The Great Dinosaur Discoveries
University of California Press, 2009
192 pp., $36.95
Dinosaur Odyssey: Fossil Threads in the Web of Life
Scott D. Sampson
University of California Press, 2009
352 pp., $85.00
John H. McWhorter
Sampson notes that these warm-blooded theropods are exactly the ones that did not trend toward the enormous. For example, the Velociraptor/Deinonychus type was among them, and there was no version the size of a building. Presumably it was because warm blood puts a limit on how big you can get. This is also the only branch of theropods that includes some varieties that went vegetarian (Naish's book, in a rare lapse, does not include a picture of these massive, pot-bellied freaks with Edward Scissorhands claws). Sampson traces this vegetarianism to warm-bloodedness as well: it's a challenge for carnivores to find enough food to keep their high metabolism going, and easier to do the trick eating plants, which have the courtesy to sit still while being eaten.
Another seemingly arcane fact gives Sampson footing for a further step past The List. In certain fossil beds, what has traditionally been seen as a plethora of coexisting animals can be shown as the result of several separate eras, each inhabited by certain of the animals and succeeded by eras with new ones. Especially interesting are the implications Sampson draws from this data for dinosaurs' famous extinction.
There are two competing hypotheses here. One is that an asteroid was the culprit. Roughly 65.5 million years ago an asteroid hit what is today Mexico, leaving a crater more than 100 miles in diameter and a layer of iridium, traceable only to meteorites, worldwide in rocks of this age. And recently, Sankar Chatterjee of Texas Tech University has argued that there is evidence for a much larger asteroid impact off the coast of India roughly 300,000 years later. The rival view suggests that the more gradual effects of volcanic eruptions and rising sea levels were also critical to the dinosaurs' extinction.
This time, Sampson argues in favor of the more "fun" hypothesis. One argument for the less dramatic view stressing gradual eclipse has been based on the claim that there were supposedly fewer kinds of dinosaur closer to their extinction than before—that is, they were apparently tapering off already before the asteroid hit. However, Sampson argues that this drop-off is an illusion. It isn't that the earlier localities were teeming with an unusual variety of creatures. Rather, what we find there is evidence of various periods during any one of which only a few species existed: even long before the asteroid hit, there were no more species around than in those fossil beds from just before the End of Dinosaur Days. The extinction, then, was truly abrupt.
These two books show the layman how almost any dinosaur on The List is significant not just as a specimen in a collection pinned like a dead butterfly among many in a box, but as a piece of evidence for a larger point about just how such fascinating creatures could ever have existed, how they lived their lives, and why today they only do so as the feathered, flying creatures we know as birds. Now dinosaurs are not just a list, but players in a fascinating drama. They encourage us to surmount tabulation and open up to cogitation.
Example: Tyrannosaur skulls are full of gaping holes. They are, in essence, conglomerations of struts, filled in the living creatures with air sacs. Conventional wisdom has been that this evolved to make their heads lighter as they became enormous. Then, Tyrannosaurs also have strangely tiny arms, of no apparent use. This, we have been told, was because these monsters were basically land sharks, "jaws on feet," with their massive heads, afforded by the skeletal lightness, rendering the arms useless and encouraging their shrinkage.
I have always been nagged by a sense that this did not qualify, in the strict sense, as an explanation. After all, there are plenty of smaller creatures with light, flexible skulls (snakes, for instance). Why would evolution not wend into a smaller rendition of "jaws on feet"?
That, for me, was merely passing query. As I write this, however, exactly such a creature has been discovered. The wires are abuzz with the discovery—in China, of course—of little Raptorex: a Tyrannosaur with all of the major features of T. Rex, but only about as big as a human.
T. Rex did not evolve its airy head to compensate for its enormous size: there actually were, we now know, junior land sharks. Features like the skull and the arms were not after-the-fact copings with the size but built in before it. This leaves the enormity by itself as a mysterious, isolated quirk—or not, if we refer back to Sampson's hypothesis that mesothermy encouraged bulk. As such, a dinosaur obsessive like me can receive Raptorex not as just one more theropod on The List, but as support for an explanation of why dinosaurs were so large.