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Paleontologia Criacionista e Mudanças

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Paleontology Meets the Creationist Challenge Daniel G. Blackburn P aleontology offers one of many bodies of evidence for the evolution of life. However, unlike technical information from molecular genetics, cladistics, and embryology, the significance of fossils is clear to a public that is acquainted with dinosaurs and other extinct forms through books, movies, and museums. Consequently, the fossil record is a major focus of the creationist attack on science. Numerous creationist books promote claims that the earth is only a few thousand years old; that rock strata indicate nothing about geological history; that fossils are remains of animals that died in a great Biblical flood; and that humans and dinosaurs coexisted in recent times. With the upsurge of creationist political activity, and the ongoing introduction of creationism into science curricula, teachers and scientists must be prepared to counter such claims with factual evidence. Fundamental to the creationist position is the proposition that the fossil record fails to document origins of the major lineages of organisms. This claim is based on purported structural and temporal gaps in the fossil record between major taxa, gaps considered to represent acts of special creation by a divine entity (Gish, 1979; Bliss, et al., 1990). From such discontinuities, many of which are real, even creationists who acknowledge that species evolve from other species deny the possibility of macroevolutionary change (Moore and Slusher, 1974; Thurman, 1978). Oft-cited examples of gaps in the fossil record include those represented as the origins of whales, birds, hominids, flowering plants, and the invertebrate phyla. In a widely-distributed book devoted to fossils, Duane Gish (1979, p. 49) offered the following challenge to biology: "whether evolution actually did happen or not can only be decided, scientifically, established by the discovery of the fossilized remains of Daniel Blackburn is an Associate Professor of Biology at Trinity College, Hartford. CT. His research and teaching focus on vertebrate zoology, neuroscience, and evolutionary biology. LICENSED TO UNZ.ORG ELECTRONIC REPRODUCTION PROHIBITED • Paleontology Meets the Creationist Challenge • representative samples of those intermediate types which have been postulated on the basis of the indirect evidence.... As a matter of fact, the discovery of five or six of the transitional forms scattered through time would be sufficient to document evolution." (emphasis added) This paper responds to the creationist challenge by considering the significance of gaps in the fossil record; by demonstrating creationist confusion over the nature of transitional forms; and by documenting specific examples of how fossil evidence has documented the origins of major groups of organisms. What Do Gaps in the Fossil Record Represent? Creationist writings notwithstanding, discontinuities in the fossil record offer no real challenge to the phenomenon of evolution. Such gaps actually reflect the nature of fossilization, as well as aspects of evolutionary change. To fossilize, an organism typically must contain hard parts that withstand action of microbes, predators, and the environment. Moreover, it must be deposited in a substrate suitable for fossilization, such as by being buried quickly in mud or sand (Raup and Stanley, 1979). After tens or hundreds of millions of years, the fossil then must be exposed at precisely the right place and time, if it is to be discovered by a human who has been trained to remove the specimen appropriately and document its discovery. Even a few weeks of wind or water erosion can destroy a specimen as a recognizable fossil. The 160 years during which our species has been seeking fossils for serious study represents less than / three millionth of 17c of the time since the first vertebrates swam the oceans. We can only speculate what a minuscule percentage of the original fossil-bearing rocks from any particular time period are, at this geological moment, accessible for observation—for such rocks cannot lie buried beneath miles of earth or ocean, and must not have disappeared over the millennia through metamorphosis, subduction, or erosion. Obviously, the chances of any given specimen having been found as a fossil are infinitesimally small. Therefore, it is to be expected that many species will not be represented among fossils that thus far have been discovered. Voluminous evidence indicates that species commonly arise as small populations that have become isolated from the parent stock—seeds of a plant that have been carried by ocean currents to a distant island, for example, or pocket mice isolated by a river. Such conditions are ideal for rapid change. Not only is the founder population unlikely to be fully representative of the parental gene pool, but random mutations are more likely to overtake a population if it is small. The more rapid the evolutionary change, and the more restricted the geographical area where it has occurred, the less likely that intermediates will be represented as fossils (Stanley, 1981). In addition, small genetic changes can have large phenotypic effects, and as a consequence, directional evolutionary change need not traverse phenotypic intermediates postulated by gradualistic scenarios (Gould, 1980). Thus, lacunae __________ __ LICENSED TO UNZ.ORG ELECTRONIC REPRODUCTION PROHIBITED • Paleontology Meets the Creationist Challenge • between species actually may indicate the mechanism of evolutionary change, rather than being an artifact of the record of fossil or living forms (Eldredge, 1985; Futuyma, 1981). A further point about discontinuities in the species record is that the number of potential gaps increases with the number of known intervening forms. Gish (1979, p. 78) takes advantage of this point in discussing the Paleozoic fish-amphibian transition. While conceding that ichthyostegids appear intermediate between crossopterygian fishes and later amphibians, he argues that a gap now exists between ichthyostegids and fishes. The fact is, however, that until every possible intermediate is discovered, the more we know about the evolution of a lineage, the more hypothetical "gaps" it will contain. By definition, so-called "missing links" are, after all, missing. Therefore, the more successful a search for transitional forms is, the more self-perpetuating that activity is likely to be. Characteristics of Evolutionary Intermediates Anti-evolutionary writings exhibit considerable confusion over the features to be expected of so-called evolutionary intermediates. Such writings commonly assume that intermediate species must appear equally transitional in all of their phenotypic features. Thus, creationists reject the proto-avian Archaeopteryx as a link between birds and dinosaurian ancestors because it had fully developed feathers (Morris, 1974a; Gish, 1979). The presence of such "reptilian" features as forelimb claws and an unkeeled sternum in Archaeopteryx are discounted by creationists because they occur in a few living birds; that such features could have re-appeared in some avian lineages during the past 70 million years is ignored (Bliss etal., 1990). The "reptilian" feature of teeth in Archaeopteryx also is dismissed by creationists, for the puzzling reason that teeth were retained in other birds of the Mesozoic (Gish, 1979). To be considered as phylogenetically intermediate by creationists, a proto-bird must have partial, not complete feathers (see Bliss et al., 1990), not to mention partial wings and partial teeth (Morris, 1974b, p. 91). Whether such a form would also have to lay partial eggs and hatch out partial chicks has not been stated. The well-known concept of mosaic evolution provides a useful framework in which to consider evolutionary transformations. Within a group of related organisms, morphological features can be considered as relatively ancestral or derived. Because features undergo evolutionary transformation at different rates and times, they can originate sequentially rather than simultaneously. As a result, any given intermediate species should be a mosaic or combination of ancestral and derived features. For example, as discussed below, Archaeopteryx is a mosaic of avian features as well as ancestral (dinosaurian) features, just as one would predict for a transitional form. 28 Creation/Evolution LICENSED TO UNZ.ORG ELECTRONIC REPRODUCTION PROHIBITED • Paleontology Meets the Creationist Challenge • Creationist publications also indicate confusion about how the stratigraphic record is to be interpreted. The creationist approach is to reject a fossil form as ancestral to a second when the two have not been found in successive rock strata (Bliss et al., 1990). However, this approach is based on the misconception that species transform progressively from one to another in a single unbroken lineage. When species arise through splitting of a filial population from a parental stock, ancestral and descendant species will exist at the same time; indeed, the former may well outlive the latter geologically. Add to these facts the spotty nature of the fossil record, and little reason exists to suppose that an ancestral species will necessarily have been found in older strata than its derivatives. Although fossil age and stratigraphic position are broadly suggestive, biology has abandoned them as precise indicators of phylogenetic relationships (e.g. Gaffney et al., 1995; Gauthier et al., 1988), in favor of cladistic analyses and molecular approaches. More Anti-Evolutionary Confusion Misconceptions in anti-evolutionary writings frequently stem from a reliance on incomplete or obsolete information. For example, Michael Denton's (1986) book offers several examples of purported gaps in the fossil record to demonstrate the implausibility of macroevolution. He illustrates the gaps by using arrows to link line drawings of skeletons of specialized descendants and putative ancestors, commonly on the basis of outmoded information. Thus, Archaeopteryx is linked to the primitive thecodont Euparkeria, in ignorance of the powerful evidence for the dinosaurian origin of birds. Likewise, the aquatic, Mesozoic plesiosaur Cryptocleidus (misspelled by Denton) is coupled to a terrestrial diapsid that predated it by 75 million years; evidence allying plesiosaurs with aquatic nothosaurs (Carroll, 1988) is not considered. Denton (1986) also links the skeleton of an Eocene bat with that of a modern shrew; with the latter drawn at twice the size of the former, the transition implied by the arrow between them appears implausible—as well it should. Another problem with the creationist approach is that purported gaps are often an artifact of its own non-technical terminology. In well-documented evolutionary transformations between two major taxa, whether biologists label a species as belonging to one group or the other can be unimportant as well as arbitrary. A good example is offered by the transition between therapsids (formerly termed "mammal-like reptiles") and mammals. This transition has been documented in such detail (Hopson, 1987) that the demarcation of "mammals" from their ancestors is arbitrary and, by consensus, is based on features of the jaw joint and middle ear. Yet, Gish (1979) argues that therapsids are not like mammals, because they lack the mammalian jaw joint! Volume 15, No. 1 29 LICENSED TO UNZ.ORG ELECTRONIC REPRODUCTION PROHIBITED • Paleontology Meets the Creationist Challenge • Similarly, in the hominid fossil record, species of Australopithecus are dismissed as aberrant "apes," and Homo erectus is recognized as a "human," although a degenerate one (Gish, 1979; Morris, 1974a). Thus, as noted by Halstead (1984), semantic trickery is used to enlarge a discontinuity. To illustrate further, Archaeopteryx could be considered a bird or a coelurosaurian dinosaur; in fact, by a cladistic classification, birds are a dinosaurian subgroup. By defining Archaeopteryx as a bird by virtue of its feathers (Gish, 1979), creationists create a gap that they exploit to discount evolution. Yet, if birds were defined by the presence of an enlarged sternum (a specialization for flight, and arguably as good a criterion as any), the supposed gap might lie between the "reptilian" Archaeopteryx and its avian descendants. The creationist preoccupation with "missing links" is retrogressive, and caricatures how paleontology actually proceeds. Indeed, the idea of intermediate forms is reminiscent of the pre-Darwinian "scale of nature," in which organisms were arranged along a continuum between protozoans and humans. Given that evolution typically involves successive branching, and that the fossil record will always be incomplete, discovery of a single common ancestor to each lineage is neither practical nor necessary. Of greater value to evolutionary biology is having enough species (extinct and extant) to detail the pattern of historical change. Thus, as Wheeler (1993) noted, a form can be considered as "intermediate" when it combines features of two distinct taxonomic groups, regardless of whether that form represents a direct link between them. Most vertebrate systematists now analyze taxa through cladistics, a method that permits detailed reconstruction of the patterns of evolution, and that offers objective judgements of the features at each successive branch point. Cladistics contrasts markedly with the creationist approach, which is to try to imagine what transitional forms should be like, and then to chide paleontology for not having found them (see Kitcher, 1982, p. 111). In view of the creationist challenge to biology, to identify even five or six transitional forms, it is enlightening to consider a few of the evolutionary intermediates that paleontology has found and analyzed. Whale Origins On the basis of details of skeletal anatomy, biologists consider whales to have originated from extinct hoofed mammals (ungulates) known as mesonychians (Carroll, 1988). Close molecular similarities between whales and living ungulates (e.g. Gemmell and Westerman, 1994), as well as cladistic analyses (Thewissen, 1994), provide independent confirmation of this interpretation. Although modern whales lack hind limbs, the limb elements develop in embryos and can be expressed as vestiges in adults (Conrad, 1983; Fezer, 1993). However, until recently, fossils documenting the transition 30 Creation/Evolution LICENSED TO UNZ.ORG ELECTRONIC REPRODUCTION PROHIBITED • Paleontology Meets the Creationist Challenge • from the mesonychians to ancient whales (archaeocetes) have been scarce. As a consequence, creationists have held up for special ridicule the idea that whales could have evolved from terrestrial, hoofed ungulates (see Edwords, 1983). The Eocene Basilosaurus has been recognized to be a primitive archaeocete whale since the 1830s. Its presence is a source of embarrassment to creationists, some of whom claim that it not only is no whale, but that it is a reptile, and a terrestrial one at that (e.g., D.T. Gish, as reported by Fezer, 1993). The recent discovery of vestigial limbs in this fossil form (Gingerich et al., 1990) provided further discomfiture, and has led some to deny that the limbs are vestigial (D.T. Gish; see Fezer, 1993), and others to challenge whether the limbs actually belonged to Basilosaurus (Johnson, 1991). Still, even if acknowledged by creationists as a whale, a significant gap exists between this genus and the terrestrial hoofed mammals. One of the most dramatic fossil discoveries of this decade is of extinct whales of the genus Ambulocetus, from Pakistan, an area formerly covered by the Tethys Sea (Thewissen et al., 1994). Like Basilosaurus, Ambulocetus dates from early Eocene strata of about 50 million years ago, and its skeleton suggests that it was the size of a male sea lion (300kg). This ancestral whale not only exhibits robust forearms, but hindlimbs with the standard complement of mammalian skeletal elements. The hind feet are very large, and each toe terminates in a convex hoof like that of mesonychians. Ambulocetus also retains the primitive, mesonychian tail structure. Thewissen et al. (1994) concluded that the animal locomoted in water by vertical undulation of the spine, as in modern whales, coupled with hind limb propulsion, as in the mesonychians. In addition, Ambulocetus evidently could also walk on land. By virtue of its skeletal structure, stratigraphic age, and mode of locomotion, Ambulocetus represents a critical intermediate between the hoofed mesonychians of the early Cenozoic and the archaeocete whales (Berta, 1994). Other important archaeocete fossils have also come to light. Well-developed hindlimbs have now been described in Eocene whales of the genera Indocetus and Rodhocetus (Gingerich et al., 1994). In Rodhocetus (as in Ambulocetus), the skeletal structure is of an animal that could locomote in water as well as support itself on land (Gingerich et al., 1994). Furthermore, the hindlimbs are somewhat reduced, but not nearly so small as in the aquatic Basilosaurus. Thus, an evolutionary reduction in hindlimb length was occurring in the Eocene, in concert with the invasion of marine habitat. In sum, recent fossil discoveries, along with molecular studies and cladistic analyses, have provided powerful confirmation that whales are closely related to ungulates. Paleontology also has revealed details of how the transition from terrestrial mesonychians to aquatic life was accomplished. Perhaps creationists will have to find some other evolutionary transformation to ridicule. Volume 15, No. 1 31 LICENSED TO UNZ.ORG ELECTRONIC REPRODUCTION PROHIBITED • Paleontology Meets the Creationist Challenge • Caecilian Origins One of the three major lineages of living amphibians (i.e., "lissamphibians"), is a group of elongated, limbless forms commonly known as caecilians or apodans (literally, "without feet"). Caecilians are located in the damp tropics. Because they lack limbs, one might be mistaken for a snake or annelid worm. However, caecilians have a spinal column and ribs, a head and brain of the vertebrate type, and such "amphibian" features as a glandular skin and a reliance on water for reproduction. Most caecilians are highly specialized for burrowing, and have small eyes, and strong skulls with which they push through the soil. About 162 species are known (Duellman and Trueb, 1986). Because caecilians share several unique, specialized features both with salamanders and with anurans (frogs and toads), they often are thought to share a common terrestrial origin with these amphibian groups (Duellman and Trueb, 1986). Some researchers consider that caecilians originated from extinct microsaurs of the Paleozoic, which had limbs, and often, elongated bodies (Carroll, 1988). Whatever their exact affinities, caecilians almost certainly originated from a terrestrial ancestor with fully-developed eyes, forelimbs, and hindlimbs. Nevertheless, among living caecilians, the eyes are reduced and no vestige of limbs or skeletal limb girdles remains. Furthermore, until recently, caecilian fossils were confined to some isolated vertebrae of the late Mesozoic and early Cenozoic (Carroll, 1988). Thus, the gap between the distant, terrestrial, limbed amphibians and the extant, limbless caecilians has been considerable, and entirely of the type that creationists use to claim that major animal groups did not arise through evolution. The recent description of an extensive series of caecilian fossils offers strong confirmation for common views of caecilian evolution. The discovery comprises 38 specimens of a new species, Eocaecilia micropodia, which dates to the early Jurassic (Jenkins and Walsh, 1993). Like modern caecilians, this fossil animal was elongate, with a compact, robust skull; remarkably, however, it had large eye openings (orbits) and both forelimbs and hindlimbs. The limb skeletal components are typical of terrestrial vertebrates; the forelimb contains a humerus, radius, ulna, and the hindlimb, a femur, tibia, and fibula. Nevertheless, the limbs are somewhat shorter relative to the vertebrae than those of most extant salamanders (Jenkins and Walsh, 1993). Thus, some degree of limb reduction apparently was underway in caecilians by the Jurassic. The Mesozoic Eocaecilia offers an ideal intermediate between the limbed amphibians of the Paleozoic and the extant caecilians, with regard to structure of the limbs, the skull, and the axial skeleton. Eocaecilia also provides valuable information about the affinities of caecilians, because of its shared similarities with microsaurs, as well as with salamanders and anurans (Jenkins and Walsh, 1993). Although caecilians comprise one of three major lissamphibian groups, the fossil find has received little public attention. 32 Creation/Evolution LICENSED TO UNZ.ORG ELECTRONIC REPRODUCTION PROHIBITED • Paleontology Meets the Creationist Challenge • However, the discovery is arguably one of the most important fossil finds of the decade, and further discredits the creationist position that major animal groups lack a fossil history. Avian Evolution The early evolution of birds has received more attention from creationists than has any other evolutionary transformation. A major reason for this attention may be that Archaeopteryx represents an ideal intermediate between birds and their presumed ancestors, small theropod dinosaurs known as coelurosaurs. Archaeopteryx is known from six Jurassic specimens from southern Germany, three of which have become known to science only in the past 25 years (Wellnhofer, 1990). Not until the 1980s were techniques of cladistics and analytical morphology applied to these fossils. Thus, much of what we know is based on recent study. Archaeopteryx is usually classified as a bird because it exhibits such derived, avian features as feathers and wings. However, it retains a host of theropod features, including teeth, forelimb claws, unfused forelimb digits, a furculum (wishbone), abdominal ribs, bipedalism, an intratarsal joint, four hindlimb digits, and a long bony tail (Carroll, 1988). Except for the furculum and features of the hind limb, most of these theropod features do not occur in modern birds. Archaeopteryx also lacks an enlarged "keel" on the sternum, which in modern flying birds, serves for attachment of the flight muscles. The fact that Archaeopteryx is a mosaic of reptilian and avian features does not, of course, imply that it was the direct ancestor to all living birds (Wellnhofer, 1990); it may well have been a cousin to such an ancestor. Nevertheless, most of its features are what one would expect a distant avian ancestor to exhibit (Carroll, 1988). So convincing is Archaeopteryx as a transitional morphotype that some anti-evolutionary works concede the point, and focus instead on other issues (Johnson, 1991). Most, however, treat the issue as too serious and too well-publicized to ignore. Thus, as discussed above, some creationists argue that Archaeopteryx is not equally transitional in all of its features -a true statement that reveals a fundamental lack of understanding of the pattern of evolutionary change. Another creationist criticism discussed is that this fossil form is a "bird" by definition, and that a gap therefore must exist between it and its reptilian ancestors (Bliss et al., 1990; Morris, 1974a). From a biological standpoint, this argument is a vacuous, semantic quibble (Ruse, 1982). More to the point are claims that a major structural gap exists between Archaeopteryx and its theropod ancestors (Gish, 1979). The large number of theropod features retained by Archaeopteryx refutes this argument. In fact, so similar is the morphology of this proto-avian form to that of a bipedal dinosaur, that one museum specimen was misclassified as a theropod for Volume 15, No. 1 33 LICENSED TO UNZ.ORG ELECTRONIC REPRODUCTION PROHIBITED • Paleontology Meets the Creationist Challenge • several years, until close examination revealed the imprint of feathers (Wellnhofer, 1990). The creationist claim that Archaeopteryx is too "avian" to be an intermediate holds a certain irony; many biologists have concluded that it was so similar to the theropods from which it was derived, that it was a bipedal runner with limited flying ability (Vasquez, 1992; Wellnhofer, 1990). Until recently, the fossil record exhibited a large discontinuity between the Jurassic Archaeopteryx and the highly specialized birds of the late Cretaceous. The latter included two groups of toothed forms: hesperomithiformes, which were flightless diving birds, and ichthyornithiformes, proficient flyers that resembled living gulls and terns (Carroll, 1988). Oddly, creationist writings do not focus on this fossil gap, perhaps under the misconception that Archaeopteryx was much like modern birds. In any case, this discontinuity now has been filled with the discovery of an astonishing array of Mesozoic forms. Many are enantiornithines, a highly successful Jurassic and Cretaceous group with a worldwide distribution (Feduccia, 1995). These bird species varied from the size of a sparrow to that of a turkey vulture; most were arboreal but some were aquatic, and others were long-legged shorebirds (Chiappe, 1995). Enantiornithines retained many primitive features of Archaeopteryx (including clawed wings and teeth), but were fully able to fly, with the specialized flight apparatus and shortened tail of modern birds (Feduccia, 1995). Bone histology suggests that these birds were not fully endothermic ("warm-blooded") as are modern birds (Chiappe, 1995). Another form, Mononykus, was a bizarre, primitive, flightless bird with very short, but stout and strong forelimbs, each of which terminated in a single finger endowed with a hooked claw (Norell et al., 1993). For what function these strange limbs were adapted is a mystery. Equally intriguing is the fact that this flightless bird had a sternal keel. Cladistic analysis indicates that this bird and its allies originated after Archaeopteryx but before the enantiornithines (Chiappe, 1995). Yet another type of bird is represented by Patagopteryx, a stout flightless animal with vestigial wings, from late Cretaceous strata. Cladistically, this bird appears to have diverged after the enantiornithines but before the more advanced hesperornithiforms and ichthyomithiforms (Chiappe, 1995). In sum, paleontological studies have provided a wealth of information about the origins and early evolution of birds. The purported gap between Archaeopteryx and its dinosaurian ancestors, so often cited by creationists, is now known to be minimal. The discontinuity between Archaeopteryx and modern birds, largely overlooked by creationists, has been eliminated through the discovery of an extraordinary diversity of Mesozoic species, a diversity that was unsuspected twenty years ago. 34 Creation/Evolution LICENSED TO UNZ.ORG ELECTRONIC REPRODUCTION PROHIBITED • Paleontology Meets the Creationist Challenge • Other Groups Many other recent discoveries bearing on the origins of animal groups can be cited. One example is a newly-described hominid, represented by 18 fossils uncovered in Ethiopia. First named as, Australopithecus ramidus, this form may warrant recognition as a new genus (White et al., 1994, 1995; Wood, 1994). To put this important find in context, over the past 25 years, paleontology has revealed the history of the hominid lineages in astonishing detail. In addition, molecular studies have suggested that hominids diverged from the common ancestor that they share with the other great apes only about 4 to 6 million years ago. The temporal and structural gap between A. afarensis (to which the "Lucy" skeleton belongs) and this common ancestor must be a small one. The new discovery of A. ramidus fills this gap. At 4.4 million years old, this species extends the hominid line backwards by half a million years, into the time range within which humans and apes probably diverged. In terms of its dentition, this species is even more apelike than other Australopithecus, and suggests a particularly close relationship with chimpanzees (Wood, 1994). Another case is offered by fossils bearing on the early evolution of turtles (chelonians). For many years, a significant gap existed between the primitive genus Proganochelys of the late Triassic (200 million years ago), and turtles with modern features, which appeared 140 million years ago in the Jurassic. The discovery of an early Jurassic turtle {Kayentachelys) has extended the record of morphologically-modern turtles back to 185 million years (Gaffney et al., 1987). Other discoveries and analyses have revealed many details about the Triassic radiation of early turtles, and have shown that Proganochelys may actually be the closest non-chelonian relative to the turtles (Rougier et al., 1995). If these interpretations are correct, the largest discontinuities in the fossil record for turtles have now been filled. Still another example is offered by the description of the oldest known lizard of the family Iguanidae (sensu Frost and Etheridge, 1989). Although this lizard family is diverse and has a wide distribution in the Americas, until recently, its oldest unequivocal fossil dated back no further than the late Pliocene. The recent description of a new fossil (genus Armandisaurus) has extended the paleontological record for this lizard family back into the Miocene (Norell and deQueiroz, 1991). Cladistic analysis has shown that this species is primitive morphologically, and lacks derived features of most other iguanids, just as one would expect for an early member of the family. Conclusion For reasons discussed above, transitional morphotypes tend to be less well represented in the species record than the major groups that they link. Nevertheless, evolutionarily intermediate forms abound. Examples described Volume 15, No. 1 35 LICENSED TO UNZ.ORG ELECTRONIC REPRODUCTION PROHIBITED • Paleontology Meets the Creationist Challenge • herein represent some of many that can be cited from recent vertebrate research. From older literature, Cuffey (1984) chronicles numerous other cases of species that show intergradations between higher taxa, including plants (angiosperms and gymnosperms), vertebrates (reptiles, therapsids, mammals, and hominids), and invertebrates (gastropods, brachiopods, crustaceans, crinoids, and ammonoids). He also documents innumerable transitional species that show intergradations within genera, and many evolutionary transformations represented as chronologically successive fossils. Other examples are found throughout the biological literature, in major reviews (e.g., Carroll, 1988), and in textbooks of zoology, botany, and paleontology. Creationists have placed themselves in a tenuous position by basing their arguments on negative evidence, i.e., the purported absence of evolutionarily intermediate morphotypes. Perhaps unintentionally, or through an overconfidence borne of ideology, they have framed their views in a way that allows them to be refuted. Falsifiability is a minimal criterion for any endeavor that aspires to intellectual respectability. However, if creationists have such aspirations, they seem to find them to be incompatible with their political and social agendas. Even the most charitable reading would find it hard to explain the self-serving distortion, deception, and obfuscation in creationist writings, as have been so abundantly documented in Creation/Evolution, and in books by biologists, philosophers, and anthropologists (e.g., Eldredge, 1982; Futuyma, 1982; Kitcher, 1982; Ruse, 1982; Montagu, 1984). The creationist challenge to biology, to find even "five or six of the transitional forms scattered through time ... " (Gish 1979, p. 49) has been met and exceeded. Thus, creationism has been refuted according to criteria of its own choosing. Unfortunately, if recent history is any indication, creationism seems likely to continue to pursue its sociopolitical agenda with little concern for mere empirical evidence. 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