The Great Fossil Enigma

by Simon J. Knell

  The first, which occurred in 1974, resulted in the discovery of unforeseen utility in the conodont fossils’ strikingly varied colors. It indicated that even when dead, buried, and fossilized, this remarkable animal could bear witness to changes going on around it. The second event was considerably more dramatic and occurred at a precise moment in 1980: An asteroid came crashing in, turning the paleontological community upside down. No one saw it coming, but no one could ignore it. Scientists from many different fields came together to understand it and its consequences. In that drama, the conodont played a bit part, valued particularly for the manner of its dying. In this period the geological community as a whole entered its most speculative and imaginative phase, and conodont workers, who were never immune to outside influence, soon developed a wonderful facility for thinking fantastic thoughts.

  In this chapter we will explore how this thinking continued to shape the animal in its world. In the next chapter we will begin the final phase of this book and start to follow the scientists as they close in on the animal itself.

  In a science so attuned to shades of brown and gray, the conodont's yellows, oranges and blacks, when combined with their beautiful translucence and miniscule but finely detailed form, had an aesthetic effect on all who studied them. These facets contributed to the objects’ attractiveness and amazingly – when you think about it – encouraged so many people to place these objects at the center of their lives. Pander had, at the outset, thought the color of conodonts both remarkable and important, but no one made anything of it until the U.S. Geological Survey's Anita Epstein, later Anita Harris, did so in the late 1960s.

  Epstein – a product of Sweet's “conodont factory” – owed rather more of her character to her origins in the tenements of the Williamsburg neighborhood of Brooklyn. The daughter of an immigrant Russian Jew, she saw geology as a means of escape from the poverty of New York. Tough and determined, she talked geology as one who lives and breathes it. “She is one sharp cookie, as we say,” Sweet remarked. “She has a photographic memory and is as hard a worker as anyone I know.” In his road trip with Epstein, described in his book In Suspect Terrain, published in 1982, John McPhee introduced the conodont, the subject of Epstein's great innovation, to his general readers using descriptive prose that seemed to be haunted by the ghost of Charles Moore: “At a hundred magnifications, some of them looked like wolf jaws, others like shark teeth, arrow heads, bits of serrated lizard spine – not unpleasing to the eye, with an asymmetrical, objet-trouvé appeal.”1

  In 1967, Epstein had noticed that there was a correlation between the color of the conodont fossils she was finding and the depth to which they had been buried. By buried, here, I mean geological burial beneath perhaps hundreds or thousands of meters of rock. As the conodonts showed a color range much like that seen in butter heated in a pan, she imagined that the color of the fossils might be used as an indicator of the maximum temperature experienced by the rocks in which they were found, as temperatures underground increase with depth. However, finding no encouragement at the Survey, she quickly dropped the idea. It was a chance meeting and conversation with Leonard Harris (later to be her second husband), six years later, that re-awoke this thought with something of a start. An oil geologist, he told her that oil companies had been using color changes in pollen and spores in this way for many years. The color range was exactly like that Epstein had seen in her conodonts. This was a moment of revelation. But when she told him that conodonts performed the same trick, it was he who was surprised.

  Epstein's discovery meant that this abundant and extraordinarily ancient time marker might possess a wholly new dimension of meaning. In an era of oil shortages, if Epstein's hunch proved correct, the conodont would offer an easy means to locate rocks that might hold oil. Oil forms from marine algae at depth in rocks exposed to certain temperatures. If the temperature is too low, oil does not form; if it is too high, oil is lost. Epstein now spent much of her spare time experimenting on relatively unaltered conodonts from Kentucky supplied to her by Stig Bergström. These were the same conodonts he and Sweet had used in their groundbreaking 1966 paper. Epstein heated the conodonts to temperatures from three hundred to six hundred degrees Celsius over a period of ten to fifty days, removing samples at regular intervals to record their color. She found that the fossils’ color altered in a “progressive, cumulative, and irreversible”2 way and was the direct product of time and temperature. Reassuringly, the colors produced in those she had cooked were exactly like those found in the field.

  Working with her husband, Jack, and Harris, she extrapolated this experimental data so that the relationship between temperature and color could be understood on the scale of geological time. Now the color of the conodonts could be used to indicate the burial history of the rocks that contained them. Announced to the world in 1974, with full results published in 1977, this discovery changed her life and expanded the meaning and importance of the fossil considerably: “This study increases their use from index fossils to metamorphic indexes and demonstrates their application to geothermometry, metamorphism, structural geology, and for assessing oil and gas potential.” The scale of color change became known as the “color alteration index,” or CAI, and it drew in new kinds of conodont workers. This was yet another case of these tiny things participating in science on the grand scale and, in this case, in science that was really useful to everyday concerns. A fascinating episode in and of itself, which opens up an entirely new story arc, we must, however, leave it here because it tells us nothing of the animal itself. CAI records only aspects of the postmortem existence of the animal. We must turn our attention now to the world of the living animal, though in doing so we will primarily concern ourselves with its death.

  The 1980 asteroid, or meteorite, that once struck earth and now, in a metaphorical sense, impacted the scientific community was delivered by Nobel laureate physicist Luis Alvarez and his geologist son, Walter, along with chemists Frank Asao and Helen Michel. It arose from work Walter Alvarez had been conducting at the Cretaceous-Tertiary boundary at the ancient town of Gubbio in Umbria, Italy. Measuring the trace element iridium, which is constantly falling to Earth as meteoric dust, he postulated that its degree of dilution in marine sediments would indicate how rapidly the sediment had been deposited. The sedimentary dilution of a constant – such as Merrill's dying and fossilized animals, discussed in the previous chapter, or meteoric dust falling to Earth – had long been used to deduce the relative rates at which rocks were laid down. The sediment that interested Alvarez marked the end of the Cretaceous, that remarkable moment when the dinosaurs and many other kinds of animal became extinct. However, it was not dinosaurs that first caused Alvarez to stop and think, but the near extinction of those tiny amoeba-like animals with delicate and intricate shells known as foraminifera. At Gubbio, a centimeter-thick layer of clay divides the extraordinarily different foraminifera of the Cretaceous from those of the overlying Tertiary. Alvarez asked, “Has this mass extinction occurred in a human timescale or a geological one?” In order to answer this question, he needed to know how rapidly the clay had been deposited.3

  As it turned out, the iridium performed better than he had hoped, for rather than simply giving Alvarez the rapidity of change, it also gave him a cause. What he found were extraordinarily high amounts of this element (in relative terms at least). After much deliberation, the team felt this could only be explained by a huge meteorite impact. Astronomy, that esoteric science of other worlds too distant to really know, now became central to understanding the history of life on Earth. Newly globalized, geology now found itself a science of planets.

  The idea was a “bombshell” that caused an explosion of papers and conferences – and some bizarre theoretical imaginings, including a death star called Nemesis and an equally deadly Planet x. Supportive speculation and doubting cynicism developed in parallel, but increasing amounts and types of data seemed to confirm this radical and seemingly improbable alien visitor.4 The
impact on science was so great that it is easy to imagine geologists now talking new talk and thinking thoughts that had never previously crossed their minds. But this was not entirely what happened. Their initial response was to work with what they knew, to marry this new idea with existing data. In this respect the asteroid became a new pair of interpretive spectacles through which to look afresh at old things. These glasses would also encourage geologists to seek out obscure and esoteric work that at one time seemed to make little sense. Perhaps it would do so now. In some cases, yesterday's nonsense and self-indulgence suddenly became prophetic. Some on the edge of the community now found themselves treated as oracles and placed at the center of this new debate.

  Otto Walliser was among those who welcomed the meteorite, or rather the change of thinking it brought about. He never found a need for the meteorite itself. It might be recalled that Walliser had played a singular role in establishing the conodont in Silurian stratigraphy in the early 1960s. With no good Silurian sections in West Germany in which to continue these studies, he simply left the field. He had moved to Göttingen, and while he retained an interest in conodonts, he did so no more than in the cephalopods that had first attracted him to the science. Trained by Otto Schindewolf who for a time was the leading light in German paleontology, Walliser had acquired an interest in the global aspects of the science long before the Alvarez meteorite hit. Indeed, back in 1954 and again in 1962, Schindewolf had suggested that cosmic radiation from a supernova could cause catastrophic extinctions of life on Earth. Such extinctions, he said, would be followed by a burst of evolution and diversification among the survivors. Schindewolf claimed that these catastrophic events could be read in the rock record because that record was sometimes complete; there were no gaps in the sequence at those moments of catastrophe. At the time, the idea of mass extinction was too much for many paleontologists. Extraterrestrial causes simply added to a sense that this was mere fantasy. To their eyes, species were lost gradually – as they seemed to be at the present day. Any apparently sudden loss was merely an artifact of missing or eroded strata. Unsurprisingly, then, Schindewolf's views did not find much support, even among those, like Norman Newell, who did much in the 1960s to demonstrate the reality of mass extinctions. Newell had taken a rather different approach and had plotted the diversity of life against time to reveal the truth of these extinctions, though the coarseness of his methods concealed the true prevalence of such events.5

  Walliser was not convinced by Schindewolf's explanation either, but he knew Schindewolf's data were good and that the phenomenon of global extinction he described was real. That fascinated him and soon began to affect the way he interpreted rocks and fossils in the field, particularly after 1965. That year he visited Iran and discovered a Devonian sequence that was lithologically and paleontologically identical to that back home. He thought this remarkable because it meant that particular changes of environment must have taken place across an extraordinarily wide area, perhaps even globally. What especially fascinated Walliser was the fact that the rocks themselves were capable of showing this change. He now understood that seemingly local phenomena, such as nodules in Devonian strata, were not local at all and that the peculiar characteristics of particular beds in one locality – such as their predisposition to slip and slide (a quality he tested by chewing the rock!) – could be global in their distribution too.6 Walliser's fascination with these unexplained global phenomena grew.

  As he traveled farther afield, particularly to Asia, so his initial findings would be confirmed. He became a connoisseur of rock sequences, increasingly convinced that nature preserved natural global markers recording moments of transformation in the earth's history. However, his views were not shared by those more utilitarian stratigraphers engaged in dividing rocks up into neat, globally recognized parcels. Walliser became an outspoken advocate for locating and using natural boundaries in rock sequences. His critics preferred to drive their boundary-defining, and metaphorical, “Golden Spikes” into sequences where nothing much happened but where the replacement of one species by another could be recognized globally. He did have some early successes, however, such as in setting the maj or Silurian-Devonian boundary at a meeting in Bonn in 1960. But such victories were rarely permanent. Mass extinctions were often associated with difficult lithologies and incomplete rock sequences; even an imaginary Golden Spike cannot be driven into rocks that are not there. Consequently, throughout the 1960s, the artificial scheme gained ground and was adopted by all the various grandly named subcommissions that sought to define and adjudicate on these global boundaries. Walliser found himself in the minority, frustrated by the victory of utilitarianism over nature.7 In the story of the conodont, of course, this is not an unfamiliar theme.

  Walliser was not entirely alone. In 1969, his friend Digby McLaren of the Geological Survey of Canada was at last willing to admit to the vital importance of these natural boundaries where a range of unrelated animals and plants became extinct, adding, pointedly, “[boundaries] which we are trying to define out of existence.” In his presidential address to the Paleontological Society that year, McLaren said these boundaries were of two types. The first was quiet and merely man-made for convenience, but the second recorded some “event” “across which something happened.” It was to explain a boundary of this second kind – a major extinction in the Devonian known as the Kellwasser Event, which he had first recognized in the 1950s – that he “landed a meteorite in the ocean with effects that had been described by [Robert] Dietz in an article in Scientific American” in 1961: “Dietz…suggests a giant meteorite falling in the middle of the Atlantic Ocean today would generate a wave twenty thousand feet high.” McLaren pondered the consequences of a similar event for his now extinct animals, and acknowledged, “This will do.”8

  Given the reception Schindewolf's alien causes had received, McLaren knew that he risked being labeled a crank. The preferred explanation for mass extinction was the rise and fall of sea level resulting from ice ages and major vertical movements in landmasses. (We need to remember that the vast majority of fossils – and therefore recorded extinctions – are of sea animals.) To this audience, then, McLaren's meteorite was unexpected and unneeded. The response to it is perhaps typified by one English contemporary, Michael House, who believed McLaren had said this “doubtless with tongue in cheek.” House viewed the problem through spectacles constructed from geology's most important guiding principle, uniformitarianism, a belief that the world of the past was created by the same gradual processes we continue to see today. Science neither needed nor had a place for catastrophes of this kind. But this was not how McLaren saw it at all: “I do not believe this explanation is farfetched…. We must look for more than everyday happenings to explain many geological features.”9

  It was, however, simply speculation; there was no evidence of an impact, even if some contemporary astronomers – beyond earshot of the geological community – thought sizable meteorites must have collided with the earth in the past.

  These late 1960s discussions of extinction caught the attention of others in the conodont community, forcing them to think new thoughts. Dave Clark, at the University of Wisconsin in Madison, for example, saw in extinction an opportunity for separating true species from their mimics and imitators. This insight arose from work that Clark's doctoral student, James Miller, was undertaking to disentangle the evolution of individual conodont elements in the late Cambrian. With an expectation of finding just four conodont elements in each kilogram of rock, Miller – who now replaced Müller as the most prolific worker on these earliest of conodonts – was nevertheless able to demonstrate that identical elements evolved in quite separate branches of the evolutionary tree. Clark recognized that Miller's great advantage was to study that first burst of evolution, as it permitted him to build upon a clean slate. It suggested to Clark that if one wanted to replicate Miller's trick anywhere else in the family tree, it would be necessary to locate a point of mass extinction and buil
d the family tree from there, in that subsequent explosion of evolution Schindewolf had recognized.10 Clark's interest in extinction reflected a utilitarian desire to be able to precisely identify his fossils; the attraction was not to indulge in imaginative speculation. He began, then, by asking when extinctions took place in the conodont world. Clark wanted a way to visualize changing diversity beyond the range charts of species and genera widely used by stratigraphers. He wanted to know not the pattern of life but the pattern of extinction.

  He began by plotting the number of form genera present in each major period of geological time together with the number that went extinct in that period. He knew the picture he drew was coarse and that his data were imperfect, and he was therefore not surprised when the technique threw up some odd artifacts of method. The Silurian, for example, appeared to be a period of crisis for the conodont animal when, really, the low number of species reflected the period's relatively short duration. As if to record the path he had traveled and perhaps prevent others from falling into the same trap, Clark published the diagram nevertheless. He then improved this picture by increasing its resolution and plotting only those species that first appeared within a given period. Now his plot showed extinction periodically overtaking evolution. In other words, there were moments of decreasing diversity – of impending crisis. Turning these two measures (new species emerging and old species becoming extinct) into ratios, he could then plot what he called an “index of evolution,” which showed graphically when the conodont animal was in crisis and when it hit boom times. It was to prove an influential study and one to which Clark would return a decade later, wondering why the conodonts supported two peaks in their life history rather than one.11