The Great Fossil Enigma (48 page)

However, what was more noteworthy about this discovery was the lack of sensation. The Scottish animal had acted like a switch: In an instant the sensational unknown had simply become an animal. Had this new Silurian animal been found a few years earlier, in the hands of a Scott or Conway Morris, perhaps it would have grabbed more headlines. It was nevertheless a topic of conversation for those one hundred conodont workers from twenty-nine countries who met at the European Conodont Symposium held jointly at Ronald Austin's University of Southampton and Dick Aldridge's University of Nottingham in July 1985. Aldridge was now adept at selling the importance of a fossil that could never, by itself, roar like
T. rex.
In the run up to the conference, he told the press, “The importance of the animal is not only that it helps solve one of geology's enigmas, but that it fills a ‘missing link' in our evolutionary knowledge – it is quite possible that conodonts had a place in our ancestry.”

This meeting was an opportunity to discuss the latest work being done on the Scottish animals and to further reflect on the meaning of those things the science had long known, such as the natural assemblages. On these latter, Briggs and Aldridge had been among the first to see new potential. Briggs had been in the States in October 1983 and met with Norby and saw some of Rhodes's specimens and Ernest Paul Du Bois's assemblages-which Briggs recognized as being outstanding. He also discovered that Norby was completely tied up with other work and unable to publish his 1976 thesis in which, according Aldridge, Norby had unscrambled the architecture of the conodont apparatus better than anyone. Briggs's solution to this problem was to suggest incorporating parts of Norby's thesis into a paper by Aldridge, Briggs, Norby, and Smith. However, he made no mention of this to Norby but understood that he would probably be amenable to such a plan. Briggs had also been chatting up other conodonts workers, such Dave Clark in Madison, and discovering other new research on assemblages. He also wondered if they might look again at Bear Gulch: “If Scott's animals were eating our creature the latter should have been preserved as well.”

Aldridge had later traveled with his postdoctoral research assistant, Paul Smith, to the United States, where they examined the Illinois assemblages, collected new specimens, and discussed the possibilities of collaboration with Rod Norby with the aim of producing a refined architectural and functional model. This work was reported at the Nottingham meeting by Aldridge, Smith, Norby, and Briggs and introduced a new orthodoxy which resulted from an elegant experiment to reconstruct the three dimensional apparatus from the flattened fossils.
¹⁴
Previously, it had been assumed, explicitly or implicitly, that the variety of assemblage patterns had resulted from muscular contraction following the death of the animal (rigor mortis) and the subsequent collapse of the apparatus on decomposition of these muscles. Aldridge and his collaborators set out to discover if the various configurations could, in fact, be explained by the simple collapse of an apparatus without invoking muscular contraction. If so, it would be possible to discover the arrangement of the apparatus in life.

Briggs and graptolite specialist Henry Williams had, a few years before, developed a method that could reverse this process of collapse – at least visually.
¹⁵
It involved building models – physical models with wood and card – and photographing them from various directions. By this process, the three-dimensional object was translated into a two-dimensional photograph, so mimicking the collapse of the fossil onto the bedding plane. Aldridge and Smith built a model conodont apparatus from modeling clay, informed principally by the arrangement of the elements in the known animals. By trial and error, they arranged and rearranged the elements and photographed them from every angle. From this, they produced a single architectural model that could, by collapse in different directions, produce all the known configurations of elements seen in the fossils. The resulting arrangement of the component elements was startlingly new. It revealed an animal that was laterally compressed and therefore unlikely to be an arrow worm, and in which that the elements were not strung out in a linear array as had long been imagined. What was particularly alarming was that the arrangement challenged their original assumptions of an animal with grasping and processing teeth. Now not all the elements opposed each other. They could not have functioned like teeth. The solution to this problem was to do as Bengtson had done and presume that the elements occupied different positions at rest and in use. The team appealed to the analogy of the hagfish, suggesting that in use the elements may have been rotated by as much as ninety degrees. While celebrating the radical new insights that had arisen from so simple a technique, Aldridge and his colleagues realized that understanding the functioning of the apparatus required more work. Smith, Briggs, and Aldridge also applied this same approach to the Waukesha specimen and concluded that the apparatus was indeed like that seen in the hagfish at least in terms of its symmetry.
¹⁶

Aldridge opened the book,
The Palaeobiology of Conodonts
, which arose from the conference, with a historical review. He did so not to revisit the myth, which seemed to belong to another era, but to empty minds of misconceptions and architectural arrangements that now had little to support them. It was an essential precursor to introducing the new. It meant the evidence of the animal need not do battle with stubborn fictions and old illusions. But he knew that to carry this argument off he would need to interrogate the past, and the objects that had produced it, with forensic precision. With eyes trained through studying the growing collection of animals, Aldridge disentangled each interpretation, rotation, re-orientation, architectural arrangement, and opposition of conodont elements. He could locate lines of influence. He noted, for example, that Schmidt had essentially reconstructed what he saw in his best specimen. Rhodes had had access to Du Bois's fine material and, influenced by one particular specimen, produced diagrammatic interpretations that reversed the direction of the component elements and included some minor repositioning. And Rhodes, through his control of the section on apparatuses in the two
Treatises
, had then turned his thinking into the orthodoxy.
¹⁷
By exposing the origins, material basis, and frailties of this thinking, Aldridge made way for the new orthodoxy he and his colleagues now presented later in this book. Like Scott before him, his possession of the animal fossils gave him a badge of authority.

Others speaking at the conference were not so fortunate. Nicoll was still making do with fused clusters, and working with Carl Rexroad re-examining Pollock's specimens from northern Indiana. Nicoll thought the rapid and preferential evolution of the platform elements at the rear of the apparatus particularly perplexing. What selective pressures could have caused this? These elements seemed to interlock or intermesh and thus must have worked in opposition. As these were fossilized together, he doubted Bengtson's suggestion that the structure was protruded when in use.
¹⁸
Nicoll imagined the conodont animal as being something like the amphioxus, which lived for up to eight years, spent two hundred days in the open ocean, and was capable of travelling over eight thousand kilometers. Amphioxus has cirri acting as a sieve and a strange ciliated wheel organ that directs food into the mouth. He imagined that the conodont's varied elements reflected similarly diverse actions in the filter-feeding animal.

Szaniawski was also at the conference. He had been undertaking comparative studies of the fine structure of conodont and chaetognath elements. This work had changed his position on conodont ancestry markedly. He no longer thought true conodonts were chaetognaths but that both groups might share a common ancestor. This did not stand in the way of the conodont animal being a chordate, he said, because there were so many uncertainties about the biological relationships of chaetognaths themselves.
¹⁹

Clark and Hearty's fossils, found in 1984, only saw the light of day in a paper two years later. These new animals confirmed earlier interpretations, turning indistinct or uncertain features into reliable characters. Much of the tentativeness of the first paper now disappeared. Segmentation was a demonstrable fact and composed of numerous V-shaped “somites,” Aldridge, Briggs, Clarkson, and Smith asserted. Embryological studies had long ago demonstrated that key structures in the vertebrate body form from these “primitive segments,” or somites, including vertebrae, muscle, and the dermis of the skin. Du Bois, whose own fossil discoveries had led him long ago to postulate an animal very like those found in Scotland, imagined different kinds of conodont element being produced in this way. This did not stop Du Bois imagining the animal as a relative of those annelid worms that were then so fashionable in Illinois, but for Aldridge and his collaborators these segments seemed to confirm the chordate. They hypothesized that the animal was, like the hagfish, a jawless craniate.
²⁰
The hagfish possesses a partial cranium or skull but no true backbone. The conodont animals simply possessed their apparatuses. The authors now took that big step and claimed that there was no longer a need to retain the conodonts as a separate phylum and drew up a shopping list of wanted diagnostic characters: the nature of the tail fins, the position of the anus, and structures of the head. The British team now began discussions with experts in cyclostomes, including Derek Yalden in Manchester and Richard Krejsa in California. Aldridge and company were looking for homologues that might connect the feeding apparatus of conodonts with those of the hagfish and lampreys. To aid discussion, they sent out proofs of the article describing their new model of the apparatus. However, Yalden told Aldridge that the model was even less convincing as the jaw apparatus of an agnathan fish than the old one.

The question of possible chordate affinities was also being independently tested by Dzik, who knew little or nothing of the newest Scottish animals. His arguments were partly based on an extraordinary Lower Devonian assemblage described by Soviet worker Tamara Mashkova in 1972. This seemed to preserve part of the three-dimensional arrangement of the apparatus. The elements were arranged symmetrically facing inward, with the long axis of each element placed vertically, the sharpest elements to the front and the most robust toward the rear. This interpretation seemed to Dzik to be corroborated by later finds and, indeed, by the first animal. But Dzik had no animals himself and thus no soft tissues from which to develop this idea, so he turned his attention to the elements. Reviewing past research into the evolution of enamel and dentine in early vertebrates, Dzik wondered if modern assumptions about bones and teeth were clouding the interpretation of the elements. Surely these materials would have evolutionary precursors. Would they have been structurally different in the past? It was this kind of reasoning that had encouraged Pander to invent his conodont fishes. Now Dzik suggested that dentine was the primitive condition of the hard parts in vertebrates and that bone evolved from it. He thought that even more primitive animals might have possessed an enamel-like substance.
²¹
There had long been discussion of a homology – an evolutionary connection – between dentine and enamel in vertebrates and base and crown of conodonts. Dzik showed that these structures developed in many different ways in primitive vertebrates and argued that later kinds of dentine and enamel should not be expected in earlier animals. This thinking put Dzik ahead of the game. He was following a logical trajectory, and one rather more reliant on technological innovation than a supply of animal fossils. Dzik was confident: The conodont was a true vertebrate, and he could imagine it evolving a catching apparatus, composed of conical elements, which was then lengthened due to selective pressures. This he thought reflected a shift from a bottom-dwelling scavenger to a carnivore of open waters.

As if to consolidate a view now in the ascendancy, in 1986, Richard Jefferies of the Natural History Museum in London, an expert in the fossil ancestors of sea urchins, published
The Ancestry of the Vertebrates
, in which he envisaged a hypothetical creature, “s,” which represented the first or “crown” vertebrate from which all others were descended. The creature, he said, was like the conodont animal!
²²
In the year Jefferies published his book he wrote to ask Aldridge for a copy of his most recent paper: “As one of the first people to advocate (though only in a speech) that conodonts should be compared with the lingual apparatus of myxinoids, I am happy to discover that I was right.” Clearly the conodont was for him a foreign land.