What are some of the problems with reconstructing changes in dinosaur populations over time? For one thing, dinosaurs are, by comparison with foraminifera for example, large beasts and, more importantly, not particularly common.1 For this reason, the possibility of developing a statistically meaningful database is impractical, and rigorous studies of dinosaur populations are very hard to carry out. Just counting dinosaurs can be difficult. Mostly, one doesn't find complete specimens, and adjustments have to be made. For example, ifyou happen to find three vertebrae at a particular site, they might be from one, or two, or three individuals. The only way to be sure that they belong to a single individual is to find them articulated. Suppose they are not; then one must speak of minimum numbers of individuals, in which case the three vertebrae would be said to represent one individual: that would be the minimum number of individual dinosaurs that could have produced the three vertebrae. On the other hand, if one found two left femora, then the minimum number of individuals represented would be two.
It would be nice to use all the specimens that have been collected in the last 170 years ofdinosaur studies in a survey of changes in dinosaur diversity. Unfortunately, dinosaur specimens are commonly collected because they are either beautiful specimens or rare; hardly criteria for ensuring that an accurate census of dinosaur populations has been performed. So any study that really is designed to get an accurate census of dino saur abundance or diversity at the end of the Cretaceous must begin by counting specimens in the field, which is a labor-, time-, and cost-intensive proposition.
Then, of course, the taxonomic level at which to count dinosaurs can create problems. Suppose that two specimens are found; one is clearly a hadrosaurid and the other is an indeterminate ornithischian. The indeterminate ornithischian might be a hadrosaurid, in which case we should count two hadrosaurids. But then again it might not (because its identity is indeterminate), in which case calling it a hadrosaurid would give us more hadrosaurids in our survey than actually existed. On the other hand, calling both specimens "ornithischians" is quite correct, but not very informative, if we hope to track the survivorship patterns of different types of dinosaurs.
Finally, within the sediments themselves, problems of correlation exist. Suppose that, in Montana, we record the last (highest level) dinosaur in the Jordan area and then record the last (highest) dinosaur in the Glendive area, about 150 km away from Jordan. Can these two dinosaurs be said to have died at the same time? How could one possibly know? Suppose that in fact these dinosaurs died 200 years apart. An interval of 200 years, viewed from a vantage point of 65 million years is literally a snap of the fingers. Yet 200 years is a long time when one is considering an instantaneous global catastrophe that ideally is measured in milliseconds.
1. How rare are dinosaurs in this part of the world? Of course, we cannot know the density of dinosaurs within the rocks, but their surface density was calculated by sedimentologist P. White and colleagues, using the Sheehan et al. database (Fastovsky and Sheehan, 1997, p. 527), reported, "White and Fastovsky calculated that 0.000056 dinosaurs are preserved per m2 of outcrop. Considered more realistically, in a statistical sense one must search a 5 m wide path of exposed rock that is 4 km long to find a single dinosaur fragment identifiable to family level (or lower)." (Fastovsky, D. E. and Sheehan, P. M. 1997. Demythicizing dinosaur extinctions at the Cretaceous-Tertiary boundary. In Wolderg, D. L., Stump, E. and Rosenberg, G. D. (eds.), Dinofest International. Academy of Natural Sciences, Philadelphia, p. 527.)
Dinosaurs. Dinosaurs are difficult animals to study (Box 15.2) and for many years, no scientific study of dinosaurs at the K/T boundary was ever carried out. Inexplicably, although no data were ever published to show this, it was long accepted that dinosaurs gradually died off about 10 million years before the boundary.
In the late 1980s and through the 1990s, field-based studies were finally designed and carried out to determine the rate of dinosaur extinction. All of them took place in the American West: two on what had been a low-lying coastal plain in what is now eastern Montana and western north Dakota, and one in an intermontane basin in what is now Wyoming, which formed in the ancestors of the present-day Rocky Mountains, rising to the west (Figure 15.14). The three studies all concluded the same thing: the dinosaur extinction was geologically abrupt.
The two studies in the coastal plains of what is now eastern Montana and western North Dakota were quantitative censuses of dinosaur diversity during the last 1.5 million years of the Cretaceous. One looked at the ecological diversity; that is, the proportion of the total dinosaur population taken by each of eight families of dinosaurs. The other counted genera of all vertebrates through the last 1.5 million years of the Cretaceous in that region, looking for changes in either abundance or diversity. Both demonstrated that, within about 150,000 years of the K/T boundary, neither ecological diversity nor abundance and generic diversity changed (Figure 15.15).
The last of the three studies, carried out in what was an ancestral Rocky Mountain intermontane basin (Figure 15.16), utilized an approach very similar to the coastal plain study of vertebrate genera described above. And the results from that study were much the same as those from the other studies: the extinction of the dinosaurs was geologically abrupt. Major extinctions occurred in most groups, but particularly in dinosaurs and mammals (Figure 15.16). A key point, however, is that none of these studies can distinguish whether the extinction took every day, or whether it took only the last minute, of that last 150,000 years of the Cretaceous.
In summary, the very limited data from the Western Interior of the USA strongly indicate an abrupt end for the non-avian dinosaurs. Only time and much further study will enable us to integrate other dinosaur-bearing localities from around the world into what is already known of North America.
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