The Panda’s Thumb (26 page)

The remarkable thing about dinosaurs is not that they became extinct, but that they dominated the earth for so long. Dinosaurs held sway for 100 million years while mammals, all the while, lived as small animals in the interstices of their world. After 70 million years on top, we mammals have an excellent track record and good prospects for the future, but we have yet to display the staying power of dinosaurs.

People, on this criterion, are scarcely worth mentioning—5 million years perhaps since
Australopithecus
, a mere 50,000 for our own species,
Homo sapiens
. Try the ultimate test within our system of values: Do you know anyone who would wager a substantial sum, even at favorable odds, on the proposition that
Homo sapiens
will last longer than
Brontosaurus?

WHEN
I
WAS
four I wanted to be a garbageman. I loved the rattling of the cans and the whir of the compressor; I thought that all of New York's trash might be squeezed into a single, capacious truck. Then, when I was five, my father took me to see the
Tyrannosaurus
at the American Museum of Natural History. As we stood in front of the beast, a man sneezed; I gulped and prepared to utter my
Shema Yisrael
. But the great animal stood immobile in all its bony grandeur, and as we left, I announced that I would be a paleontologist when I grew up.

In those distant days of the late 1940s, there wasn't much to nurture a boy's interest in paleontology. I remember
Fantasia
, Alley Oop, and some fake-antique metal statues in the Museum shop, priced way above my means and not very attractive anyway. Most of all, I recall the impression conveyed in books:
Brontosaurus
, wallowing its life away in ponds because it couldn't support its weight on dry land;
Tyrannosaurus
, fierce in battle but clumsy and ungainly in motion. In short, slow, lumbering, pea-brained, coldblooded brutes. And, as the ultimate proof of their archaic insufficiency, did they not all perish in the great Cretaceous extinction?

One aspect of this conventional wisdom always bothered me: why had these deficient dinosaurs done so well—and for so long? Therapsid reptiles, the ancestors of mammals, had become diverse and abundant before the rise of the dinosaurs. Why didn't they, rather than dinosaurs, inherit the earth? Mammals themselves had evolved at about the same time as dinosaurs and had lived for 100 million years as small and uncommon creatures. Why, if dinosaurs were so slow, stupid, and inefficient, did mammals not prevail right away?

A striking resolution has been suggested by several paleontologists during the past decade. Dinosaurs, they argue, were fleet, active, and warmblooded. Moreover, they have not yet gone the way of all flesh, for a branch of their lineage persists in the branches—we call them birds.

I once vowed that I would not write about warmblooded dinosaurs in these essays: the new gospel had gone forth quite adequately in television, newspapers, magazines, and popular books. The intelligent layperson, that worthy abstraction for whom we write, must be saturated. But I relent, I think, for good reason. In nearly endless discussions, I find that the relationship between two central claims—dinosaur endothermy (warmbloodedness) and dinosaurian ancestry of birds—has been widely misunderstood. I also find that the relationship between dinosaurs and birds has provoked public excitement for the wrong reason, while the right reason, usually unappreciated, neatly unites the ancestry of birds with endothermy of dinosaurs. And this union supports the most radical proposal of all—a restructuring of vertebrate classification that removes dinosaurs from Reptilia, sinks the traditional class Aves (birds), and designates a new class, Dinosauria, uniting birds and dinosaurs. Terrestrial vertebrates would fit into four classes: two coldblooded, Amphibia and Reptilia, and two warmblooded, Dinosauria and Mammalia. I have not made up my own mind about this new classification, but I appreciate the originality and appeal of the argument.

The claim that birds had dinosaurs as ancestors is not so tumultuous as it might first appear. It involves no more than a slight reorientation of a branch on the phyletic tree. The very close relationship between
Archaeopteryx
, the first bird, and a group of small dinosaurs called coelurosaurs has never been doubted. Thomas Henry Huxley and most nineteenth-century paleontologists advocated a relationship of direct descent and derived birds from dinosaurs.

Archaeopteryx
GREGORY S. PAUL

But Huxley's opinion fell into disfavor during this century for a simple, and apparently valid, reason. Complex structures, once totally lost in evolution, do not reappear in the same form. This statement invokes no mysterious directional force in evolution, but merely asserts a claim based upon mathematical probability. Complex parts are built by many genes, interacting in complex ways with the entire developmental machinery of an organism. If dismantled by evolution, how could such a system be built again, piece by piece? The rejection of Huxley's argument hinged upon a single bone—the clavicle, or collarbone. In birds, including
Archaeopteryx
, the clavicles are fused to form a furcula, better known to friends of Colonel Sanders as a wishbone. All dinosaurs, it appeared, had lost their clavicles; hence, they could not be the direct ancestors of birds. An unimpeachable argument if true. But negative evidence is notoriously prone to invalidation by later discovery.

Still, even Huxley's opponents could not deny the detailed structural similarity between
Archaeopteryx
and the coelurosaurian dinosaurs. So they opted for the nearest possible relationship between birds and dinosaurs—common derivation from a group of reptiles that still possessed a clavicle, subsequently lost in one line of descent (dinosaurs) and strengthened and fused in another (birds). The best candidates for common ancestry are a group of Triassic thecodont reptiles called pseudosuchians.

Many people, on first hearing that birds might be surviving dinosaurs, think that such a striking claim must represent a complete discombobulation of received doctrine about vertebrate relationships. Nothing could be further from the truth. All paleontologists advocate a close affinity between dinosaurs and birds. The current debate centers about a small shift in phyletic branching points: birds either branched from pseudosuchians or from the descendants of pseudosuchians—the coelurosaurian dinosaurs. If birds branched at the pseudosuchian level, they cannot be labeled as descendants of dinosaurs (since dinosaurs had not yet arisen); if they evolved from coelurosaurs, they are the only surviving branch from a dinosaur stem. Since pseudosuchians and primitive dinosaurs looked so much alike, the actual point of branching need not imply much about the biology of birds. No one is suggesting that hummingbirds evolved from
Stegosaurus
or
Triceratops
.

The issue, thus explicated, may now seem rather ho-hum to many readers, although I shall soon argue (for a different reason) that it isn't. But I want to emphasize that these twists of genealogy are of utmost concern to professional paleontologists. We care very much about who branched from whom because reconstructing the history of life is our business, and we value our favorite creatures with the same loving concern that most people invest in their families. Most people would care very much if they learned that their cousin was really their father—even if the discovery provided few insights about their biological construction.

Yale paleontologist John Ostrom has recently revived the dinosaurian theory. He restudied every specimen of
Archaeopteryx—
all five of them. First of all, the main objection to dinosaurs as ancestors had already been countered. At least two coelurosaurian dinosaurs had clavicles after all; they are no longer debarred as progenitors of birds. Secondly, Ostrom documents in impressive detail the extreme similarity in structure between
Archaeopteryx
and coelurosaurs. Since many of these common features are not shared by pseudosuchians, they either evolved twice (if pseudosuchians are ancestors of both birds and dinosaurs) or they evolved just once and birds inherited them from dinosaur ancestors.

Separate development of similar features is very common in evolution; we refer to it as parallelism, or convergence. We anticipate convergence in a few relatively simple and clearly adaptive structures when two groups share the same mode of life—consider the saber-toothed marsupial carnivore of South America and the placental saber-toothed “tiger” (see essay 28). But when we find part-by-part correspondence for minutiae of structure without clear adaptive necessity, then we conclude that the two groups share their similarities by descent from a common ancestor. Therefore, I accept Ostrom's revival. The only major impediment to dinosaurs as ancestors of birds had already been removed with the discovery of clavicles in some coelurosaurian dinosaurs.

Birds evolved from dinosaurs, but does this mean, to cite the litany of some popular accounts, that dinosaurs are still alive? Or, to put the question more operationally, shall we classify dinosaurs and birds in the same group, with birds as the only living representatives? Paleontologists R. T. Bakker and P. M. Galton advocated this course when they proposed the new vertebrate class Dinosauria to accommodate both birds and dinosaurs.

The Telltale Wishbone. With permission from
Natural History
, November, 1977. © American Museum of Natural History, 1977

A decision on this question involves a basic issue in taxonomic philosophy. (Sorry to be so technical about such a hot subject, but severe misunderstandings can arise when we fail to sort formal questions in taxonomy from biological claims about structure and physiology.) Some taxonomists argue that we should group organisms only by patterns of branching: if two groups branch from each other and have no descendants (like dinosaurs and birds), they must be united in formal classification before either group joins another (like dinosaurs with other reptiles). In this so-called cladistic (or branching) system of taxonomy, dinosaurs cannot be reptiles unless birds are as well. And if birds are not reptiles, then according to the rules, dinosaurs and birds must form a single, new class.

Other taxonomists argue that branching points are not the only criterion of classification. They weigh the degree of adaptive divergence in structure as well. In the cladistic system, cows and lungfishes have a closer affinity than lungfishes and salmon because the ancestors of terrestrial vertebrates branched from the sarcopterygian fishes (a group including lungfishes) after the sarcopts had already branched from the actinopterygian fishes (standard bony fishes, including salmon). In the traditional system, we consider biological structure as well as branching pattern, and we may continue to classify lungfishes and salmon together as fish because they share so many common features of aquatic vertebrates. The ancestors of cows experienced an enormous evolutionary transformation, from amphibian to reptile to mammal; lungfish stagnated and look pretty much as they did 250 million years ago. Fish is fish, as an eminent philosopher once said.

The traditional system recognizes unequal evolutionary rates after branching as a proper criterion of classification. A group may win separate status by virtue of its profound divergence. Thus, in the traditional system, mammals can be a separate group and lungfishes can be kept with other fish. Humans can be a separate group and chimps can be kept with orangutans (even though humans and chimps share a more recent branching point than chimps and orangs). Similarly, birds can be a separate group and dinosaurs kept with reptiles, even though birds branched from dinosaurs. If birds developed the structural basis of their great success after they branched from dinosaurs, and if dinosaurs never diverged far from a basic reptilian plan, then birds should be grouped separately and dinosaurs kept with reptiles, despite their genealogical history of branching.

Thus, we finally arrive at our central question and at the union of this technical issue in taxonomy with the theme of warmblooded dinosaurs. Did birds inherit their primary features directly from dinosaurs? If they did, Bakker and Galton's class Dinosauria should probably be accepted, despite the adherence of most modern birds to a mode of life (flight and small size) not wonderfully close to that of most dinosaurs. After all, bats, whales, and armadillos are all mammals.

Consider the two cardinal features that provided an adaptive basis for flight in birds—feathers for lift and propulsion and warmbloodedness for maintaining the consistently high levels of metabolism required by so strenuous an activity as flight. Could
Archaeopteryx
have inherited both these features from dinosaur ancestors?

R. T. Bakker has presented the most elegant brief for warmblooded dinosaurs. He rests his controversial case on four major arguments: