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
Robert T Bakker Page 13
