The Great Fossil Enigma (52 page)

The news was of such significance that it made the cover of
Science
, while inside the team announced a shake up; this was “unequivocal evidence for the inclusion of conodonts within the vertebrates.” Repeating Pander's original assertion, they claimed it pushed back the appearance of vertebrate hard parts by forty million years, back to the Late Cambrian. Important to this interpretation was the researchers' attachment to Jefferies's recent reorganization of the vertebrates, which made them synonymous with the craniates, a point rather buried in the footnotes. It had long been believed that vertebrate hard parts first evolved as external armor, which later developed into specialist feeding apparatuses; now they suggested the reverse was true.
¹⁰
They also contradicted the contemporary belief that dentine appeared in the fossil record before bone; these fossils appeared to contain no dentine, only cellular bone, enamel-like tissues, and cartilage.

The news spread even more rapidly than that concerning the discovery of the animal. It was reported on the front page of the
Times
under the headline “Razor-toothed fish bites into human history.” It read, “Mankind's oldest ancestor was a tiny, eel-like fish with a formidable set of razor-teeth that lived 515 million years ago.” And it ended, “It is still not clear what these creatures looked like. Mr Sansom said the teeth suggested carnivores. If alive today, they would give a bather a nasty nip.” The paper even reflected on the discovery in its editorial. The
Independent
, a British broadsheet paper, took a more mundane line and was rather less certain of the significance of the discovery. “Limestone yields the oldest set of teeth” began “British scientists have discovered the oldest known set of teeth in the world. They have been embedded in a lump of limestone for about 500 million years. The teeth belonged to a small, ferocious marine animal that probably spent much of its time eating other members of its species. Scientists are excited about the find because it means they have identified the human race's oldest vertebrate ancestor.” The
Telegraph
gave a more informed, sensation-free account in “Teething troubles”: “A wriggling, toothed, eel-like animal that evolved 520 million years ago was the earliest vertebrate, scientists have reported.” The
New York Times
was even more grown up, devoting half a page to the discovery under the heading “Vertebrates found to be much older.” Here readers were advised that “conodont” should be pronounced
“KAHN
-o-dahnt,” perhaps translating it into a New York accent. Doug Palmer, writing in
New Scientist
, felt the discovery transformed the significance of the conodonts and gave the research Sansom was undertaking great urgency. The article carried an illustration of one of Sansom's etched teeth. The caption read, “First vertebrate: this tooth-like fossil, only 2 millimetres in section, is the forerunner of all animals with backbones.”
¹¹

Discover
magazine, writing about the top fifty science stories of 1992, proclaimed, “If Sansom is right, vertebrates are as old as the oldest conodonts – that is, at least 515 million years old.” Here Sansom recounts the moment of first discovery in Newcastle: “We were sitting there, and these images appeared that were quite spectacular…. It was very, very clear we were looking at enamel, bone, and cartilage.” He continued: “It came as quite a shock…. The structures were actually clearer in the conodonts than in human teeth.” The
Northern Echo
, a British regional paper, gave the story the headline “Meet your great, great, great, great…grandad.”
¹²

Those pondering the origins of vertebrate life could no longer ignore the conodont. Indeed, conodont workers of all persuasions might have felt reason to celebrate the suddenly elevated status of their fossils. Chris Barnes, who was immersed in events in Earth history, wondered if a particular event in the Ordovician had marked an important moment in the evolution of vertebrate tissue. For those already advancing on the vertebrate from other directions, especially the Leicester team, there could be no better piece of news; it would form a vital component in their arguments for years to come. Briggs, who had been asked by
Science
to write a “Perspectives” piece to accompany the article, immediately celebrated the impact of this discovery, which had thrown ideas regarding early vertebrate evolution into disarray and in an instant expanded the number of known vertebrate genera in the Cambrian-Ordovician from 5 to 150 (nearly all of which were conodonts). “In any event the vertebrates can now be added to the list of major metazoan taxa that appeared during the Cambrian radiation,” Briggs observed. Aldridge now felt he could at last state, without fear of contradiction, that “conodonts are extinct primitive soft-bodied vertebrates” and even go so far as to suggest “the soft tissue evidence now confirms their place among our earliest ancestors.” Henry Gee, an editor of
Nature
and an important sounding board for the British workers, later reflected on the implications of finding this new piece in the vertebrate jigsaw. He recalled that almost since the beginning of paleontology, it had been believed that vertebrate life began with sluggish armored fishes, but now these were as nothing compared to the diversity of conodonts that joined them and in most cases were known only by their tooth-like hard parts: “Like so many geological Cheshire cats leaving their smiles to posterity.” “Could later vertebrates have had instead a common ancestor among the conodonts animals?” he wondered. Reflecting on his childhood inspirations and the bizarre armored fishes that once populated his mind, he reimagined the natural history museum of that time: “The Hall of Fossil Fishes might have presented a truer picture of the Age of Fishes had the bulk of its space been devoted to conodonts, with just one small case, in a dark corner, devoted to pteraspids, sarcopterygians and their relatives.” But precisely where conodonts fitted into the vertebrate family tree remained unresolved: “The mystery continued: it was a bizarre Borgesian plot made real, of future palaeontologists obliged to reconstruct human history based on several million sets of dentures, and nothing else.” For an object vanishingly small, its impact was never less than remarkable. Even Gee, overcome as he was with lyrical metaphors, found himself affected.
¹³
The discovery was a gauntlet thrown at the feet of those who were shaping the picture of early vertebrate evolution. It demanded that science return to its microscopes and that biologists consider fossils.

The question of whether conodont elements really were vertebrate teeth was now being tackled on two fronts. While in Birmingham, Smith, Sansom, and their collaborators were revealing the fossils' material affinity, in Leicester, Mark Purnell was discovering if they could have functioned like teeth. Purnell had drawn a line in the sand at 1982 and corralled everything before that year, including all those functional models produced in the 1970s, into a period of prehistory he called “pre-animal.” The animal discoveries had provided sufficient new and quite specific data on many key aspects of apparatuses that had previously been matters of speculation. In a paper with Canadian conodont worker Peter von Bitter, Purnell attempted to reinterpret the function of the pair of robust blade-like elements that sat at the back of the apparatus. Were they covered in soft tissue? Did they roll particles of food between them? Did they bite, shear, or operate like a gate? Noticing some small nodes at one end of each blade, they reasoned that these nodes were set opposite each other, offering a rasp-like functionality in a scissor-like action. By implication this meant that the animal must have grasped “large” food particles with the anterior part of the apparatus rather than used these elements as filter devices.
¹⁴

It had been while still working in Canada that Purnell had concluded that a fundamental objection to all previous functional models was a reliance on a test of “plausibility.” Now he asked, could this be avoided? Were there other aspects of the assemblage that might more objectively point to function? At its most fundamental, the argument over function that had developed in the 1970s had revolved around whether elements were teeth or components in a filtering apparatus. Zoologists had long known that different parts of an organism grow at different rates according to the animal's needs, that growth patterns in some respects reflect mode of life. Comparative data had suggested to Purnell that a filter-feeding animal would need to preferentially grow the net – the comb-like elements near the opening of the mouth. What he found, however, was that the robust mashing elements grew more rapidly. This suggested to him that they had a molar-like function. The other elements grew at the same rate as, or slightly more slowly than, the animal as a whole. Purnell could also quash rumors that conodont teeth were, like those in the hagfish, replaced in life or lost or reabsorbed by the living animal. He deduced that the conodont animal must have been predatory and used the front of its apparatus to grasp and the rear elements to “chew.”
¹⁵

Bob Nicoll, however, was unimpressed both by the vertebrate and these food-munching teeth; he continued to defend his filtering model.
¹⁶
Hit lacked fully formed jaws, Nicoll argued, how could the animal – with a muscular arrangement something like that in amphioxus – exert sufficient force to cut?

Meanwhile, in Bristol, Briggs was crafting yet another interpretive lens. He had set his postdoctoral researcher, Amanda Kear, to watch over the dead; to gather information on the decay of amphioxus (the lancelet or
Branchiostoma
). Their aim was to better understand the preservation of the conodont animals and other so-called primitive vertebrates.
¹⁷
Lancelets were considered possible “living descendents of the ancestors of vertebrates.” Briggs and Kear's experiments revealed that the noto-chord was resilient to decay, recording its former existence in two lines like those seen in the Scottish fossils. This work permitted interpretation of these specimens to move beyond a priori assumptions about the nature of the animal and even explain some of the inadequacies in the fossils themselves. It altered expectations and clarified interpretations; it, too, moved arguments away from the test of plausibility.

These were heady days for a science that would soon be reaching the peak of its activity. In 1993, and now in possession of ten Granton specimens, the three original authors, plus Paul Smith and Neil Clark, published the final, definitive, anatomical account of the Scottish animals in the
Philosophical Transactions of the Royal Society.
¹⁸
Aided by discoveries in South Africa, the two lobe-like head structures were now interpreted as hollow sclerotic cartilages that once supported the eyes. One specimen also seemed to preserve the animal's hearing apparatus. These were capsules that Aldridge and Briggs had once thought eyes. They had had their minds changed by the discovery of similar structures in the Mazon Creek lamprey,
Mayomyzon
, and hagfish
Myxinikela.
They also believed they could see faint traces of gill structures like those described in the Silurian relative to the lamprey,
Jamoytius.
The twin lines running down the body of the animal were now compelling evidence of a notochord, and in one heavily phosphatized specimen the area between these lines displayed a fibrous structure that just might indicate the notochord sheath. Again something similar had been seen in the Mazon Creek chordate
Gilpichthys.
Some aspects, however, remained frustratingly indefinite – a whisker away from being the proof they needed. The dorsal nerve cord was among these hazy features. But with each new specimen the beautifully preserved V-shaped segmentation could not be doubted. It formed “one of the most compelling pieces of evidence for chordate affinity.” The separation of the chevrons could now be attributed to postmortem shrinkage as could be seen in Briggs and Kear's decay experiments. In one specimen, Aldridge's team even wondered if they could see evidence of original muscle fibers. Their conclusions were emphatic: “The evidence of the soft-part anatomy, together with features of element histology, show that the conodonts are vertebrates.” The Birmingham team's corroborative work had given Aldridge and his colleagues a sense of certainty: “Other hypotheses that have been forwarded in recent years can now be refuted.”

Sweet's tests had, in part at least, been answered. The notochord had gained solidity, and the nerve cord and gill structures tentatively interpreted. Sweet's suggestion that other invertebrate groups might preserve the trunk structures seen in the conodont animal had already been firmly denied. Again, Conway Morris and Janvier provided important supporting voices for believing the animal a chordate. Indeed, for many, including Aldridge and his colleagues, the question had since 1987 become “which chordate?” Nowlan and Carlisle, for example, had been amongst those who had placed the animal with amphioxus. However, new certainty regarding the eyes suggested that interpretation was incorrect.