Robert T Bakker

by The Dinosaur Heresies (pdf)

  volving hundreds of new thin sections. I've cut a few fossil thin

  sections myself, but de Ricqles is the unchallenged bone-slicing

  champion of all time. He has cut and polished samples from nearly

  every type of prehistoric vertebrate. And the evidence he pro-

  vides for warm-blooded growth patterns in dinosaurs is over-

  whelming and incontrovertible.

  After de Ricqles and I had published our first papers, several

  biologists and paleontologists published critical reactions that were,

  to be polite, difficult to take seriously. I had cited Enlow and

  THE WARM-BLOODED TEMPO OF THE DINOSAURS' GROWTH I 353

  Brown's argument that Haversian canals were evidence for warm-

  blooded dinosaurs. A student from Duke University disagreed,

  arguing that some primitive cold-blooded reptiles had Haversian

  canals and therefore that the presence of them in some later di-

  nosaurs proved nothing about warm-bloodedness one way or the

  other. But this critic had missed the point. Enlow and Brown hadn't

  been convinced by a few isolated Haversian canals, they were im-

  pressed by the enormous abundance of them found in some dino-

  saurs, an abundance far exceeding that typical of reptiles and

  matched only in big mammals. Dinosaur canal systems are often

  so tightly intergrown that the thin slice of bone looks like whole

  clusters of those onions cut in cross section. Now, some old large

  crocodiles develop a few scattered Haversian canals, and the very

  primitive fin-backed reptiles from the Permian often showed a few.

  But only mammals and dinosaurs possess whole swarms of Ha-

  versian systems. All through their adult life these animals grow new

  canals. The dense crowding of the systems forced the newly grow-

  ing canals to cut into the old ones. No cold-blooded animal, past

  or present, has ever evolved such densely packed Haversian sys-

  tems.

  Although Haversian canals are somehow connected to high

  metabolism, no one knows precisely how they work. Adult hu-

  mans display very densely packed canals, but in young people they

  aren't as abundant. It can be argued, then, that densely packed

  systems are needed more by adults than by the young. Yet some

  mammals and some dinosaurs have no Haversian canals at all, even

  when they are fully mature. In general, the canal systems are bet-

  ter developed in meat-eaters and omnivores than in strictly vege-

  tarian species, but there are many exceptions. Haversian canals keep

  dissolving and redepositing bone mineral as they form, so maybe

  their purpose is to maintain some of the minerals in a fluid state

  so that the calcium ions can enter the bloodstream quickly if some

  bodily organ needs calcium in a hurry. Whatever their role, densely

  packed Haversian systems are clearly marked "for warm-bloods

  only."

  The argument from Haversian systems for warm-bloodedness

  is only one part of the case that can be made from bone texture.

  Some dinosaurs lacked Haversian canals, as do some big mam-

  mals. But all dinosaurs show direct evidence of fast growth rates.

  354 | THE WARM-BLOODED METRONOME OF EVOLUTION

  Bone consists of two materials: (1) the bone mineral, crystals of

  calcium phosphate; and (2) strands of tough connective tissue called

  collagen (the same material that also gives strength and elasticity

  to our skin and muscles). When bone grows slowly, the collagen

  fibers are wrapped in layers one atop the other, all around the

  outside surface of the bone. In any one layer, all the strands tend

  to lie parallel to one another, but the direction of the strands al-

  ternates from one layer to the next. Bone mineral forms within

  the collagen as long, pointy-ended crystals that lie parallel to the

  strand. The geometric result of slow growth is what de Ricqles called

  "lamellar" bone: each subsequent layer of fiber contains densely

  packed crystals all oriented in one direction. When this type of

  bone is cut in thin section across its grain and put under the mi-

  croscope, the alternations in the direction of the fibers catch the

  rays of polarized light and show up as alternating circles of bright

  and dark—quite a pretty light show. Crocs and turtles, and most

  other big reptiles, display this texture. If orthodoxy were correct,

  dinosaurs should also have this cold-blooded style of texture in their

  bones. But they don't.

  Fast-growing bone has quite a different microtexture. When

  a young bird or mammal goes through the characteristic warm-

  blooded spurt of growth, its bones grow so quickly that the col-

  lagen fibers aren't given the time to be laid out in neat parallel

  rows. They are thrown together in an irregular jumble of loosely

  packed bundles going every which way. De Ricqles called this

  "woven bone," because under the microscope the crystal rows re-

  semble a loosely woven fabric. Did dinosaurs have such woven

  bone? Absolutely. Fossils of young dinosaurs routinely display the

  texture characteristic of fast growth. And dinosaurs must have kept

  growing fast until nearly full-sized, because woven bone is the

  dominant microstructure found in most subadult specimens as well.

  Brian McNab is a very good, very thoughtful environmental

  physiologist at the University of Florida. He has published classic

  work on how animals of different sizes use their metabolism to

  meet the challenges of climate. He has written, for example, a su-

  perb paper on the world's smallest mammal, the pygmy shrew, a

  dynamo weighing two grams (one fifteenth of an ounce), ten times

  smaller than the average white mouse. But when it came to eval-

  uating the texture of fossil bone, McNab was misled by theories

  THE WARM-BLOODED TEMPO OF THE DINOSAURS' GROWTH I 355

  of mass homeothermy. He wrote a paper claiming that the only

  reason dinosaurs displayed a mammal-style bone texture was that

  they were so big, their bulk alone allowed them to maintain their

  body temperature more or less constantly high without the need

  for warm-blooded physiology. His argument completely ignored

  the fact that giant cold-blooded crocs and turtles never develop a

  fast-growth bone texture. Really huge living crocodiles can weigh

  half a ton, as big as the average Allosaurus. But in the wild they

  never possess fast-growth bone texture and never have densely

  packed Haversian systems as adults. Giant tortoises never develop

  fast-growth bone either. It is therefore impossible to argue that

  the texture of the dinosaurs' bones was simply the result of their

  great size.

  McNab's argument also overlooked all the dinosaurs that were

  not gigantic. Many—both vegetarian and carnivorous—reached adult

  size between ten and a hundred pounds, no larger than scores of

  modern croc and turtle species. All these medium-sized dinosaurs

  also had a mammal-style bone texture, whereas crocs and turtles,

  and snakes of the same bulk don't. In fact, all dinosaurs of all sizes

  had a mammal-style bone texture, while all crocs, turtles, and liz-

  ards of all sizes have typically re
ptilian textures.

  The final argument from the texture of bones derives from

  the growth rings. Most people are familiar with growth rings in

  oak and pine: thin, dark lines are winter wood; wide, pale bands

  are summer wood. Probably not too many people know that growth

  rings also form every year in animals. When winter comes, the

  snapping turtles burrow into the pond bottom to escape freezing.

  The bones nearly stop growing and lay down a thin, dark layer.

  The following spring, the turtles start eating and growing and lay-

  ing down a thick, light layer of growth in their bones. Deer also

  stop growing in the winter, and the slowdown is marked by a thin,

  dark line in the bone. Game wardens, in fact, use growth rings in

  the roots of teeth and in bones to enforce laws against shooting

  underage bears, moose, coyotes, and beaver. If the warden sus-

  pects foul play, he can have the growth rings counted at a lab, and

  obtain a conviction on the basis of them. Winter isn't the only cir-

  cumstance that can stimulate growth rings in bones or teeth; any-

  thing that cuts off food or water will have the same effect.

  Could growth rings tell whether extinct animals were warm-

  356 I THE WARM-BLOODED METRONOME OF EVOLUTION

  LEFT: At birth, a brontosaur was about one fifth adult height and about one

  one-hundredth adult weight. Bone texture shows that growth was faster than

  elephants today. (The brontosaur young were too big to be laid in eggs, so

  the newborn probably passed alive through the mother's large pelvic outlet.)

  R I G H T : Protoceratops laid relatively large eggs, and the young grew as fast as

  ostriches do today.

  Largest growth gap—a hatchling

  duckbill weighed only one sixteen-

  thousandth as much as its mother,

  but bone texture shows that growth

  was so fast, adult size was reached

  in a few years.

  THE WARM-BLOODED TEMPO OF THE DINOSAURS' GROWTH | 357

  A few nondinosaurs had fast-growth bone texture in the Mesozoic. The fish-

  lizards—the lichthyosaurs—were fast growers. (Shown here is the twenty-five-

  foot Temnodontosaurus from the Early Jurassic attacking Plesiosaurus.)

  blooded or cold-blooded? Maybe, under careful analysis. Both

  warm-blooded and cold-blooded animals today can develop growth

  rings in habitats where winter becomes severely cold. But in warm

  climates where the dry seasons aren't too extreme, cold-blooded

  species tend to have better-developed rings than warm-blooded

  species. So, if fossils came from an ancient habitat with a warm

  climate, it could be expected that warm-blooded animals would have

  more poorly developed rings—on average. It must be remem-

  bered only the average condition is really significant because some

  warm-bloods will have well-developed rings. Now, in many of the

  bones Armand de Ricqles cut from the primitive reptiles and am-

  phibians of the Coal Age in Europe and North America, he found

  growth rings. The Coal Age environment was warm, tropical. He

  therefore concluded these growth lines were the products of typ-

  ically cold-blooded physiology. But growth rings were much less

  common in dinosaurs, and so he concluded that dinosaurs must

  have had a more mammal-style rhythm of growth.

  Yet some dinosaurs did have yearly rings. A pair of Canadian

  paleontologists found them in the teeth of duckbill dinosaurs and

  tyrannosaurs and loudly declared their evidence proved the dino-

  saurs were cold-blooded. Their conclusion was hardly justifiable

  since they hadn't taken into account the fact that growth rings are

  very common in the teeth of some warm-blooded mammals living

  in tropical habitats (—lions and hyenas in East Africa have such

  rings—) and that these mammals usually have more sharply de-

  fined rings in their teeth than in their bones. Moreover, if we

  compare the average condition of crocs and dinosaurs from any one

  habitat, the crocs invariably have better-defined growth rings and

  more of them, just as East African crocs today exhibit better rings

  than the mammals in the same locale. Finally, the Canadian dino-

  saurs actually showed the mammal-style pattern: rings in the teeth

  but not in the bones.

  Some other scientists have found growth rings in the limb

  bones of dinosaurs—in one specimen of Allosaurus, in one bron-

  tosaur from England, and in another excavated in Madagascar. A

  great deal was made of each of these specimens with rings, but all

  the hundreds of dinosaur specimens with no rings whatever were

  ignored. Were some dinosaurs cold-blooded, then, while others

  were warm-blooded? A theoretical possibility. But the evidence

  THE WARM-BLOODED TEMPO OF THE DINOSAURS' GROWTH I 359

  from growth rings certainly does not prove, as orthodoxy would have

  it, that any dinosaur was cold-blooded. Growth rings merely prove

  that growth stopped during one part of the year. The only useful

  way to derive evidence from them must come from a broad sur-

  vey: If, on average, dinosaurs were more warm-blooded than crocs

  or turtles, then in any one fossil habitat more and better-defined

  growth rings should be found in the crocs and turtles. And that is

  exactly what is found. At Como Bluff, all the turtles and crocs dis-

  play sharply defined growth rings, but the dinosaurs only rarely.

  The same is true in the Late Cretaceous deltas of Montana and

  Alberta.

  Paleontology's treatment of the evidence from bone texture

  is an example of what I call the "harrumph-and-amen" syndrome.

  Enlow and Brown and others pointed to many dinosaurs with a

  warm-blooded type of bone texture, and the orthodoxy snorted,

  "Harrumph—all that means nothing." But when a few growth rings

  were discovered in dinosaurs, then orthodoxy responded with a

  fervent "Amen, we knew it all the time—dinosaurs were cold-

  blooded reptiles."

  A piece of fossil bone is rich in textural meaning—a labyrinth

  of canals left by blood vessels, a three-dimensional basketwork of

  crystals, a diary of the animal's life written in the layers of mineral

  fabric. Good times and bad are written there, seasons of plenty

  and seasons of drought. These ancient diaries can be opened and

  the stories of dinosaur lives read, their youthful exuberance in

  growth, the pulse of blood flow in maturity. Ever since the 1830s

  these diaries have been telling the scientific community about di-

  nosaurs' growth and their life style. And the message is clear—not

  the story of one or two isolated cases, but the chronicle of whole

  dynasties. Defenders of orthodoxy may quibble over a growth ring

  here or an isolated Haversian canal there. But the overall point

  cannot be ignored. Dinosaurs grew mammal-fashion; they grew fast

  and bred early. And their dynamic approach to quick maturity must

  have been one of the most powerful weapons in their adaptive ar-

  senal.

  360 I THE WARM-BLOODED METRONOME OF EVOLUTION

  17

  STRONG HEARTS, STOUT

  LUNGS, AND BIG BRAINS

  Paleontology
is a hard science. The objects of our study are

  crisp, solid bones, free of any soft bits such as blood vessels or

  muscle tissue that will only rarely indicate their existence in fossil

  remains. But the soft bits, especially the heart, lungs, stomach, in-

  testines, and brains, do have their place in studying dinosaurs. The

  gizzard stones and intestinal digestive systems have already been

  discussed in an earlier chapter. Here we must investigate the other

  soft organs of the Dinosauria—the heart, the lungs, and the cere-

  bral equipment.

  The important thing about hearts and lungs is that evolution

  designs them to withstand the stress of prolonged intense activ-

  ity—what physiologists call "exercise metabolism." Metabolic rates

  during exercise are always many times higher than the rate of av-

  erage standard metabolism. When we humans sit doing nothing,

  our metabolism works our heart-lung apparatus at only one twen-

  tieth of the maximum capacity of a well-trained athlete. Human

  hearts and lungs are powerful organs, and to set them going full

  throttle we have to engage in prolonged, strenuous exercise—cross-

  country skiing, long-distance swimming, or intense gymnastic ex-

  ercise (bowling doesn't do it). Then our thickly muscled heart and

  minutely compartmentalized lungs extract oxygen at maximum rates

  and send it to all the exercising organs at full speed. A human lung,

  or a dog's or horse's, is full of tiny cells so that the tissue area is

  STRONG HEARTS, STOUT LUNGS, AND BIG BRAINS | 361

  Dinosaurs with two brains? Stegosaurs—like these Kentrurosaurus from the

  Late Jurassic of Tanzania—had an enlargement of the spinal cord in the

  sacrum.

  maximized for the exchange of gas from air to blood, and vice versa

  (oxygen must be vented in, carbon dioxide must be vented out of

  the bloodstream). Such a system is necessary for surviving among

  the vigorous confrontations of a fully warm-blooded ecosystem. The

  physical arrangements of heart and lung among birds are often

  unique, but the avian system has the same high capacity for exer-

  cise found in the most advanced mammals.

  Modern lizards have no need of a heart-lung system with any-

  where near as much capacity. An iguana's lung is a simple sac, a