The Great Fossil Enigma (53 page)

The worm – the worm that had grown out of necessity in response to the discomfort caused by Pander's vertebrate – was now very dead and the reinstatement of Pander's imagined fish in full swing. However, for many involved in the debate, elevation into the hallowed realms of human sisterhood depended not simply on the fossil but also on what one could
call
a vertebrate. This had long been disputed territory, and the current view promulgated by Janvier was that the vertebrates consisted of animals with a cranium and vertebral elements; those which just possessed the cranium were then referred to as craniates and not considered vertebrates. It was, however, a messy division. What Aldridge and his collaborators required was a chink in Janvier's armor, and they found that chink by appealing to “common usage,” which placed Janvier's craniates in with the vertebrates. The argument, which came from developmental biology, drew upon a shared feature present in the embryos of all vertebrates: the neural crest, a feature from which vertebrae, cranium, and other defining elements emerged. Aldridge remained convinced that the conodonts were anatomically close to the hagfish.

One important hurdle to winning this argument seemed to be to persuade Janvier, but he and Peter Forey of the Natural History Museum in London were not convinced, and certainly not by Sansom's work. They argued that the similarities between the structure of conodont elements and enamel were superficial; they looked similar but there were serious differences. They wanted dentine: “There appears to be a total absence of dentine, which is unexpected if conodonts are vertebrates. Dentine is universal in vertebrates and is thought to be the most primitive of vertebrate hard tissues.” They were not alone; others also doubted Sansom and his colleagues.
¹⁹

Most vociferous among the vertebrate paleontologists objecting to the conodont vertebrate was Sue Turner, a fossil fish specialist at Queensland Museum in Australia. She and Alain Blieck were two of the organizers of a major international symposium that both commemorated the work of Walter Gross and discussed the problem of “Palaeozoic microvertebrates” at Walliser's university in Göttingen in August 1993. For Paul Smith, Sansom, and Purnell, this was their first opportunity to present their views on the conodont animal to a traditional vertebrate paleontology community, and they soon discovered they had driven headlong into a brick wall. Paul Smith later recalled, “One lunchtime we realized that things were a bit quiet around the conference venue, only to discover that an impromptu meeting had been called to discuss how this community could counter the conodont hypothesis. It was at a reception at the end of that meeting, in Otto Walliser's house, that Sue Turner told us that ‘no evidence we could ever provide' would convince her that conodonts were vertebrates.” When
Nature's
Henry Gee heard this, he christened the Birmingham-Leicester team, the “the Pommie Bastard Conodont Group,” an epithet these workers then used ironically to identify themselves. Gee suggested that instead of attempting to overcome the seemingly immovable obstacle of at least some in vertebrate paleontology, they concentrate on convincing the broader and less politically organized biological community. Turner, Blieck, and a number of others at this meeting would now hold their resolve indefinitely. They would be joined by conodont workers who, while not working directing on the biology of the animal, were nevertheless unconvinced by the British animal.

Sansom, with Paul Smith and Moya Smith, continued his assault on Janvier's stronghold. This time they focused on
Chirognathus
, a conodont genus with delicate elements that Branson and Mehl had described as fibrous, alluding to their frayed, wood-like structure. At one time these fossils were considered fundamentally different from other kinds of conodont, but Lindström and Ziegler had shown this structure simply resulted from a lack of white matter. Branson and Mehl had described the basal filling as bone-like and with Stauffer they marveled at the survival of fossils with such “delicate, sharp-pointed, hand-like crests with the fibrous structure.” Sansom and his co-workers found that the base of these fossils did not contain the globular cartilage he had seen in the previous study but a form of dentine. This diversity of tissue was not unknown in early fish remains, and in 1994 they concluded, “It seems that both conodonts and agnathans experimented with many tissue types and tissue associations in the early history of vertebrate biomineralization.” The discovery of dentine was, they believed, particularly significant. It seemed to offer “additional and conclusive evidence of the vertebrate nature of conodonts.”
²⁰

Anne Kemp and Bob Nicoll responded to the earlier paper by searching for collagen in the conodont elements. Collagen occurs in bone, cartilage, and dentine but is not – in living animals – found in enamel. Using a stain that reacts to the presence of collagen, they investigated the organic components within the elements and found the lamellae making up the crown of the conodont element took the stain, but not the white matter or basal tissue. This suggested that the lamellae did not consist of enamel and that the white matter and basal tissues were not bone or dentine. These results flatly denied those of Sansom and his co-workers. Kemp and Nicoll continued to favor amphioxus as the most appropriate comparator organism and saw no evidence of vertebrate affinity.
²¹
The British, however, doubted that material this old could preserve chemically active collagen capable of responding to the stains used by biologists working on living animals.

Purnell now stepped in, supporting Sansom and his colleagues with what he believed was unequivocal evidence that conodont elements did function as teeth; food was “crushed” and “sheared” and had left the marks to prove it.
²²
From studying completely preserved apparatuses, it was now possible for him to know which surfaces came together in processing food, and in yet another short paper, rapidly published in
Nature
in April 1995, he reported how he had looked for signs of wear like those found in mammal teeth. Purnell knew that such evidence did not simply support the tooth theory, it opened up entirely new possibilities for understanding the animal as a living entity. Mammal teeth, he wrote, show three types of wear: polishing, scratching, and pitting. He found these same distinctive features on his tiny conodont elements. His platform element showed pitting but no scratching and suggested a crushing mechanism. A blade element showed polishing, which suggested that area was not exposed to abrasive food. Other elements did show scratching, but to variable degrees, suggesting exposure to different kinds of food. And most interestingly, but problematically, these wear patterns were found on young conodont elements too, meaning that the elements were in use while they were still growing. The elements were, it seemed, periodically enclosed for repair but spent most of their time exposed and functioning.

With a little orchestration behind the scenes, the very next paper in this issue of
Nature
revealed Gabbott's Soom Shale animal. Remarkably, the animal had preserved in fine detail evidence of muscle-fiber and its component fibrils. Preservation of muscle around the eye suggested more advanced “encephalization” or “brain” development. This, together with other preserved features, indicated that the conodont animal might have been more advanced than the hagfish or the lamprey. Although anatomically similar to the Scottish animals,
Promissum
possessed a rather different apparatus, indicating that it belonged to a different group of conodont animals. It had, of course, come from much older rocks, and its preserved muscle tissue was now the oldest in the vertebrate fossil record. It was something like the muscle fiber found in amphioxus, but more like that found in slow agnathan fish. It suggested that the animal could cruise rather than race.
²³

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.