminous text is matched by lucid drawings rationally placed. . . . A great teaching monograph, the book will charm lay readers of fossil history. For virtually every college & public collection."—Scitech Book News
|Publisher:||University of Chicago Press|
|Sold by:||Barnes & Noble|
|File size:||12 MB|
|Note:||This product may take a few minutes to download.|
Read an Excerpt
The Evolution of Vertebrate Design
By Leonard B. Radinsky
The University of Chicago PressCopyright © 1987 The University of Chicago
All rights reserved.
Introduction: Fossils and Phylogeny
The history of vertebrate life is a fascinating story, one that chronicles the rise and fall of numerous groups of organisms, including strange and bizarre types like nothing alive in the world today. It is a story of major evolutionary transformations over millions of years, of long periods of little or no change, and of disastrous worldwide extinctions and great evolutionary radiations. And, of course, the story includes our own evolutionary history, which can be traced from our early ancestors among ancient jawless fish. This book will look at the important changes in basic body organization that have occurred over the past 500 million years and at the periodic evolutionary radiations that produced a stunning diversity of life forms at various points in time. Emphasis is placed on explaining the functional significance of evolutionary changes in anatomical structure. We will look at the evidence and methods of analysis used to obtain those explanations, so that you can see how we have come to our current understanding of ancient life forms.
Sources of Information
We have two main sources for the evolutionary history of the vertebrates: the fossil record and the diversity of living vertebrates. The fossil record is very incomplete, for it preserves evidence of only a very small percentage of all the species that ever existed, and it usually provides information only on their hard parts or skeletons. However, it is our only source of information about extinct species. The fossil record is set in the framework of geological time and therefore can provide direct evidence of actual evolutionary transformations at fairly specific periods through time. Furthermore, with careful analysis, skeletal remains can provide much insight into the ways of life of extinct animals.
Fossils are the traces or remains of extinct organisms, sometimes just imprints in rocks that were once soft sediments but often the actual mineralized remains of the hard parts of the body. Two questions commonly asked about fossils are, How they are found? and How they are dated? Most fossil localities are found more or less by accident — in the course of geological explorations or commercial excavations or stumbled upon by someone who notices an unusual looking bone lying on the ground. Once they are discovered, rich fossil localities are usually visited time and again by paleontologists. Some of the most productive localities today are places that were first discovered in the middle of the last century, and they have been worked on and off for over a hundred years. More rarely, fossil localities are found by paleontologists who deliberately explore regions where rocks of the desired age and of the right kind to preserve fossils are exposed at the surface, bare of vegetation and soil.
Fossils are dated by radiometric "clocks," that is, elements with unstable forms that change (decay) spontaneously and at a regular rate compared with other elements. The two most important clocks for fossil dating are uranium 238, which decays to lead 206, and potassium 40, which decays to argon 40, at known and very slow rates. When a rock solidifies from a liquid state (such as molten lava), its atoms are frozen into the lattices of mineral crystals and the decay products of unstable elements begin to accumulate next to the as yet unchanged atoms. Thus from the amount of decay product relative to unchanged parent element, and with a knowledge of the rate of decay, we can calculate the length of time that has passed since the rock solidified. Now, fossils are not preserved in rocks that solidified from a molten state (igneous rocks). Rather, they are found in sedimentary rocks that form from the compaction and cementation of layers of sediments, such as mud, silt, and sand. Such rocks can be dated only indirectly, by their position above or below igneous rocks that contain radiometric clocks. For example, the oldest known direct human ancestor, Australopithecus afarensis, is dated at between 3.8 and 3.6 million years old because it was found in sedimentary rocks sandwiched between lava flows of those dates.
Living species are our other source of information on the evolutionary history of vertebrates. They can provide a vast amount of data on anatomy, physiology, and behavior. These data can be used to infer the pathways of evolutionary transformation through geological time that resulted in the diversity of modern forms of life. A common approach to the study of evolutionary transformations begins with the selection of a group of living species assumed to represent an evolutionary sequence — for example, a fish, an amphibian, a reptile, and a mammal — and to use that series to study the transformation of a given organ or anatomical system of the body. We call such a series a scala naturae, or "scale of nature," because it is assumed to represent an ascending sequence of organisms, from primitive to more advanced.
However, a potential problem with this approach is that all living species, being end products of unique, individual evolutionary histories, are mosaics of primitive and advanced features. We cannot always be sure that a particular feature in one living form represents an ancestral stage of that feature as seen in another living form. For example, mammals evolved from reptiles, and modern reptiles are generally considered to be more primitive than modern mammals. Therefore it is often assumed that modern reptiles represent an evolutionary stage that the ancestral mammals passed through. However, although mammals evolved from reptiles, they did not evolve from the sort of reptile that exists today. Therefore, we would be misled if we considered the modern reptile brain, for example, to represent an early evolutionary stage of the mammal brain. The forebrains of modern reptiles are dominated by an advanced integrating center (called the dorsoventricular ridge) that is not found in the brains of modern mammals. Instead, mammals developed a different part of the forebrain (the cerebral cortex) as a major integrating center. In dealing with presumed evolutionary sequences based on living species alone, we must always consider the possibility of mosaic evolution and look for independent evidence that the presumed primitive condition of the system under study is indeed primitive.
A more productive approach to understanding historical transformations of morphology from living species is to compare changes between forms that are most closely related. But first the relationships among the organisms in question have to be established. Systematics is the special discipline in biology that studies the evolutionary relationships among organisms. Such genealogies or historical hypotheses of relationship are called phylogenies. Phylogenies or evolutionary relationships of groups that are presented in this book are constructed from a study of the distribution of primitive and derived (or advanced) character states.
For any character or trait of an animal, there are often several conditions or character states. For example, if the heart were a character being studied in vertebrates, the different character states might be defined by how many chambers the heart has in different groups. All character states have polarity; that is, one character state has evolved sequentially from another. It is the direction of this polarity that establishes which character state is primitive and which is derived. Using the heart example again, we are interested in establishing whether a four-chambered heart is primitive and a two-chambered heart is derived (or advanced), or vice versa. Sometimes a character state is considered primitive when it is the most widely distributed state among members of other groups of organisms than the ones being studied. This method of establishing polarity is called outgroup comparison. Another method for establishing polarity is to examine the ontogenetic development of the character. The adult character state that appears first during development is sometimes considered to represent a more primitive condition than a state appearing later. Using our heart example once again, if during development the heart starts out with two chambers and changes to four chambers later in ontogeny, the two-chambered heart might be considered primitive.
The polarities for a large number of characters are established for any phylogenetic analysis. Once the character states for each character have been determined to be either primitive or derived, the number of shared derived character states are used as the basis for determining degree of relationship among the organisms being studied. Those organisms with the greatest number of shared, derived character states are considered to be most closely related. They are placed adjacent to each other on a branching diagram.
As mentioned above, an important approach for reconstructing evolutionary transformations from living species is through the examination of ontogeny, or the course of an individual's embryonic development. The late nineteenth-century biologist Ernst Haeckel proposed as a "law" that ontogeny repeats or recapitulates phylogeny, meaning that the individual, in the course of its embryonic development (ontogeny), goes through the same stages that its ancestors did in their evolutionary history (phylogeny). Over the past century, this idea has been variously in and out of favor. Currently it is the subject of renewed interest, with the understanding that it is ancestral developmental stages rather than ancestral adult stages that can be seen in modern embryonic development. A classic example of the value of this approach is the demonstration from developmental series that two of the tiny bones in the middle ear of mammals (the malleus and the incus) were derived from the jaw bones (articular and quadrate) of our reptilian ancestors. That is, the early developmental stages of ear bones in mammals resemble early stages of jaw development in reptiles. This inferred evolutionary transformation of reptile jaw bones to mammal ear bones was later confirmed with the discovery of "missing link" fossils that display the intermediate condition in adult stages.
Of our two sources of evidence on the evolution of vertebrates, fossils preserve only a small amount of the total anatomy of the once-living animal but can be dated and placed in temporal sequences. It is important to remember, however, that the majority of extinct forms are likely to be a mosaic of primitive and derived features just like most living species. The geologically oldest representatives of a group do not necessarily have only primitive character states. For this reason, scientists also rely on embryonic data and outgroup comparison to establish which character states are primitive and which are advanced. Often the out-groups used for comparison include fossils, but living groups are part of the analysis as well.
Phylogenetic relationships inferred from outgroup comparison and embryonic data are independent of the time sequence known from collecting and dating fossils. The time-sequence relationships can then be used as an independent "check" or test of the proposed evolutionary sequence. Figure 1.1 shows a proposed phylogeny for mammals and mammal-like reptiles based on the analysis of 200 characters for which primitive and derived character states have been defined from outgroup comparison and embryonic data. Figure 1.1 also shows a phylogenetic sequence of the same organisms based on the geological age of specimens. You will see that, in this case, the two diagrams are in good accord. The same evolutionary relationships have been obtained from the time sequence of fossils as from outgroup comparison and embryonic data on living and fossil species. Throughout this book, groups are referred to as primitive or advanced. These terms are applied on the basis of the relative number of primitive and derived or advanced character states found in the group and on the basis of examination of the time sequence of change from the fossil record.
Methods of Analysis
There are two main approaches to interpreting the functional significance of evolutionary changes and the ways of life of extinct species: form-function correlation and biomechanical design analysis. Form-function correlation involves looking for the behaviors or functions that are correlated with a particular anatomical form in living species and then extrapolating the correlation back to extinct species in order to infer function from their form. For example, fast-running animals today, such as horses, antelope, and deer, have relatively long legs and very long feet. From this association of long legs and speed in living animals, we can infer that extinct species with relatively long legs may well have been fast runners. The form-function correlation approach works well as long as we have living species with the anatomical features in question, but many fossil species display features unknown in the modern world, and in these cases we must use the second approach, biomechanical analysis.
Biomechanical design analysis involves looking at an anatomical structure from a biomechanical or engineering perspective and inferring from its shape and construction how well it would perform a given function. For example, one group of dinosaurs, the dome-headed dinosaurs, or pachycephalosaurs, had enormously thick and dense skull caps overlying their brains. There is nothing like that in living animals for comparison, but a biomechanical analysis of the shape of the skull shows that it meets design criteria for absorbing large forces or shocks from head-on blows. Given shock absorption as the probable function of the thickened skull caps, a likely interpretation of their significance with respect to biological role, or way of life, is that pachycephalosaurs lived in social groups and used head butting to establish dominance hierarchies, just as mountain goats and mountain sheep do today.
Sometimes we compare a particular morphology with a theoretical optimal design for a given function, or we compare different morphologies with each other and with an optimal design. Optimality is defined by the amount of energy it takes to perform a specific function. When these kinds of comparisons are made, we sometimes talk about the relative efficiency of a given design. Efficiency is a term that is used repeatedly in this book to discuss morphological transformations or changes. One structure is considered to be more efficient than another if it is closer to the biomechanically defined optimal design, that is, if it requires less energy to perform a given function. Energetic efficiency can be measured by comparing mechanical efficiency of a system (see chap. 7 for more detail) or by measuring the amount of oxygen it takes for two animals with different morphologies to perform a given function.
Ideally, form-function correlation and biomechanical design analysis approaches are used together to examine a given problem or question. Take, for example, the long legs and fast running correlation we have just considered. Biomechanically, one of the ways to increase running speed is to have a longer stride length. Larger steps mean that an animal will cover a greater distance for a given limb movement or expenditure of energy. The longer legs seen in fast runners are consistent with biomechanical predictions of how to increase speed.
Historically, these approaches to the functional analysis of form have not always been rigorously applied. As a result, early form-function studies often resulted in erroneous conclusions. For example, the earliest primates (the order of mammals including lemurs, monkeys, apes, and humans) had enlarged pointed incisor (front) teeth, superficially similar to those of modern rodents, such as squirrels and mice. The enlarged incisors of modern rodents seem superficially well suited for nibbling on seeds, nuts, and berries, and one can see squirrels and other rodents eating such food in parks and zoos. Therefore, for many years, biologists assumed that the enlarged incisor teeth of the earliest primates indicated that they fed on such plant food. The origin of the order Primates was therefore thought to be correlated with a shift in diet from the primitive insectivory to herbivory. However, a more careful, detailed analysis of the diets of many different rodent species revealed that many modern rodents are actually omnivores. They feed on insects and other animal material as well as on vegetation. Further, a recent form-function analysis of the correlation between diet and the size and shape of molar teeth in many living species showed that the type of molars seen in the earliest primates is correlated with a basically insectivorous diet. Thus the original, superficial analysis of form-function correlation that rodent incisor shape necessarily indicated an herbivorous diet proves incorrect. Our current understanding of the origin of the order Primates is that no shift in diet was involved and that, like their ancestors, the earliest primates were basically insectivorous. The moral of this story is that it is not enough to have superficial observations on possible form-function correlations or to make intuitively reasonable assumptions. We must carefully and rigorously gather data to establish form-function relationships.
Excerpted from The Evolution of Vertebrate Design by Leonard B. Radinsky. Copyright © 1987 The University of Chicago. Excerpted by permission of The University of Chicago Press.
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.
Table of ContentsPublisher's Note
1. Introduction: Fossils and Phylogeny
Sources of Information
Methods of Analysis
Vertebrates in Geological Time
2. Evolution: Patterns and Process
The Process of Evolution
3. The Basic Vertebrate Body Plan
4. The Earliest Fossil Vertebrates: The Ostracoderms
5. The First Jawed Fishes
6. Early Bony and Cartilaginous Fishes
7. Later Cartilaginous Fishes
8. Later Bony Fishes
9. Designs for Land Life
10. The Amphibians
11. Early Reptiles
12. The Great Reptile Land Radiations
14. Reptile Designs for Marine Life
Other Aquatic Reptiles
15. Reptile Designs for Flight
16. The Origin of Mammals
17. Diversity of Mammalian Design
18. The Evolutionary Radiations of the Mammals