Robert T Bakker Read online

  foot brontosaur head, with only a handful of pencil-size front teeth,

  had to feed twenty or thirty tons of body. Obviously, the standard

  orthodoxy has it, the brontosaur's extreme microcephaly imposed

  severe dietary restrictions. Only the most nutritious and softest of

  water vegetation would have met the stringent requirements. And

  even with a superabundant supply of such green mush, the bron-

  tosaur's metabolism would still had to have been incredibly low—

  somewhere between the level of a tortoise's and a cactus's—for

  the great beast to survive at all.

  This argument has been repeated hundreds of times by

  schoolteachers and Ivy League professors alike. A recent issue of

  National Geographic featured a long piece by a respected curator

  at a university museum. Typically, this author scoffed at the idea

  of any brontosaur's having a high metabolism. He dismissed any

  such notion with a single fact: its head was too small. In a 1984

  article in a technical journal, a young paleontologist presented a

  mathematically reasoned argument that proved beyond the least

  GIZZARD STONES AND BRONTOSAUR MENUS I 125

  Yard-wide gizzard of a

  Brontosaurus. With its

  thick muscular walls and

  lining of hard rocks, the

  brontosaur gizzard could

  grind enough tough

  leafage to fuel a warm-

  blooded body.

  doubt that the big brontosaur's meager cranial apparatus was

  hopelessly undersized to provide for any sort of high metabolism.

  Several years before, a graduate student from Yale lecturing be-

  fore an enthralled audience at Harvard used the rate at which moose

  chew water lilies to prove irrefutably that a twenty-ton brontosaur

  simply could not support anything but the most subdued and slug-

  gish life style. Documentary proof. Irrefutable logic. The giant

  brontosaurs could only have spent all their lives in a somnolent

  state of semi-torpor, just barely moving their long necks to reach

  into the lukewarm water, poking slowly about for the softest part

  of the Jurassic swamp salads.

  But all these arguments, both popular and professional, leave

  out important pieces of the brontosaur puzzle: gizzards, stones, and

  moas.

  A white mouse sacrificed to a hungering alligator posthu-

  mously provides a most important clue. The bones of the mouse

  show up quite clearly in the alligator's stomach on the laboratory's

  television X-ray monitor. But the mouse's bones are not alone.

  The alligator's after-stomach is lined with hard, dense objects—

  gizzard stones. The gizzard stones are convulsed by sudden mus-

  cular contractions of the gizzard's walls. The monitor clearly shows

  the mouse is being chewed, not by teeth in the mouth but by stones

  in the gizzard.

  Naturalists who study big 'gators and crocs in the wild find

  huge masses of gizzard stones when they cut open the animals to

  126 | THE HABITAT OF THE DINOSAURS

  study their feeding habits. The stones are found only in one cham-

  ber of the stomach—the gizzard—and this one chamber has walls

  with grooves and folds to permit expansion and contraction. Even

  without X-ray monitoring, it is obvious that this stomach chamber

  is a churning compartment designed to crush and pulp the prey's

  body after the gastric juices begin their preliminary chemical

  treatment. Crocs usually select very hard stones—quartz and gran-

  ite pebbles, for example—to line their gizzards. If such materials

  are lacking in their native streams, they may use angular bits of

  hard wood, pieces of glass bottles, or whatever else is available. I

  have also seen one or two near-perfect fossil alligator skeletons

  containing a neat bundle of hard pebbles clustered between the

  ribs precisely where the gizzard was in life. These fossilized gastric

  mills demonstrate plainly that gizzard stones have been an essen-

  tial functional component of crocodilian food processing for many

  millions of years. And the study of crocodilian gizzards leads to

  some intriguing conclusions about evolution both in birds and in

  the Dinosauria.

  Zoos mislead their visitors by the way the species are housed.

  Birds are in the Bird House, of course, and crocodiles are always

  segregated to the Reptile House with the other naked-skinned,

  scale-covered brutes. So the average visitor leaves the zoo firmly

  persuaded that crocodilians are reptiles while birds are an entirely

  different group defined by "unreptilian" characteristics—feathers

  and flight. But a turkey's body and a croc's body laid out on a lab

  bench would present startling evidence of how wrong the zoos are

  once the two stomachs were cut into. The anatomy of their giz-

  zards is strong evidence that crocodilians and birds are closely re-

  lated and should be housed together in zoological classification, if

  not in zoo buildings.

  Both birds and crocs have the identical plan to their special-

  ized gizzard apparatus, and this type of internal food processor is

  absent in the other "reptiles"—lizards, snakes, and turtles. In both

  birds and crocs, the gizzard is a thick-walled, muscular, crushing

  compartment with two great tendons reinforcing the walls of mus-

  cle (these are the shiny sheets of tough tissue you cut off the tur-

  key gizzard before cooking it). In both birds and crocs, the muscular

  gizzard is just aft of the thin-walled glandular stomach where food

  is softened by gastric juices.

  This croc—bird digestive system makes a lot of mechanical

  GIZZARD STONES AND BRONTOSAUR MENUS | 127

  sense. We humans chew our food first, then pass it to the glan-

  dular stomach, where it is softened by stomach juices. Our system

  makes our teeth do the heavy work; they must crunch up the food

  as it comes directly through the lips. If the human diet is a civi-

  lized one, full of soft TV dinners and tender cuts of meat, our teeth

  don't wear much. But in primitive human societies the natural foods

  are often tough and gritty—the Anasazi Indians of ancient New

  Mexico wore their teeth down to the gums because tiny bits of

  sand got mixed into their cornmeal when it was ground on stone

  matates. Even horses wear out their huge molars if they have to

  feed on grass growing in gritty soil. But consider the advantages

  of the croc—bird system. They swallow without chewing and pass

  their food directly into the glandular stomach, where the food rests,

  softened by the gastric biochemistry. Then sphincter muscles act

  as gastric gatekeepers, letting the food pass on to the gizzard where

  it is chewed. The "teeth" of this system (the gizzard stones) don't

  begin their crunching work until the food has been rinsed, soaked,

  and softened.

  Crocs have powerful digestive processes. However, no croc

  species eats vegetation purposely; sometimes weeds are swallowed

  accidentally when the croc swallows turtles or fish. So crocs don't

  provide a complete picture of how a gizzard might work in an her-

  bivorous dinosaur lik
e Brontosaurus. Fortunately many species of

  bird are plant-eaters, and vegetarian birds perform some truly

  spectacular gastric feats with their rock-lined gizzards. Ducks and

  geese shovel up hard nuts and grains and even live clams, chug

  them down to the gizzard, and crunch them up with the gizzard's

  lining. Clamshells, acorns, and corn kernels are all equally cracked

  into small pieces by this formidable gastric mill. Fruit pigeons do

  even better; their gizzard is especially tough and contains horn-

  covered "teeth" growing from the inside lining. Even the hardest

  of tropical nuts are swallowed hole, passed into the gizzard, and

  cracked with an audible thunk. Ostrichs shot in the wild have giz-

  zards lined with the hardest rocks—usually those rich in quartz—

  available in the countryside. And a large bird can carry around as

  much as a double handful of these stong gastric tools.

  Now the problem of tooth wear in nature is not a minor one.

  When wild species wear out their adult teeth and can't replace them,

  they die. Elephants possess huge adult teeth, the largest ever

  128 I THE HABITAT OF THE DINOSAURS

  evolved. But every elephant eventually wears out its last molar and

  wastes away along some swampy shore, attempting to gum soft

  water plants for nourishment. Having a continuous supply of teeth

  in each socket, as was the case for dinosaurs, eases the tooth-wear

  problem but doesn't remove it entirely. The basic adaptive diffi-

  culty is that the hardest material in a tooth—the enamel—is still

  much softer than the grit that covers most foods in nature. Wind-

  blown dust generally contains tiny specks of silica. Silica is natural

  glass, a very common material in rocks and soil. Plants growing in

  natural soils become coated with windblown grit and with dirt

  containing silica particles.

  Not only do soil and wind tend to make plant food gritty, but

  plants themselves sometimes evolve silica armor to discourage the

  plant-eaters. Horsetails are one such armored type of plant, an an-

  cient group dating back to long before the dinosaur. Modern

  horsetail species are sometimes called "scouring rushes" because

  peasant housewives used to scrub pots with horsetail stems. They

  scour well because evolution has provided them with special cells

  that concentrate silica from the soil. The silica cells armor the en-

  tire stem with row after row of glass-hard microlumps. A plant-

  eater learns quickly that a diet of horsetails will erode its teeth

  down to the gumline.

  Gizzards not only give plant-eaters an edge in their evolu-

  tionary struggle with plants. They also confer the freedom to do

  other things besides constant chewing. Pity the poor plant-eater

  with neither gizzard nor ruminating stomach—a zebra, for exam-

  ple. The zebra must chew each lump of grass directly, without

  soaking or softening. Zebra heads are large for their bodies and

  are provided with huge molars—twelve on each side of the mouth

  (twice the number humans have). Even with this dental armory,

  when grass is tough and sparse, zebras are nonetheless forced to

  spend nearly all their working hours plucking and chewing. All this

  chewing demands that the zebras remain out on the plains, ex-

  posed to rain, wind, and constant danger from lions and hyenas.

  What would happen if a zebra were supplied with a hypo-

  thetical gizzard? Such a zebra could pluck up grass quickly, with-

  out masticating, fill its forestomach chamber, and retreat to the

  shade and safety of a bush-covered hill to let its gizzard do all the

  work of mastication. Gizzards also free the animal's mouth for other

  GIZZARD STONES AND BRONTOSAUR MENUS 129

  activities—such as sex. With its gizzard doing all the work of

  chewing, the zebra could use its mouth to snort and whinny and

  make all sorts of elaborate noise display to attract mates and frighten

  sexual rivals. Ever wonder how tiny songbirds can afford to spend

  so much of their time singing? Little birds are notorious for their

  high metabolism, but when do they find the time to chew? They

  don't. As the warbler sings, its gizzard and forestomach are doing

  the food processing without interfering with the music.

  Cud-chewing mammals have evolved a soak-and-soften mech-

  anism almost as good as the gizzard. A cow or deer plucks a

  mouthful of gritty grass, swallows it without chewing, and passes

  the lump of grass to a series of special stomach chambers. These

  chambers are fermentation vats where gastric juices and yeastlike

  microorganisms clean the wad of food and break down the tough

  plant fiber. Only after the lump of grass has soaked and softened

  is it passed back up to the mouth to be chewed by the molars. The

  technical name for this stomach vat system is "rumen," and such

  cud-chewing mammals are called ruminants.

  The ruminant system must be reckoned as one of the best de-

  vices mammals have evolved for coping with tough plant food. Most

  of today's successful big plant-eating mammals are in fact rumi-

  nants—all the cattle, sheep, goats, antelope, deer, giraffes, and

  others. But the gizzard system must be considered superior.

  Imagine a twenty-ton Brontosaurus equipped with an ad-

  vanced, avian-style, rock-lined gizzard. A two-hundred-pound os-

  trich may possess a gizzard four inches across and a pound in weight.

  A roughly proportionate gastric grinder would provide a twenty-

  ton brontosaur with a gizzard of approximately one hundred pounds.

  One hundred pounds of tough muscle contracting a lining of big

  quartz pebbles could crush up Jurassic vegetation at a rate more

  than adequate to supply any level of metabolism. A hundred pounds

  of gizzard muscle weighs more than four times the jaw muscles of

  a five-ton African elephant. So four elephants, totaling twenty tons,

  possess less chewing power than the single hypothetical brontosaur.

  But what about that tiny head—would a brontosaur be able

  to engorge enough food to keep a giant gizzard apparatus going at

  full capacity? That question can be answered by turning to New

  Zealand, where up until a few centuries ago a giant, long-necked,

  pinheaded herbivore waddled about the landscape plucking leaves

  130 | THE HABITAT OF THE DINOSAURS

  from trees and crushing them with its gizzard. This native New

  Zealand plant-grinder was the moa—or more precisely, the moa

  family, a group of flightless species of bird that achieved a weight

  of half a ton. New Zealand's ecosystems evolved without any na-

  tive land mammals, so the role of large plant-eater was filled by

  the evolution of these big ground birds. Unfortunately for mod-

  ern science, the Polynesian colonists, the Maoris, who arrived in

  New Zealand about A.D. 1300, found the moas tasty and easy to

  kill, so moas were extinct before Western civilization could meet

  them alive.

  But moas created a sensation when they first turned up as

  fossils in New Zealand bogs and stream gravels. European zoolo-

  gists already knew ostriches well, because
they had been circus fa-

  vorites from the time of the Caesars. But no one suspected that a

  plant-eating bird as large as a small buffalo could have existed. In

  1838, Sir Richard Owen, Queen Victoria's favorite anatomist, re-

  ceived a packet from New Zealand containing a curious bone

  fragment the size of an ox's femur. Owen was such an accom-

  plished comparative anatomist that he instantly recognized the

  fragment as from a bird—a bird possessing a body five times heav-

  ier than any previously known. With a courage few other young

  scientists might display, Owen publicly announced his discovery.

  Six-foot Maori hunter and

  the great moa, Dinornis

  GIZZARD STONES AND BRONTOSAUR MENUS

  131

  Based on this single fragment, he deduced the existence of huge

  birds rivaling the mammals in size. Owen's name for the extinct

  bird was an emphatic superlative: Dinornis maximus "enormous

  terror bird."

  Owen's announcement met with skepticism, but his judgment

  was vindicated by more and better discoveries in New Zealand:

  partial hind limbs, vertebrae, and then astonishingly complete

  skeletons found standing upright, buried in quicksandlike depos-

  its. Owen's deductions, based on one thigh fragment, were on tar-

  get—the greatest species of moa were twelve feet tall and must

  have weighed a half a ton. Other species varied down to pony size.

  Moa anatomy was full of surprises for biomechanical anatomists.

  The wings were nearly totally absent—unlike ostriches, moas re-

  tained not even a tiny feathered remnant. And the moa's head was

  tiny—a twelve-foot-high moa carried a skull no bigger than a

  poodle's.

  Moas are delightful objects of study in their own right, but

  their importance in the present discussion lies in the pinheaded

  configuration of their head and neck. At a distance, moas would

  have appeared as microcephalic as any brontosaur, with the tiny

  moa skull perched atop a very long, gracefully tapering neck. And

  moas were without doubt herbivores; their beaks were con-

  structed like those of living leaf-eating species. Unassailable evi-

  dence for their food preferences subsequently came from skeletons

  found in bogs and caves, where the stomach contents from the giant

  birds' last meal were mummified with the bones. These fossil meals