There is a major problem with the asteroid extinction hypothesis of the dinosaurs that fails to convince Bakker of its soundness. He notes that the fossil record when looked at more closely actually indicates something other than a sudden disappearance of dinosaurs. In fact, the fossil record shows a period of gradual decline in the evenness of dinosaur species beginning early in the Late Cretaceous. By "evenness" Bakker is referring to the ratio of one species to another. In most types of healthy ecosystems the varying species should be approximately equal to one another. It's once you have a disproportionate amount of one or two species over others that the ecosystem begins to degrade. Bakker found in the dinosaurs of the Lance faunas examples of a loss of evenness in the variety of dinosaur species some two million years before the end of the Cretaceous. For example, "the genus Triceratops made up 70 to 80 percent of the finds of large dinosaurs." (437) In other words, several millions of years prior to the end of the Cretaceous there were many dinosaur species already in major decline, some extinct even. Thus, Bakker says, an asteroid impact occurring at the end of the Cretaceous can not fully explain the extinction of most dinosaurs. At best it might have served as "the coup de grace to a dying ecosystem already suffering from massive problems." ( 439)
Bakker's own solution has two interrelated components. For the first part of his explanation Bakker adopts what is known as the "drain-mix-and-cool theory of extinction for the ocean" or for short the "shallow seas theory." (439). This theory states that abnormally warm periods on earth cause shallow seas on land to drain off the continents into the ocean causing, paradoxically, an overall cooling trend for the planet. Bakker on the consequences of these changes for the ecosystem:
"The best answer for the extinction of the great sea animals is that their favorite haunts disappeared when the warm, shallow seas drained off the continents. And the best answer for the extinction of the open-water, deep sea creatures is that surface water becomes colder and more thoroughly mixed with deep water when the shallow seas drain off." (439)
According to Bakker these changes are well documented. He asserts that climatologists have in fact determined that during the middle Cretaceous there were many shallow seas on the present day continents. (For example, present day North America during this period had a huge in land sea extending from the arctic down through central America. See picture to the right.) Moreover, they have also noted that towards the latter portion of the Cretaceous these shallow seas were in fact rapidly disappearing. So then we have a reasonable explanation for the extinction of some dinosaur species via documented climate changes. However, as Bakker notes this explanation is incomplete since the resulting overall cooling trend of the planet would likely not have killed off the big land dinosaurs that were not already in some fashion dependent on the inland seas.
Thus, the second component to Bakker's explanation:
"Let us observe the historical sequence that unfolds on the land during the mass extinctions. Shallow seas drain off, so that land areas once underwater become dry and regions that had been separated from each other become connected by land bridges or island chains. At the same time, mountain-building forces weaken so that there are fewer barriers dividing the terrestrial regions...But the net result is a more homogenized ecosystem where species can pass more easily from one end of a continent to another, and from one continent to another. Such easy intercontinental exchange can be found precisely at the end of the Cretaceous. Until late in the Cretaceous, Mongolia had supported quite a different fauna from that of North America. There were many advanced mammals and protoceratopsid dinosaurs in the Central Asiatic Highlands not found in Alberta, Montana, and Wyoming. But very late in the last epoch of the Cretaceous Period, the Asiatic mammals and dinosaurs began appearing in North America...Could such interchanges over the continents cause extinction?" (441-42)
Bakker answers in the affirmative noting that "one of the unshakable tenets of animal geography is that the most extreme consequences are possible when foreign species move into a new region." (442) Every species carries its own set of parasites and disease organisms that it has over time adapted to. But when these species are introduced into an area with other species that have never had any contact with such diseases and parasites the result is often the decimation of the latter. History offers many examples of this from the Black Plague (introduced from Asia to Europe), to the destruction wrought by Rinderpest (an Asian cattle disease introduced into Africa by Lord Kitchner), to the settling of Old World peoples with Old World diseases (such as smallpox) into the New World causing the devastation of the vast majority of New World populations, and many more. But the dangers of interspecies mixing doesn't just come from unfamiliar germs and bacteria. The introduction of larger animals into a new ecosystem can also wreak havoc as the Australians learned when rabbits, a relatively minor nuisance in Europe, were first brought to the land down under where they now have become a major pest doing much ecological harm to the Outback.
Likewise then with the dinosaurs concludes Bakker:
"The Late Cretaceous world contained all the prerequisites for this kind of disaster. The shallow oceans drained off and a series of extinctions ran through the saltwater world. A monumental immigration of Asian dinosaurs streamed into North America, while an equally grand migration of North American fauna moved into Asia. In ever region touched by this global intermixture, disasters large and small would occur. A foreign predator might suddenly thrive unchecked, slaughtering virtually defenseless prey as its populations multiplied beyond anything possible in its home habitat. But then the predator might suddenly disappear, victim of a disease for which it had no immunity. As species intermixed from all corners of the globe, the result could only have been global biogeographical chaos." (443)
(Note: this intermixing depends on the majority of dinosaurs being warm blooded; cold blooded animals would be unable to travel long distances.)
So to recap, Bakker's position on the extinction of most of the dinosaurs runs as follows:
1.) The fossil record indicates that millions of years before the end of the Cretaceous many dinosaurs species were declining, a few actually becoming extinct. An asteroid impact at the end of the Cretaceous cannot fully account for this data.
2.) In the latter part of the Cretaceous the shallow seas began to drain off the continents causing the extinction of major sea animals as well as opening up massive new areas of land travel.
3.) The resulted intermixing of new species of dinosaurs introduced various agents of extinction such as new germs, parasites, bacteria, etc, and other ecological changes that eventually killed off many of the dinosaurs.
4.) The remaining few species of dinosaurs (excepting a few) may have finally been killed by the impact of a heavenly body of some kind at the end of the Cretaceous.
I for one find Bakker's thesis very persuasive. However, while most of what Bakker put forward in this book has since become part of mainstream paleontology, his views on dinosaur extinction has, to my knowledge at least, yet to be wholly accepted. I suspect this is because Bakker's explanation is so mundane. Most dinosaurs died out because they mixed with other, previously, isolated species of dinosaurs?! In other words, Bakker's explanation fails to dazzle us like the traditional view that most of the dinosaurs were killed off in one fell swoop by a gigantic asteroid. We prefer big, amazing explanations for things, especially mysteries. And for this reason I doubt many people will be convinced that most of the dinosaurs actually died off in a rather unexciting manner. Yet sometimes the truth, whether we like it or not, can be rather, well, banal.