The Great Fossil Enigma (49 page)

By 1986, everyone seemed to agree that the chaetognath was off the menu and some went so far as to imagine the chordate as a mere waypoint as the animal rose to join that most exclusive of clubs, the vertebrates. The idea seemed to be gathering momentum, though Godfrey Nowlan and David Carlisle were of the opinion that craniates were a step too far. They believed the evidence suggested conodonts might be better placed with amphioxus.
²³
But then, as if from nowhere, Simon Tillier and Jean-Pierre Cuif, from the Centre National de la Recherche Scientique
(CNRS)
in Paris, threw a group of poorly known worm-like mollusks with spiny coats into the ring. These were aplacophorans. Janvier, who had been taken with the chordate, now changed his tune; this new group looked just as convincing.
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He wrote to Briggs to inform him of his conversion. “I have let myself be convinced by one of my colleagues,” he admitted. “Actually his arguments are striking.” But having just seen Briggs and company's latest paper, in which the conodont animal now had a “very convincing” tail and “very large notochord,” he was in something of quandary. He admitted, “Your new evidence make[s] me hesitate again…. ‘Entre les deux mon coeur balance!' [Between the two my heart balances].” He then joked, “Of course, craniate affinities would be a more noble pedigree than mollusc affinities, and the Society of Vertebrate Paleontology would suddenly increase by hundreds, but, seriously, I think we are coming now very near to a definitive solution, and the chaetognath affinities (at least for true conodonts) can now be ruled out.” However, he enclosed a paper, shortly to be published with Tillier, on the molluskan alternative. He told Briggs it was “intended to provoke you and raise a discussion (if not a debate), and we would like you to respond [to] it.” Briggs, Aldridge, and Smith did so. Indeed, they saw it as unhelpful speculation and moved fast to close it down. Bengtson added his support, placing their rebuttal on the opening page of the next issue of
Lethaia.
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These bizarre and unexpected leaps by outsiders, which drew the animal into associations with the unknown, exotic, or rare, would remain a constant hazard for the conodont while the animal retained traces of its former enigmatic self. The history of the animal had, at its most superficial level, merely been a realization of that human capacity to see resemblances in things. Briggs and his colleagues were convinced that Tillier and Cuif's arguments also rested on the same kind of looka-like illusions. However, Alain Blieck of the Université de Lille wrote to Clarkson expressing his doubts about the recent interpretation of the animal as a chordate. He felt the histology of the conodont elements simply did not support this interpretation and thought Tillier's exotic slug much more plausible. He accused Briggs and his colleagues of pursuing superficial resemblances.

Janvier admitted to playing devil's advocate in the hope of forcing upon Briggs, Aldridge, and their collaborators a desire to find more convincing evidence. The British team was out in the open and now actively defending a view that the conodonts were chordates. They dug in, their defensive position shored up with new material brought in from the rear. Here they were aided by the fortuitous rescue of blocks of the Granton Shrimp Bed in 1985. When Clark worked through this material, between May 6 and 8, 1987, he once again astounded his colleagues by finding another five conodont animals. Although remaining largely in the background, Clark's staggering talent for finding these fossils made him the most important cog in a British engine that was now trying to separate fact from fiction. As Aldridge, Briggs, Clarkson, and Smith studied these specimens, the question that concerned them was whether these new fossils possessed the items on their shopping list.

Before these new fossils could be loaded into arguments, Walt Sweet's monograph,
The Conodonta
, appeared. It was the first solo treatise on these fossils since Lindström's creative little book of 1964.
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Like Lind-ström's book, Sweet's was personal, provocative, and a little experimental. It was, however, published in an entirely different era. In 1964, there were great chasms in the understanding of these fossils. The young Lindström was at the start of his career and writing the book forced him to develop a more holistic knowledge of his subject. While many of the Swedish old guard thought book writing an inappropriate activity for a scientist, his German colleagues had welcomed it. And for all its individualism, Lindström's capacity for careful study and lateral thinking had produced a number of immensely useful ideas. When Sweet's book was published in 1988, the science of conodonts was fully formed; never had it been so certain of itself. Sweet was then in the closing years of his career and had found himself collared at a conference by a very convincing woman from Oxford University Press. (Lindström had been similarly collared.)

No one could doubt Sweet's command of his field or the remarkable and forthright contributions he had made. Like Lindström's, his had been a shaping hand. But he was, unlike Lindström, willing to fight for what he believed was scientifically right. The publisher may have assumed that Sweet was going to write a summary textbook – a modern treatise, but he had never been associated with pedestrian science. He was used to thinking afresh and quite happy to take the controversial path, if he thought it was the right one to take. He had no patience with lazy or poor science. In 1988, he was still looking forward, still wishing to shape and influence. He set about writing a book that reflected how he saw the conodont world: “I was just trying desperately to put a lot of things together, and some of it was half baked and some of it was not.”

Sweet would never claim to be a
palaeobiologist
, even if his work on statistical assemblages paved the way for the science to recover its biology. However, no author of a book on conodonts could ignore the animal. It was the hot topic of the moment and he could not deny the exciting new evidence provided by the Scottish animals, but he said that on many key points – points on which Briggs and company had been only too happy to draw conclusions – these objects remained “mute.” Sweet willingly conceded to tentative conclusions that the fossils might preserve segmentation akin to the muscle blocks (myotomes) found in amphioxus and fish. What he found objectionable was the leap of interpretation that turned them into somites, with all that that might mean for possible vertebrate affinity. He thought it a conclusion “in no way required by the evidence.” This leap, he suggested, led into a “much more subjective discussion of anatomy.” The British workers had located a “head,” but where was the brain? There was no evidence for advanced head structures, and Sweet preferred a less suggestive terminology. He simply could not understand why Aldridge, Briggs, and others had seen a chordate animal.

Sweet suggested that “chordates are not the only organisms that might yield such V-shaped impressions on compression.” Could these impressions not belong to a flatworm (nemertine)? The modern nemer-tine had many of the characteristics preserved in the Scottish specimens. Sweet was not, of course, the first to suggest this relationship. And why had the British workers interpreted the lines running down the center of the body as they had? There were other possibilities here too. He felt that “Aldridge et al. allowed their interpretation of the Scottish specimens as primitive chordates to restrict their survey of anatomic possibilities” and permitted them to preferentially see “a nerve chord, notochord or dorsal aorta.” Might the preserved “lines” at the front of the animal represent the nemertine's eversible proboscis, he wondered, while those preserved at the rear perhaps record the gut? If so, then that gut reached the tail of the animal. This was an invertebrate feature as it exits the body before the tail in vertebrates.

Sweet also took issue with Jefferies' mysterious “s”: “marine habit, eel-shaped body, head-trunk-tail, rasping teeth, ?lensless paired eyes, ?somites.” How many other animals offered this model? “I suggest that, as Jefferies intimates, the resemblance between the hypothetical animal ‘s' and the Scottish conodonts maybe so striking because the latter were used as a general model for the former.”

Sweet shared the view that the teeth possessed by hagfish merely resemble proper teeth; they bear no evolutionary relationship. They could not assist in giving the conodont chordate affinity, nor could the phosphate, the element shape, basal material, chevron impressions, or models of element function. All were unreliable. The proof, he said, was to find the notochord, gill slits, or dorsal nerve cord.

Sweet's outlook was framed by
Wonderful Life
spectacles. He had had a chance to discuss conodonts with Stephen Jay Gould on two occasions when lecturing at Harvard. The thrust of Sweet's book, which promoted the idea that conodonts were a distinctive form of life, arose from these conversations. To him, the attempts of Briggs and friends to pigeonhole the conodont in the chordates was simply illogical. He believed paleontology was moving in the opposite direction, away from blinkered classifications and presentist attitudes: “It has several times been suggested that even the venerable Chordata is possibly no more than a shaky confederation of invertebrates and vertebrates.”

The key message in Sweet's arguments was not, however, about the particulars of the animal's biology but the boundaries and frailties of interpretation. In this respect his arguments read like Richard Owen's chastisement of Pander. Owen simply could not comprehend, given all the evidence to the contrary, why the Russian had thought the animal a vertebrate.

We don't know what Pander thought of Owen's opinion, but Aldridge and Briggs were certainly not the kind to take these criticisms lying down. They had been accused of producing mere “waffle.”
²⁷
When Sweet saw that Aldridge and Briggs had together written a six-page review of his book, he must have felt a little concerned. Book reviews are almost without exception written by individuals, not duos. Aldridge and Briggs were, however, gentle. Aldridge certainly knew, and had the greatest respect for, Sweet. And he was probably not that surprised that Sweet had produced a book that was as “provocative” as it was “controversial.” It was, they said, food for thought.
²⁸
But by the third page, their sleeves were rolled up: “Having been involved in presenting these candidates [the Scottish animals] for consideration and in using them to develop a hypothesis of chordate affinity…we have a particular interest in how they are treated in this milestone in the conodont literature.”

Their assault began by countering Sweet's
Wonderful Life
view of the distant past. Now became clear the significance of Briggs's involvement in the review. It was Briggs, with Conway Morris, who had populated Gould's world with a myriad new life forms, which in turn shaped Sweet's outlook. But the tide was on the turn; Briggs admitted that some of these problematic animals were indeed finding a place in long-established phyla. This gave validity to his and Aldridge's attempts to understand the conodont animal's chordate affinity and suggested that one should not buy into Gould's vision unquestioningly.

Now the five newest animals entered the debate for the first time, for they confirmed the “V-shaped” structures that the two men were convinced were unlikely to be found in preserved nemertines. They were very like chordate muscle blocks seen in amphioxus and hagfish. Sweet had, of course, made his arguments without possessing the animals – old or new – and thus it was rather easy for Aldridge and Briggs to undermine his interpretations; the fossils simply said otherwise, they said: “The characters of the conodont animals, such as phosphatised elements, a transversely operating feeding apparatus, V-shaped somites, a laterally flattened trunk, and ray-supported posterior fins extending further along one margin than another, all bear comparison to chordates.” The new specimens enhanced this view. The problem, they felt, was that Sweet had become wedded to the view that the conodonts represented a separate group and thus he looked for differences rather than similarities with other forms of life. Of course, only a few years earlier, confronted with the ambiguities of the first fossil, this had also been their refuge. In just a few years, however, the science had fundamentally changed and, they believed, it was no longer appropriate to think these old thoughts.