It was now possible to think freely again about the animal, to reason from what was known. The field was heating up and the animal was once again a hunted species. But with no fresh discoveries to hand, the hunt took place in acts of quiet contemplation. It was time to imagine.
Julian Priddle, a young zoologist at the University of Reading in the UK, reflected on the similarity of some conodonts to the horny eversible “tongue” of the hagfish. He thought conodont elements acted as skeletal supports beneath a layer of secreting flesh which produced a horny surface layer (figure 12.4b). While hagfish lacked this underlying structure, lampreys possessed it. And while the lamprey showed a radial arrangement of these elements, in the hagfish they were arranged symmetrically. Priddle also noted that the structure of pits and striae revealed by recent SEM work was strikingly similar to that found in unworn mammalian teeth. In mammals, this pattern was caused by the cells responsible for the secretion of enamel.
A model of concise reasoning, Priddle seemed to suggest that one could have one's cake and eat it: “This reconciles the internal nature of conodonts with a tooth function (whereas it was previously generally assumed that to function as teeth the elements had to be external…).” Priddle sent Scott a manuscript copy of his paper while the paper itself was in press, but Scott was now in defensive mode. Implicitly, Priddle's model doubted Melton and Scott's discovery, and Scott responded, believing Priddle's mind had been contaminated by the “canard”: “I want to assure you that the animals are for real and that the assemblage of conodont elements occurs precisely where it is reported.” Scott had a series of animal fossils now that showed that the conodont elements grew in size as the animal grew. “For these reasons,” he argued) “I think the search for the animal is over. The questions now are, how did they work and what is their evolutionary significance?”13
Conway Morris's fossil had emerged in some senses from a blind spot – it had been collected and placed in a museum but had not been studied or analyzed. Günther Bischoff, now at Macquarie University in Australia, also thought he had uncovered a blind spot. He puzzled over some long-known spiked bars and ribbed plates. Walliser had noticed that these spikes mirrored the interior shape of the conodonts’ basal cavity. That encouraged Bischoff to develop the idea that these bars were “conodont supporting elements,” which joined together into ribbed plates and together made up the mysterious Conulariids. These animals existed over the same time period as the conodonts and remained difficult to interpret. They are considered part of a group of animals that includes corals and jellyfish. The conodont elements were to Bischoff, then, a covering of the exterior shell of this animal.14
Jan Hofker at The Hague in the Netherlands compared the structure of conodont apparatuses to those found in a diverse group of microscopic animals known as aschelminthes, which include the tiny soildwelling rotifers. By careful rearrangement he tried to show that they were homologous,15 using that same visual trick executed by Macfarlane half a century earlier. Few conodont workers would be fooled by such lookalike reasoning.
These views indicated that the enigma was again reaching beyond the closed community of conodont workers and again generating solutions from those who had little sense of how sophisticated the field had become. Within the conodont community the search continued for new ways to look at what was already possessed. For example, the base of the conodont had remained, since the time of Gross, largely ignored. In part this was understandable, for no basal body was generally associated with those later conodonts that had most interested conodont workers. But Cambrian conodont specialist Stefan Bengtson, in Uppsala, thought the base ripe for renewed investigation and thought it might throw new light on how the conodont elements grew.16 This had, after all, been one of Gross's main objections to the tooth hypothesis. Pointing his SEM at the problem, Bengtson developed a model that saw the “crown” of the conodont (the conodont fossil as commonly understood) attached to the base and sitting within a pocket in secreting tissue. Bengtson's key innovation was to suggest that when in use, the conodont protruded through the tissue; when at rest, it did not – it was protected and could continue to grow or be repaired. It was a neat solution that had parallels in the anatomy of arrow worms. Bengtson arrived at this model, however, not by analogy to any living animal but by studying those Cambrian fossils that looked very similar to conodonts. Within these Cambrian forms he distinguished two types on the basis of their growth patterns: a newly named group he called “protoconodonts” and Müller's paraconodonts. Both had a rather contentious relationship to true conodonts, but Bengtson suggested these other fossils were ancestral to them. With this in mind he considered how their growth differed. His protoconodonts, for example, appeared to have grown on the surface of the secreting tissue and were progressively extended by additions to the base. In the paraconodonts, these structures began to form within the tissue, emerging as they grew. The true conodonts, however, could not grow in this fashion as additions were made to the whole of the structure and not just to the base. To resolve this problem he had placed them in fleshy pockets. It was an elegant model, not least because it drew upon real fossils and made no appeal to modern analogy (figure 12.4a). While the more speculative animals were met with silence, at least in press, Bengtson's model stimulated considerable debate, forcing Bengtson himself to make accommodations.
12.4. Conodont teeth: (a) In the mid-1970s, Stefan Bengtson gave conodont workers reason to believe again that conodont elements might be teeth. The series of images on the left, bottom to top, show an evolutionary relationship between protoconodonts, paraconodonts, and true conodonts based on changing patterns of growth. The true conodonts were enclosed in pockets from which they protruded when in use. The argument had a beautiful logic that convinced many. (b) Inspired by the anatomy of the hagfish, Priddle had earlier imagined conodont elements covered in flesh that carried a horny surface layer. (c) Landing suggested that the simple cone-shaped paraconodonts and protoconodonts might have operated together as superteeth. Reproduced with permission from S. Bengtson, Lethaia 9 (1976); J. Priddle, Geological Magazine 111 (1974); E. Landing, Journal of Paleontology 51 (1977). SEPM (Society for Sedimentary Geology).
Perhaps the most important outcome of this model was to give the conodont workers back their teeth, which for so long had be relegated to a supporting role.17 The implications were considerable, for teeth connect the animal to its environment, permitting the interpretation of feeding habit, ecology, and lifestyle. Teeth are fundamentally more exciting than hidden supporting structures of unknown function. They are visible and comprehensible – more so than any other skeletal component. When Hass first pushed the conodont beneath the “skin,” giving it only a vague function, it was as though something had been lost from the science.
Sweet thought Bengtson's solution ingenious. Almost immediately conodont workers began to imagine teeth again. As had been seen repeatedly in this animal's history, it was not the evidence of fossils alone that shaped the animal in the heads of scientists, for this evidence was never unambiguous; the animal that had thus far lived in those minds was the result of reason, familiarity, ignorance, imagining, and perhaps even desire.
Bengtson's solution was little affected by Müller and Dietmar Andres's discovery of a fused “proto conodont” cluster from the Upper Cambrian shortly afterward. The cluster's most interesting feature was the size range seen in its six pairs of simple curved and cone-shaped elements. It suggested that elements might have been added during life – a radical and potentially disrupting notion, particularly for those who built apparatuses statistically. A year later, Peter Carls interpreted these same fossils rather differently, suggesting elements might also be lost.18
Ed Landing at the University of Michigan also possessed some thirty-five fused clusters of essentially the same type. These came from a conodont animal that was unique for being known from individual elements, natural assemblages, and fused clusters.19 Landing's finds, however, forced him to see these
fossils differently. The elements were placed so close together they seemed to form two opposing “super-teeth” capable perhaps of grasping and capturing prey (figure 12.4c). He imagined these superteeth grew by the addition of conodont elements, as Müller and Andres had suggested, and thought they might be a model for the paraconodonts and protoconodonts but not for the conodonts proper. Landing also poured cold water on Conway Morris's creation, doubting that any animal could possess so small a lophophore in comparison to its size or that true conodonts had actually evolved at that time. If these were conodont-like objects they would have to be protoconodonts.
Just as lines of support had once formed behind the fish and worm, so now a division appeared between those who welcomed the return of the tooth and those attached to the filter. The latter group was headed by Lindström and supported by Conway Morris. They were joined by Robert Nicoll, who found a complete conodont apparatus in the gut of a fossil fish from the Canning Basin. Nicoll painstakingly extracted the conodont elements, each complete with a basal plate. He then argued against Bengtson's pockets, believing the conodont element was covered in tissue and that the basal plate had been attached to muscle fibers. Reversing the direction of the apparatus – something Jeppsson had done earlier-Nicoll proposed a ciliated filter feeder.20 Situated in a groove beneath the animal's head, the different element types picked up food, directed water currents, and sent food into the mouth (figure 12.5a).
Shortly afterward, Victor Hitchings and A. T. S. Ramsay from the University College of Swansea in Wales joined this group by reinterpreting Schmidt's apparatus arrangement. They imagined Schmidt's jaws opening and closing, while his gill elements became a food-filtering basket (figure 12.5b).21 Scott was also thinking along similar lines (figure 12.5c) and gave Ronald Austin an unpublished illustration in 1978, before these British workers had published, that also imagined a filtering mechanism. Presumably, this filter was to exist within the Melton and Scott animal. Within a few years, then, science had furnished the animal with every possible filtering arrangement. The kind of reasoning that had led Lindström to produce his animal now infected others.
On the opposing side, the tooth theory had been strengthened considerably by the evidence of objects that were not true conodonts but that Bengtson said evolved into them, the paraconodonts and protoconodonts. As yet, no one had confronted Lindström's important sticking point that conodonts were simply too complex and fragile to have functioned as teeth. Then fellow Swede Lennart Jeppsson entered the debate. He had long pictured teeth. Indeed, he would have preempted Bengtson's model by several years had not an editor required him to cut an inordinately long and complex paper down to size. The pages he removed were insufficient to form another paper, so they were put on a “to do” pile for later. With Bergström now back in the United States, the young Jeppsson was geographically and intellectually isolated. This made it difficult for him to judge the significance of his unpublished work. He eventually published his thoughts on the functioning of the conodont assemblage in 1979, and here he followed a familiar logic: Conodonts look like shark teeth, and like teeth in general they have a base and a crown.22 Why should they be so divided if merely internal structures? Jeppsson carried through his arguments using photographs as indisputable visual proofs. Even the most complex of conodont elements – those with problematic multiple planes of growth – seemed to have equivalents in the dental elements of fish. In essence, Jeppsson was saying if element shape is a product of function, how can we deny that conodonts are teeth if their shape so perfectly matches that of known teeth?
12.5. Conodont filters: (a) Nicoll's filter is here seen head on and slightly tilted so as to show the three groups of conodont elements one behind the other. These would have been covered in flesh and perhaps tentacles. Those nearest to us picked up the food, those in the middle directed water currents, and those furthest back directed the food into the mouth. (b) Hitchings and Ramsay's filter turned Eichenberg's gill-supporting elements into a food-absorbing basket. (c) Scott's filter is similarly arranged, though drawn the other way up: food entered through the lower opening. Reproduced with permission from R. S. Nicoll, BMR Journal of Australian Geology and Geophysics 2 (1977). © Commonwealth of Australia (Geoscience Australia) 2012. This material is released under the Creative Commons Attribution 3.0 Australia License; V. H. Hitchings, and A. T. S. Ramsay, Paleogeogeography, Palaeoclimatology, Palaeoecology 24 (1978). Scott's filter courtesy of Ronald Austin.
His concept of “teeth” was necessarily broad, including not just those of vertebrates but also the mechanical food-processing units of a range of invertebrates. He felt these functional analogies could be taken even further. The different shapes of shark teeth, for example, reflect their different functions – slicing, crushing, grasping. Perhaps it was possible to use this information to better understand the functioning of individual conodont elements and the apparatus as a whole. Grasping teeth in sharks, for example, had to resist forces operating in a number of directions. This seemed to be replicated in conodont assemblages, and it was the presence of the mineral apatite that permitted these elaborate constructions in both animals. Jeppsson's arguments were about understanding function, not, as had so frequently been the case, an argument for affinity on the basis of similar shape. His paper called for the conodont tooth to again be taken seriously; its destruction had been premature.
This paper now started a debate in the pages of the palaeontological journal Lethaia. Conway Morris was the first to enter the fray. His filter-feeding animal could not coexist with Jeppsson's theorizing or with Landing's supertooth. For Conway Morris, the “peeling back” of tissue to reveal even a simple cone – as Bengtson suggested – posed space issues. He found Jeppsson's case for teeth suggestive only as a result of selective reasoning and argued that the significant differences of scale in the objects he was comparing would affect their function. With Scott-like certainty he claimed that his was “the only fossil in which soft parts have associated conodont-like elements arranged in a pattern that is consistent with a viable hypothesis of conodont function and as such merits attention.” Jeppsson thought this latter assertion something of an overstatement – there was no certainty that Conway Morris possessed a conodont animal or had interpreted the fossil correctly. There was a resemblance but nothing more. Jeppsson simply could not see any underpinning logic in many of Conway Morris's assertions and felt they did nothing to undermine the tooth. Bengtson thought Conway Morris's arguments “traditional” (in other words, addressed in the past) and his animal improbable.23 He argued that evolutionary evidence was growing to support his model and that common examples from zoology could be used to counter the argument about space.
Hubert Szaniawski from the Polish Academy of Sciences now teased apart the curved, spine-like protoconodonts from which Ed Landing had made his supertooth and built from them an ancestral arrow worm or chaetognath. Müller and Andres had noted how chaetognath-like they looked but thought this to do with convergent evolution. To Szaniawski, even the internal structure of the protoconodonts was arrow worm-like. There were other actual and hypothetical similarities too, as arrow worms also have their spines in two groups and enclosed in a sheath when at rest. Only the phosphatic composition seemed to argue against the elements belonging to chaetognaths, but Szaniawski could show that this was a common condition in animals at the time. As Conway Morris had noted, in the early Cambrian there was an excess of phosphate in the environment. With the seemingly constant problem of chemistry dealt with, the case for the arrow worm seemed remarkably strong. The supertooth may have been demolished, but it had flagged up the difference between these more primitive forms and conodonts proper. The question remained whether Bengtson's model of evolution was still plausible and thus whether chaetognaths and conodonts shared a common ancestor. The research landscape had again shifted. Old objects had been renewed; once again there were new things to prove and disprove. Walt Sweet found himself “particularly attracted” to Szaniawski's arg
uments: “And, with the appearance of Szaniawski's elegant study, many concluded that we had finally discovered the conodonts’ roots.”24
Doubtless many conodont workers dug into their bookshelves to discover a little more about these relatively obscure animals. Ralph Buchsbaum's classic Animals without Backbones included this vignette: “In the open ocean we find transparent, slender animals, usually 1–3 inches long, that look like cellophane arrows as they dart after their prey. Though at certain seasons they occur in incredible numbers, and at such times form a large part of the food of fish, the arrow worms are members of a phylum, the CHAETOGNATHA, which has relatively few species. The name means ‘bristle-jawed’ and refers to the curved bristles, on either side of the mouth that aid in catching prey. The body is divided into head, trunk, and tail and has finlike projections, which probably serve as balancers. The brain is well developed, and there is a set of eyes. The anus is situated at the junction of trunk and tail, about a third of the way from the posterior end. The three body regions are separated internally by transverse partitions, and there is also a longitudinal partition which separates the coelom into right and left halves. The animals are hermaphroditic: both male and female sex cells arise from the lining of the coelom. The body plan is so different from that of other groups that it is difficult to say what relationships they have to other invertebrates. In certain details of development the chaetognaths resemble some of the members of the phylum to which man belongs.”25 One could imagine the conodont animal again.
By the end of the 1970s, the enigma of the conodont had reached the peak of anticipation. The desire for, and elusiveness of, this real animal, had caused the production of numerous imaginary ones. In paleontology courses around the world, lecturers would regale their students with the tale of the science's great enigma. It demonstrated better than anything else that nature still possessed great mysteries despite the best efforts of science to defrock them. In the middle of this decade of imagining, the second Treatise was written, devoted this time wholly to conodonts. In it, Müller provided data essential to the conodont myth: In the course of 120 years of study the conodont had generated fifty-three possibilities for what it might be. All were in some respects products of the imagination, invented forms of life. As Müller concluded, the true nature of conodonts remained “one of the most fundamental unanswered questions.”26 He could not know then that the answer was just around the corner.
The Great Fossil Enigma Page 33
