Merrill knew that the shales were produced near the shore while the limestones represented fully marine conditions farther out to sea. The rocks representing these two different environments were stacked one upon the other, which Merrill suggested represented fluctuating sea levels. As the sea level rose, the shoreline would move inland and these two environments and their conodonts would follow.
Merrill then asked himself how rapidly these changes took place. He could answer that question by using an old idea that required a little lateral thinking. On the presumption that animals were dying at a constant rate and falling to the seafloor to form fossils, high concentrations suggest that the rate of sedimentation has been low and a thin layer of rock represents a long period of time. Finding these fossils heavily diluted by sediment could then be interpreted as evidence of rapid sedimentation. Thus in a rock section, time may be both condensed and stretched. A rough measure it may be, but it permitted Merrill to consider how rapidly the sea had transgressed the land. He found this often occurred so rapidly that it prevented the shale fauna from developing locally. The shallowing (regression) of the sea caused by the growth of a delta was, however, slower, and in these circumstances the expected fauna could develop. Merrill thought he was looking at shallow and very shallow water communities that were not greatly distinguished in terms of water depth. Instead, he believed salinity a controlling factor as the shales contained brackish-water fossils while the limestones were fully marine. He deduced that the animals actively maintained a link to the environment and did not, like chaetognaths, simply waft around in the currents: “The conodont governed its own occurrences in a much more direct manner…. It was nektonic; an active organism fully capable of exerting important controls upon its own distribution, depth and destiny.” Seddon and Sweet's model simply did not fit the bill.
Merrill believed that the richness of modern data was permitting conodont workers to see subtlety and complexity, displacing beliefs in the universal. He put it metaphorically: For so long it had been possible only to hear the orchestra; now they were beginning to hear the individual instruments themselves, even if “the composition and the composers are unknown.”
In the 1970s the science possessed for the first time the makings of a global database of information on conodont distribution in space and time. It was already altering perceptions. For example, simple cone-shaped conodont fossils, which were considered mere contaminants in the Devonian in 1957 and questioned as such two years later, were in 1973 mapped by as a true but progressively declining component in that fauna.25 In this incremental way the whole data set was changed and extended, knowledge shifted, and interpretations were reformulated according to new and, as we shall see, increasingly spatially aware criteria.
This new trend in conodont studies really took off as geology in general embraced the unifying theory of plate tectonics. It was a theory that gave the earth a fluid geography of mobile continents and spreading oceans. It encouraged the sometimes near-sighted paleontologist to think big as it gave new explanation to the comings and goings of oceans and the appearance and disappearance of animals and plants over time. The idea that the continents drifted over the surface of the globe had been debated for much of the twentieth century, but it was only in the late 1960s that all the components of the theory were locked together into a convincing whole. It became the new paradigm. But not everyone felt the need for an imposing theory. The science had, after all, gained nearly all its intellectual possessions by hard graft in the field or through instrumentation. Conodont worker Anita Harris – a onetime PhD student of Walt Sweet – certainly belonged to this proud tradition. Back when the theory was still fresh, she told popular geological writer John McPhee, in a book in which she was the main actor, “The plate-tectonic model is so generalized and used so widely…. People come out of universities with PhDs in plate tectonics and they couldn't identify a sulphide deposit if they fell over it. Plate tectonics is not a practical science. It's a lot of fun and games but it's not how you find oil. It's a cop out. It's what you do when you don't want to think.”26 Conodont science was indisputably practical, and Harris had by then, as we shall see, made a major contribution to that practicality. She had done so by paying her dues in the field, not by theorizing. Her comments indicate how fundamentally geology was changing, and she in time would adjust to it once all the hullabaloo settled down. The new tectonic theory rapidly found its way into the interpretative armory of practical geologists, particularly those concerned with the spatial dynamics of the earth. Leading paleoecologist James Valentine at the University of California in Davis, for example, combined the new theory with evolutionary concepts to revolutionize the way paleontologists thought about the distribution of life over time.27 Drawing on these new theoretical and interpretive resources, it was again possible to think differently. Geology received a shot in the arm as the developing theory gave explanation to phenomena that had long been puzzling. Perhaps it was coincidence, but just as the conodont workers began to possess global data about their fossils, so the geology as a whole started to think global thoughts.
Chris Barnes of the University of Waterloo in Ontario was certainly among those who thought these new theoretical ideas powerful interpretive tools. Valentine's work was in his mind when he teamed up with Rexroad and Cambrian specialist James Miller of the University of Utah to consider the global pattern of conodont communities and provinces during the period when these communities first appeared and experienced explosive diversification.28 They reasoned that for this diversification to have occurred there must have been ecological niches available for exploitation by different species. Valentine's refined definitions of province and community suggested a spatial continuum of environments and a way to relate diversity to a changing world. It gave them a means to reinterpret Bergström and Sweet's provinces and subprovinces and attempt to locate communities within them. These communities, they believed, were distributed as a series of lateral bands parallel with the shore and extending into deeper water. Using the brand-new understanding of continental margins, they set these bands in the context of plate-tectonic theory. It gave their ecological niches a theoretical rationale and a new global context. But Sweet and Bergström simply could not agree and recalled a paper in press to add a short addendum critical of this interpretation. It was, however, a sign of the times: “The concepts and ideas of the theory of global tectonics have proven to be a virtual panacea for geologists.”29
Many of the ideas developed in this first paper were extended and given greater clarity when, three years later, Barnes and Lars Fåhræus of Memorial University of Newfoundland in St. John's reviewed the growing literature to propose a “unifying model of the major habitats” of Ordovician conodonts.30 Seddon and Sweet had imagined a weakly swimming or floating animal, but Barnes and Fåhræus reassessed the evidence and suggested a bottom-dwelling animal that was in control of its own movements. The lateral banding of fossils was so clear, they felt it was possible to predict neighboring communities. There were, they admitted, some simple species that transgressed these bands that probably represented swimming forms, but these were less numerous (figure 9.1e).
Barnes and Fåhræus then took Bergström and Sweet's two provinces and rotated them in order to understand their relationship in Ordovician times. East-west now became north-south. The Appalachian or North Atlantic Province was understood to represent a normal marine environment with a “virtually cosmopolitan” fauna. The midcontinent fauna was, by contrast, seen as being restricted to a “fairly narrow equatorial belt” and adapted to higher temperatures and salinities.
Using these ideas, they now reimagined Bergström and Sweet's panoramic view of mobile populations of animals with ideas that integrated time and community with the province. The controlling factor was a relative change in sea level. In the lowermost rocks of the midcontinent, the animals held simple cone-shaped elements. They showed no lateral segregation. However, during the Middle Ordovician, major transgressi
ons flooded the midcontinent from the direction of the Appalachian province. Habitats and communities diversified and ecologically specialized faunal belts developed. In this manner, Barnes and Fåhræus gave a sophisticated and detailed reading of the history of a group of animals experiencing environmental change. They, too, were sure that the form and shape of individual conodont elements and assemblages reflected particular lifestyles. Increasingly, they felt it was possible to talk with reasonable certainty about the life requirements of particular genera and species. Drawing upon a sizable literature, they had pieced together a rich, complex, and changing conodont world. One could almost imagine the animal.
Barnes and Fåhræus's paper appeared in April 1975. A month later, Barnes played host to a paleoecology-themed meeting of the Pander Society in Waterloo, Ontario. The meeting showed how the field had diversified.31 The idea that environments could be read merely from the shape of these tiny fossils was then in the ascendancy.
One of the more extraordinary papers was by Jeppsson, who had been inspired by science fiction writer and acclaimed Ice Age mammal specialist Björn Kurtén. Kurtén had attempted to go beyond the descriptive essentials of paleontology, measuring bones in order to interpret communities. Jeppsson transferred the method to the deep past and took to measuring almost immeasurably small processes on these tiny fossils. He considered the possibilities of the animal having a spawning season and seasonal migrations.32 Barnes thought the paper both speculative and intriguing. It was typical of Jeppsson's individualistic approach to his science but it also marked a wider desire to give these animals biological clothes of some kind.
While an increasing number of conodont workers were becoming rather theoretically minded, Dick Aldridge of the University of Nottingham in the UK was not alone in treating his piece of geological time with historical specificity. Believing each period to be distinctive and affected by particular influences, Aldridge thought the existing models rather simplistic in their assumptions of cause and effect when so many environmental factors were known to be at play.33 Many others discussed the various models, though the latest by Barnes and Fåhræus was simply too new to attract much attention. Opinion remained mixed; some testing had taken place, but it was inconclusive.
Karsten Weddige and Willi Ziegler, being dissatisfied with the explanatory power of the models produced to date, came up with yet another (figure 9.1d). They did not believe that depth and distance from shore were responsible for the observed distributions of fossils. Indeed, the models seemed too static; the ocean was a dynamic environment and other factors were at play. Icriodus, they said, preferred turbulent, oxygen – and carbonate-rich waters, while Polygnathus preferred quieter conditions associated with muddier sediments.34
Riding the wave of ecological optimism, Fåhræus and Barnes rushed a second paper into print in the distinguished science journal Nature. Their vision was nothing short of oceanographic. The conodont was to be a tool in “studies involving the extent and relative depths of sedimentary basins; for unraveling patterns of transgressions and regressions; as aids in the recognition of depositional environments characterized by raised temperature and salinity; and in understanding palaeogeographic and tectonic changes.”35 They demonstrated this by examining the relative abundance of two genera thought to occupy adjacent and overlapping communities: Phragmodus, which occurred offshore, and the nearshore form, Plectodina. The changing relative abundance of these in a geological section would indicate transgressions and regressions. It was a view not far removed from Merrill's. But Fåhræus and Barnes now saw the conodont as a precision tool for plotting major global change: “Thus, initial destruction of this ancient continental margin can be dated with considerable precision.” In taking this view, Barnes acknowledged the debt they owed Sweet and Bergström whose data they had turned to their own ends.
Barnes and Fåhræus were not alone in considering major events in the earth's history. Across the Atlantic, Otto Walliser was having similar thoughts. The science was continuing to change, and as it did so it would move the cutting – or at least fashionable – edge away from ecology. The late 1970s marked a point of reflection for those who had looked at palaeoecology as holding promise. The conodont workers certainly felt they had achieved new understanding, but this was not the view of Smithsonian foraminifera specialist Martin Buzas when he reluctantly took up the task of reviewing the book that came out of the Waterloo conference. In that review, titled “On the edge of the unknown,” he admitted to knowing nothing of conodonts or of the rocks in which they are found. He could, however, reflect upon the distribution of foraminifera in rocks, and he remarked how little indication this gave of their distribution in life. He wondered, given the data they possessed, how conodont workers could deduce an open-water swimming lifestyle and distinguish it from an animal that lived on the seafloor: “I conclude conodonts were some strange animals but don't venture a guess as to where they lived.”36
The initial optimism that had accompanied the rise of paleoecology had begun to wane. One major textbook noted, “Paleoecology, which during the 1960s occupied center stage in the paleontologic theatre has matured as a subdiscipline but has also lost some of its luster; appreciation of the incompleteness of the invertebrate fossil record has led to a general narrowing of goals in the study of ancient marine communities.” Joel Hedgpeth, who had pioneered the field in the 1950s, found ecology undergoing significant change and palaeoecology often in possession of old ideas, unable to keep up.37
In 1978, Gil Klapper and James Barrick also asked whether it really was possible to infer lifestyle from the distribution of conodont fossils in the rocks.38 Reviewing a number of marine animals, they became convinced one could not. Seddon and Sweet's favored chaetognath analogue, for example, did not show simple depth stratification after all but reflected the complexities of temperature and salinity, which actually produced lateral variation. Indeed, bottom dwellers and swimmers were capable of leaving the same record. Only the presence of conodonts in black shales, which were devoid of bottom dwellers, strongly suggested a swimming or floating animal of the open sea. They concluded that since conodonts seemed to be confined to the continental shelf, it would be better to draw on modern analogues to visualize the ways in which they might have been distributed in life. These reflected changes in key environmental variables in relation to the coast (figure 9.1f). Like many living animals, conodonts had a few specialist species able to survive the difficult conditions of the nearshore; their diversity increased away from the shore. This was another model, but it removed the need for a bottom-dwelling lifestyle yet could still produce the observed lateral changes and overlaps. It also captured that reciprocal relationship between genera, which Weddige and Ziegler, Merrill, and even Rexroad had written about.
9.1. Modeling the ecology of the animal. Deducting lifestyle from the distribution of fossils in the rocks. The triangles represent the sea in section, showing increasing depth and distance from shore. The patterns represent different genera of conodont animal: (a) Druce saw lateral distribution in his reef limestones; (b) Seddon introduced a one-way depth-controlled filter; (c) Druce adopted Seddon's model but argued that nearshore population densities were higher; (d) Weddige and Ziegler saw water clarity and oxygenation as differentiating Icriodus (left) and Polygnathus (right); (e) examining other environments, Barnes and Fåhræus saw clear banding of animal communities, suggesting they were bottom dwellers; and (f) reviewing the evidence in 1978, Klapper and Barrick showed that the distribution of marine animals was governed by complex factors and that the record of the rocks could be explained in multiple ways.
There were, then, repeated attempts to adjust the theory to the limitations of the data but no wholesale retreat from a line of enquiry that had certainly delivered results. New data continued to reinforce Bergström and Sweet's provinces. Indeed, provinces became established in other periods, too, but turned out not to be universal. In the Devonian, at least, it was understood that conodonts were pr
obably confined to tropical latitudes. Of these, Icriodus returned to favor, finding its own parallel zonation to mirror the one Ziegler had developed using Palmatolepis. This work revealed that Icriodus had become extinct rather earlier than assumed and that an impostor had evolved from Pelekysgnathus. The first Icriodus was quite well distributed in the upper, highly illuminated waters of the coastal zone, but its impostor, known as “Icriodus,” was more restricted in its distribution.39 A reversal of expectation had taken place. Having believed in universalism, workers increasingly expected environmental control.
Near it in the field, I remember, were three faint points of light, three telescopic stars infinitely remote, and all around it was the unfathomable darkness of empty space. You know how that blackness looks on a frosty starlight night. In a telescope it seems far profounder. And invisible to me because it was so remote and small, flying swiftly and steadily towards me across that incredible distance, drawing nearer every minute by so many thousands of miles, came the Thing they were sending us, the Thing that was to bring so much struggle and calamity and death to the earth. I never dreamed of it then as I watched; no one on earth dreamed of that unerring missile.
H. G. WELLS,
The War of the Worlds (1898)
TEN
The Witness
THROUGH THE 1970S, PALEONTOLOGY ACQUIRED AN increasingly global outlook as geology as a whole embraced the unifying ideas of plate tectonics. The conodont workers felt this sense of the global even more profoundly as its field of study spread to every corner of the earth. In this period, the living animal became a mobile entity inhabiting clearly defined niches and repeatedly evolving similar anatomies to deal with the return of particular environmental conditions. Progress for the conodont workers, as for most of paleontology, had been logical and incremental. But then two unexpected events forced them to look and think differently, and even to imagine the unimaginable.
The Great Fossil Enigma Page 24
