Robert T Bakker

by The Dinosaur Heresies (pdf)

  sort of limp-walled balloon that can supply oxygen at rates only

  one tenth as high as a dog's or chicken's of comparable body size.

  Iguana hearts too are of low capacity, much smaller than a heart

  from a mammal with the same size of body. Some other lizards—

  the monitors, especially—have more thickly walled hearts and more

  complex lungs than do iguanas, but no present-day reptile can match

  362 I THE WARM-BLOODED METRONOME OF EVOLUTION

  the heart-lung system typical of active, modern, advanced mam-

  mals and birds.

  How, then, was the heart and lung system of the dinosaurs

  arranged? According to orthodoxy, the dinosaurs' physical exer-

  tions alternated between episodes of stolid shuffling and long pe-

  riods of somnolent inertia. But the dinosaurs' limbs, as we have

  already seen, were not constructed for that kind of movement.

  Tyrannosaurus's legs were built for speed, vigorous prolonged ex-

  ercise. To keep such huge muscles functioning, tyrannosaurs would

  have needed powerful hearts. There may even be some direct evi-

  dence for the size of those hearts. Today, whenever a species has

  a very small heart, the front end of the ribcage can be extremely

  narrow, because the only internal organ filling it is the heart mus-

  cle. Iguanas are a good example: their hearts are tiny and their rib-

  cage seems very constricted in the front end. The ribcages of

  dinosaurs, by comparison, tended to be very deep, and the ante-

  rior ribs were often thick and long. What filled those deep noble

  chests? Quite probably thickly walled hearts of heroic dimensions.

  To maintain bodily activity at high levels, dinosaurs would have

  needed lungs with a capacity to match the output of their hearts.

  And those lungs have left some clear traces in their skeletal anat-

  omy. Many dinosaurs had hollow cavities in their vertebrae. A sin-

  gle bone of a Brontosaurus's spine is so full of holes and indentations

  that the actual bony tissue is reduced to thin partitions, often a

  few millimeters thick, folded and convoluted many times to pro-

  duce the major structural contours. Allosaurus and other meat-eat-

  ers also had such hollowed-out vertebrae, though to a lesser extent

  than in the brontosaurs. What filled the vertebral holes and hol-

  lows is not difficult to see because very similar hollow backbones

  can be found in today's birds. In them, the hollows are filled by

  air sacs connected by tubes to the lungs. Avian lungs are excep-

  tionally efficient, better at extracting oxygen than mammal lungs,

  and this is due to the air sacs and the resultant pattern of air flow.

  The lungs of mammals and lizards have one fundamental flaw—

  they're dead-end organs. Air must be sucked into the alveolar sacs

  and then squeezed directly out again. Such a method is inefficient.

  The lung would work better if air flowed in one direction only,

  across the lung's surfaces and then out the throat. Birds solve this

  problem by means of their complex system of air sacs. They draw

  STRONG HEARTS, STOUT LUNGS, AND BIG BRAINS I 363

  the air first into the system of air sacs. Then they pass it from the

  sacs through the lung tissue proper, so that the exchange of gases

  between air and blood can happen as the air flows in one direc-

  tion, on its way out. Physiologists call this a "countercurrent ex-

  change" (the blood in the lung's surfaces flows in the direction

  opposite to the flow of the air). Countercurrent exchange allows

  oxygen extraction and carbon dioxide venting at much greater ef-

  ficiencies than are possible with our mammalian dead-end lungs.

  The dinosaurs' vertebral hollows are so similar to birds' that

  there can be little doubt an avian-style system of air sacs was at

  work in these Mesozoic animals. Moreover, the holes in the bones

  represent only the periphery of the total system. Birds locate their

  largest air sacs between their flight muscles and in their body cav-

  Deep dinosaurian lungs. Brontosaurus had a very deep chest that must have

  enclosed large lungs and a large heart. A crocodile of the same weight would

  have a much shallower chest and far weaker cardiac and pulmonary

  machinery.

  364 | THE WARM-BLOODED METRONOME OF EVOLUTION

  ity. Allosaurus and Brontosaurus must have had huge air sacs dis-

  tributed throughout their viscera. And it's a good supposition that

  they had evolved lungs fully capable of intense, prolonged exer-

  cise. No typical cold-blooded reptile has ever evolved a dinosaur-

  bird type system of air sacs.

  Some dinosaurs (duckbills, horned dinosaurs) exhibited no

  vertebral hollowings, but I suspect they had located air sacs fully

  within their body cavity. Most primitive dinosaurs had some hol-

  lowed-out vertebrae, so the ancestral dinosaurs were probably

  equipped with air sacs. Many modern species of bird that have air

  sacs have solid vertebrae without any hollows (like the duckbills

  and horned dinosaurs). In these avian species, the big air sacs re-

  main nonetheless well developed within their body cavity. So

  duckbill and horned dinosaurs, though they lacked peripheral sacs

  inside their vertebrae, may well have had the main system of sacs

  operating between the other internal body organs.

  Myths about dinosaurs die hard. As a child, I first heard the

  tale of the dim-witted, double-brained Stegosaurus, a fable created

  in the 1880s and still popular a century later. This story had two

  sources: the pea-sized brain of Stegosaurus, and its legendary after-

  brain.

  Let us consider the problem of brain size. Large animals need

  large brains, because the mass of their active cells requires many

  nerve channels to carry signals to and from the brain, and because

  the brain needs adequate capacity for processing information about

  all this physiological activity. An elephant is roughly as "smart" as

  a dog—they have equivalent powers of learning. But the elephant

  requires a three-pound brain to achieve this intellectual level while

  the dog needs only four ounces. To gauge a stegosaur's intellect,

  therefore, it is only necessary to compare its brain size to that of

  a mammal of the same body size. It's not hard to measure brain

  size in dinosaurs. Their braincase was nearly completely enclosed

  by thick bone. When the first stegosaur skulls were discovered by

  Professor Marsh in 1878, scientists at Yale simply sawed open the

  braincase, and measured its volume. Calculating the brain's live

  weight was straightforward. In big animals, the brain fills about half

  the braincase; the other half is filled out by connective tissue, a

  sort of cerebral padding, packed between the outer surface of the

  brain and the inner walls of the braincase.

  STRONG HEARTS, STOUT LUNGS, AND BIG BRAINS I 365

  Brain and sacral "brain" cavities

  in an elephant and a stegosaur.

  Stegosaurus's live brain size occupied about half its braincase.

  Calculations made on the basis of that suggested Stegosaurus had

  been monumentally underbrained compared to modern mammals

&n
bsp; of the same size. This dinosaur had been endowed with two ounces

  of brain cells at most. An elephant has a brain at least thirty times

  larger. So dim-wittedness is a judgment hard to avoid when think-

  ing about Stegosaurus. But the cranial end was only half the story

  366 I THE WARM-BLOODED METRONOME OF EVOLUTION

  of its neurological system. Between its hip sockets, deep within

  the backbone where the pelvis attaches to the vertebral bodies,

  was a huge enlargement of the spinal cord, a swelling of nerve tis-

  sue thirty times larger than the volume of the brain itself—the

  stegosaur's "second brain." Professor Marsh found sacral enlarge-

  ments in other dinosaurs, too, but none as spectacular as in Stego-

  saur us.

  What did Stegosaurus accomplish with this afterbrain? En-

  largements of the spinal cord came as no surprise to experienced

  anatomists—nerves enter and leave all along its length from head

  to tail. Each vertebral segment is endowed with its own set of out-

  going and incoming signal lines, and wherever organs are espe-

  cially big and complex, the cord is swollen by additional nervous

  tissue to help organize preprogrammed reflexes. For example, ex-

  tra nerve centers are needed to regulate the sequence that makes

  all the muscles operate in the proper order to carry out the smoothly

  coordinated movements of a leg or a tail. Stegosaurus had huge hind

  legs and especially huge tail muscles, complexly subdivided and

  capable of immensely powerful movements. Enlargements of the

  spinal cord in the hip area therefore made perfect sense. Ostriches

  are similarly heavy in the area of their locomotor muscles, because

  their wings are atrophied and most of their contractile tissue is

  concentrated in the hind legs. And ostriches have an enlargement

  of the spinal cord inside the hip vertebra. Stegosaurus weighed

  twenty times more than the largest ostrich, so its hind-end de-

  mand for coordinated nervous activity was far greater. The other

  large-tailed, big-rumped dinosaurs also tended to develop very

  sizable sacral enlargements— Brontosaurus was especially well

  equipped in that regard.

  Big-rumped dinosaurs certainly did not "think" with their af-

  terbrain. "Thinking" is usually defined as the highest level of neu-

  rological exercise, encompassing analysis of incoming stimuli in the

  context of experience stored in the memory, and decision making

  that draws upon the inborn instincts and the immediate percep-

  tion of circumstance. In all the Vertebrata, only two types of brain

  tissue carry out these functions, and both types are found only

  within the braincase, inside the skull. Mammals think with their

  cerebrum, the part of the midbrain that first evolved to handle in-

  formation coming in from the sense of hearing. Most present-day

  mammals have enlarged cerebral compartments, so large that the

  STRONG HEARTS, STOUT LUNGS, AND BIG BRAINS | 367

  cerebral lobes, which grow upward from the midbrain, expand

  forward to cover the underlying forebrain completely. Birds think

  with expanded midbrain tissue too, but the avian intellectual ap-

  paratus develops from a different layer, the corpus striatum. Both

  bird and mammal thinking organs, however, look alike from the

  top. And if a bird's brain is dissected, its overall shape strongly

  resembles a mammal's: a pair of expanded "thinking" lobes cov-

  ering most of the brain stem.

  Never, absolutely never, does "thinking" tissue develop in the

  sacral enlargements of modern birds or mammals. If the midbrain

  lobes were removed from an ostrich, it could still run in circles for

  a while, because the system of muscle-coordinating relays would

  still be intact. But it certainly couldn't think, learn, remember, and

  most certainly couldn't make decisions. Therefore the sacral ner-

  vous tissue in the stegosaur's rump would have helped it move

  gracefully and swing its spiked tail with dangerous precision. But

  this "afterbrain" wouldn't have added even one small storage area

  to its capacity for remembering and deciding.

  As Professor Marsh's laboratory staff examined brain after

  brain, from dinosaurs, fossil mammals, alligators, he espied a gen-

  eral trend in cerebral history, a common thread running through

  400 million years. His observation became known as Marsh's Law.

  It stated that on average, any evolutionary line of birds' or mam-

  mals' brains grew steadily in size over millions of years. And, on

  average, for any given body size, present-day species were brainier

  than their ancient ancestors. The modern jaguar has twice the brain

  size of the jaguar-sized saber-toothed cat that stalked the Ne-

  braska woodlands thirty million years ago. And the modern loon

  possesses twice the brain of the loonlike birds Marsh excavated

  from the Cretaceous sediments of Kansas. Most of this evolution-

  ary upgrading was focused in the centers of higher learning—in

  mammals, the cerebral lobes had enlarged under the guidance of

  natural selection; in birds, it was the corpus striatum. Primates—

  the monkey-ape clan—scored especially high in the cerebral

  sweepstakes through the ages. A chimpanzee has a brain four times

  larger than a jaguar of the same weight. But the example ne plus

  ultra of Marsh's Law is ourselves. The average human brain is seven

  or eight times larger than that of the average modern mammal of

  the same size. And our bulging cerebral lobes are forty times the

  368 | THE WARM-BLOODED METRONOME OF EVOLUTION

  size of those found in the average 120-pound mammal of the Pa-

  leocene Epoch, sixty million years ago. Since all the dinosaurs'

  higher intellectual functions had to be carried out inside their tiny

  braincase, Stegosaurus and all the other large dinosaurs must have

  been single-brained and dim-witted—or so it must be supposed.

  But Marsh's law doesn't apply to true Reptilia. The brains of

  living alligators are no larger than those of their most ancient

  crocodilian ancestor of Mid Triassic times, over 200 million years

  ago. Turtles, too, took the low road in the cerebral race. Frogs,

  snakes, salamanders, and most fish have been content with the low-

  capacity mental equipment that was in vogue back in the Coal Age,

  300 million years ago. Since turtle, lizard, frog, and snake species

  together outnumber mammal species three to one, it must be con-

  cluded that dimwits can be part of an ideal adaptive mode, at least

  for small creatures.

  Dinosaurs—with few exceptions—showed few evolutionary

  tendencies toward developing greater intellectual prowess. Most

  dinosaurs had brains no greater in size than a turtle or croc of the

  same bulk. And the pin-headed dinosaurs with their tiny skulls

  relative to their body mass had outstandingly small brains.

  Big-headed dinosaurs, such as Triceratops, had larger brains than

  stegosaurs but were still far short of the cerebral capacity of any

  large modern mammal. Were dinosaurs then incredibly stupid? Most

  popular works today still say so. And it is difficu
lt to deny that

  most dinosaurs would seem dullards compared to a good Labrador

  retriever, circus elephant, or jaguar. There is a rough correlation

  between brain size and intelligence today, and there must have been

  in the Mesozoic. Humans, chimps, large dogs, and adult alligators

  are roughly the same size. Humans, if they're careful, can outwit

  chimps. Chimps routinely outwit large dogs. And large dogs can

  learn more and make cleverer decisions than can alligators. Most

  dinosaurs probably would have fallen in the category of the alli-

  gators, and no living species with brains as small as the dinosaur's

  would be called clever.

  As soon as the dinosaurs' average brain size became widely

  known, some paleontologists jumped to the conclusion that tiny

  brains proved cold-bloodedness. At first sight, this notion seems

  reasonable. No living cold-blooded reptile has a big brain. Birds

  and mammals are the only warm-blooded vertebrates in the pres-

  STRONG HEARTS, STOUT LUNGS, AND BIG BRAINS I 369

  ent ecosystem, and both have high average brain-to-body weight.

  Some physiologists even went so far as to claim that large brains

  caused warm-bloodedness—presumably the physiological coor-

  dination required to balance heat production, sweating, panting,

  and blood flow implied large areas of cerebral circuitry. But this

  hypothesis is demonstrably wrong. Mammals and birds don't use

  their expanded midbrain lobes for thermoregulation; that chore is

  carried out in the brain stem, the oldest part of the brain in evo-

  lutionary terms. There would be no difficulty in guiding a warm-

  blooded system with an alligator-sized brain stem.

  The link between being warm-blooded and having big brains

  must be, at best, an indirect one. Brain tissue is vulnerable to

  changes in temperature. Human brains addle when heated to 108°F

  even for a few minutes, and higher cerebral functions become er-

  ratic when the brain is chilled below 90°F. So warm-bloodedness

  and a constant body temperature are prerequisites for a large brain.

  It may well be that warm-bloodedness evolved first, and the evo-

  lution of large brains followed. Warm-bloodedness would have been

  an advantage even to an animal with an alligator's brain power,

  because a constant high body temperature speeds growth, accel-