We can now see how fundamentally Bergström and Sweet's remarkable paper challenged everything to which Ziegler aspired just at that moment when he had achieved global success. Not only did it suggest he needed to document the evolution of assemblages, rather than isolated elements, but it also put into question the very notion of the universal conodont.
Bergström and Sweet continued to develop increasingly sophisticated and subtle interpretations of provincialism in their animals. Soon they were “tracing and matching faunal ‘tongues’ that represent the shifting of provincial and subprovincial boundaries in time.” In his paper with Kohut, Sweet could talk of Phragmodus undatus retreating northwards while other conodonts were “more tolerant” of change and stayed put. Increasingly he began to think about environments, and as he did so, particular conodont species acquired lifestyles. One, for example, “seems to have flourished in a nearshore, shallow-water environment, perhaps on a tidal mud flat that was periodically exposed to the atmosphere.” In contrast Phragmodus undatus was “an inhabitant of deeper waters.” Gerald Webers, influenced by the Ohio workers, also began to think along these lines. In Ohio, at least, and in the Ordovician in particular, the universal conodont was dead.11 Necessity had also spawned a rather different approach to stratigraphy. There were no neat time markers here. Rather, time seemed to be marked by the very ebb and flow of life.
In the language of the 1970s, Sweet and Bergström were applying a generalized conceptual “model” – that of the province – and using its perceived attributes to interpret their data. It was a model so enshrined in geological practice that few questioned it. However, at a London conference in 1969, it became apparent that this universal concept was far from concrete. And as the delegates at the meeting deliberated its meaning, so Peter Sylvester-Bradley became increasingly depressed: “I am afraid that we can claim to have answered no problems in this symposium. Quite the reverse. We have dredged up from the stores of knowledge many old problems that had been shelved away and almost forgotten.” Those who had organized the conference had to agree: There was no agreement on what a province was, how it could be recognized, or even whether such things existed in the geological record.12 This did not invalidate Bergström and Sweet's interpretation, but it did mean that the province remained hypothetical.
At that time, the descriptive natural sciences were adjusting to the logic of the computer and the imagined objectivity of numbers. Its world was about to be redrawn in systems diagrams of the type used by programmers and systems analysts in the computer industry. Among those taking a lead was another of Sweet's former students, Tom Schopf, whose Models of Paleobiology performed as an evangelical tract for this new way. Schopf argued for a retreat from short-sighted realism and precision, suggesting that generalized theoretical models should be developed to predict, and be tested by, data. He was then at the University of Chicago, and was the latest in a line of academics there who sought to lift paleontology out of those habits of which Ager had been so critical.13
The conodont, which in Bergström's and Sweet's minds seemed to exist as a great underwater swarm of expanding, contracting and shifting life, was then being captured by other modelers who made the animal's distribution rather more structured. The origins of this close-up view of conodont distribution lay in the paper Glenister and Klapper wrote on conodonts from the Canning Basin in Australia, which implanted Ziegler's Upper Devonian zonation there. When Glenister and Klapper came to make this correlation, they drew upon a recent study of the basin by Phillip Playford and D. C. Lowry of the Geological Survey of Western Australia. In order to understand these complex rocks, Playford and Lowry developed a model suggesting that the rocks there represented four different reef environments: the back-reef, reef, fore-reef, and inter-reef. To Glenister and Klapper, the distribution of the Australian conodonts suggested they were ecologically controlled, for, like the cephalopods, these fossils were found in the fore – and inter-reef areas but were rare in the reef itself and absent from the back-reef. They were also rare in beds containing brachiopods.14
This understanding of ecological control became more specific when English emigrant Ed Druce investigated the conodont fauna of the Bonaparte Gulf Basin in the far north of Australia. Druce was a veteran of the field: “He enjoyed the subtle beauty of the outback and camping appeared to be part of his nature. His mobile conodont lab and field processing of samples meant that field seasons could easily be up to 3 months long. As long as the tea, beer and meat were there, the work would go on.” In the Bonaparte, he found conodont faunas restricted to particular parts of the Devonian reef complex. And although Glenister and Klapper had reported no conodonts from the back-reef, Druce found a fauna there dominated by Pelekysgnathus and other conodonts not utilized in Ziegler's standard. The fore- and inter-reef were in contrast populated with high numbers of Palmatolepis, Polylophodonta, and Scaphignathus. Here Pelekysgnathus was absent.15 Although these findings were present to some degree in his published data, it was only later that he gave these distinctions clarity. Not long after completing this work, Druce entered the Canning Basin, collecting material from the Devonian reef complex for a doctorate supervised by Frank Rhodes, who had recently moved to the University of Michigan.
As Druce's study of the Bonaparte Gulf went to press, George Seddon was completing a four-year consultancy for WAPET on the Canning Basin.16 Seddon was unusual in being a member of the Departments of Geology and Philosophy at the University of Western Australia. He would later be celebrated for the range and significance of his work, but in 1970, his mind was on conodonts.
Both Druce and Seddon sought to resolve the outstanding strati-graphic problems of a region that could be linked to Europe but within which many rocks remained uncorrelated because of complexities presented by the presence of reefs. Seddon's aim was not simply to superimpose Ziegler's system on these Australian rocks but to consider the significance of those conodonts that had gone unmentioned in the German scientist's work. Seddon soon came to understand, as Glenister and Klapper had before him, that the distribution of conodonts was controlled by the different environments that made up a reef. Like Druce, but independently, he found two distinct, environmentally controlled, faunas characterized by different form genera. One fauna was typified by Branson and Mehl's favorite, Icriodus, together with Polygnathus and Pelekysgnathus. The other was dominated by Ziegler's most useful Palmatolepis, along with Ancyrodella and Ancyrognathus.17 The Palmatolepis fauna contained examples of the Icriodus fauna but in lower abundance. With few exceptions, the Icriodus fauna did not include those key genera from the Palmatolepis fauna. Seddon imagined the existence of a one-way filter through which Icriodus and associated forms could pass but through which Palmatolepis could not, though at the time he did not identify what that filter might be. The Icriodus fauna was found in the fore-reef but close to the reef itself, while the Palmatolepis fauna occurred seawards, from the fore-reef to the inter-reef.18 Palmatolepis was, of course, the key to Ziegler's universal standard, but Icriodus was not. However, Seddon's discovery of a filtering mechanism enabled him to relate the two and determine the age of his rocks. To achieve this he created a parallel Icriodus-based chronology recognizing that this represented a rather specialist environment. Seddon's paper was published in 1970. It acknowledged Druce's helpful critique of his work, though it seems possible that Seddon knew little of Druce's thinking.
In May that year, both Seddon and Druce presented their findings at the Michigan meeting of the Pander Society. Druce could now talk generally about the depth control of his two faunas and locate parallel examples in the lowest Carboniferous (figure 9.1a).19 The “exotic and bizarre,” he noted, were restricted to deeper waters but had evolved from shallow-water stock. These deeper-water forms were generally more diverse, and it was for this reason they had been so useful to Ziegler and Helms. Druce also noted the same one-way mixing of faunas. That Ziegler's system was again called into question was not lost on the audience, particularly o
n Ziegler himself. Druce advised his colleagues to record Icriodus and other shallow-water forms in order to counter these weaknesses in Ziegler's standard.
Thus far Seddon and Druce had been working in parallel, and broadly with the same geographic, stratigraphic, and intellectual goals. By 1970, conodont ecology was becoming a hot field, not least because it tested the idea of the universal conodont. Increasing numbers of workers began to develop similar and overlapping topics of study. Competition and debate increased. This parallel production of data and knowledge meant that individuals tended to know different things, and this did not depend solely on personal networks and research groups. Some would gather new data or ideas by attending meetings while others would not receive this information until the paper was published – which might be years later or not at all. Some important studies were published merely as short abstracts of orally presented papers. The abstracts were hardly scientific arguments, but they acted as important markers and were often referred to. In 1970, both Druce and Seddon heard each other speak and both published short abstracts of their papers.20 Druce made a commitment to write up his paper for the book arising from the conference, but this book would have such a catastrophically delayed gestation that the paper would not appear for three years. And when it did, it was not the paper Druce presented at the meeting but one that reflected upon that meeting and Seddon's presentation there. We shall come to that paper shortly.
Seddon, who produced a rather different interpretation of the data for the 1970 meeting, chose not to publish in the conference volume. Instead, he teamed up with Sweet to explain his filtering mechanism in more detail. In order to do so the two men looked for a “likely ecologic analogue” and chose the chaetognaths, or arrow worms. These are typically carnivores, three centimeters long, “that spend their entire existence floating or swimming in the water without relation to the bottom.”21 Although Seddon and Sweet suggested no direct relationship between conodonts and these animals, they could clearly talk of the analogue darting forward to capture prey and visualize their own animal doing the same. There were other similarities too, as one authority considered the chaetognaths something of an enigma: “They may be the most isolated group in the animal kingdom.” Doubtless their “paired batteries of anterior and posterior teeth, and grasping spines” were sufficiently unique to support Seddon and Sweet's contention that they had before them a “suggestive analogy.” In doing so, they drew upon a number of books to act as their authorities on these animals, many of which came from the 1950s. These sources told them that the animal's wide distribution was controlled by temperature, salinity, and available food, and that species were vertically stratified: Most species occurred in water depths of less than two hundred meters, but more specialized forms could be found between two hundred and one thousand meters, and a few even beyond that depth (figure 9.2b). This suggested a possible mechanism for the operation of the biological filter that affected conodont distribution. It had long been suspected that Icriodus was a shallow-water form but now this genus could be visualized as occupying surface waters, overlying the deeper waters where Palmatolepis swam or floated. On death, conodonts elements would sink to the seafloor. Those that accumulated in deeper waters would as a result contain both shallow- and deep-water forms.
Drawing upon the contemporary literature, Seddon and Sweet could generalize further and suggest that the shallow zone contained just a few unspecialized conodont species, while at depth, where the environment was more stable, there was greater diversity. Sweet's Ordovician then became the testing ground for this new depth-stratification model, but here Seddon and Sweet could not call upon the relative simplicity of the reef model to infer water depth or relationships between communities. They were, of course, dealing with fundamentally different genera, but they still felt that within the different provincial faunas it was possible to detect this two-way, depth-controlled division of genera. Indeed, they thought the ratio between two particular genera might provide a crude index of water depth, which could be confirmed by lithological data. As Seddon and Sweet were developing this idea, others reported Seddon's distinctive Icriodus fauna in various parts of the United States, demonstrating its wide distribution.
When Druce's paper finally did appear, two years after Seddon and Sweet's, it showed the influence of Seddon's work. It further distinguished a third (Belodella) fauna, which Seddon had included with Icriodus but Druce thought occurred still closer to the reef. Druce sought to test and extrapolate his model across the whole of the Upper Paleozoic and into the Triassic, aware that very similar apparatus architectures and element morphologies had repeatedly evolved across long periods of time. Homeomorphy, the repeated appearance of identical forms as a result of environmentally determined convergent evolution, suggested this close link between the form of an animal and the nature of the environment.22 This kind of morphological convergence had been seen across the natural world in everything from mollusk shells to teeth. It was reasonable to expect it in conodonts even if the animal remained unknown and unimaginable. How wonderfully informative these tiny fossils would be if this was the case; find a microscopic fossil, and there, in your hand, you have the key to understanding the world in which it lived. Druce was not alone in thinking in this way; Ronald Austin compared Walliser's Silurian species to his Carboniferous forms and, surprised by how similar they were, had similar thoughts.
Ecological models reflected this desire for generalization: They emerged almost subliminally from the data and would then perform as spectacles through which others would look at fossils and the rocks that held them. These workers might accept this new way of seeing, cast these spectacles aside, or make improvements to them. Druce chose to do the latter. He was sure that Seddon's model was better than his own but still not perfect. He knew the Palmatolepis fauna was more widespread than that of Icriodus, but Seddon's model predicted the opposite. So Druce refined it, concentrating populations toward the shore and effectively superimposing his earlier model on Seddon's (figure 9.1c).
In 1970, the appearance of Icriodus and other Devonian conodonts in various parts of the world raised a number of questions about the faunas Ziegler was finding. His collections inexplicably lacked these fossils. Whichever of the models was correct, there was good reason to believe these conodonts should also occur in Germany, and not simply because the conodont was celebrated there as the universal fossil. Druce wondered if some kinds of conodont had not been recorded because they were less exotic, often long-ranging, and therefore less useful to stratigraphy. Austin and Rhodes had similar suspicions and began to doubt Ziegler's data. They had found that British, Irish, and Belgian conodont faunas were similar to one another but different from those Ziegler had published. (Later these faunas would turn up in Pakistan, Russia and China.) Ziegler found himself increasingly under assault on this point, and attention now focused on the methods he used to process his fossils. Doubts had been raised about Beckmann's method, which Ziegler used, as early as 1964. At the time, Beckmann, Ziegler, and all the other British, American, and German heavyweights in the field closed ranks and rejected these claims. But with the identification of statistical assemblages a few years later it became increasingly obvious that Ziegler's faunas were incomplete. The arrival of this new shallow-water Icriodus fauna simply accentuated these doubts.
Ziegler remained resistant to change until Lindström moved to Marburg and used Beckmann's method on his Ordovician conodont samples. To his great surprise, Lindström found the conodonts seriously affected. Indeed, the results were quite spectacular. From two hundred grams of limestone, using Beckmann's method and leaving the sample bubbling away for two weeks, Lindström obtained just six badly corroded cone fragments. Repeating the exercise but recovering the residue every two days, he obtained eight hundred identifiable specimens, a few of which were corroded. This dissolution of conodonts was selective, bar-like fossils being particularly vulnerable. This seemed to explain why these elements were missing from Ziegler's coll
ections. In contrast, the platforms, which so interested Ziegler, were much more resilient. But surprisingly, this discovery did not greatly alter Ziegler's use of acids; he simply removed his fossils from the acid solution more regularly. He believed this was fine for stratigraphic work in which only the platforms were required but that where more detailed paleobiological studies were to be carried out, acetic acid should be used.23 Ziegler was perhaps typical of most conodont workers: If they could find sufficient specimens, they tended to believe the method was fine.
Others were more inclined to frown at Ziegler's disregard for producing accurate samples. One in particular, Lennart Jeppsson, would take the opposite approach and go to great pains to improve his methods and reduce the risks of loss or distortion. Jeppsson needed to process huge quantities of limestone to get just a few specimens. Sometimes he was literally looking for needles in haystacks.
The ecological models that soon became well known in the conodont research community did not go unchallenged. Glen Merrill, for example, had argued from the early 1960s that conodonts were environmentally controlled. For him, the universal animal was an illusion resulting from inadequate sampling. Having submitted a paper on the subject to the same volume that would eventually publish Druce's, he felt compelled to recall it so he could criticize Seddon and Sweet's model. Merrill's contribution was to point to a phenomenon he had seen in Pennsylvanian faunas and that Chalmer Cooper had briefly mentioned back in 1947. (Indeed, it was not unlike that distribution of fossils Rexroad had spotted in the late 1950s.) Merrill could show that alternative abundances in two platform genera depended upon lithology.24 One was dominant in shales, the other in limestones. The relationship was both consistent and remarkable.
The Great Fossil Enigma Page 23
