The Great Fossil Enigma (35 page)

Walliser's agenda for the group was subtle and complex. It revealed the degree to which so much was unknown or unclear about these natural divisions in the record of life. It was, in this respect, a million miles away from the sound-bite science of asteroid impacts, but inevitably, his group had its asteroid chasers too. Soon the number of extinction events began to swell and geologists began to debate the cause of extinction like never before. Paleontologists remained largely skeptical of extraterrestrial causes at first, though some, like Gould, who saw the meteorite as supporting his punctuated equilibrium, were rather more welcoming.
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While meteorites and huge volcanic eruptions grabbed the headlines as the most spectacular theories went head-to-head, many paleontologists remained wedded to a more conservative view of a planet with its constantly reconfiguring continents, its wobbly rotation and changing climate, its ice ages and fluctuating sea levels. This view suggested that there was no need for alien visitors. Nevertheless, the meteorite was felt everywhere. It could not be ignored. Phillip Playford, for example, whose reef model had helped shape George Seddon's interpretation of the Canning Basin, hooked up with McLaren and others and went in pursuit of Kellwasser Event iridium there. They were drawn to the Canning Basin by the secure timescale supplied by its conodonts. Collecting in New York and Belgium had failed to detect a Kellwasser iridium spike, but the Canning produced one at twenty times the background level. This did not, however, land the expected meteorite because they also discovered a concentration of the fossil cyanobacterium,
Frutexites
, which was known to be capable of concentrating this and other elements in its filaments.
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Back in the 1920s, Schindewolf and Hermann Schmidt had distinguished Upper and Lower Kellwasser horizons. Now the former was recognized as one of the five greatest mass extinction events in Earth history. It became an important test of theory and of intense interest to Walliser's group. Associated with the Upper Kellwasser horizon are a number of black shale sequences thought to represent anoxic conditions in deep water. As sea level was raised and lowered, so the area covered by this anoxic water expanded and contracted, though perhaps not as simply as this explanation suggests. Walliser imagined that such changes in water depth would have ripple effects throughout the food chain and into other environments.
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It suggested a silent menace in the depths of the ocean. Perhaps the sky watchers were looking in the wrong direction?

Walliser knew the Kellwasser possessed a complexity that defied easy explanation; there were a multitude of possible causes and many were probably acting simultaneously. Different groups of animal had not been affected in the same way, and both extinction and successive radiation seemed to be stepped. This complexity convinced Walliser that it could not result from McLaren's asteroid impact, pointing out, in 1984, that the iridium anomaly was younger than the main Kellwasser Event.
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For Walliser, change was slow and accompanied by a few global events, none of which alone had a universal effect on life.

In that same year, Willi Ziegler entered the Kellwasser debate and Walliser's project. Like Walliser, he could now look at Helms's iconic diagram and see a moment of extinction prior to the rapid evolutionary diversification of
Palmatolepis.
Collaborating with Charles Sandberg of the
USGS
in Denver and Roland Dreesen of the Institut National des Industries Extractives in Liège, they sampled contrasting paleoenvironments in Utah, Nevada, Germany, and Belgium, where they found surprising agreement in the changes taking place. All the sequences signaled a switch from deep-water to shallow-water forms, suggesting a relative fall in sea level. However, there were also some unusual mixtures of shallow – and deep-water species, which encouraged the team to postulate that the Kellwasser Event had caused tsunamis that washed the
Icriodus
-dominated faunas into deeper water.
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Walliser was not convinced by this sea level-based explanation, even with its rather dramatic finale; if one looked beyond the evidence of conodonts, he said, the picture was far more complex.

Given Ziegler's longstanding commitment to utilitarian stratigraphy, it is unsurprising that he, like Walliser, was interested in events for their evolutionary implications, which could then be fed into stratigraphic study. Ziegler and Richard Lane now re-examined their collections through spectacles made for them by Clark and Walliser. They looked for “conodont evolutionary cycles.” Following an extinction event, the conodonts showed a period of low diversity, followed by a short “innovative phase.” Here innovative conodonts tended to have larger than normal basal openings. These then formed the rootstock for an evolutionary flowering or a “radiative phase.” Finally evolution entered what they called a “gradualistic phase” before yet another extinction event.
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In the period of time between the Late Silurian and Middle Carboniferous, they believed they could detect seven of these three phase cycles, showing that the conodont was peculiarly adept at providing these high-resolution pictures of extinction and evolution.

Sandberg, Ziegler, and Dreesen continued to work on an explanation for these crises. They came to believe that the extinction that led to
Palmatolepis's
blossoming “occurred in far less than 20,000 years and more likely within a few years or days.” Accommodating the complexity recognized by Walliser and others, and using data from six sites across Europe and America, they now pictured a sequence of twelve steps extending over the Kellwasser Event. This event began with a sea-level rise and the drowning of reefs. The sea level then fell, and they theorized that a large “bolide” (an object capable of a huge impact but without presuming to know whether it is a rocky or metallic asteroid or icy comet) passed close by or a small one impacted the earth. Faunas then became re-established on mud mounds but reefs did not make a reappearance. There then followed another couplet of sea-level rise and severe fall. The rise had led to the development of stratified seas and the spread of anoxic conditions in deeper waters. The seas continued to shallow, and now a large bolide impacted, causing storms recorded in the Belgian rocks and the widespread extinction of conodont species. As the seas continued to shallow, inevitably shallow-water conodonts were able to spread. Tsunamis then preceded a new transgression of the sea over coastal lands, but glaciation in the Southern Hemisphere soon caused yet another period of shallowing seas in the north. This was followed by yet another mass extinction later in the Devonian. Of those conodonts that survived the extinction event, there were some, such as
Palmatolepis praetriangularis
, which were “opportunistic” and began a process of repopulation. Two species of
Icriodus
– “strong survivors” – also made it through. These workers located a five-centimeter-thick conodont-free shale situated clearly between their conodont zones in Schmidt Quarry, near Ense in Germany. It suggested that the Upper Kellwasser Event could have lasted no more than 12,500 years, though the preference remained for just a few days.
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Those reading this paper may have been rather skeptical of the drama it portrayed, but even Walliser thought it an important contribution to working out the precise chronology of the event. This chronology would be corroborated by later workers.
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On the matter of cause, however, the meteorite possessed a sticky aspect. It was hard to shake off and it seemed to attach itself to almost any theory; Sandberg and his colleagues certainly became attached to it. Walliser was both amused and exasperated, recalling that as his project came to an end, many of those who had previously objected to it found themselves increasingly won over. Maurits Lindström admitted to being a late convert. Sandberg, who with Ziegler and Dreesen had merely “theorized” a bolide, became increasingly interested in the Alamo Impact site in Nevada discovered in the early 1990s. Dated by conodonts and then radiometrically, that impact was too early to account for the Kellwasser crisis, but by 2005, Sandberg and his former student Jared Morrow could demonstrate that it was simply one of a number of impacts around this time. Others were known from Germany and Sweden, and the latter seemed to be timed very closely to the Kellwasser Event. This evidence led Sandberg, Morrow, and Ziegler to postulate that in the Devonian's turbulent history comet showers were a probable trigger for major changes in life on the planet. It was while engaged in taking this work further that Ziegler died in 2003. Appropriately enough, for the planet's most single-minded and dedicated conodont specialist, he had conferred with Sandberg from his hospital bed just ten days before.

The full implications of this events thinking are perhaps revealed most strongly in the work of Lennart Jeppsson, a Lund conodont worker who gained his insights and inspiration in circumstances that were in some respects the reverse of those that shaped Walliser's. Both are known for their Silurian work, but Walliser became increasingly convinced of the truth of global events as a result of travel. In contrast, for Jeppsson the world often arrived in the form of scientific papers, and his inspiration came from essentially staying put and working his patch. For most of his career, Jeppsson's focus has been Gotland, the second largest island in the Baltic Sea. With some fifty-seven thousand people living there, this is no Robinson Crusoe retreat but an ancient, sunny, and beautiful place to spend one's life in the field. Here, based at the Allekvia Field Station, Jeppsson spent two or three weeks each summer digging into the Silurian strata and carrying vast quantities of it across the island to the Slite cement works, where it was stored and would eventually gain passage across the Baltic to Limhamn, some twenty-five kilometers from Lund.

The attraction of Gotland for the geologist is the considerable thickness of the Silurian rocks – some five hundred to seven hundred meters. Largely free of the corrupting influences of metamorphism, folding or faulting and gently dipping to the southeast, they preserve almost perfectly a shallow-water platform which in the Silurian lay just south of the equator. Through rapid deposition, the island's thick rock sequences effectively stretch geological time, making it possible for Jeppsson to understand events in the past with the geologist's equivalent of a stopwatch.

Jeppsson came to conodonts as a result of a lecture by Stig Bergström; like Bergström, he had begun his scientific life as a botanist. His curiosity pricked by the mystery of these strange conodont fossils, in 1965 Jeppsson found himself studying Silurian conodonts for the equivalent of a master's degree under Bergström's supervision. He acquired Gotland as a kind of inheritance. The conodonts there had been studied by Anders Martinsson at the University of Uppsala in the mid-1950s, though not as his main interest. Martinsson, who was a regular visitor to Lund and a close friend of Bergström, had set up the field station and remained a leading light when Jeppsson began his first steps in the field. Jeppsson must have shown talent, for Bergström spoke to Martinsson and asked if Jeppsson might take on the Gotland conodonts in a project Martinsson was then running. However, not everything was plain sailing. On arriving on Gotland, he had found himself in competition with the young Lars Fåhræus, who soon took the lead by publishing on the Gotland conodonts in the late 1960s.
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Fortunately, this turned out to be a temporary problem for both men.

Jeppsson's bigger problem was technical: He discovered that the fossils were far fewer and far poorer than those he had already studied. This meant his task was going to be rather more difficult than he had imagined. The relative rarity of conodont fossils in these time-stretched strata meant that processing vast quantities of rock did not guarantee sufficient material for interpretation. His situation was entirely the opposite of that of Bergström and Sweet, who seemed to be drowning in these fossils. Rather than sifting through hundreds of thousands of specimens, Jeppsson often found himself looking for a single species recorded by a single specimen. It was a problem that forced Jeppsson and his dedicated technical team to repeatedly study and improve their methods. In 1983, small collections were produced from processing 0.5-kilogram samples. A decade later they were dissolving 20- to 80-kilogram samples of rock in acid. Progressively they reduced the number of conodont elements being destroyed, and soon conodonts were found in rocks Jeppsson once thought barren. His overall productivity increased a hundredfold. Nevertheless, at only ten to one hundred specimens per kilogram, his huge collections of fossils speak of considerable effort and a particular devotion to his subject.
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Initially, Jeppsson's task was to describe the conodont faunas. But in such a thinly populated country with few paleontologists, no scientist could consider his subject so narrowly, and from the outset Jeppsson became interested in every aspect of the animal. His research took a particular turn when he discovered that his fossil species were showing episodic disappearance and reappearance, as if rising from the dead – so – called “Lazarus species.” He was not unaffected by the ecology and provincialism debates of the 1970s, but by the early 1980s he thought the ecological models of no significance to his work, and like others he doubted the existence of faunal provinces in the Silurian. He gave thought to the repeated and almost instantaneous evolution of identical forms from more conservative stock; perhaps the Lazarus species were really imposters? He admitted this remained a possibility, but he possessed no evidence to support it. Instead he began to consider other factors, such as water chemistry, aware of its extraordinary influence in the modern day Baltic. In 1984, this seemed the most profitable line of inquiry.
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