America is a nation of volunteers. Management expert Peter Drucker has pointed out that one out of every two adults is a volunteer; when they are included, the nonprofit sector is the nation's largest "employer." Without their corps of stalwart, dedicated, (and unpaid), volunteers, the museums of America (and most of the rest of the nonprofit sector) would be but pale shadows of themselves.
Sheehan and his colleagues at the Milwaukee Public Museum realized that the dinosaur specimens in museum and university collections had been amassed not to solve the puzzle of extinction, but for a variety of other reasons—typically because the specimens were large, or rare, or unusual in some way. They reasoned that in order to get a better handle on the question of dinosaur extinction, a new, large sample collected specifically for that purpose was needed. If enough fossils from the upper Cretaceous could be collected, scien tists might be able to tell whether the dinosaurs were already declining well before the boundary as the anti-impactors claimed, or whether they were found right up to the boundary. At least they might be able to shrink the ghastly gap.
But who would do all the work of collecting the required large number of new dinosaur specimens? To the museum professionals, the answer came at once—the volunteers] Sheehan spent three summers in North Dakota and Montana collecting in the Hell Creek formation, accompanied each time by 16 to 25 carefully trained and closely supervised volunteers from the "Dig a Dinosaur" program of the Milwaukee Public Museum, who paid $800 for the privilege. They spread out in "search parties," scouring the Hell Creek terrain for any sign of a dinosaur fossil. When a volunteer found a specimen, a paleontologist went over to make the identification, which was then logged into the computer. Almost all specimens were left in place rather than being collected and removed. To reduce the effect of different sedimentary environments, the collectors restricted their efforts to one of three sedimentary facies (distinct rock types that sedimentologists can identify). The volunteer workers logged the amazing total of 15,000 hours of careful fieldwork and found 2,500 dinosaur fossils. A key point of the study, one that differs from earlier work, was that they recorded not only whether a given species persisted at a certain level, but how many times it occurred. In other words, they measured not only taxonomic diversity (how many species are present no matter how rare), but what they called ecological diversity (how many individuals are present). This is a distinction with a difference: A species that was almost but not quite eliminated would leave taxonomic (naming) diversity unchanged—only a single individual would retain the taxon's name on the list. On the other hand, ecological diversity (number of individuals) would have plummeted, showing that it is obviously the more informative measure for tracing patterns of extinction.
The Milwaukee crew divided the Hell Creek formation into three units of approximately equal thickness, with the top one reaching up to the K-T boundary, and measured the number of dinosaur families in each third. Their most diligent search found dinosaur fossils within 60 cm of the K-T boundary, thus shrinking the ghastly gap over which Luis Alvarez and Clemens had tangled far into the Berkeley night. Their focus on ecological diversity allowed them to conclude: "Because there is no significant change between the lower, middle, and upper thirds of the formation, we reject the hypothesis that the dinosaurian part of the ecosystem was deteriorating during the latest Cretaceous. These findings are consistent with an abrupt extinction scenario."51
Not surprisingly, Clemens and Archibald, among others, disagreed. Clemens, for example, citing studies by Peter Dodson,52 which he said showed a decline in diversity, held firm: Any viable hypothesis of the causal factors of dinosaurian extinction must account for the evidence of decrease in generic diversity."53 Sheehan and Fastovsky countered by also quoting Dodson: "There is nothing to suggest that dinosaurs in the . . . Maastrichtian were a group that had passed its prime and were in a state of decline."54
The work of the good folks from the Milwaukee Public Museum drives a nail in the coffin of arguments for the gradual decline of the dinosaurs. In an interview published in 1994, looking back, Clemens appeared to doubt the earlier evidence: "The 'Ghastly blank,' the un-fossiliferous meter or so separating the stratigraphically highest dino-saurian bones and the iridium-enriched layer, might well be the product of leaching of fossils from the uppermost Hell Creek by acidic ground waters derived from the widespread Tullock Swamps."55
Reviewing this debate, a territorial chauvinism becomes obvious. Dinosaurs lived on every continent, yet the entire argument about their extinction is based on evidence from one small area in eastern Montana where, as Dale Russell pointed out in his review of Archibald's book (Dinosaur Extinction and the End of an Era"), "tabulations of dinosaur species are based on about 100 incomplete skeletons."57 On the basis of this limited sample, from a geologically complicated and minute fraction of dinosaur-land, some paleontologists have made the most categorical statements about dinosaur extinction. Yet surely, as we saw at Zumaya, local conditions can cause a blank. Perhaps for unknown reasons, the dinosaurs simply left that part of the Hell Creek area. In the Raton formation in Colorado, the tracks of duck-billed hadrosaurs are found only 37 cm below the iridium layer.58 Since tracks are not reworked and required a living, breathing dinosaur, this proves that the dinosaurs lived to within a few thousand years of the K-T boundary. In Mongolia, more dinosaur species are found in the latest Cretaceous than below it.59 In China, dinosaur fossils are found so close to the K-T boundary that some say they actually transcend it.60 In the Deccan, dinosaur eggshells are found in an intertrappean bed just at the boundary.61 All this evidence shows that the dinosaurs did not go extinct well before the K-T boundary, but lived right up to it. If a blank does exist, it is not so ghastly after all, but merely, once again, an artifact of the Signor-Lipps effect.
In Chapter 9 I listed the two major predictions for the fossil record made by the Alvarez theory. How well has the dinosaur evidence met them?
prediction 1: Prior to the K-T boundary, the dinosaurs were not already going extinct for some other reason. Their extinction was sudden and right at the boundary.
Dinosaur expert Peter Dodson, and the work of Sheehan and colleagues, indicate that the first part of this prediction is met: There was no gradual decline. The dinosaurs did not become extinct well before the K-T boundary, but lived right up to it.
PREDICTION 2: Dinosaur fossils are not found above the iridium horizon.
With a few unconfirmed exceptions, this prediction is also met. The "Tertiary dinosaurs" from Hell Creek and China may not be that at all, but instead result from misplacement of the K-T boundary or from reworking. If it does turn out that a few dinosaurs survived into the Tertiary, they will not be sufficiently common to falsify this prediction or to figure importantly in earth history.
In addition to these two, there is a question that can be asked, even if it does not lend itself to a third prediction: Can the Alvarez theory help to explain the selectivity of the K-T extinction? Since the days of Baron Cuvier, the French father of taxonomy and paleontology, at the turn of the eighteenth century, the extinction at the end of the Cretaceous has been known to have been strangely selective. Marine reptiles, flying reptiles (including the pterodactyl, named by Cuvier), and the dinosaurs died out, as did many marine invertebrates, including the ammonites and most of the planktonic foraminifera. But many terrestrial vertebrates—snakes, crocodiles, turtles, and mammals—and some plants, survived. The impact-extinction theory ought to make it easier to explain this peculiar pattern. On the other hand, if the theory does not help, it may not be because it is wrong but rather because we lack knowledge and imagination. After all, no one has yet been able to explain under any theory why the crocodiles and turtles survived and the dinosaurs did not. Were the impact theory also to prove wanting, it would be no worse off than the theories that geologists have traditionally pre-ferred—those theories cannot explain the selectivity either. But certainly, an ability to explain the selectivity of the K-T extinctions would immeasurably strengthen the Alvarez theory.
Was this article helpful?