The Great Fossil Enigma

by Simon J. Knell

  All these discoveries were sufficiently in tune with the real world to attract the interest of journalists and consequently several made the national and popular scientific press in 1994 and 1995. Janvier's sometime collaborator, Peter Forey, of the Natural History Museum in London, admitted to New Scientist, “For many years, we as vertebrate paleontologists did not want anything to do with conodonts.”24 Now the evidence was irresistible. Janvier's resistance to the conodont craniates had been crumbling since early 1994, and in this same issue of Nature, faced with the weight of mounting evidence, he finally capitulated. In an article titled “Conodonts join the club,” he wrote, “I think it is time for me to stop playing the Devil's advocate against this theory. There are still peculiarities in the structure of the ‘conodont animal’ that are at odds with what we know of classical early vertebrate fossils, but its tail, arrangement of the muscle blocks and large eyes are strikingly vertebrate-like.” He was, however, a reluctant convert. Sansom and company's assertions regarding tooth structure seemed to him to be an act of “shoehorning,” though he was rather more convinced by the recent evidence of dermal bone and dentine. The conodont remained problematic and, like the hag-fish, he suggested that it might have evolved from a conventional early vertebrate. Like Sweet, he was puzzled by the things that had not been found. Why was there no evidence of the dense organic matter associated with gills (“no other fish lacks gills”)? This was associated with another oddity: the positioning of the eyes in front of the mouth. Perhaps the front of the animal was missing, he said, and the conodont elements were associated with the pharynx and gill apparatus rather than the mouth, “like the pharyngeal denticles of jawed vertebrates”?25 This would actually make the conodonts more vertebrate-like and explain the lack of gills: “Although the conodont animal is reconstructed as a small, frail, almost transparent animal that reflects the classical ‘archetype’ of the ancestral vertebrate, it may, after all, turn out to be a highly specialized close relative of the jawed vertebrates that developed suction feeding and pharyngeal denticles to grasp and shear prey.”

  The focus of Janvier's argument had now shifted to the positioning and functioning of the apparatus. In Leicester, Aldridge and Purnell had recruited a new doctoral student, Phil Donoghue, to take this study forward by looking for evidence in the elements themselves. Part of the problem concerned how the elements grew: How could they function and show wear and yet still grow? This, more than anything, challenged the notion that these really were teeth, particularly now that Bengtson's pockets looked increasingly untenable. Donoghue's study, which reviewed and built upon the complete literature examining the interior structure of the fossils, is remarkable testament to the seriousness with which the biology of the animal was being considered.26 The detail of its probing was unparalleled, even in comparison to those pioneering light and scanning electron microscope studies of just a few decades before. Now these belonged to a distant past. “Without any degree of constraint over affinity this proved an unprofitable line of research, resulting in a series of esoteric accounts of hard tissue ultrastructure,” Donoghue reflected. “In retrospect, it would never have been possible to reach an unequivocal conclusion regarding conodont affinity just by analyzing element morphology and internal structure alone.”

  The science had never put this level of resource into such a seemingly tiny question, but now rather more rested upon its conclusions. The arguments were detailed, but there was a belief that there was a possibility of discovering still more. So Donoghue examined growth across a range of element types, looking for patterns that might separate analogy from homology. The blades, he found, began by growing lamellae but almost immediately began secreting white matter, which formed the core of all denticles, the number of which was extended during growth. Later growth was concentrated in the lower portion of the element, changing its shape. The final stage saw the crowns “finished off” with lamellar tissue and growth of the white matter was halted. Other types of element showed rather more complex and varied growth patterns.

  Donoghue was also interested in understanding how these growth processes had changed through the course of the creature's evolution. As to what his data meant, Donoghue permitted his interpretations to be shaped by the “current consensus,” which said that conodonts were chordates. But he took issue with Sansom and colleagues’ suggestion that the animal experimented with tissue structure. Instead he argued that dentine itself was so structurally variable there was no need to believe this variety resulted from experimentation. The conodonts’ unique white matter remained difficult to interpret, but Donoghue thought it rather more dentine-like than like cellular bone. White matter, it seemed, had its own complex evolutionary pattern; it had not been adopted in the same way, or even at all, across the conodont group as a whole. Clearly it strengthened the element, but it was difficult to say more.

  As to the problem of growth, he thought the answer lay not in considering vertebrate teeth – which cannot repair the functional enamel once erupted – but in scales. He noted that in many fish, the scales are drawn back into the dermis for additional growth before re-erupting. Conodont elements were not, then, teeth in the strictest sense, even if they functioned as such. He also postulated that, like the lamprey and hagfish, the conodont animal probably had a skeleton composed of cartilage. As cartilage is rarely preserved in the fossil record, this assertion seemed to him quite reasonable. His interpretation repeated suggestions made by Aldridge and his collaborators back in 1986, but they could not prove them then.27

  Meanwhile, Purnell had been involved in reconstructing the Soom Shale animal's apparatus using the modeling methods Aldridge and his co-workers had exploited in the mid-1980s.28 In 1997, along with Donoghue, Purnell presented an operational model of the animal's apparatus. This produced an arrangement of elements suggesting an analogy, in terms of its operation, to the jawless lampreys and hagfish. They imagined a hideous “eversible lingual apparatus,” once held together by muscle and cartilaginous plates.29 Its spiteful fine-toothed elements were arranged to project outward in a grasping motion, redolent of and perhaps influenced by Ridley Scott's formidable creature in Alien (1979) (figure 14.2).30

  In 1995, the Leicester and Birmingham groups could celebrate a victory. The Leicester workers en masse did so in an article titled “Conodonts and the first vertebrates” in the popular science journal Endeavour. “Some time ago,” they began, “probably during the early part of the Cambrian Period (520 million years ago), a new type of animal appeared. It was small, a few centimeters in length, and elongate; it had no hard skeleton, but a stiffening rod of cartilage along its back and V-shaped blocks of muscle along its sides; it had paired eyes, a brain and tail fins. It was the first vertebrate.”31 This notion of the ancestral vertebrate was not the conodont animal, for all that it looked very like one. Rather, it was a long-established and widely held concept, stemming originally from drawing a line from amphioxus, which was a vertebrate-like invertebrate, to the hagfish, seen by some as the most primitive living vertebrate. Recent work by British workers had shown that the conodont did not lie on this line but was more advanced – more a vertebrate – than the hagfish. Now the Leicester team explained the significance of the conodont animal and the debate that surrounded it in plain language. The textbook view was that the first vertebrates were suspension feeders living lives comparable to amphioxus and larval lampreys. According to this explanation, they only became predators a hundred million years later. But Purnell considered a predatory condition was necessary for the evolution of eyes, muscles, and skeletons. On this latter point, they too would assert that the vertebrate skeleton began not with defensive armament but with teeth. Although there were still some anatomical uncertainties about the animal, these new vertebrates nevertheless had impact for they had existed for some three hundred million years and had left an unparalleled record of their time on Earth. The article was illustrated with a photograph of conodont elements on a pinhead. One of the most strikin
g images the science had ever produced, it at last permitted the general reader to visualize these obscure fossils in everyday terms. It soon appeared on magazine covers and websites, wherever the science needed to explain itself to a wider public (see this book's frontispiece).

  14.2. Alien jaws. Purnell and Donoghue's reconstruction of the animal's formidable apparatus seen head on. Photo: Mark Purnell, University of Leicester.

  Purnell translated this Endeavour piece into an article for children called “Armed to the Teeth.”32 Here, too, the “weird” conodont was a new and exciting clue to that mysterious vertebrate ancestor from which we sprang. “What, I hear you ask, is a conodont? It's a good question, and until a few years ago no-one really knew the answer.” The conodont was made for this dinosaur-loving audience: “One of our early ancestors was a small but very vicious killer!” That killer was illustrated, though it unfortunately lacked Alien menace.

  In another article, “The conodont controversies,” Aldridge and Purnell disseminated their new vertebrate among evolutionary biologists, most of whom knew little or nothing of the animal. Gee had remarked that modern evolutionary biologists used methods that encouraged a presentist view of the past; they made their decisions based on what was now living rather than on an understanding of the extinct. It was a problem Simpson had recognized back in the 1930s and it had not gone away. But modern biology could not invent the conodont. For one and half centuries the animal had fought for its place among the vertebrates simply by virtue of its tooth-like remains, struggling against doubts that suggested it was simply too old and that its teeth were not as tooth-like as they might superficially seem. As the conodont entered this new world of vertebrate biology, it lost none of its controversy; it appeared from nowhere possessing a rich evolutionary history supported by millions of specimens, and it sought to significantly rewrite history. Of course, Aldridge and Purnell's biggest problem was a lack of certainty about where the conodonts fitted in the vertebrate family tree. But this was true of many of those animals lacking a backbone yet applying for admission to the vertebrate camp. In an attempt to court this new community, Aldridge and Purnell paraded six possibilities for placing the conodont in the tree of vertebrate life. In their Endeavour article, they had restricted themselves to one. Of course, a great deal depended on what one might call a vertebrate, but they argued that “authors who have worked on the conodont animal specimens are united in the view that the conodonts belong within the Craniata (= Vertebrata, if the myxinoids are included in vertebrates [)].” They did not point out that these authors belonged to one extended family, but that mattered little. If other scientists were to adopt the conodont, they would do so on their own terms, just as Janvier did. Aldridge and Purnell's paper revealed that there was still much to play for. As Gee noted, “On this evidence, at least, it seems clear that conodonts represent a very early radiation of ‘jawed’ vertebrates, possibly distinct from the radiation of more familiar jawed vertebrates…. In which case, they need not tell us very much about vertebrate origins as such.” But on this, he admitted, the jury was still out.33

  14.3. The conodont animal as it is imagined today. The animal has been depicted in a range of guises, both comic and factual. In 1993, an image appeared that had the animal possessing huge eyes, an error introduced by a picture editor who flattened a perspective drawing of the animal swimming toward the viewer. This encouraged Forey and Janvier to suggest that the animal might be a larval fish. Nevertheless, the eyes in the Scottish animal were relatively large, which Purnell explained as representing the minimum possible size for functioning vertebrate eyes. The strap-like eye muscles in Gabbott's beast suggested the animal could rotate its eye – a feature only found in vertebrates. Positioned on the front of the head, the eyes also suggested to Purnell that the animal was an active predator. New reconstruction of the animal by scientific illustrator, Debbie Maizels. ©Debbie Maizels/Simon Knell.

  These new interpretations were also finding their way into standard textbooks. The author of many, Bristol paleontologist Mike Benton, referred to the conodont as a microvertebrate: “The first fishes date from the Late Cambrian, and the commonest group then, and in the Ordovician, were the conodont animals.”34 Textbooks produce orthodoxies. Students learn and never forget that first act of learning. The conodont was a vertebrate: established in science, won by campaign, and evangelized across the media.

  Of course, the battle was not entirely won, even if it had become the orthodox view. There was still the matter of tree building and negotiating that boundary where life could be said to become “vertebrate.”35 The territory that had been won for the conodont animal in this campaign continued to be defended and strengthened by those who had fought for it.36 The new orthodoxy continued to be consolidated. At the conodont's sesquicentennial birthday celebrations in Leicester in 2006, Dick Aldridge suggested it was time to stop beating around the bush and call conodont elements “teeth.” He believed the vertebrate skeleton originated in conodonts and that they were our earliest “stem ancestors.” The concept of stem ancestors here meant the conodonts were the first branch on the bough of an evolutionary tree which had jawed vertebrates at its end.

  Among the two hundred or so active conodont workers then on the planet, beliefs varied about what the animal was. Each mind held a slightly different animal. Nearly all believed in the chordate, though some preferred amphioxus as a model.37 Only a tiny group had taken these thoughts into press. In 2006, the conodont controversies had shifted slightly more to consensus, not least because the mountain on which the conodont had been placed seemed unassailable – its defense was too strong. But as the heat dissipated, those who had built and defended that mountain became drawn into new projects and other animal groups. With Aldridge only a few years from retirement, was the science again to change?

  Not long after this meeting, I returned some borrowed papers. Dick Aldridge looked at me with a wry smile on his face. “This will be good for your book,” he said. He showed me a short paragraph in Ordovician News that reported that three Australian protagonists – Anne Kemp, Susan Turner, and Carole Burrow – had “initiated a polemic paper on the relationships of conodonts with vertebrates, to develop the idea that conodonts are NOT vertebrates. This is agreed in majority by the early vert experts, and perhaps also by many (if not the majority of) conodont experts. This paper will be co-authored by a group of people including Turner, S., Burrow, A., Hanke, G. F., Männik, P., Nowlan, G. S., Reif, W.-E., Rexroad, C. B., Trotter, J., Viira, V. & Young, G. C. (provisional list).”38 Sometime later, he sent me the abstract of a paper by Alain Blieck, Susan Turner, Carole Burrow, Hans-Peter Schultze, Wolf-Ernst Reif, Carl B. Rexroad, and Godfrey Nowlan's paper explaining “why conodonts are not vertebrates.” It stated, “Excessive self-citation, mis- and over-interpretation by the ‘British School’ in association with a blanket of publication and communication have wrongly established an acceptance by the scientific community in journals, textbooks, websites and popular articles that conodonts are vertebrates.”39 It was a manifesto for political change rather than a scientific argument. The list of signatories included a cross section of the conodont community, many of whom had never engaged in debating the animal's biology. Others had been longtime objectors and had proposed a number of alternative solutions to the enigma. They now formed a confederation that was the equal of the British school. The controversy of the animal was once again alive, and had Christian Pander somehow returned to Earth, he might well have laughed at the irony of it all. For more than 150 years there had been so much scientific effort – and so much dreaming, imagining, and arguing – and yet here we were back at the beginning, or so it seemed to some. As had been the case so many times before, two groups faced off against each other. One, possessing the evidence of real things, assumed Pander's imaginative self-assurance. The other, by contrast, felt it possessed the truth of the known world. It now adopted Owen's uncompromising stare.

  In keeping with its enigmatic qualities
, the conodont animal truly was as slippery as an eel. At the slightest provocation it seemed to retreat into a Kafkaesque world of disorientation and uncertainty. Aldridge's smile said a lot. He had delighted in the science but also in the debate and controversy. The animal had been an event. It had been fun. And, most important, it had lost none of its enigmatic charm. That too appealed to Aldridge. Was it possible that El Dorado was not here but somewhere in the distance? Some seemed to think so. Perhaps it is still possible to dream of solving the riddle of the conodont, to imagine that somewhere, in “some layer of sub aquatic volcanic ash,” one can find El Dorado.

  It was so small, such a tiny, early, transitional mass, a coagulation of the unsubstantial, of the not-yet-substantial and yet substance like, of energy, that it was scarcely possible yet – or, if it had been, was now no longer possible – to think of it as material, but rather as mean and border-line between material and immaterial.

  THOMAS MANN,

  The Magic Mountain 1928

  AFTERWORD

  The Progress of Tiny Things

  THIS BOOK TELLS THE STORY OF A SCIENTIFIC JOURNEY OF TWISTS and turns through assertions and denials, past alien monsters and incoming asteroids, through a world of unexpected discoveries and real utility, which ultimately arrives at an animal that, rather surprisingly, seems to say something about our own ancestry. In the course of all this traveling, countless animals formed in scientific minds only to dissolve, replaced by new apparitions. The fossils themselves were so small, that seeing them – really seeing them – was no easy matter. Indeed, the millions of these things present in collections around the world today might, if poured like so many grains of sand, fit into a few shoe boxes. We could be forgiven, then, for not knowing the conodont. But my aim in writing this book has not simply been to record a famous episode in the folk history of a science. Being so tiny and evocative, these fossils possessed a chameleon aspect. El Dorado–like, they pulled their victims into mirror-filled rooms, ensnaring their thoughts with mirages and illusions. Vanishingly small and impossibly ambiguous, they occupied that borderline between material and immaterial, never wholly one or the other. If one was not careful, it was possible to see in them what one wanted to see. If that was the case, then what they saw was no more than a thought, an imagined thing conjured into existence. If what the conodont workers thought they saw was then transcribed onto paper, one has, in these representations of the fossil, a tiny lens on the minds of those who found themselves inextricably entangled with these enigmatic material things. It was with this thought that I began this book, and consequently my focus has been on fossils of the mind rather than their physical counterparts, for it was only in the scientific mind that these objects acquired their magical properties.