We wish to learn how the dinosaurs died, not how they lived; therefore our interest is in their last few million years during the late Cretaceous. How many dinosaur species were living then, where are their remains, and what do they tell us? According to paleontologist Peter Dodson, only about 2,100 articulated bones of dinosaurs have been collected, and they span 160 million years.40 If spread evenly, we would have one specimen for each 75,000 years, but in fact the discovered remains of dinosaurs are highly clustered in time and space. All the Tyrannosaurus rex specimens, for example, come from Montana and the Dakotas. Therefore in the rock record there are spans of millions of years during which we know dinosaurs lived but of which we have no trace. Dodson reported that 336 recognized dinosaur species have been identified, but that nearly 50 percent are known only from a single fossil specimen. The 336 species belong to 285 genera (remember that genus—plural, genera—is the next taxonomic grouping above the species level), of which over 70 percent occur only in one rock formation. Paleontologists have learned that a typical genus has several species and that the species:genus ratio therefore is usually well above 1:1. To find it so close to 1:1 (336:285) for the dinosaurs indicates that sampling has barely scratched the surface. Recent experience confirms this conclusion, for new dinosaur discoveries seem to pop up in the press every few months. Surely many more dinosaur genera await discovery. Dale Russell estimated that we have found only about 25 percent of the genera that lived during the late Cretaceous alone, which is by far the most studied period.41
Although dinosaur specimens are few, they occur in late Cretaceous rocks on every continent and at dozens of sites around the world. To study their extinction, all we need are dinosaur-bearing sections that extend from the late Cretaceous up to the K-T and at least a short distance beyond it into the Tertiary. At how many places in the world can such sections be found? The answer, shockingly, is three: Alberta, Wyoming, and Montana. The Hell Creek formation near Glendive, Montana, is by far the best studied. Dinosaur research is continuing in other countries today, in Argentina and in China, for example, and in time more sites will meet the criteria. But up to now, to provide the litmus test for dinosaur extinction theories, paleontologists have had no alternative but to rely on fossils from the upper Great Plains, and from the Hell Creek formation in particular.
What, then, do we know about the dinosaurs from the Great Plains? At Snowbird I, Tom Schopf pointed out that our knowledge of the Maastrichtian (latest Cretaceous] dinosaurs derives from only 16 known species, which have in turn been identified from only 200 individual specimens.42 No dinosaur has captured our imagination better than the horrific Tyrannosaurus rex, yet only a handful of complete skulls has ever been found.
When this paucity of hard evidence is added to the problems of gaps, migration, Signor-Lipps effect, dissolution, location of boundaries, channel cutting, reworking, and so on, it is clear that any confidently definitive statement about the demise of the dinosaurs based on scarce fossil evidence is apt to be wrong. A bang can easily be mistaken for a whimper, and vice versa.
The Hell Creek formation achieved notoriety in 1902 when famed dinosaur hunter Barnum Brown discovered there the first, magnificent Tyrannosaurus rex. Over the more than nine decades since, fossil hunters and serious paleontologists have returned again and again to northeast Montana and Hell Creek to collect and to decipher.
How did the Hell Creek rocks come to be? In late Cretaceous times, a shallow sea stretched across western North America from Canada to Mexico. As it shrank and retreated, the sea left behind a sequence of sediments formed in different marine and nearshore environments. Subsequent deposits entombed these older sediments, which were lithified (turned into rock) by the resulting heat and pressure; erosion later exhumed and exposed them for us to see today. By studying the composition and features of sedimentary rocks and their fossils, sedimentologists and paleontologists can reconstruct the history of ancient landscapes in great detail. They know that the oldest formation in the Hell Creek area, the Bearpaw Shale, formed from muds deposited on the floor of the ancient sea. Above the Bearpaw lies a sandstone that formed from the beaches left behind as the seaway retreated. It in turn is overlain by two formations deposited by streams that meandered back and forth across marshy, nearshore floodplains: the Cretaceous Hell Creek formation and higher up, the Tertiary Tullock formation. Near the base of the Tullock lie several thin beds of coal that formed in the reducing conditions in the coastal swamps. Geologists had come to accept, as the K-T boundary in Montana, the lowermost of these coals, the "Z coal," in part because no dinosaur remains occur above it. (Admittedly, a certain amount of circular reasoning is going on here.) The uppermost dinosaur fossils were thought, especially by Clemens, to lie about 3 m below the Z coal and no closer, suggesting that the dinosaurs had gone extinct scores of thousands of years before the boundary.
From decades of study, vertebrate paleontologists had come to conclusions about the end of the dinosaurs that the Alvarez theory was initially unable to shake. For example, although 36 dinosaur genera occur in rocks dating some 10 million to 11 million years before the K-T boundary, those immediately below the boundary contained only about half that many genera. To most paleontologists, this was a clear indication that the dinosaurs were on the way out well before the end of the Cretaceous and that, if anything, impact had delivered only a coup de grace. Dale Russell pointed out, however, that the higher number of genera from the older rocks was a total obtained by adding together all those found at 25 locations around the world, whereas the smaller, later number had come from only the three North American sites. This suggested that the difference might be only a sampling effect. Nevertheless, by the mid-1980s, most paleontologists who had studied the Hell Creek fossils had firmly concluded that the dinosaurs had already died out some 20,000 to 80,000 years prior to the K-T boundary, before the putative arrival of any meteorite. Some paleontologists presented evidence that the dinosaurs had already started to be replaced by mammals well down in the Cretaceous, and claimed that this showed the dinosaurs had started to disappear well before the K-T boundary.43
In the mid-1980s, the confidence in this picture began to wane. It was shown then that the mammals occurred not in rocks of true Cretaceous age, but in Tertiary channel deposits cut down into the Cretaceous rocks (see p. 134], thus evaporating the evidence for early replacement of dinosaurs by mammals. Standing the argument on its head, others have reported finding dinosaur remains in Tertiary rocks in the Hell Creek region and claim that the dinosaurs did not become extinct at the K-T after all!44
Further work at Hell Creek eroded the attempts at precise correlation of the rock strata there. According to Jan Smit, previous workers had placed the K-T boundary at Hell Creek between 2 m and 12 m too high (the same point Smit made for the sediments at Mimbral).45 When a boundary is set too high, species immediately below it appear to have gone extinct earlier than they actually did. By placing the K-T boundary too high, according to Smit, the false impression was created that the dinosaurs had gradually disappeared and had been replaced by mammals well before the end of Cretaceous time. The Z coal beds, thought to mark the bottom of the Tertiary, were shown not to be of the same age at different locations, eliminating their usefulness as a time marker.Finally, the Hell Creek strata were found to be riddled with gaps. At the 1995 meeting of the Geological Society of America, J. K. Rigby, Jr., of Notre Dame, reported paleomagnetic studies which showed that 300,000 to 500,000 years of the rock record were missing there.48 The modern view is that the rocks of Hell Creek cannot be matched from one spot to another with sufficient resolution to make the precise chronology of dinosaur extinction clear.
As sometimes happens, the more a phenomenon is studied, the more questions are raised and the less confident the answers become. The Hell Creek strata now appear so complicated and full of gaps that it is hopeless to attempt to use them as the litmus test of dinosaur extinction. Some scientists realized that they needed to take an entirely different approach from the traditional, one that provided large enough samples so that statistics could be employed, and which did not depend on the precise location of the K-T boundary at Hell Creek. This view gave rise to two important studies.
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