The Great Fossil Enigma (36 page)

Over time his interpretive frame shifted, affected by the massive swell of publications that sought to understand the past globally. Influential in this change of thinking was Princeton's Al Fischer and Michael Arthur. They reviewed data relating to the oceans of the past and considered how radically the science's conceptions of Earth had changed even before the meteorite had hit. Oceans were now transient features, their distributions, depths, submergence of continents, and circulation patterns quite unlike those of the present-day planet. These shifting configurations took place as the chemistry of the atmosphere changed and as Earth gained its succession of animal and plant populations.
²⁶
Fischer saw the possibility of rediscovering Earth history as a singular and specific narrative. The belief that the present alone could be used to interpret the past seemed too constraining; the past was different and exotic.

Yet despite his belief in a singular and particular narrative, Fischer felt he saw an underlying structure to the history of life: “There runs through earth history an orderly thread of changes in the state of the earth as a whole – a thread of complexly interwoven filaments that produce rhythmical patterns.” He and Arthur suggested a pattern reflecting alternating periods when oceans were rich or poor in biological diversity. Fischer noted that there was evidence for four ice age periods over the last seven hundred million years, including the one we live in today. In these periods, Earth was in what he called an “icehouse state,” with low sea levels (because so much water was locked up in the icecaps), a strong temperature gradient from equator to pole, and an ocean circulation where cold polar waters sank to form the deep ocean. In the intervening periods, Earth was in a “greenhouse state.” Ocean waters were warmer and did not sink at the poles. Rather, the deeper waters resulted from the sinking of heavier, more saline waters at rather lower latitudes. Fischer proposed a sophisticated plate tectonic-driven model for the production of these two fundamentally different planetary states, with a key ingredient being changing quantities of atmospheric carbon. He suggested that these changes took place with regularity, that there were supercycles of hundreds of millions of years, with smaller cycles of thirty million years and ten thousand to one hundred thousand years superimposed on them that were reflected in changing faunas, alternating lithologies, and moments when anoxic black shales became widespread.

At Berkeley, Pat Wilde and William Berry were also considering how climatic change could affect ocean conditions. They believed that as glacial or icehouse conditions formed or broke down, so ocean stratification would collapse and result in the upwelling of potentially toxic or anoxic waters and mass extinctions in the life-rich surface waters.
²⁷
They saw these as long-range cycles; most of the time the earth's climate was warm.

These models of the past stimulated some to begin library-based number crunching of the record of life in search of cycles. Others sought to reinterpret their understanding of the record in the field using cyclical eyes. McLaren was certainly one who took Fischer's cycles seriously when interpreting the Kellwasser in the early 1980s. And as we have seen, cycles were also in the minds of Ziegler and his collaborators. Indeed, this cyclical view became embedded in the literature, often picked up by readers who never referred back to the original work.

In 1987, Jeppsson explained a single extinction event in the Silurian, first mentioned by Dick Aldridge a decade earlier, in terms that owed something to this debate: “The extinction…seems to have coincided with a change from more oxygenated oceanic conditions to ‘greenhouse' conditions, with a final fauna before extinction that thrived during an episode of widespread abnormal sedimentary conditions.”
²⁸
By then Jeppsson had global data showing that lithological and faunal changes had worldwide distribution, though not without some local variation. The key seemed to be a global decrease in the deposition of carbonate rocks (limestones) and an increase in the formation of muddy ones. American workers had seen this change at a regional level and interpreted it as a tongue of muddy sediment spreading from an unknown landmass to the east. But now Jeppsson could demonstrate that the phenomenon was much more widespread and had a considerable impact on the conodonts. He believed that the climate in the Silurian was not the consistently equitable greenhouse that had long been imagined. Perhaps conodonts were sensitive indicators of ocean conditions in rather more subtle and sophisticated ways than previously considered. With information on four anomalous episodes in the Silurian, Jeppsson was also of the view that what happened in each was different and could only be resolved using high-resolution stratigraphy. In this, of course, he had the advantages of Gotland's thick strata and the incomparable conodont.

By 1988, Jeppsson had produced a model to explain the effects he saw in the field. The context of his theorizing was the literature speaking of cycles and climatic change. Perhaps inevitably his digestion of field data, and new oceanographic data from workers such as Pamela Hallock, with whom he by chance came into correspondence, led to conclusions much like those of Fischer and others. Jeppsson's P (later Primo) and S (Secundo) episodes were essentially identical to Fischer's icehouse (later O [= oxygenated]) and greenhouse (G) states. In 1990, Jeppsson published his model in full (
figure 10.2
) and shortly afterward joined with Dick Aldridge and Ken Dorning to apply that model and name the episodes and events in the Silurian it seemed to describe.
²⁹

10.2.
Jeppsson's oceanic models. Some characteristics of the two states in Lennart Jeppsson's oceanic model: P state (
top
) and S state (
bottom
). Redrawn with permission from L. Jeppsson,
Journal of the Geological Society of London
147 (1990).

As with Fischer's, the key to Jeppsson's model was the carbon cycle. The cold ocean waters absorbed carbon dioxide and oxygen from the atmosphere and took them down into deep water, effectively lowering atmospheric CO
₂
and thus lowering temperatures. In these Primo episodes, higher humidity and rainfall at lower latitudes resulted in runoff, and erosion, with an increase in muddy sediments and the formation of fewer pure limestones. These muddy sediments would foul reefs and lead to the formation of black shales. These were glorious times for conodonts as they diversified to occupy a range of specialized habitats. Not all life was so fortunate: The upwelling of nutrients from depths would kill off any limestone reefs. The low levels of CO
₂
in the atmosphere would also mean that rain and surface water would erode limestones on land.

But over time the ocean would be able to absorb decreasing amounts of carbon dioxide leading to a rise in the amount in the atmosphere. Greenhouse conditions would consequently begin to develop and temperature would rise. Now the Secundo episode began, perhaps with a faltering start. The oceans became warmer and cold waters at high latitudes were now less dense than the more saline waters farther south. Consequently, these southern waters began to sink to form the deep water, and because they possessed less oxygen and carbon dioxide, they created low oxygen conditions at depth, resulting in the formation of black shales. Yet, in shallower waters, and in a now drier climate, less muddy sediment was flowing into the sea and reefs and limestones could develop in relatively low nutrient conditions. In these seas, conodonts were rare. But this growth in the production of limestones drew in increasing quantities of carbon dioxide, locking it in the rock and lowering concentrations in the atmosphere. As a consequence temperatures began to decline, leading eventually to a resumption of Primo conditions.

The change from warm to cold oceans would cause a contraction of the water mass and a lowering of sea levels; when warm oceans were reinstated, sea levels would rise again.

The model had an undeniable logic and seemed to unite his new evidence of global changes in sediments with the widely discussed importance of the carbon cycle. To this Jeppsson added the Milankovitch cycles – variations in the orbit, tilt, and wobble of Earth – which had been key explanations of cyclical climate change since the mid-1970s. It seemed that the regular changes in the tilt of the Earth matched the changes Jeppsson saw in the field, and he imagined the Milankovitch effect nudging one unstable system into another but in complex ways. By the time Jeppsson published his first paper with Aldridge and Dorning, he was very busy drawing out aspects of his model in order to show its applicability outside the Silurian.

Of the events discussed by Jeppsson, none was more important than the Ireviken Event, Jeppsson's paper on which, although submitted for publication in 1990, languished in press for seven long years. The Ireviken was the Silurian's most impressive event, killing off 80 percent of conodont species worldwide and 50 percent of trilobite species on Gotland. In Jeppsson's mind it marked a period when Earth switched from Primo to Secundo conditions. The event lasted two hundred thousand years, and in most geological sections it occupies just one meter of rock. On Gotland this stretched to nineteen meters. Using conodonts he could at times attain a resolution of one two-hundredth of the overall event. It meant he could talk in terms of periods of ten thousand years, the geological equivalent of microseconds. He now understood that different conodont lineages reflected preferences for different ocean conditions. The stepwise extinction of conodonts produced eight variably spaced datum points he believed he could see in papers describing other sections in Alaska, Canada, California, New York state, Virginia, Greenland, Britain, and Australia. This high-resolution work was soon taken up by two Estonian colleagues, Peep Männik and Viive Viira, and before long Jeppsson was pressing ahead with establishing a new conodont zonation based on extinctions rather than appearances. Soon Jeppsson's model was appearing in textbooks as an exemplar of what could be achieved with high-precision paleontology, and by the mid-1990s it became a standard explanation for change in the Silurian.
³⁰

In life, in death, and in dying, conodonts bore witness to extraordinary events. That at least seemed certain. The nature of those events remained rather more a matter of interpretation and personal conviction, but they undoubtedly made possible the seeing of new things and the thinking of new thoughts. In this respect, they acted as catalysts or interpretive spectacles. In themselves, however, their most important aspect was to make possible connections between data: data of different kinds and data produced at different locations. This made a small contribution that permitted the science to generate new hard evidence. This science did indeed discover large numbers of meteor craters and make geologists think of their subject in planetary terms. Major events in Earth history became a reality in the science's thinking.

All this work, of course, added only small, and rather implicitly received, knowledge concerning the animal itself. That discussion was going on elsewhere. Following Walter Gross's destruction of the fish in the 1950s, the animal had not been much discussed. Instead, the conodont workers had put their heads down and worked hard to develop a very practical and connected science. However, at the end of the 1960s, the animal again began to emerge and this aspect of conodont studies once again became a hot field. It is now time to unveil the animal.

 

MANY WHO HAD HEARD THE CONODONT'S STORY DOUBTLESS
imagined that impossible day when the animal would be found. In a corner of so many minds, there was intense curiosity about these tiny things. They were, after all, as Maurits Lindström had put it in 1964, “the biggest and most important group of fossils about which the zoological relationships are entirely unknown.”
¹
The fossils seemed to defy comprehension. That special day did, however, come. It was September 5,1969. That, at least, was the day of realization. Less than two months after Neil Armstrong stepped onto an alien world, paleontology produced its own alien and it was utterly bizarre. Before long, the news spread across the same networks that had covered the moon landing, though rarely warranting more than a column inch.

The animal's reception – at least in the scientific world – was, appropriately enough, that reserved for aliens in those classic American sci-fi films of the 1950s: From the moment of its innocuous arrival, mankind, so it seemed, sought the alien's destruction. And in keeping with that tradition, it seems appropriate that our hero is also our antihero, his integrity doubted, his actions condemned. His name was Harold Scott. It will be recalled that he, as a young man in the 1930s, had made that giant leap to reveal the animal's complexity – a discovery that ultimately turned the science on its head. In 1969, he was at the other end of his long career and nearing retirement, yet his role in the drama was no different. As in the 1930s, Scott was again making assertions few could, or wanted to, believe. His critics thought this latest conodont animal was the product of a fertile imagination. Scott however, had good reason to believe that he would again prevail.

The great moment had been billed to take place at a meeting of the North Central Section of the Geological Society of America in Iowa in May 1968, but this proved to be a false dawn. Rumors circulated suggesting that Brian Glenister possessed a communication from Scott that would reveal all. It was to be read out at the final session of the meeting. The room was packed to standing. Anticipation was intense. As Glenister began to speak, the room became silent, but the atmosphere soon changed as the audience began to realize that this was not the great moment at all. The disappointment was palpable. What Scott had found, in those same Montana rocks that had furnished him with his assemblages, were “blebs,” carbonaceous or asphalt patches two square millimeters in size. He had found eighty of them and they contained conodonts. Seemingly impossible to photograph or draw well, the Iowa audience looked and saw the same rather patchy material their intellectual parents had seen when Croneis had presented Scott's results thirty-four years earlier. As with the previous disappointment, Scott was not there to witness the cynicism, and he pressed on regardless with a paper that claimed “Discoveries bearing on the nature of the conodont animal.”
²

When Scott looked at those blebs or blobs, he saw the animal's head: “These conodonts have been held in the cartilaginous material of the head of an animal, not in gills; the cartilaginous head material has been thicker and stronger than the remainder of the body and upon death the conodont teeth remained ‘stuck' in the cartilaginous substance. As the cartilaginous material altered to a bituminous or asphaltic base the conodonts became twisted, intertwined, and occasionally broken.” He continued, “We cannot fully judge the position in the mouth, but the evidence points more and more to a circum-oral arrangement as rights and lefts rather than uppers and lowers and functioning as strainers in a mouth-esophagus rather than as gill-rakers.”
³
But the teeth in the blebs were puzzlingly small. The “black, glossy, asphaltic patches” looked, to Scott, like skin: “The blebs do tell us that the head of the conodont bearing animal was at least partially covered with reticulated skin. Also, they tell us that at least a portion of the head consisted of cartilaginous material, and that this cartilage was thick and strong enough to be preserved) thereby raising hopes of future discoveries.”
⁴
Of all this, Scott was quite certain – as certain as he had been, decades earlier, that the animal was a worm. Now he was willing to speculate that it was a centimeter-long ancestor to the primitive jawless fishes, the lamprey and hagfish.

Confirmation of Scott's recent discoveries soon arrived in a paper from German paleontologist Friedrich-George Lange. It said that Lange had found fossilized excreta or coprolites containing conodont assemblages, but as we learned in
chapter 8
, Lange had only used this explanation in order to get his paper published, as Ziegler had objected to the interpretation of these fossils as accurately preserved apparatuses. Of course, Scott did not know this, and on reading Lange's paper, Scott was convinced that Lange was mistaken, for Lange's finds, like his own, each contained a single unmixed assemblage. This made Lange's coprolite theory bizarre. Was Lange suggesting that the animal “waited to excrete the remains of the one victim prior to eating another”? Scott had seen coprolites and knew they were stained “lumps” containing broken conodonts; they were not like Lange's finds.
⁵
Scott now used Lange's fossils to support his own ideas, believing that evidence was mounting and that he was on the brink of a major discovery. This was, however, a new direction for him, for although he had published a few important papers three or four decades earlier, conodonts had not become his life. Now he was back but rather out of touch with all that had gone on since. At the start of 1969, he applied for a National Science Foundation
(NSF)
grant to continue the search for the animal. But he was unaware that others were now on his patch, and it would be they who would take the next big step.

In March 1968, an undergraduate student, Douglas Wolfe, brought two fine fossil fishes to Bill Melton, curator at the Geology Department of the University of Montana in Missoula. They had been found by quarry owner Charles Allen and Ralph Hartin in the local Bear Gulch Limestone, a rock that lay, so they thought, within Scott's Heath Formation.
⁶
Melton was impressed. Complete fish of this age were rarities, so he gathered up some students and returned to the quarry in search of more specimens. Another fish popped out, along with other fossils. What Melton had come across was a “Konservat-Lagerstätte,” a deposit containing exceptionally well preserved fossils. Melton thought the discoveries unusual and significant, and when Eugene Richardson of the Field Museum of Natural History in Chicago read about them, he thought so too. Richardson was then organizing sessions for the first North American Paleontological Convention due to take place at the museum in early September 1969, and he invited Melton to give a paper on these new finds.

Melton spent the summer before the convention in the field with his assistant, Jack Horner, looking for fishes in what was known, or became known, as Surprise Quarry. They found sixty-five. But these were not all they discovered. In the first week, Melton later recalled, “we found a curious, carbonized impression of an animal that I could not identify in the field. Several others were found in the next five weeks.”
⁷
When they at last got these enigmatic fossils back to the laboratory, they discovered they contained conodont elements.

Melton turned up at the convention with his fishes and strange fossils. Soon fish specialists were swarming over them, and one of these, noticing the conodont-bearing specimens, called for conodont specialists from the audience. Huddle, Collinson, Lane, Scott, and others crowded in for a brief look. “Plans were quickly changed,” Melton recalled, “and a photograph of a specimen was made.” Melton then presented his paper, titled, appropriately enough-but with more understatement than he knew – “Unusual fossils.” It was a prime spot: The first paper of this first convention, and in it he made mention of the “soft-bodied animals.” However, the full impact of the discovery was not realized until Scott took to the stage immediately afterward. Apparently nominated by his fellow conodont workers because of his seniority in years and his groundbreaking work in Montana in the 1930s, Scott's appearance was already a break from the published program. He did what it was necessary to do on this great occasion, something conodont workers had rehearsed hundreds of times before. He told the story. With the enormity of the enigma in the audience's mind, Scott then said, “I have just seen the conodont animal!” To some in this audience the news must have seemed more remarkable than the moon landing. “It was a most exciting discovery,” Scott later told a friend. Stephen Jay Gould was there, “a wet-eared, first year professor” impressed by the paleontological talent in the room: “If the Russians – or the Chinese, or whoever wanted to destroy this entire profession, one bomb…”
⁸
One young Japanese scholar, Kenji Konishi, told Scott five years later of “the still unforgettable pleasure” of hearing Scott's opinions on “this epoch-making find.”This was September 5, 1969. The “conodont animal” had landed (
figure 11.1
).

11.1.
The beast of Bear Gulch. Melton's headless animal was extraordinary – it was like nothing previously seen. The tail is on the left. Reproduced with permission from S. Conway Morris,
Philosophical Transactions of the Royal Society, Series B
327 (1990).

That evening, delegates gathered around the piano for a singsong: