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
Read together, Naish's and Sampson's new books give us a sense of the vast array of dinosaurs as keys to puzzles in a science proceeding ever forward. For example, back in the day, a dino fan knew a rather static fact: the meat-eating bipedal kinds of dinosaur (theropods) came in two types. Some were the big kind, like Tyrannosaurus Rex, and some were smaller, such as the poison-squirting Dilophosaurus in Jurassic Park. Early on, so the story went, one branch of theropods got big while the others stayed small, and The List was a matter of cataloguing a burgeoning list of Big Ones and Small Ones.
But over the past two decades, scholars concluded that this classification was merely impressionistic. Various strains of giant theropods evolved from smaller ones at different times. There were latter-day relatives of Dilophosaurus, unknown until the 1980s, that got as massive as Tyrannosaurus. Meanwhile, Tyrannosaurus was one offshoot of a branch that included critters of moderate size, such as Velociraptors.
Until the past decade, the point was largely made through arcane skeletal likenesses between certain Small Ones and Big Ones. But two additions to The List of late (Dilong and Guanlong), covered in Naish's book, demonstrate it especially clearly. There were, we now know, compact little proto-tyrannosaurs, not yet massive like T. Rex but well on their way to its defining features.
The point that creatures like this illuminate, beyond bulking up The List, is that giantism was an ever-tempting "strange attractor" for theropods. If it were just that once, one line that happened to opt for enormity, we would be left with a happenstance of little interest beyond that which amazes a kid impressed by "monsters." However, enormity was apparently a trend especially likely of dinosaur biology, which leads to a general question: why, really, did dinosaurs get so big? There has never been any such trend among mammals or even modern reptiles.
Sampson shows that the massiveness in question was connected to, of all things, the Dinosaur Renaissance-era theory that dinosaurs were warmer-blooded than lizards. The idea here was that dinosaurs were warm-blooded like mammals and were possibly even covered with fur. Today, however, that dramatic notion is rather "1975." Evidence suggests that, as so often, the truth was in between: dinosaur metabolism was zestier than lizardness (ectothermy) but rested before the tipping point of catness (endothermy). Sampson terms it mesothermy, under what he calls the Goldilocks hypothesis: not too cold and not too hot but just right.
In itself, this neither-fish-nor-fowl conclusion is incommensurate with the notion of dinosaur science as an "odyssey," as Sampson's title has it. Where is the fun in the mundane truth that dinosaurs weren't cold-blooded like snakes but weren't warm-blooded like Saint Bernards either? Sampson argues, however, that this very fact was exactly why dinosaurs could get so large.
As ectotherms' metabolic rate increases, their energy rate increases as well. Two main results of this increase in energy are the generation of heat internally instead of depending on the sun and, notably, the building of bodily mass. Hence giantism is a possibility. Along for the ride also come active childrearing and vigorous courtship strategies, evidence for which has arisen for dinosaurs over the past thirty years.
However, at a certain tipping point in the metabolic rate, so much energy goes into the generation of bodily heat—making, for example, a warm cat on your lap—that there is less available for building bodily mass: there have never been cats the size of trucks. Only mesothermy, wishy-washy as it seems, can create something as staggeringly massive as Brontosaurus, or its even larger relatives that reached lengths of over one hundred feet.
Sampson thinks mesothermy explains a lot of other things about dinosaurs that otherwise seem just so. For example, one branch of theropods that yielded fecund subbranches (i.e., countless additions to The List) were apparently actually warm-blooded. This has become clear from gorgeously detailed fossils unearthed in China of late, revealing feathers, not for flying but for warmth. However, this does not mean that feathers were a quirk of dinosaurs in China: relatives of the same theropods have long been known from elsewhere but not as exquisitely preserved, meaning that the living creatures had feathers, too.