Vertebrate Life 9th Edition PDF

1.2 Classification of Vertebrates The diversity of vertebrates (more than 63,000 living species and perhaps a hundred times that number of species now extinct) makes the classification of vertebrates an extraordinarily difficult task. Yet classification has long been at the heart of evolutionary biology...

1.2 Classification of Vertebrates The diversity of vertebrates (more than 63,000 living species and perhaps a hundred times that number of species now extinct) makes the classification of vertebrates an extraordinarily difficult task. Yet classification has long been at the heart of evolutionary biology. Initially, classification of species was seen as a way of managing the diversity of organisms, much as an office filing system manages the paperwork of the office. Each species could be placed in a pigeonhole marked with its name; when all species were in their pigeonholes, the diversity of vertebrates would have been encompassed. This approach to classification was satisfactory as long as species were regarded as static and immutable: once a species was placed in the filing system, it was there to stay. Acceptance of the fact that species evolve has made that kind of classification inadequate. Now biologists must express evolutionary relationships among species by incorporating evolutionary information in the system of classification. Ideally, a classification system should not only attach a label to each species but also encode the evolutionary relationships between that species and other species. Modern techniques of systematics (the evolutionary classification of organisms) have become methods for generating testable hypotheses about evolution. Classification and Names Our system of naming species is pre-Darwinian. It traces back to methods established by the naturalists of the seventeenth and eighteenth centuries, especially those of Carl von Linné, a Swedish naturalist, better known by his Latin pen name, Carolus Linnaeus. The Linnaean system employs binominal nomenclature to designate species and arranges species into hierarchical categories (taxa, singular taxon) for classification. Binominal Nomenclature The scientific naming of species became standardized when Linnaeus’s monumental work, Systema Naturae (The System of Nature), was published in sections between 1735 and 1758. Linnaeus attempted to give an identifying name to every known species of plant and animal. His method assigns a binominal (two-word) name to each species. Familiar examples include Homo sapiens for human beings (Latin hom = human and sapien = wise), Passer domesticus for the house sparrow (Latin passer = sparrow and domesticus = belonging to 8 CHAPTER 1 the house), and Canis familiaris for the domestic dog (Latin canis = dog and familiaris = of the family). Why use Latin words? Latin was the early universal language of European scholars and scientists. It has provided a uniform usage that scientists, regardless of their native language, continue to recognize worldwide. The same species may have different colloquial names, even in the same language. For example, Felis concolor (Latin for “the uniformly colored cat”) is known in various parts of North America as cougar, puma, mountain lion, American panther, painter, and catamount. In Central and South America it is called león colorado, onça-vermelha, poema, guasura, or yaguá-pitá. But biologists of all nationalities recognize the name Felis concolor as referring to a specific kind of cat. Hierarchical Groups Linnaeus and other naturalists of his time developed what they called a natural system of classification. The species is the basic level of biological classification, but the definition of a species has been contentious, partly because criteria that have been used to identify extant species (e.g., reproductive isolation from other species) don’t work for fossil species and don’t always correspond to genetic differences. Similar species are grouped together in a genus (plural genera), based on characters that define the genus. The most commonly used characters were anatomical because they can be most easily preserved in museum specimens. Thus all doglike species—various wolves, coyotes, and jackals— were grouped together in the genus Canis because they all share certain anatomical features, such as an erectile mane on the neck and a skull with a long, prominent sagittal crest on the top from which massive temporal (jaw-closing) muscles originate. Linnaeus’s method of grouping species was functional because it was based on anatomical (and to some extent on physiological and behavioral) similarities and differences. Linnaeus lived before there was any knowledge of genetics and the mechanisms of inheritance, but he used characters that we understand today are genetically determined biological traits that generally express the degree of genetic similarity or difference among groups of organisms. Genera are placed in families, families in orders, orders in classes, and animal classes in phyla (singular phylum). 1.3 Phylogenetic Systematics All methods of classifying organisms, even preLinnaean systems, are based on similarities among the included species, but some similarities are more The Diversity, Classification, and Evolution of Vertebrates significant than others. For example, nearly all vertebrates have paired limbs, but only a few kinds of vertebrates have mammary glands. Consequently, knowing that the species in question have mammary glands tells you more about the closeness of their relationship than knowing that they have paired limbs. You would thus give more weight to the presence of mammary glands than to paired limbs. A way to assess the relative importance of different characteristics was developed in the mid-twentieth century by Willi Hennig, who introduced a method of determining evolutionary relationships called phylogenetic systematics (Greek phyla = tribe and genesis = origin). An evolutionary lineage is a clade (from cladus, the Greek word for a branch), and phylogenetic systematics is also called cladistics. Cladistics recognizes only groups of organisms that are related by common descent. The application of cladistic methods has made the study of evolution rigorous. The groups of organisms recognized by cladistics are called natural groups, and they are linked in a nested series of ancestor-descendant relationships that trace the evolutionary history of the group. Hennig’s contribution was to insist that these groups can be identified only on the basis of derived characters. “Derived” means “different from the ancestral condition.” A derived character is called an apomorphy (Greek apo = away from [i.e., derived from] and morph = form, which is interpreted as “away from the ancestral condition”). For example, the feet of terrestrial vertebrates have distinctive bones—the carpals, tarsals, and digits. This arrangement of foot bones is different from the ancestral pattern seen in lobe-finned fishes, and all lineages of terrestrial vertebrates had that derived pattern of foot bones at some stage in their evolution. (Many groups of terrestrial vertebrates—horses, for example—have subsequently modified the foot bones, and some, such as snakes, have lost the limbs entirely. The significant point is that those evolutionary lineages include species that had the derived terrestrial pattern.) Thus, the terrestrial pattern of foot bones is a shared derived character of terrestrial vertebrates. In cladistic terminology, shared derived characters are called synapomorphies (Greek syn = together, so synapomorphy can be interpreted as “together away from the ancestral condition”). Of course, organisms also share ancestral characters— that is, characters that they have inherited unchanged from their ancestors. These are called plesiomorphies (Greek plesi = near in the sense of “similar to the ancestor”). Terrestrial vertebrates have a vertebral column, for example, that was inherited from lobe-finned fishes. Hennig called shared ancestral characters symplesiomorphies (sym, like syn, is a Greek root that means “together”). Symplesiomorphies tell us nothing about degrees of relatedness. The principle that only shared derived characters can be used to determine evolutionary relationships is the core of cladistics. The conceptual basis of cladistics is straightforward, although applying cladistic criteria to real organisms can become very complicated. To illustrate cladistic classification, consider the examples presented in Figure 1–3. Each of the three cladograms (diagrams showing hypothetical sequences of branching during evolution) illustrates a possible evolutionary relationship for the three taxa (plural of taxon, which means a species or group of species), identified as 1, 2, and 3. To make the example a bit more concrete, we can consider three characters: the number of toes on the front foot, the skin covering, and the tail. For this example, let’s say that in the ancestral character state there are five toes on the front foot, and in the derived state there are four toes. We’ll say that the ancestral state is a scaly skin, and the derived state is a lack of scales. As for the tail, it is present in the ancestral state and absent in the derived state. Figure 1–3 shows the distribution of those three character states in the three taxa. The animals in taxon 1 have five toes on the front feet, lack scales, and have a tail. Animals in taxon 2 have five toes, scaly skins, and no tails. Animals in taxon 3 have four toes, scaly skins, and no tails. How can we use this information to decipher the evolutionary relationships of the three groups of animals? Notice that the derived number of toes occurs only in taxon 3, and the derived tail condition (absent) is found in taxa 2 and 3. The most parsimonious phylogeny (i.e., the evolutionary relationship requiring the fewest number of changes) is represented by Figure 1–3(a). Only three changes are needed to produce the derived character states: 1. In the evolution of taxon 1, scales are lost. 2. In the evolution of the lineage including taxon 2 + taxon 3, the tail is lost. 3. In the evolution of taxon 3, a toe is lost from the front foot. The other two phylogenies shown in Figure 1–3 are possible, but they would require tail loss to occur independently in taxon 2 and in taxon 3. Any change in a structure is an unlikely event, so the most plausible phylogeny is the one requiring the fewest changes. The second and third phylogenies each require four evolutionary changes, so they are less parsimonious than the first phylogeny we considered. Phylogenetic Systematics 9 (a) 1 2 3 Scales→ no scales 5 toes→ 4 toes Tail→ no tail (b) 2 Tail→ no tail 1 Scales→ no scales 3 Tail→ no tail 5 toes→ 4 toes (c) 3 1 Tail→ no tail Scales→ no scales 2 Tail→ no tail 5 toes→ 4 toes Figure 1–3 Three cladograms showing the possible evolutionary relationships of three taxa. Bars connect derived characters (apomorphies). The black bar shows a shared derived character (a synapomorphy) of the lineage that includes taxa 2 and 3. Colored bars represent two independent origins of the same derived character state that must be assumed to have occurred if there was no apomorphy in the most recent common ancestor of taxa 2 and 3. The labels identify changes from the ancestral character state to the derived condition. Cladogram (a) requires a total of three changes from the ancestral condition to explain the distribution of characters in the extant taxa, whereas cladograms (b) and (c) require four changes. Because cladogram (a) is more parsimonious (i.e., requires the smallest number of changes), it is considered to be the most likely sequence of changes. A phylogeny is a hypothesis about the evolutionary relationships of the groups included. Like any scientific hypothesis, it can be tested when new data become available. If it fails that test, it is falsified; that is, it is rejected, and a different hypothesis (a different cladogram) takes its place. The process of testing hypotheses and replacing those that are falsified is a continuous one, and changes in the cladograms in suc10 CHAPTER 1 cessive editions of this book show where new information has generated new hypotheses. The most important contribution of phylogenetic systematics is that it enables us to frame testable hypotheses about the sequence of events during evolution. So far we have avoided a central issue of phylogenetic systematics: How do scientists know which character state is ancestral (plesiomorphic) and which is The Diversity, Classification, and Evolution of Vertebrates 8. Batrachia 7. Lissamphibia 12. Archosauria 11. Diapsida 10. Sauropsida Eutheria (placentals) Metatheria (marsupials) Monotremata (platypus, spiny anteaters) Aves (birds) Crocodilia (alligators and crocodiles) Anura (frogs) Caudata (salamanders) Gymnophiona (caecilians) Dipnoi (lungfishes) Actinistia (coelacanths) Actinopterygii (ray-finned fishes) Chondrichthyes (sharks, skates, rays, ratfishes) Petromyzontiformes (lampreys) Myxiniformes (hagfishes) Outgroups (tunicates and cephalochordates) Evolutionary lineages must have a single evolutionary origin; that is, they must be monophyletic (Greek mono = one, single) and include all the descendants of that Lepidosauria (tuatara, lizards, snakes) 1.4 The Problem with Fossils: Crown and Stem Groups Testudinia (turtles) ancestor. The cladogram depicted in Figure 1–4 is a hypothesis of the evolutionary relationships of the major living groups of vertebrates. A series of dichotomous branches extends from the origin of vertebrates to the groups of extant vertebrates. Cladistic terminology assigns names to the lineages originating at each branch point. This process produces a nested series of groups, starting with the most inclusive. For example, the Gnathostomata includes all vertebrate animals that have jaws; that is, every taxon above number 2 in Figure 1–4 is included in the Gnathostomata; every taxon above number 3 is included in the Osteichthyes (bony fishes); and so on. Because the lineages are nested, it is correct to say that humans are both gnathostomes and osteichthyans. After number 6, the cladogram divides into Lissamphibia and Amniota, and humans are in the Amniota lineage. The cladogram divides again above number 9 into two lineages, the Sauropsida and Synapsida lineages. Humans are in the Eutheria, which is in the synapsid lineage. derived (apomorphic)? That is, how can we determine the direction (polarity) of evolutionary transformation of the characters? For that, we need additional information. Increasing the number of characters we are considering can help, but comparing the characters we are using with an outgroup that consists of the closest relatives of the ingroup (i.e., the organisms we are studying) is the preferred method. A well-chosen outgroup will possess ancestral character states compared to the ingroup. For example, lobe-finned fishes are an appropriate outgroup for terrestrial vertebrates. 14. Theria 13. Synapsida (including Mammalia) 9. Amniota 6. Tetrapoda 5. Rhipidistia 4. Sarcopterygii ? 3. Osteichthyes 2. Gnathostomata 1. Vertebrata Figure 1–4 Phylogenetic relationships of extant vertebrates. This diagram depicts the probable relationships among the major groups of extant vertebrates. Note that the cladistic groupings are nested progressively; that is, all placental mammals are therians, all therians are synapsids, all synapsids are amniotes, all amniotes are tetrapods, and so on. In a phylogenetic classification lineages can be named at each branching point, although it is not necessary to do so. The dashed line and question mark indicate uncertainty about the branching sequence for hagfishes. The Problem with Fossils: Crown and Stem Groups 11 This method of tracing ancestor-descendant relationships allows us to decipher evolutionary pathways that extend from fossils to living groups, but a difficulty arises when we try to find names for groups that include fossils. The derived characters that define the extant groups of vertebrates did not necessarily appear all at the same time. On the contrary, evolution usually acts by gradual and random processes, and derived characters appear in a stepwise fashion. The extant members of a group have all of the derived characters of that group because that is how we define the group today; but, as you move backward through time to fossils that are ancestral to the extant species, you encounter forms that have a mosaic of ancestral and derived characters. The further back in time you go, the fewer derived characters the fossils have. What can we call the parts of lineages that contain these fossils? They are not included in the extant groups because they lack some of the derived characters of those groups, but the fossils in the lineage are more closely related to the extant group than they are to animals in other lineages. The solution to this problem lies in naming two types of groups: crown groups and stem groups. The crown groups are defined by the extant species, the ones that have all the derived characters. The stem groups are the extinct forms that preceded the point at which the first member of the crown group branched off. Basically, stem groups contain fossils with some derived characters, and crown groups contain extant species plus those fossils that have all of the derived characters of the extant species. Stem groups are paraphyletic (Greek para = beside, beyond); that is, they do not contain all of the descendants of the ancestor of the stem group plus the crown group because the crown group is excluded by definition. 1.5 Evolutionary Hypotheses Phylogenetic systematics is based on the assumption that organisms in a lineage share a common heritage, which accounts for their similarities. Because of that common heritage, we can use cladograms to ask questions about evolution. By examining the origin and significance of characters of living animals, we can make inferences about the biology of extinct species. For example, the phylogenetic relationship of crocodilians, dinosaurs, and birds is shown in Figure 1–5. We know that both crocodilians and birds display extensive parental care of their eggs and young. Some fossilized dinosaur nests contain remains of baby dinosaurs, suggesting that at least some dinosaurs may also have cared for their young. Is that a plausible inference? Obviously there is no direct way to determine what sort of parental care dinosaurs had. The intermediate lineages in the cladogram (pterosaurs and dinosaurs) are extinct, so we cannot observe their reproductive behavior. However, the phylogenetic diagram in Figure 1–5 provides an indirect way to approach the question by examining the lineage that includes the closest living relatives of dinosaurs, crocodilians and Archosauria Dinosauria Saurischia Aves Lepidosauria Phytosaurs Crocodilians Pterosaurs Ornithischian dinosaurs Saurischian dinosaurs Most recent common ancestor of crocodilians and birds Figure 1–5 Using a cladogram to make inferences about behavior. The cladogram shows the relationships of the Archosauria, the evolutionary lineage that includes living crocodilians and birds. (Phytosaurs were crocodile-like animals that disappeared at the end of the Triassic, and pterosaurs were the flying reptiles of the Jurassic and Cretaceous.) Both extant groups— crocodilians and birds—display extensive parental care of eggs and young. The most parsimonious explanation of this situation assumes that parental care is an ancestral character of the archosaur lineage. 12 CHAPTER 1 The Diversity, Classification, and Evolution of Vertebrates Birds birds. Crocodilians are more basal than pterosaurs and dinosaurs and birds are more derived; together crocodilians and birds form what is called an extant phylogenetic bracket. Both crocodilians and birds, the closest living relatives of the dinosaurs, do have parental care. Looking at living representatives of more distantly related lineages (outgroups), we see that parental care is not universal among fishes, amphibians, or sauropsids other than crocodilians. The most parsimonious explanation of the occurrence of parental care in both crocodilians and birds is that it had evolved in that lineage before the crocodilians separated from dinosaurs + birds. (We cannot prove that parental care did not evolve separately in crocodilians and in birds, but one change to parental care is more likely than two changes.) Thus, the most parsimonious hypothesis is that parental care is a derived character of the evolutionary lineage containing crocodilians + dinosaurs + birds (the Archosauria). That means we are probably correct when we interpret the fossil evidence as showing that dinosaurs did have parental care. Figure 1–5 also shows how cladistics has made talking about restricted groups of animals more complicated than it used to be. Suppose you wanted to refer to just the two lineages of animals that are popularly known as dinosaurs—ornithischians and saurischians. What could you call them? Well, if you call them dinosaurs, you’re not being phylogenetically correct, because the Dinosauria lineage includes birds. So if you say dinosaurs, you are including ornithischians + saurischians + birds, even though any seven-year-old would understand that you are trying to restrict the conversation to extinct Mesozoic animals. In fact, there is no correct name in cladistic terminology for just the animals popularly known as dinosaurs. That’s because cladistics recognizes only monophyletic lineages, and a monophyletic lineage includes an ancestral form and all its descendants. The most recent common ancestor of ornithischians, saurischians, and birds in Figure 1–5 lies at the intersection of the lineage of ornithischians with saurischians + birds, so Dinosauria is a monophyletic lineage. If birds are omitted, however, all the descendants of the common ancestor are no longer included; and ornithischians + saurischians minus birds does not fit the definition of a monophyletic lineage. It would be called a paraphyletic group. The stem groups discussed in the previous section are paraphyletic because they do not include all of the descendants of the fossil forms. Biologists who are interested in how organisms live often want to talk about paraphyletic groups. After all, the dinosaurs (in the popular sense of the word) differed from birds in many ways. The only correct way of referring to the animals popularly known as dinosaurs is to call them nonavian dinosaurs, and you will find that and other examples of paraphyletic groups later in the book. Sometimes even this construction does not work because there is no appropriate name for the part of the lineage you want to distinguish. In this situation we will use quotation marks (e.g., “ostracoderms”) to indicate that the group is paraphyletic. Another important term is sister group. The sister group is the monophyletic lineage most closely related to the monophyletic lineage being discussed. In Figure 1–5, for example, the lineage that includes crocodilians + phytosaurs is the sister group of the lineage that includes pterosaurs + ornithischians + saurischians + birds. Similarly, pterosaurs are the sister group of ornithischians + saurischians + birds, ornithischians are the sister group of saurischians + birds, and saurischians are the sister group of birds. Determining Phylogenetic Relationships We’ve established that the derived characters systematists use to group species into higher taxa must be inherited through common ancestry. That is, they are homologous (Greek homo = same) similarities. In principle, that notion is straightforward; but in practice, the determination of common ancestry can be complex. For example, birds and bats have wings that are modified forelimbs, but the wings were not inherited from a common ancestor with wings. The evolutionary lineages of birds (Sauropsida) and bats (Synapsida) diverged long ago, and wings evolved independently in the two groups. This process is called convergent evolution. Parallel evolution describes the situation in which species that have diverged relatively recently develop similar specializations. The long hind legs that allow the North American kangaroo rats and the African jerboa to jump are an example of parallel evolution in these two lineages of rodents. A third mechanism, reversal, can produce similar structures in distantly related organisms. Sharks and cetaceans (porpoises and whales) have very similar body forms, but they arrived at that similarity from different directions. Sharks retained an ancestral aquatic body form, whereas cetaceans arose from a lineage of terrestrial mammals with well-developed limbs that returned to an aquatic environment and reverted to the aquatic body form. Convergence, parallelism, and reversal are forms of homoplasy (Greek homo = same and plas = form, shape). Homoplastic similarities do not indicate common ancestry. Indeed, they complicate the process of deciphering evolutionary relationships. Convergence Evolutionary Hypotheses 13 Bornean Clouded Leopard Sumatran Clouded Leopard Mainland Clouded Leopard Mainland Clouded Leopard Leopard Snow Leopard Lion Jaguar Sumatran Clouded Leopard Tiger Bornean Clouded Leopard Domestic cat (a) (b) (c) (d) Figure 1–6 An example of the value of phylogenetic analyses in conservation. (a) A cladogram of clouded leopards and (b) a map showing the geographic locations of surviving populations. The clouded leopard found on the Asian mainland, Neofelis nebulosa (c), is as distant genetically from the species of clouded leopard found on Sumatra and Borneo, Neofelis diardi (d), as lions are from tigers. The island forms of Neofelis diardi are more closely related to each other than either is to the mainland form. Nonetheless, the genetic differences that distinguish the leopards on Sumatra from those on Borneo are large enough to be separated in captive breeding programs for the two forms. and parallelism give an appearance of similarity (as in the wings of birds and bats) that is not the result of common evolutionary origin. Reversal, in contrast, conceals similarity (e.g., between cetaceans and their four-legged terrestrial ancestors) that is the result of common evolutionary origin. Phylogeny and Conservation Combining genetic analysis with cladistic analyses can provide an important tool for biologists concerned with conservation (Figure 1–6). For example, some of the new species of mammals described in section 1.1 14 CHAPTER 1 were identified by comparing their DNA with the DNA of related species. When a genetic difference is large, it means that the two forms have been reproductively isolated from each other and have followed different evolutionary pathways. From a conservationist’s perspective, lineages that have evolved substantial genetic differences are Evolutionarily Significant Units (ESUs), and management plans should protect the genetic diversity of ESUs. For example, a genetic study published in 2007 revealed that the clouded leopards on the islands of Borneo and Sumatra (Neofelis diardi) and those on the The Diversity, Classification, and Evolution of Vertebrates Asian mainland (Neofelis nebulosa) separated between 1.4 and 2.8 million years ago, and the three forms have been following independent evolutionary pathways since then. The genetic mainland form and the island forms are genetically different. Furthermore, the island populations are reproductively isolated from each other, and the clouded leopards on Borneo and Sumatra are genetically distinct. Thus, the three forms represent three ESUs, and conservation plans should treat the mainland species and the two island species separately. Before this study the three forms were grouped together, and a portion of the genetic diversity of clouded leopards was lost through interbreeding in captivity. 1.6 Earth History and Vertebrate Evolution Since their origin in the early Paleozoic, vertebrates have been evolving in a world that has changed enormously and repeatedly. These changes have affected vertebrate evolution both directly and indirectly. Understanding the sequence of changes in the positions of continents, and the significance of those positions regarding climates and interchange of faunas, is central to understanding the vertebrate story. These events are summarized inside the front cover of the book, and Chapters 7, 15, and 19 give details. The history of Earth has occupied three geological eons: the Archean, Proterozoic, and Phanerozoic. Only the Phanerozoic, which began about 542 million years ago, contains vertebrate life, and it is divided into three geological eras: the Paleozoic (Greek paleo = ancient and zoo = animal), Mesozoic (Greek meso = middle), and Cenozoic (Greek cen = recent). These eras are divided into periods, which can be further subdivided in a variety of ways, such as the subdivisions called epochs within the Cenozoic era from the Paleocene to the Recent. Movement of landmasses, called continental drift, has been a feature of Earth’s history at least since the Proterozoic, and the course of vertebrate evolution has been shaped extensively by continental movements. By the early Paleozoic, roughly 540 million years ago, a recognizable scene had appeared. Seas covered most of Earth as they do today, large continents floated on Earth’s mantle, life had become complex, and an atmosphere of oxygen had formed, signifying that the photosynthetic production of food resources had become a central phenomenon of life. The continents still drift today—North America is moving westward and Australia northward at the speed of approximately 4 centimeters per year (about the rate at which fingernails grow). Because the movements are so complex, their sequence, their varied directions, and the precise timing of the changes are difficult to summarize. When the movements are viewed broadly, however, a simple pattern unfolds during vertebrate history: fragmentation, coalescence, fragmentation. Continents existed as separate entities over 2 billion years ago. Some 300 million years ago, all of these separate continents combined to form a single landmass known as Pangaea, which was the birthplace of terrestrial vertebrates. Persisting and drifting northward as an entity, this huge continent began to break apart about 150 million years ago. Its separation occurred in two stages: first into Laurasia in the north and Gondwana in the south, and then into a series of units that have drifted and become the continents we know today. The complex movements of the continents through time have had major effects on the evolution of vertebrates. Most obvious is the relationship between the location of landmasses and their climates. At the end of the Paleozoic, much of Pangaea was located on the equator, and this situation persisted through the middle of the Mesozoic. Solar radiation is most intense at the equator, and climates at the equator are correspondingly warm. During the late Paleozoic and much of the Mesozoic, large areas of land enjoyed tropical conditions. Terrestrial vertebrates evolved and spread in these tropical regions. By the end of the Mesozoic, much of Earth’s landmass had moved out of equatorial regions; and, by the mid-Cenozoic, most terrestrial climates in the higher latitudes of the Northern and Southern Hemispheres were temperate instead of tropical. A less obvious effect of the position of continents on terrestrial climates comes from changes in patterns of oceanic circulation. For example, the Arctic Ocean is now largely isolated from the other oceans, and it does not receive warm water via currents flowing from more equatorial regions. High latitudes are cold because they receive less solar radiation than do areas closer to the equator, and the Arctic Basin does not receive enough warm water to offset the lack of solar radiation. As a result, the Arctic Ocean is permanently frozen, and cold climates extend well southward across the continents. The cooling of climates in the Northern Hemisphere at the end of the Eocene epoch, around 34 million years ago, may have been a factor leading to the extinction of archaic mammals, and it is partly the result of changes in oceanic circulation at that time. Another factor that influences climates is the relative levels of the continents and the seas. At some Earth History and Vertebrate Evolution 15 periods in Earth’s history, most recently in the late Mesozoic and again in the first part of the Cenozoic, shallow seas flooded large parts of the continents. These epicontinental seas extended across the middle of North America and the middle of Eurasia during the Cretaceous period and early Cenozoic. Water absorbs heat as air temperature rises, and then releases that heat as air temperature falls. Thus, areas of land near large bodies of water have maritime climates—they do not get very hot in summer or very cold in winter, and they are usually moist because water that evaporates from the sea falls as rain on the land. Continental climates, which characterize areas far from the sea, are usually dry with cold winters and hot summers. The draining of the epicontinental seas at the end of the Cretaceous probably contributed to the demise of the dinosaurs by making climates in the Northern Hemisphere more continental. In addition to changing climates, continental drift has formed and broken land connections between the continents. Isolation of different lineages of vertebrates on different landmasses has produced dramatic examples of the independent evolution of similar types of organisms, such as the diversification of mammals in the midCenozoic, a time when Earth’s continents reached their greatest separation during the history of vertebrates. Much of evolutionary history appears to depend on whether a particular lineage of animals was in the right place at the right time. This random element of evolution is assuming increasing prominence as more detailed information about the times of extinction of old groups and radiation of new groups suggests that competitive replacement of one group by another is not the usual mechanism of large-scale evolutionary change. The movements of continents and their effects on climates and the isolation or dispersal of animals are taking an increasingly central role in our understanding of vertebrate evolution. On a continental scale, the advance and retreat of glaciers in the Pleistocene caused homogeneous habitats to split and merge repeatedly, isolating populations of widespread species and leading to the evolution of new species. Summary The more than 63,000 species of living vertebrates span a size range from less than a gram to more than 100,000 kilograms. They live in habitats extending from the bottom of the sea to the tops of mountains. This extraordinary diversity is the product of more than 500 million years of evolution, and the vast majority of species fall into one of the two major divisions of bony fishes (Osteichthyes)—the aquatic ray-finned fishes (Actinopterygii) and the primarily terrestrial lobe-finned fishes and tetrapods (Sarcopterygii), each of which contains more than 25,000 extant species. Phylogenetic systematics, usually called cladistics, classifies animals on the basis of shared derived character states. Natural evolutionary groups can be defined only by these derived characters; retention of ancestral characters does not provide information about evolutionary lineages. Application of this principle produces groupings of animals that reflect evolutionary history as accurately as we can discern it and forms a basis for making hypotheses about evolution and for designing management plans that conserve the genetic diversity of evolutionary lineages. Earth has changed dramatically during the half-billion years of vertebrate history. Continents were fragmented when vertebrates first appeared; coalesced into one enormous continent, Pangaea, about 300 million years ago; and began to fragment again about 150 million years ago. This pattern of fragmentation, coalescence, and fragmentation has resulted in isolation and renewed contact of major groups of vertebrates on a worldwide scale. Discussion Questions 1. Why don’t phylogenetic (cladistic) classifications have a fixed number of hierarchical categories like those in a Linnaean classification? 16 CHAPTER 1 2. What aspect of evolution does a phylogenetic classification represent more clearly than a Linnaean classification does? The Diversity, Classification, and Evolution of Vertebrates