The farther back we try to go in time, the more we feel the effects of our geological myopia. Indicators become more and more fragmentary, and harder to decipher. So let us start with the least far-off of these mysteries, the one where there has not yet been enough time to eliminate all trace of the culprit - the KT crisis. What can paleontologists tell us about this last great crisis that struck our planet?
To start with, what about the famous dinosaurs, those dragons resurrected from oblivion who continue to support astonishing waves of advertising but also plainly still provide interest or amusement for a great many people? Some experts say the last remains are clearly older than the KT boundary, possibly by 200,000 years; others, that they are 200,000 years more recent! But the "evidence," which comes from Montana, is hotly contested. The fossils may have been displaced by later geological events. The fossils of the largest of these animals are rare, the picture of their disappearance very fuzzy in any case and, in fact, differs from one continent to another. Sometimes a great distance separates the last dinosaur bones from the first Cenozoic mammal bones. It has not yet been possible to establish for certain whether the last great saurians disappeared simultaneously. The picture on which many paleontologists seem to be converging, however, is of a gradual decline in the diversity of dinosaur species over the last few million years of the Mesozoic, with an undoubted acceleration several hundred thousand years before the boundary. So far as the dinosaurs are concerned, we cannot (yet?) speak strictly of a sudden mass extinction.
Other terrestrial vertebrates were affected, among them the flying reptiles and the marsupials. But freshwater fish and amphibians, turtles and crocodiles, and snakes and lizards were almost untouched, and placental mammals, whose fate particularly concerns us since our ancestor was among them, survived. In the seas, one group of large reptiles, the mosasaurs, died out; over half the sharks and rays disappeared, but the rest lived on. In general, it was the larger and the more "specialized" animals that vanished, while the smaller ones and the "generalists" pulled through rather well.16 Those with the broadest geographical distribution in the most varied environments survived better than the others.
The evolution of vegetation close to the KT boundary seems confused. Some experts speak of a gradual decline that started a few million years before; others emphasize the discovery, in North America, of an uncommon abundance of fern spores. These "opportunistic" plants are the first to recolonize a forest after a fire. They may mark the reconquest of a devastated world from which we know that many flowering plants, the angiosperms, had disappeared. Yet a few hundred kilometers farther north, in Canada, we find no further trace of this "fern peak," and the effects of mass extinction seem greatly reduced.The French paleontologist Eric Buffetaut stresses this selective, nonuniform aspect of extinctions in the continental context. To his way of thinking, a severe deterioration of climate or a simple size effect (the disappearance of the largest forms) cannot by themselves be a cause of extinction. The crocodiles, for example, which according to him are as sensitive to cold as the dinosaurs were, survived. Large crocodiles "made it" across the boundary, while many small marsupials did not. Noting that freshwater communities did not suffer too much, and that it was the large plant-
16 This paleontologic situation does not necessarily seem to apply to MDs . . .
eaters that disappeared, Buffetaut suggests that a crisis in the plant kingdom interrupted the food chain, thus wiping out the herbivorous dinosaurs and by consequence their carnivorous predators. Meanwhile the small carnivorous, insectivorous, or omnivorous vertebrates, and the freshwater organisms, whose food chains did not depend so heavily on the plant kingdom survived.
For his part, the American scientist Robert Bakker, an original and controversial specialist in dinosaurs, has long contended that a great number of these saurians, and particularly the largest and most active, were warm-blooded. So the comparison with crocodiles and other cold-blooded animals would no longer apply. Bakker believes that the extinction of his favorite animals was a prolonged event, quite simply caused by the low sea level at the end of the Cretaceous. This made it possible for the more mobile species, the more prodigal expenders of energy, to migrate over long distances, increasing the risk that they might succumb to diseases to which they were not resistant; by comparison, the smaller animals (among them our ancestors) and the cold-blooded species, being less mobile, would not have traveled far from their original habitat. This idea goes back to one of the fathers of the study of dinosaurs, Owen (1804-92), who was struck at the devastation caused by the introduction of bovine leprosy in Africa and at the adverse implications for kangaroos when rabbits were brought to Australia.
However, the continental paleontologic record by itself does not permit us to determine either the duration of the crisis, or its first causes. How can we evaluate the influence of changes in rock type and rock preservation (which may extend to a total absence of some periods)? How can we assess the local, regional, or global value of a given observation? How can we study scale in space and time or seek the cause of crises, whether fluctuations in climate or in sea level?
The marine environment, where sedimentation is generally more regular than in the continental context, offers more hope. The hard parts of the bodies of marine animals fall to the bottom and are rapidly covered. But 90% of geological sections from the KT boundary are incomplete and give the appearance of a single and abrupt mass extinction. Detailed analysis of the few very continuous sections (those with high sedimentation rates) where the rock and its fossils have been studied centimeter by centimeter yields a very different spectacle.
The marine invertebrates, such as the mollusks, do not furnish a very clear picture. Their diversity and abundance decline a few hundred thousand years before the boundary,17 and then at the boundary itself. Some generalist species of simple morphology survive into the start of the Cenozoic. Ward's work with ammonites in the Basque country first showed a decline in species' diversity long before the boundary. But new fossils discovered in nearby sections a few years later have now attested the presence of some species of ammonites within a few meters of the boundary. And in the summer of 1996 the geochemist and astrophysicist Robert Rocchia even found a beautiful mold of an ammonite shell only 7 cm below the boundary! Today, Ward believes that a gradual extinction, caused by the slow drop in sea level at the end of the Cretaceous was followed by a final, abrupt extinction.
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