CSI Cretaceous Part 2

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.    

CSI Cretaceous Part 1

Many posts ago I blogged about the present day genetic evidence which conclusively indicates that modern day birds are in fact descended from some of the very few dinosaurs that managed to survive the mass(?) extinction event(s?) that occurred at the end of the Cretaceous. Wanting to know more I picked up Robert T. Bakker's The Dinosaur Heresies which has turned out to be one of the better reads I have had so far this year.

Prior to writing this book Bakker along with a few other paleontologists had been challenging what had by mid-20th century become the orthodox position on dinosaurs. A view that represented dinosaurs as cold blooded, scaly, slow moving, and dim witted, i.e., reptiles par excellence. The thrust of Bakker's work was to show how this consensus was mostly wrong-headed, principally by arguing for the warm blooded nature of most dinosaurs. The Dinosaur Heresies then was meant to serve both as platform for getting these ideas out to the general public and as a place where Bakker could cumulatively present the evidence he had accrued through years of research. I'm no paleontologist of course but in my opinion Bakker succeeded on both counts.

But it doesn't really matter whether I found his arguments persuasive because since the book was first published in 1986 Bakker (and others) have been repeatedly vindicated so much so that what was then a revolutionary position has now become the general consensus among today's paleontologists. These ideas would come to influence Steven Spielberg with Bakker coming to serve as one of Jurassic Park's scientific advisers. Thankfully so since we might have been left with a plodding Tyrannosaurus rex who procured most of his food from scavenging rather than through hunting. The result would have been a decidedly less exciting film. (Note: there are still a few paleontologists who maintain that the T-rex was a scavenger. However, this remains a marginal view. Go here for more.)

The book itself is simply too rich to attempt a proper review here. Plus, I'm not really qualified to do so. But in the next couple of posts I want to talk about one chapter in the book that I found particularly illuminating.

One of the more fascinating mysteries when it comes to the dinosaurs has to do with their extinction. (Well, this is a bit of a misnomer since dinosaurs technically still exist as modern day crocs and birds.) Species of course come and go all the time, but what makes the extinction of the dinosaurs special according to Bakker is that "no new wave of species appeared to replace those that had died out." (436) And as Bakker notes the solutions given to this mystery have been many. Perhaps it's best to quote him at length here:

"...it is suggested that dinosaurs died out 'because the weather got too hot'; 'because the weather got too cold'; 'because the weather got too dry'; 'because the weather got too wet'; 'because the weather became too hot in the summer and too cold in the winter'; 'because the land became too hilly'; 'because new kinds of plants evolved which spread deadly diseases'; 'because new kinds of mammals evolved which competed for food'; 'because new kinds of mammals ate the dinosaur eggs'; 'because a giant meteor smashed into the earth'; 'because a supernova exploded near the earth'; 'because cosmic rays bombarded the earth'; or, 'because massive volcanoes exploded all around the earth.'" (425)

The view that came to be accepted in Bakker’s time and that is generally still held today (with some modification) is that a heavenly body of some kind, most likely an asteroid, impacted the earth some 65 million years ago wiping out most of the dinosaurs in one fell swoop. The primary evidence for this position has to do with a thin layer of iridium that features prominently in the geological record around the time of the end of the Cretaceous. This is significant because iridium is a noble metal that is “extremely rare on the earth’s surface, but much more abundant in celestial bodies such as meteors, asteroids, and dead stars.” (432) The idea then is that this iridium layer found in the geological record can best be explained by theorizing the impact on earth of a large extraterrestrial body some 65 million years ago that heaved up massive amounts of rich iridium clouds into the atmosphere which then eventually settled back onto the ground to form the iridium layer that geologists today have identified.

When this hypothesis is then viewed in light of the fossil record which indicates that most dinosaur species died out around the same time it’s hard not to see a correlation between the two. Additionally, mass extinction by an asteroid would explain why a few dinosaurs did manage to survive. For example, the ancestors of modern day crocodiles because of their cold blooded composition would have been able to hibernate so as to wait out the environmental changes such a collision would have caused. Similarly, small theropod dinosaurs and tiny mammals could have burrowed and waited out the changes as well only to later emerge and evolve into birds in the one case and our mammalian ancestors in the other. So then it is easy to see why this theory of dinosaur extinction has persuaded most and continues to be promulgated in various dinosaur media today.  

But there is a problem with this hypothesis that keeps a few like Bakker from fully adopting it. This, along with Bakker's peculiar solution, we will look at next time. 

The Chicken or the....Dinosaur???

I was about to turn 11 when I saw Jurassic Park for the first time. The experience is something that I vividly remember because I had always had a fascination with dinosaurs. In fact, my brother and I's room was typically scattered with all kinds of toys, many of which were dinosaurs of one sort or another, especially of the Tyrannosaurus Rex. And the earliest books that I recall reading were books about dinosaurs. Supposedly, according to many members of my family, my brother and I were experts often having to correct others on their mistaken identification of some dinosaur species. In addition, I watched as many movies and TV shows that I could at the time about dinosaurs. So naturally when I heard about Spielberg making a dinosaur movie called Jurassic Park I begged my mother to take us to opening night which she graciously did. I was of course awe struck by the entire film but specifically I remember being fascinated by Dr. Grant's implication early in the film that birds evolved from dinosaurs as well as the short discussion he had with Timmy about a book he wrote on the same topic. Now of course I didn't really have any idea what evolution was or how it worked then; nevertheless the mere suggestion that some dinosaurs became modern day birds captivated me.

The theory that birds originated from dinosaurs isn't all that novel having been proposed as far back as 1861 upon the discovery of Archaeopteryx, a winged and feathered dinosaur. However, it took time for the theory to gain any traction, partly because of the intense controversy it engendered among evolutionary biologists. But the hard to deny vast similarities between the bones of some dinosaurs (such as the Velociraptor) and that of modern day birds, the discovery of many more fossils linking the two such as the Chinese feathered dinosaurs, and advances in modern genetics has persuaded most of the viability of the theory. It's especially the advances in genetics that has convinced most. Amazingly, what geneticists are able to do today, while a chicken and/or bird is an embryo, is reactivate some of its atavistic genes to give it teeth, remove its feathers, and even grow it a tail among other modifications. (Note that this isn't just convincing proof that birds evolved from dinosaurs; it's compelling, indeed conclusive, evidence for the theory of evolution itself). I had stumbled upon this some time ago while watching a program on the Discovery Channel but had forgotten about it until I recently watched a TED video presented by famed paleontologist Jack Horner (who was actually partly the inspiration for the character of Alan Grant in Jurassic Park). I am thus compelled to share this video which does a good job summarizing the gains geneticists have made in "retro-engineering" a dinosaur from a chicken. Enjoy.

(For the impatient lot of you I would suggest starting at about 10 mins into the presentation.)