Saturday, May 19, 2012

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:

" 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. 

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