Scientific Methodologies in Collision. The History of the Study of the Extinction of the Dinosaurs
MICHAEL J. BENTON
Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, U.K.
[Cite as: Benton, M. J. 1990. Scientific methodologies in collision: the history of the study of the extinction of the dinosaurs. Evolutionary Biology, 24, 371-400.]
The extinction of the dinosaurs is a major topic in modern paleontology, and indeed in a variety of disciplines that touch on the history of the earth and the history of life. It has also become a major theme in popular accounts, in news paper and TV reports of science, and in museums. However, this has not always been the case.
About 70 years ago, the extinction of the dinosaurs was regarded as a minor hiccup in the progression of life, no more significant than, say, the extinction of the labyrinthodont amphibians, or the origin of the mammal-like reptiles. It acquired a certain notoriety in the 1950s and 1960s because of its popular appeal, and a vast array of hypotheses was presented, many of them rather bizarre in retrospect. The methods of research and the criteria of hypothesis testing during these years were often very loose.
Finally, in the 1970s and 1980s, major advances in paleobiological methodology and in geochemical and astrophysical research focused strong attention on the question, both from scientists and from the press. The three phases of study of dinosaurian extinction this century may be designated very broadly as follows: (1) the nonquestion phase (up to 1920); (2) the dilettante phase (1920-1970); (3) the professional phase (1970 onward).
These three phases are not temporally exclusive, but are based on the majority of publications on the subject each year. General opinions and approaches to the question of the extinction of the dinosaurs have changed in a progressive way from phase 1 to phase 2 to phase 3, and clearly the majority of scientists have followed the broad trends. However, a number of individuals approached this subject in new ways long before the majority of researchers did. For example, Nopcsa (1911, 1917) was considering reasons for dinosaurian extinction in a "dilettante" way long before this became the common approach, in the 1930s and 1940s. At the other end of the spectrum are those individuals who persisted in the old way long after the main body of their science had moved on. So, for example, several paleontologists in the 1950s and 1960s regarded the extinction of the dinosaurs as a nonquestion, while even today many regard this as a subject not capable, nort worthy, of serious scientific analysis. On the other hand, several highly significant serious studies were carried out in the 1960s. Hence, the dates given above are only very approximate indications.
The aims of this chapter are to review these stages in the study of the extinction of the dinosaurs and to illustrate the key aspects of each with published examples. Most attention is given to the present, professional phase, the roles of biology and physics in the debate, and the role of the press.
The extinction of the dinosaurs was not regarded as a major event by most 19th century scientists for two reasons. Before 1840, very few dinosaurs had been described, and their uniqueness had not been recognized, so that the early 19th century catastrophists did not focus their attention on them. After 1840, and the naming of the Dinosauria, catastrophism was replaced by uniformitarianism in geology, and Darwinian natural selection emphasized the gradual and continuous nature of extinction. It was assumed then that the dinosaurs had dwindled in a progressive fashion during the Cretaceous. Thus, most 19th century and early 20th century paleontologists did not regard the extinction of the dinosaurs as a particular problem.
The first dinosaurs were described in 1824 and 1825, but they were initially interpreted simply as curious giant lizards. The idea of extinction at the species or the genus level was generally accepted by 1825, and there seemed to be little unusual in finding a few such curious extinct reptiles. The theological arguments of the 18th century, that to admit the possibility of extinction was to accuse God of having made a mistake in Creation, were no longer made (Mayr, 1982; Bowler, 1984 ; Buffetaut, 1987), since pragmatic arguments against extinction had been effectively rebuffed. Discoveries of mammoths and mastodons from 1750 onward convinced many that there were extinct species. Nevertheless, as late as 1771, Thomas Pennant could write about these fossil elephants, "as yet the living animal has evaded our search ; it is more than possible that it yet exists in some of those remote parts of the vast new continent, unpenetrated as yet by Europeans" (Pennant, 1771). New discoveries of the remains of large fossil animals that had never been found alive the giant ground sloth, the giant Irish deer, giant cattle, and the much older plesiosaurs and iuchthyosaurs allowed Cuvier and others to demonstrate the reality of extinction to the satisfaction of most savants by 1825.
Cuviers work on fossil vertebrates from 1799 onward convinced him that many large mammals had become extinct in the not too distant past. He was also able to demonstrate successive earlier faunas of vertebrates and invertebrates in the Tertiary sediments of the Paris Basin. Cuvier invoked a catastrophic explanation for the extinction of these earlier faunas:
Life in those times was often disturbed by these frightful events. Numberless living things were victims of such catastrophes: some, inhabitants of the dry land, were engulfed in deluges; others, living in the heart of the sea, were left stranded when the ocean floor was suddenly raised up again; and whole races were destroyed forever, leaving only a few relics which the naturalist can scarcely recognise. (Cuvier, 1825, Vol. 1, p. 9.)
Cuvier did not, however, extrapolate from his theory of localized catastrophic extinctions to the global scale. He imagined that physical catastrophes acted within basinal areas only, since life must have survived elsewhere in order to provide the continuity of the fossil record. Cuvier also expressed views on the vertebrates of the Mesozoic and Cenozoic. At that time (Cuvier, 1825), the dinosaurs were barely known, and the only Mesozoic reptiles were aquatic formsJurassic crocodilians, ichthyosaurs, and plesiosaurs, Cretaceous turtles, and older Permian lizardlike animals. Cuvier, then, argued that, until the end of the Mesozoic, vertebrates were essentially marine, or at most swamp-dwellers. He regarded the Tertiary mammals as the first fully dry-land forms. Cuvier recognized a major faunal change, then, at the Cretaceous-Tertiary (K-T) boundary, and he related it to a supposed major phase of regression when the great Mesozoic seas retreated.
The idea of a major shift in vertebrate evolution at the K-T boundary from the sea to the land was rejected by 1830 because of increasing evidence of terrestrial reptiles and mammals in the Mesozoic. Mantell (1831) heralded this in a popular paper entitled, "The geological age of reptiles," and this transition from an Age of Reptiles to an Age of Mammals has held sway ever since.
Buckland (1823) and others more explicitly extrapolated Cuvier's ideas to develop the fully catastrophist geology in which truly global calamities had occurred. Buckland developed his catastrophist hypotheses from his studies of mammal bones in Pleistocene caves. He argued that these "relics of the Flood" had been wiped out by a universal deluge, and extrapolated this back to cover earlier major faunal replacements. Critics of catastrophism, such as Fleming (1826), pointed out that most of the mammals in the Pleistocene caves could have been hunted to death by early humans, and he also indicated that earlier extinctions might also have been caused by predators. Increasing knowledge of the fossil record led to the view by 1849 that at least 29 catastrophes were needed (Bourdier, 1969), each corresponding to a major stratigraphic boundary.
The causes of these catastrophes were not clear, but Buckland wrote, in reference to the sudden change in climate after the last catastrophe (the Flood), "What this cause was, whether a chang in the inclination of the earth's axis, or the near approaeh of a comet, or any other cause or combination of causes purely astronomical, is a question the discussion of which is foreign to the object of the present memoir" (Buckland, 1823, pp. 47-48). These remarkably modernsounding speculations gave way later to the proposal that the cooling earth had induced the major catastrophes (Buckland, 1836).
The noncatastrophist view was presented by Lyell (1832). He also regarded extinction as important, but more at the species level than necessarily globally. His view was that the species present at any time depended on the environmental conditions, and when such conditions became hostile, extinction inevitably followed.
In the 1820s and 1830s, extinction was viewed broadly in five ways. One view was that extinctions had occurred catastrophically a number of times when most species were wiped out (Buckland). Agassiz took this view one step further, and considered that all species were killed off at times of extinction, to be replaced by a fresh divine Creation. The third view was that extinctions occurred essentially at the local level owing to catastrophic physical effects (Cuvier). The fourth and fifth views were that extinctions occurred at the species level: because conditions became unsuitable (Lyell), or because the species had outlived its natural span.
By 1842, several genera of dinosaurs had been described, including Megalosaurus, Iguanodon, and Hylaeosaurus, and in that year, Richard Owen recognized them as a distinctive order for which he introduced the term Dinosauria (Owen, 1842). Desmond (1979) has argued that Owen "invented" the Dinosauria for a very specific purpose that had little to do with descriptive paleontology, but a great deal to do with then-current views of the history of life. Between 1830 and 1855, there was an active debate about the idea of progression in the history of life (Bowler, 1976). One view was that life had progressed from simple to more and more complex forms through time, as argued by Robert E. Grant and others, a view emanating from Buffon and Lamarck in the 18th century. Richard Owen opposed this simple notion, and sought to argue that the fossil record showed that degeneration was really the order of the day. Owen's new Dinosauria were described by him (Owen, 1842) as animals "which in structure most nearly approach Mammalia," and which "from their superior adaptation to terrestrial life, [may be concluded] to have enjoyed the function of such a highly organised centre of circulation in a degree more nearly approaching that which now characterizes the warm-blooded Vertebrata." Owen then used these "mammal-like" dinosaurs as direct evidence against the progressionist doctrines of Lamarck, Grant, and others, arguing that these earliest reptiles were the most advanced of all, and that the stock had degenerated to leave only the poor crocodiles, lizards, snakes, and turtles of today.
Owen (1842) argued that the Creator had chosen the Mesozoic Era as suitable for the dinosaurs because of its different atmospheric conditions. He believed that the air then was deficient in oxygen, and that this suited the dinosaurs. As reptiles, they had lower metabolic rates than the birds and mammals and could survive on less energy. He argued that oxygen levels rose during the Mesozoic and that the atmosphere became more "invigorating." The world then became uninhabitable for the huge saurians, and they died out, together with the giant marine reptiles and the flying pterosaurs. This argument was essentially circular, since Owen's evidence for low oxygen levels in the Mesozoic was simply the presence of dinosaurs and other prehistoric reptiles and the virtual absence of mammals and birds. He was explaining the presence of these early reptiles in Mesozoic rocks, very different from the dominant mammals and birds of today, in terms of precise matching by a benevolent Creator of living things and physical environments. He was not primarily trying to explain why the dinosaurs died out when they didfor him, that would have been a preordained event in the Creator's plan.
The progressionists and the supporters of degeneration models for the history of life did not regard the extinction of the dinosaurs as a particular issue, since there were so few of them known, and since they had far more pressing controversial issues to consider: the role of the Creator, the possibility of progression, transmutation (evolution), and so on. The progressionists would probably have interpreted the disappearance of the dinosaurs in terms of the isolated death and replacement of each of the three species then known. The nonprogressionists would have had a number of attitudes. Owen's view, that living reptiles are degenerate forms compared to the dinosaurs and other extinct reptiles, is an example of a theory of "discontinuous progression" (Bowler, 1976), in which a group is created and subsequently remains constant or declines. Another non-progressionist view, espoused by Charles Lyell, was that living things matched the available habitats and they changed in response to physical changes. Thus, if conditions were right at some time in the future, the dinosaurs would return to inhabit the earth. In none of these views did suprageneric extinction really come into question.
The advent of the Darwinian theory of evolution by natural selection in 1859 and subsequent studies of phylogeny based on the fossil record by Haeckel, Huxley, Cope, Marsh, and others did not lead to any real discussion of mass extinctions, nor of the extinction of the dinosaurs in particular, as issues worthy of explanation. Indeed, Darwin (1859) saw the rapid disappearance of the ammonites at the end of the Cretaceous as probably as much to do with gaps in the fossil record as with any real phenomenon. In his paper "On the classification of the Dinosauria . . ., " Huxley (1870) noted the 16 genera of dinosaurs that were known at that date, spanning most of the Mesozoic Era, and based on discoveries in Europe, North America, Africa, and Asia. However, neither here nor elsewhere did he have any statements to make regarding their extinction. Marsh (1882) listed 46 dinosaur genera, and simply noted that they "continued in diminishing numbers to the end of the Cretaceous period, when they became extinct." In a later work, Marsh (1895) listed 68 genera of Dinosauria, but made no mention of their extinction at all.
In the late 19th and early 20th centuries, many paleontologists and biologists moved to non-Darwinian viewpoints, such as orthogenesis and finalism (Bowler, 1983). These models assumed that evolution was directed in some way and that there are regular patterns laid out along which evolution proceeds. The dinosaurs could be viewed as primitive lumbering beasts that had to give way to the more advanced mammals.
This then led to views of racial senilitythe belief that certain long-lived groups of animals became old and their store of evolutionary novelty dried up. There was a parallel here between the life-span of an individual plant or animal and that of an evolutionary stock. Youth and early racial vigor were equated and seen to be just as inevitable as the old age and death of an individual and the racial senescence and eventual extinction of a major phylogenetic group. According to this view, the dinosaurs had been around for a long time, and they simply ran out of the genetic variability that was necessary to survive. The remarkable horns, frills, and spines of some late Cretaceous dinosaurs were occasionally cited as evidence for this racial senility, but in general the extinction of the dinosaurs remained a nonquestion.
Standard textbooks of general paleontology and vertebrate paleontology of the latter half of the 19th century and the first decades of the 20th, like the technical papers, barely mention the extinction of the dinosaurs. Woodward (1898, p. 213) notes that "toward the close of the Mesozoic period [sic] the Dinosaurs gradually became extinct," and later (p. 418), he states that the dinosaurs of the Cretaceous "became more specialised and almost fantastic just before they disappear." Von Zittel (1902, 1932), Hutchinson (1910), Jaekel (1911), Williston (1925), and Kuhn (1937) barely refer to this topic at all. Even into the 1950s and 1960s, several authors on vertebrate palaeontology continued virtually to ignore the extinction of the dinosaurs as a topic (e.g., von Huene, 1956; Orlov, 1964; A. H. Müller, 1968).
I hope to show below that the study if the extinction of the dinosaurs went through several phases in the 20th century. There was a long time, from about 1920 to 1970, when it was seen generally as a moderately interesting (and moderately amusing) topic, attracting only a few pages of scientific commentary each year. Then, after 1970, the level of interest increased, as did the rates of publication.
An attempt has been made to survey everything that has been published on the extinction of the dinosaurs in order to provide a basis for this survey of attitudes to this topic. I searched through my reprint collection, noting all technical papers, books, and popular works that dealt solely with the extinction of the dinosaurs, or that covered the Cretaceous-Tertiary (K-T) boundary event, or that devoted at least a few pages to the question. I supplemented these figures with references given in the various Bibliographies of Vertebrate Palaeontology (indexed 1928-1973). The total number of publications is 597. When these are plotted according to the year of publication (Fig. 1), it is clear that the rate of publication has been erratic, but has risen in the 1960s and again, dramatically, in the late 1970s and 1980s.
The total number of publications per annum range from 0 to 7 (mean 1.8) for 1910-1959, from 3 to 11 (mean 7.8) for 1960-1969, from 8 to 36 (mean 15.2) for 1970-1979, and from 21 to 85 (mean 50.1) for 1980-1986. Until 1960, most of the papers on the subject of the extinction of the dinosaurs were isolated publications that did not give rise to any real debate, but some, such as the work of Cowles and Bogert in the 1940s on reptilian therrnoregulation (e.g., Cowles, 1945, 1949; Colbert et al., 1946), gave rise to brief flurries of papers. In the 1960s and 1970s, more sustained work on dinosaurian biology and on paleoclimates led to a steady number of four or five papers every year.
FIG. 1. The increasing rate of publication on dinosaur extinction, and the K-T event, from 1900 to 1986. Figures are plotted Year by year, based on a compilation of publications that are concerned solely or mainly with dinosaur extinction, with the K-T event (including stratigraphic accounts), and with broad questions of extinction that relate to the K-T event (shown as "others"). Since 1980, most of the publications that fall in the last category concern the periodicity debate, and these are also ploned separately in the inset. Two seminal papers, in 1980 and 1984, are noted in the diagram.
The main boosts in rates of publication seem to have come from non-dinosaurian quarters. Indeed, the annual number of papers devoted to the extinction of the dinosaurs alone has remained fairly constant (Fig. 1) as a figure between 0 and 8 per annum (mean 2.0 per annum in the 1960s; 2.8 per annum in the 1970s; 3.7 per annum in the 1980s). The total number of papers has been boosted by general interest in the K-T boundary events, by the rise of catastrophic extraterrestrial theories [particularly in the 1970s, and after the Alvarez et al. (1980) paper], and by the more recent interest in the periodicity of mass extinctions following the publication of Raup and Sepkoski (1984). Papers devoted essentially to the K-T event, but mentioning dinosaurian extinction, and papers about periodicity of mass extinctions are indicated separately in Fig. 1. It can be seen that these two categories constitute most of the overall increase in number of publications over the past few years. Overlaid on the general rising pattern are considerable annual fluctuations in number of papers; the peaks generally correspond to the publication of symposium volumes on the K-T event (1960, 1979), on catastrophic models for mass extinction (1977, 1982 [two]), and on general aspects of mass extinction and diversity in the fossil record (1977, 1981, 1984 [three], 1985 [two], 1986 [three]).
The pattern of publication about the extinction of the dinosaurs may be divided, rather artificially, as has already been noted, into two main phases, the first running from about 1920 to 1970, and the second from 1970 to the present day.
During the early part of the 20th century, a small number of authors began to treat the extinction of the dinosaurs as an event, and as an event that was worth explaining. A typical early account was given by Arthur Smith Woodward (1910) in his address to the British Association for the Advancement of Science. He argued that the end of the dinosaurs was largely the result of racial senility. He pointed to the great spinescence, excess growth, and loss of teeth of the later dinosaurs as evidence. Loomis (1905, p. 842) wrote about the dorsal bony plates of the stegosaurs: "with such an excessive load of bony weight entailing a drain on vitality, it is little wonder that the family was short-lived." Racial senility as a theory of extinction held sway in some quarters for a long time. Schuchert (1924, p. 12) wrote: "when races are senile, or overspecialized, or are giants of their stocks, they are apt to disappear with the great physiographic and climatic changes that periodically appear in the history of the earth." These kinds of views were also expressed by Beurlen (1933) and Swinton (1939), for example, although they had been earlier rejected by Stromer von Reichenbach (1912), Hennig (1922), and Audova (1929).
Ideas of orthogenesis and racial senility of the dinosaurs were effectively demolished by the advent of the modern synthesis or neo-Darwinian model of evolution in the 1930s and 1940s. There was no longer any place for preordained patterns in this view of evolution. However, the notions of racial senility and of dinosaurs as inferior, lumbering monsters still live on, even if rather subconsciously, in the minds of scientists and the public alike. There seem to be two aspects of this lingering "crypto-orthogenesis": (1) we would like to believe in some form of progress as an optimistic view of the meaning and aim of life; and (2) we would like to believe that Homo sapiens, and mammals in general, are at the top of the tree, and that they somehow proved it by overwhelming the dinosaurs. Many evolutionists have wrestled with the idea of progress in neo-Darwinism (e.g., Julian Huxley, George Gaylord Simpson, Francisco Ayala, Theodosius Dobzhansky), or in punctuated macroevolution (e.g., Steven Stanley), but they have met with difficulties (Gould, 1985; Benton, 1987). It has proved virtually impossible to define "progress" in a modern view of evolution in anything other than a Whig manner [i.e., we can only see it from our present standpoint, and judge it with present values(Schopf, 1979)]. The lumbering dinosaur doomed to extinction is an insistent metaphor that refuses to die.
Biotic and Physical Factors
A number of authors, however, eschewed racial senility, and concentrated on biotic and physical factors that might have caused the extinction of the dinosaurs. Baron Franz Nopcsa was one of the first. He noted, for example (Nopcsa, 1911, p. 148) that the great amount of cartilage that he believed was necessary for growth to huge size "perhaps was one of the causes for the rapid extinction of the Sauropoda." Later Nopcsa (1917, p. 345) summarized a number of views on the extinction of the dinosaurs: their "low power of resistance," their huge size, a shortage of food, or "a reduction in their sexual functions." He hints that a key factor may be the supposed "increase in function of the hypophysis" (pituitary gland) which was related to their very large body size. Secretions from the pituitary caused giantism, partly by the production of large masses of cartilage as precursors of bone, and partly by a form of acromegaly, or pathological excess thickening and overgrowth of limb bones and facial bones. He notes that "the increase in weight of the limbs in the dinosaurs recalls the eunuch condition."
William Diller Matthew (1921) presented early evidence for a model of dinosaurian extinction that involved gradual topographic change and progressive replacement by mammals. His study of the late Cretaceous and Paleocene in North America suggested that there was extensive mountain building and continental uplift. The dinosaur faunas, which were adapted to lowland and marsh situations, were displaced, and the placental mammals, which were adapted to upland zones, moved in. The new mammal faunas apparently had their origins in Asia. Nopcsa (1922, pp. 110-113) presented a modified version of this model, although he laid more stress on floral changes and the loss of marsh-type vegetation.
Other suggestions of these times were that climatic cooling was the cause (Jakovlev, 1922), that disease levels had risen markedly in the late Cretaceous dinosaurs (Moodie, 1923), that early mammals ate all of the dinosaur eggs (Wieland, 1925), or that volcanic eruptions were responsible (L. Muller, 1928).
Audova (1929) reviewed the whole question of the extinction of the dinosaurs in some detail in a remarkable 61-page paper in the short-lived German journal Palaeobiologica. He rejected racial senility and simple natural selection as explanations, and focused on environmental change. His favored view, after surveying geological evidence on paleotemperatures and physiological evidence on the thermoregulation of modern reptiles, was that temperature had declined gradually, and that this acted directly on the dinosaurs and other Mesozoic reptiles by preventing proper embryonic development.
This approach typified much of the literature on dinosaurian extinction from the 1930s to the 1960s. Only a small number of papers was published on this topic, ranging from detailed investigations of dinosaurian physiology (e.g., Colbert et al., 1946) and of patterns of diversity and extinction (e.g., Newell, 1952; Simpson, 1952), to brief suggestions and musings on the subject [e.g., de Laubenfels (1956): "Dinosaur extinction : One more hypothesis"].
A Survey of Theories for the Death of the Dinosaurs
It is not possible to discuss all of the hypotheses for dinosaurian extinction that were proposed during the dilettante phase. Jepsen (1964) listed 40 separate hypotheses, and there are doubtless many more than that. I summarize these hypotheses, together with key references where possible, and add numerous others that I have tracked down. In order to bring some semblance of order into this list, I have tried to classify the hypotheses under major headings. The list is probably more comprehensive than any survey yet published, but it is probably still far from complete.
In addition, I have attempted to distinguish the theories that were presented as deliberate jokes (marked with an asterisk) from those that were intended seriously (no asterisk), and those that, in retrospect at least, would appear to be testable at least in part (T) from those that seem to offer no hope of refutation (no T). Hypotheses marked (T?) were presented as speculative ideas which would require some assumptions and extensive testing, while those marked (T) were supported by some evidence and had been tested to some extent.
I. Biotic causesA. "Medical problems"
B. Racial senility ("phylogeronty")
A. Terrestrial explanations
Problems with the "Dilettante" Approach
Certain of these suggestions are perfectly reasonable ideas on the basis of present knowledge, but the obviously ludicrous nature of many has had two consequences. First, many paleontologists were led to believe that this approach was the only one to a study of mass extinction, and therefore that mass extinctions were of little importance to a serious paleontologist. Second, the whole approach was apparently so easy and such fun that everyone felt that they had the opportunity, if not the duty, to solve the question of why the dinosaurs died out. Many of the ideas listed above were presented by nonpaleontologists, and certainly most of the authors had little first-hand knowledge of the late Cretaceous fossil record of dinosaurs - hence the "dilettante" soubriquet. A large number of the theories, most of which were published in standard scientific journals by scientists who were no doubt expert in their own fields, show a remarkable relaxation of scientific standards. It was as if, at the mere mention of "dinosaur extinction," scientists breathed a sigh of relief and felt freed from the straitjacket of normal scientific hypothesis-testing.
I believe that there are four main arguments in support of this view.
Fortunately, scientific approaches to the question of the extinction of the dinosaurs, the K-T event, and extinctions in general have improved markedly over the past 20 years or so.
The professional phase, in which investigators attempted to study the pattern of events at the K-T boundary in detail and to present testable hypotheses for the extinction of the dinosaurs, began in the mid 1960s. Sloan and Van Valen (Sloan, 1964, 1970; Van Valen and Sloan, 1972, 1977) attempted to analyze in detail the changes in vertebrate faunas across the K-T boundary in Montana. L. S. Russell (1965), Ostrom (1970), and Bakker (1971, 1972) began detailed studies of dinosaurian physiology and its relation to extinction, while Axelrod and Bailey (1968) and others surveyed paleobotanical evidence of climatic change. Newell (1962, 1967), Valentine (1969, 1974), and Raup (1972) began general studies of the fossil record and the identification of mass extinctions. Catastrophic terrestrial and extraterrestrial scenarios for extinction were explored by Terry and Tucker (1968), Hatfield and Camp (1970), Crain (1967), Hays (1971), D. A. Russell and Tucker (1971), and Urey (1973), while explanations involving sea-level change were presented by Ginsburg (l964), Newell (1967), and others. Studies in the 1970s and 1980s have generalized from tackling the question of dinosaur extinction alone to the problem of the whole K-T event.
None of the proposed scenarios for K-T extinction, or the extinction of dinosaurs alone, is conducive to a single test. However, each entails a number of hypotheses, some of which are testable. For example, tests can be made of the pattern of a particular event [what was its duration, synchroneity worldwide, synchroneity for all taxa, what did and did not go extinct (any size, habitat, stratigraphic correlations?), absolute and relative taxic effects] as well as many particular predictions made by the different scenarios (e.g., synchroneity of geochemical anomalies worldwide and with the extinction events, terrestrial and extraterrestrial sources of geochemical spikes, matching of timing of extinction events with other physical changes - topography, sea level, climate, chemical composition of seawater, temperature, atmospheric composition).
The Current Scenarios
The two main current scenarios to explain the K-T event (including the extinction of the dinosaurs) are the "gradualist" ecological succession model of Van Valen and others and the "catastrophist" extraterrestrial impact model of Luis Alvarez and others. Recent reviews include Van Valen (1984), Officer et al. (1987), and Hallam (1987) on the one side, and L. Alvarez (1983, 1987) and W. Alvarez (1986) on the other side of the issue. There is a considerable amount of evidence of different kinds for both scenarios: mainly paleontological and stratigraphic for the ecological succession model, and mainly geochemical and astrophysical for the extraterrestrial impact model. A "catastrophist" would envisage that the main extinction event lasted less than, say, 1 year, while a gradualist would regard the time span as somewhat more than, say, 1000 years (Van Valen, 1984). At these levels of distinction, the stratigraphy is not good enough to distinguish these time spans: although very different on a biological time scale, they are both geologically "instantaneous." Gradualists typically view the events as very long-term; Sloan et al. (1986) suggest 7 million years for the extinction of the dinosaurs, with an acceleration in the rate in the last 0.3 million years. On the other hand, catastrophists have offered a range of timings. Alvarez et al. (1980, p. 1099) showed durations of 1-5000 years for the whole event, with the main extinctions occurring in 1-10 years. However, Hsu (1984) has indicated that the knock-on effects of an impact could have lasted for more than 1000 years. Other analyses of the geochemical evidence have given durations of 0.1-0.25 million years (Rocchia et al., 1984; Officer and Drake, 1985), figures that overlap the timings of many gradualist models.
The recent proposal of "stepwise extinction" patterns (Kauffman, 1986; Hut et al., 1987) expands the time scale for extraterrestrially induced mass extinction to 3.5 million years or more. The proposal is that showers of comets arrive on the surface of the earth over intervals of typically 1-3 million years and that pulses of cometary impact cause a major global extinction event in three or four steps. This notion could be seen as a compromise between the instant catastrophe models and the gradualist models (Raup, 1986), a kind of additive long-term catastrophe. However, the stepwise extinction model derives from the extraterrestrial catastrophe models, and it explains mass extinctions by bolide impact, even if by several impacts spaced over 1-3 million years. This may be a necessary modification to the single-impact scenario, as a result of more detailed paleontological evidence, but it is in no way concordant with the earthbound gradualist models as regards the ultimate causation of extinction.
The "Gradualist" Models
The gradual ecosystem evolution model has been largely based on the progressive appearance of a mammal community (the Protungulatum Community) of distinctive Paleocene aspect in the last 300,000 years of the Cretaceous in Montana. As the mammals increase in abundance, the dinosaurs apparently decline until they disappear altogether (Van Valen and Sloan, 1977; Sloan et al., 1986). This gradual replacement is explained in terms of diffuse competition between dinosaurs and mammals set against a major change in habitats. The lush subtropical dinosaur habitats were apparently giving way to cooler temperate forests which favored the mammals. It has been argued, however (Fastovsky and Dott, 1986), that the occurrence of dinosaurs and mammals in these particular cases are in channel sediments that cannot be dated as either Cretaceous or Paleocene.
The gradualist scenario has been extended to cover all aspects of the K-T events. Thus, Perch-Neilsen et al. (1982) note that the planktonic extinctions took 10,000 years, and various groups were already declining well before the boundary (Signor and Lipps, 1982). A variety of sea-bottom livers and filter-feeders died out, but sea-bottom predators and detritus-feeders were little affected. Extinction patterns of many marine groups show gradual declines throughout the late Cretaceous (Kauffman, 1984). Many gradualists also link the extinctions to marine regressions, as well as to declining temperatures (e.g., Ginsburg, 1964; Hallam, 1984, 1987). Gradualists also argue that the fact that many groups did not go extinct at the K-T boundary is hard to understand in the face of some of the devastating catastrophist scenarios (Buffetaut, 1984). On land, placental mammals, birds, lizards, snakes, crocodiles, champsosaurs, and tortoises and other freshwater organisms were little affected, and the plant record shows only modest and gradual changes (Hickey, 1981).
The "Catastrophist" Models
There is a huge literature now on the catastrophist extraterrestrial scenarios [for review see L. Alvarez (1983, 1987), Hsu (1984), W. Alvarez (1986)]. Basically, these postulate the impact of an asteroid or a comet on the earth. The impact caused mass extinctions either by throwing up a vast dust cloud which blocked out the sun and prevented photosynthesis (Alvarez et al., 1980), by releasing cyanide (Hsu, 1980), by flash heating of the atmosphere on entry (Hsu, 1980), by excessive cooling of the atmosphere (Turco et al., 1983), by releasing poisonous arsenic and osmium (Hsu et al., 1982), by global wildfires (Wolbach et al., 1985), or by a combination of darkness, extreme cold, and acid rain (Prinn and Fegley, 1987). These chemical and darkness models are postulated to explain most of the extinctions, while the extinction of the dinosaurs is often ascribed to thermal stress (Hsu, 1984).
The best evidence for the impact hypothesis is said by some to be the iridium (Ir) enhancement at the boundary, now recorded from nearly 60 locations worldwide (Alvarez, 1983), while others emphasize the importance of shocked quartz (Bohor et al., 1987). Further evidence includes the occurrence of spherules in a few sections (Smit and Kyte, 1984), the similarity of the ratios of elements in boundary clays to those of chondrites, isotopic changes in O and C, and the actual presence of clays at the boundary in so many sections. A catastrophic extinction is also indicated by abrupt shifts in pollen ratios at some K-T boundaries (Tschudy et al., 1984; Wolfe and Upchurch, 1986), as well as abrupt plankton and other extinctions in certain sections (Alvarez et al., 1984a; Surlyk and Johansen, 1984).
In opposition to the catastrophist explanations, Van Valen (1984) and Hallam (1987) note the following criticisms: supposed extraterrestrial material below the K-T boundary, absence of the effects of elimination of stratospheric ozone, and problems with the darkness, cooling, acid rain, and other predicted results of impact. Officer and Drake (1985) and Hallam (1987) have further argued that the Ir could be of terrestrial volcanic origin, and that the Ir spikes lasted from 10,000 to 100,000 years. This idea of a relatively long Ir spike and the evidence for long-terrn decline of dinosaurs and certain marine groups could suggest a combination of the gradualist and catastrophist views in which certain groups were already declining when a bolide impact, or intense vulcanism, finished them off - a kind of "last-strawist" view point (cf. Buffetaut, 1984).
Physics versus Paleontology
The two main models for dinosaurian extinction are based on rather different kinds of data, as mentioned above: essentially paleontological and stratigraphic for the gradualist models, and mainly geochemical and astrophysical for the catastrophic models. This has inevitably meant that it has been hard for the proponents of one view to assess the evidence that supposedly favors the other view, as noted by Van Valen (1984). There is, however, apparently a more fundamental source of potential conflict between certain biologists and physicists, or "soft" scientists and "hard" scientists, as Raup (1986, p. 212) describes the pecking order.
The initial publications by Alvarez et al. (1980) were greeted skeptically by many paleontologists and geologists with long-term expertise on aspects of the K-T boundary. No doubt, they resented the intrusion into their subject by a group of physicists, and Luis Alvarez's (1983, p. 632) lengthy catalogue of his team's credentials (a physicist, two nuclear chemists, and a geologist) may not seem so unusual in view of this resentment: "suddenly I realised that we combined in one group a wide range of scientific capabilities, and that we could use these to shed some light on what was really one of the greatest mysteries in science - the sudden extinction of the dinosaurs." Walter Alvarez (1986) characterizes the two main geological prejudices that the impacters faced: a deeply held belief in uniformitarianism and gradualism among many geologists, and detailed objections from paleontologists who saw no general large-scale instant catastrophe in the fossil record.
The crux of the dispute was outlined by Jastrow (1983, p. 152): "Professor Alvarez was pulling rank on the palaeontologists. Physicists sometimes do that; they feel they have a monopoly on clear thinking. There is a power in their use of math and the precision of their measurements that transcends the power of the softer sciences." The very titles of their papers could be seen to exemplify this: Alvarez et al. (1980) is "Extraterrestrial cause for the Cretaceous-Tertiary extinction - Experimental results and theoretical implications," while Alvarez (1983) is "Experimental evidence that an asteroid impact led to the extinction of many species 65 million years ago." Van Valen (1984, p. 122) commented that "to call [the Alvarez] evidence "experimental" is misleading propaganda; it refers merely to the fact that some observations were made in the laboratory rather than in the field, not to an active experimental test."
Luis Alvarez is surprisingly revealing throughout his 1983 paper. He is dismissive of his critics:
I think the first two points - that the asteroid hit, and that the impact triggered the extinction of much of the life in the sea - are no longer debatable points. Nearly everybody now believes them. But there are always dissenters. I understand that there is even one famous American geologist who does not yet believe in plate tectonics People have telephoned with facts and figures to throw the theory into disarray, and written articles with the same intent, but in every case the theory has withstood these challenges. (Alvarez, 1983, p. 67.)
He later outlines the advantages of physics in comparison with paleontology: "The field of data analysis is one in which I have had a lot of experience" (Alvarez, 1983, p. 638), and "In physics, we do not treat seriously theories with such low a priori probabilities" (p. 640). He writes, "That is something that made me very proud to be a physicist, because a physicist can react instantaneously when you give him some evidence that destroys a theory that he previously had believed But that is not true in all branches of science, as I am finding out" (p. 629). Public utterances from Luis Alvarez about his "opponents" have frequently been more critical than these examples to the point of being libellous (e.g., Browne, 1988). Van Valen (1984, pp. 136-137) complains about how Alvarez "makes fun" of paleontologists, while Halstead (1986) and Archibald (1987) argue that it is wrong to argue that physics is necessarily better than paleontology - it is only different.
On the other hand, much of the distrust of the physicists by certain paleontologists has surely been unfounded, as Raup (1986, pp. 104-105) notes. He quotes at length a statement by Robert Bakker, a dinosaur paleobiologist, originally published in the New York Times:
The arrogance of those people is simply unbelievable. They know next to nothing about how real animals evolve, live and become extinct. But despite their ignorance, the geochemists feel that all you have to do is crank up some fancy machine and you've revolutionized science. The real reasons for the dinosaur extinctions have to do with temperature and sea level changes, the spread of diseases by migration and other complex events. But the catastrophe people don't seem to think such things matter. In effect, they're saying this: "We high-tech people have all the answers, and you paleontologists are just primitive rock hounds."
The impact hypothesis, being a new idea, was initially at a disadvantage, as Raup (1986, pp. 195-197) argues. In order to displace the established wisdom, the impacters would have to present overwhelmingly strong evidence which would be much more critically scrutinized than the gradualists' evidence. Indeed, the catastrophist hypothesis was on trial in a way that the gradualist hypothesis was not. As Clemens et al. (1981) wrote of the proposed asteroid impact, "analyses of the paleobiological data suggest that such an event is not required to explain the biotic changes during the Cretaceous-Tertiary transition."
Styles of Argumentation
Elisabeth Clemens (1986) has analyzed the nature of the debate about "asteroids and dinosaurs," and she argues that there are many nonscientific undercurrents, such as styles of argumentation and the role of professional and popular publication.
First, the broadly-based research enterprise that has developed around the question of the extinction of the dinosaurs - geologists, paleontologists, chemists, physicists, astronomers - is not a single community. It is a body consisting of several factions, each going in different directions, and with very little communication, a point noted also by Van Valen (1984, p. 121). Clemens goes on to suggest that the Alvarez theory gained rapid notice and acceptance in many quarters because catastrophism in geology was becoming intellectually fashionable. Catastrophic theories for mass extinction had been madc for years (e.g., de Laubenfels, 1956; Terry and Tucker, 1968; McLaren, 1970; D. A. Russell and Tucker, 1971; Urey, 1973; Reid et al., 1976), but they contradicled the strictly gradualistic "geological dogma" of the day. However, the supernova theories (Terry and Tucker, 1968; D. A. Russell and Tucker, 1971; Béland et al., 1977) probably did prepare the way for the asteroid theory.
Clemens (1986) argues that it was the mode of presentation of the Alvarez hypothesis that won it such wide attention and acceptance: "In a sense, the problem of the K-T boundary was framed so as to be amenable to the methods of particle physics." The bulk of the long 1980 paper (14 pages in all) was confined to the geological and physical evidence for an impact, and the physical results of the impact. The discussion of the biological results of the impact occupies only half a page. The paper was restricted then to a rather simple astrophysical hypothesis which could be tested in many ways, and the more complex aspects of stratigraphic imprecision and complexity of the evolution of biological communities were largely omitted. These issues had to be taken on board later, however. Alvarez et al. (1984b) allowed from 104 to 105 years for the overall length of time involved in the extinctions, while Alvarez et al. (1984a, p. 1135) note that "the paleontological record thus bears witness to terminal-Cretaceous extinctions on two time scales: a slow decline unrelated to the impact and a sharp truncation synchronous with and probably caused by the impact." The recent proposal of a stepwise extinction model involving comet showers (Kauffman, 1986; Hut et al., 1987) extends the catastrophists' time scale to 3.5 million years or more. However, by 1984, the simplicity of the "instant-extinction" model of 1980 had ensured its general acceptance by many scientists. The later modifications noted here are seen by Clemens (1986, pp. 434, 441) as rather ad hoc qualifiers that tend to protect the impact theory from refutation by stratigraphic or paleontological evidence. The extension of the impact scenario. to "nuclear winter" models in 1981, to theories of extraterrestrially induced periodic mass extinctions in 1983, and to the comet shower model of 1987 further helped to cement its professional and popular appeal.
The Role of the Professional and Popular Press
Clemens (1986) then goes on to argue that the nature of the professional and popular press has largely shaped the development of models of dinosaurian extinction since 1980. She points out that the 1980 Science article (Alvarez et al., 1980) was twice as long as such articles usually are, and it was published in a prominent position, at the start of the issue. This one article gained a very wide readership, particularly in the United States, whereas articles that presented similar theories at the same time (Smit and Hertogen, 1980; Hsu, 1980; Ganapathy, 1980) were much less widely read (Hoffman and Nitecki, 1985). Since 1980, it has been alleged, pro-impact papers have been much favored by the editorial board of Science, and the argument has spilled over into the commentary and review sections of leading journals and into the newspapers (e.g., Lewin, 1983, 1985a,b; Maddox, 1985; Hoffman, 1985; Browne, 1985, 1988). Clemens (1986) suggests that the very format of publication has had a restrictive effect, since most of the debate has been carried on so far in the pages of Science and Nature, both of which normally publish only very short papers of three or four pages in length, and both of which require papers to be readily understandable to a wide audience. It is easier to present a simple hard view, such as the impact, she argues, than to argue about the imprecision of present methods for dating rocks, or the complexity of biological communities.
There are clearly a number of layers to the "catastrophists versus gradualists" controversy, ranging from purely scientific aspects, to matters of style, modes of argumentation, and the nature of publication. These all add spice to the controversy, but do not necessarily lead to progress toward its resolution. At present, it is hard to see how the two viewpoints will be fused, since there can only be one correct explanation for the extinction of the dinosaurs, whether "gradualistic," "catastrophic," or a bit of both. From its rather modest image only 20 or so years ago, the extinction of the dinosaurs has now become one of the most studied unique events in the history of life. Indeed, there is now a sort of "bandwagon" effect, as new topics are spawned nuclear winter, periodicity, comet showers which keep public interest alive, and which keep levels of funding for research at record levels. In general, the controversy has been good for the historical sciences (geology, paleontology), and it can only be hoped that the uncomfortable grating between physicists and paleontologists will eventually lead to a more satisfactory and fully cooperative research effort.
I thank Peter Bowler, Steve Yearley, Max Hecht, and anonymous reviewers for helpful remarks.
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