Robert T Bakker Read online

  contrast in bioengineering these two beaked clans displayed. The

  Triceratops's snout was a mammoth set of pincers, with a sharply

  edged upper and lower beak, narrow from side to side, and cov-

  ered with horn. Such deep, powerful beaks must have given those

  horned giants the power to slash and cut long, tough fronds and

  branches—fodder probably too coarse even for the wide beak of

  duckbills.

  After he had completed his unorthodox treatment of duck-

  bills, John Ostrom, in 1963, attacked the problem of Triceratops's

  diet. To maximize their biting strength, jaw muscles require three

  biomechanical properties: muscles must be thick, they must be long,

  and they must have great leverage. Great leverage can be devel-

  oped by designing the muscles so that their line of pulls is located

  far from the jaw joint. John Ostrom showed that Triceratops and

  its horned relatives evolved high bony cranks on their lower jaws

  to move the line of muscle pull up and thereby increase leverage.

  Triceratops chewing design

  168

  THE HABITAT OF THE DINOSAURS

  The evolution of duckbills remodeled the lines of muscle pull in

  the same way, but the horned dinosaurs went far beyond them when

  it came to muscle thickness and length. All primitive dinosaur heads

  had some basic design problems that caused difficulties when evo-

  lution tried to enlarge the biting muscles. Most of the jaw muscles

  in the skull were housed inside of bony compartments located be-

  hind the eye sockets, so the outer bony walls of the skull tended

  to limit possible muscle size. If the muscles grew too big, they would

  bulge out during a strong bite with a force that would burst open

  the skull bones—certainly maladaptive.

  But Triceratops % ancestors required even larger jaw muscles

  because those dinosaurs were locked into an evolutionary path

  leading to diets of ever tougher, thicker foods. They needed an

  escape from the limitations imposed by the architecture of the older

  skulls—and they found one. On the top of the primitive dino-

  saur's head, behind the eye socket, were a pair of holes, covered

  in life by a tough membrane. Holes like these evolved many times,

  probably because the stresses of chewing were concentrated along

  a certain few trajectories in the head bones. The most effective

  way to construct a head was thus to evolve thick bone where stresses

  were great and holes were stresses were minimal. Large upper-rear

  head holes (formally called "temporal fenestrae" in anatomical

  parlance) gave the horned dinosaurs their escape route to freedom

  for the design of their jaw muscles. As the head evolved, the rear

  rim of the hole grew upward and backward, forming a gigantic frill.

  Since the jaw muscle was attached to the membrane that covered

  the rear rim, as the rim grew backward, so did the muscle, and the

  entire mass of the jaw muscle could enlarge to unprecedented

  proportions. Marks on the Triceratops's frill illustrate how far the

  muscles had enlarged in both length and width: On a big skull, the

  distance across the mass of muscles is often three feet and the

  maximum tract of muscle fiber often three and a half feet in length.

  These muscles must have delivered an astounding bite in life, with

  a force greater by far than any other land herbivore in life's his-

  tory.

  Triceratops could use this prodigious biting power either for

  nipping branches at the beak end or for cutting up the fodder into

  smaller chunks with its teeth. Horned dinosaurs had cheek pouches

  and could employ tongue—cheek coordination to keep chopping

  DINOSAURS AT TABLE I 169

  Jaw muscles in Psittacosaurus (above)

  and Protoceratops (below)

  the bolus into ever finer slices. The geometry of their teeth re-

  sembled the duckbills'—several rows of teeth were packed tightly

  together so that enameled ridges provided a self-sharpening cut-

  ting mosaic. But the horned dinosaurs' teeth were arranged to

  provide more of a vertical slicing action and less fine shredding

  than those of the duckbills.

  What did Triceratops eat? Ostrom suggested cycadeoid fronds,

  probably a good guess. Cycadeoids were plants with large fronds,

  their leaves resembling the cycads popular today in Florida as dec-

  orator shrubs. Cycadeoids were so common that the Cretaceous is

  known as the Age of Cycads. Both cycads and cycadeoids had fronds

  two, three, even four feet long, characterized by especially strong

  fibers and prickly pointed leaflets, so that cutting such leaves was

  a nasty business. But a rich source of protein and calories lay in

  those Cretaceous fronds, awaiting any beast that could evolve the

  proper chewing armament. Horned dinosaurs were late arrivals;

  they didn't make their evolutionary debut until halfway through

  the Cretaceous. But once they got going, they developed with ex-

  plosive success, proliferating species by the dozen, the result of

  their mechanical prowess in chopping the previously inaccessible

  fronds.

  Beaked dinosaurs featured another adaptive device in their

  plant-eating repertoire, an extra-long digestive tract for soaking and

  fermenting stubborn plant tissue. Paleontologists usually dismiss

  any theorizing about the soft parts of dinosaurs. Stomachs rot, in-

  testines decay . . . both disappear without a trace in the fossil.

  Ergo, all speculation about gastrointestinal tracts in the Dinosauria

  is futile. This is a serious problem because obviously there's no

  hope of understanding the dinosaurs' approaches to plant eating

  without at least some knowledge of their innards. Some skepti-

  cism about the study of digestive systems is justified—only very

  rarely do sediments preserve direct evidence of inner architec-

  ture. Digestive structures possess neither bones nor other hard

  tissue, so the only way their outlines can be preserved is on rare

  occasions when mud fills the stomach and intestines before the tis-

  sue rots (a few Coal Age amphibians were preserved that way, but

  no dinosaurs).

  Christine Janis, a fellow graduate student at Harvard in the

  mid-seventies, was the first to excite my interest in the digestive

  DINOSAURS AT TABLE | 171

  organs. She pointed out that teeth and jaws tell only half the story.

  Nearly all plant-eaters have fermenting vats, enlarged chambers

  where food sits and soaks while microbes attack it with powerful

  enzymes. Janis stressed how enormous is the variation in location

  chosen by natural selection for the fermenting site. Ruminants—

  the deer—cattle—antelope family—chose a forward site and re-

  modeled their stomach into a complex multi-chambered rumen

  where the bolus is soaked by enzymes. Since the rumen is located

  in the forward stomach compartment, a deer, antelope, or buffalo

  can crop leaves, wad them up into a bolus, pass it down for pre-

  softening, then pass it back up to the teeth for a thorough chew

  after the leaves have been softened. Forward locations offer sub-

 
stantial advantages—teeth are saved from unnecessary wear when

  all food is pre-soaked and softened. Horses, rhinos, and ele-

  phants, on the other hand, chose a rearward location, a pocket

  evolved far back in the intestine or colon. Rearward location has

  one major disadvantage—the bolus can't make any sort of return

  to the mouth. But since the rear of the body cavity is spacious, its

  advantage is that rearward fermenting vats can be huge.

  A dinosaur's fossilized ribcage can reveal a great deal about

  the organs it housed—information largely ignored until recently.

  For one thing, how big the dinosaur's digestive chambers were can

  be gauged by the size of the ribcage. Orthodoxy maintains that

  many dinosaurs were too weak-toothed to eat tough plants; but

  large digestive tracts could compensate for weak teeth. Janis made

  a point often ignored by bone-and-teeth paleontologists: The bet-

  ter the enzyme soak given to food, the fewer the teeth needed to

  deal with any specific food texture. A good case in point: Today's

  herbivorous lizards usually have relatively small, weak teeth and

  until recently had the reputation of being inefficient plant-eaters,

  but recent experiments show that some lizards carry out very ef-

  fective rearward fermentation in their extra-long intestines. Giant

  ground birds—rheas and ostriches—have tiny heads and no teeth

  whatever, yet these birds successfully employ rearward fermenta-

  tion on a large scale.

  The precise details of any dinosaur's plumbing cannot be de-

  termined, but the overall body contours outlined by the ribcage

  and hips do show how large the entire digestive apparatus was and

  in what locations. Humans don't have ribs in their abdominal sec-

  172 | THE HABITAT OF THE DINOSAURS

  tor—their ribs end at the posterior edge of the lung compartment.

  But dinosaurs had long ribs attached to every segment of the

  backbone from chest to hip, so the cross section of the digestive

  tract is preserved by the skeletal architecture. Brontosaurs clearly

  had short, deep, crowded gastric tracts, because the abdominal

  ribcage was compact front to back, and the ribs over the belly arched

  widely outward from the backbone like barrel staves. In general

  configuration the brontosaur's intestines followed the proportions

  of modern elephants. And, just as in elephants, the front edge of

  their hip bone (ilium) was flared outward to support their wide

  belly. Elephants are big rearward fermenters, and brontosaurs must

  have been so too. Some brontosaurs had larger digestive organs

  than others; Brontosaurus itself had a very short torso from front

  to rear and must have had a less voluminous intestinal apparatus

  than Brachiosaurus with its long torso. Equipped with both gizzard

  stones and rearward fermenting vats, brontosaurs could have tackled

  really tough plant food.

  Early beaked dinosaurs were bipeds, using hind legs alone for

  their fast locomotion. That presented a special design problem: They

  had to evolve a large digestive tract without upsetting the balance

  necessary for bipedal walking. In most primitive dinosaurs, the

  digestive tract ended where it butted against the wide pubic bones,

  which formed a rear bulkhead for the entire abdominal cavity.

  Located above the pubic bones was a narrow passage through the

  hips, through which must have passed all the animal's outlets—

  colon, urinary tube, and birth canal. This pubic bulkhead was an ob-

  stacle to redesigning the intestines: in order to lengthen the gastro-

  intestinal tract, the pubic bulkhead had to be pushed backward and

  with it the entire pelvis. So a longer digestive system implied a

  longer, heavier body cavity before the hips—a shape hard to

  balance. Primitive beaked dinosaurs were forced to face this design

  problem squarely from the very beginning of their evolution-

  ary development, because nearly all the earliest species were very

  long-limbed and lively, built for ultrafast bipedal running. Fast lo-

  comotion placed special strain on the back, and a long, heavy

  stomach in front of the hips would be difficult to support. Evolu-

  tion was thus pulling in two opposed directions—shorter torsos

  provided better balance for running, but longer torsos were re-

  quired by the need to deal with the problem of digesting leaves.

  DINOSAURS AT TABLE | 173

  How shifting the pubis backward lengthens

  the guts in beaked dinosaurs. Arrow

  points to pubis.

  Escape from this biomechanical dilemma was engineered by a

  clever bit of anatomical sleight-of-hand. Peter Galton was the first

  to work out the details in his Ph.D. thesis in 1966. If beaked di-

  nosaurs managed to shift their digestive system beneath the hip

  bones, its length could be increased without relocating the hip joint.

  That effect was obtained by bending the pubic bones backward from

  the point where they already attached to the other hip bones, so

  that the intestines could lengthen while the hip joint remained as

  it was. In their new position, the pubic bones slanted downward

  and backward instead of straight down as before. Beaked dino-

  saurs finally shifted the lower end of the pubic bones so far back

  that they were far behind the hip socket. In this remodeled posi-

  tion the thick coils of intestines and the colon continued without

  interruption backward from the belly, to below the hip socket, and

  all the way to the base of the tail.

  174 | THE HABITAT OF THE DINOSAURS

  The solution was elegant. The digestive system could be

  lengthened and bipedal balance improved at the same time, for

  more intestinal tubing located to the rear of the hip joint clearly

  helped to balance the weight of the body before it. Although the

  beaked dinosaur classes are totally extinct nowadays, a quite vivid

  picture of their intestinal arrangements can be obtained from a

  chicken, turkey, or any modern bird. In birds, the gastrointestinal

  tract passes right under the hips all the way to the end of the pubis

  below the base of the tail. And birds obtain the same advantages

  for balance and strength as did the dinosaurs. Birds—like early

  beaked dinosaurs—are bipeds when they walk, and their intestinal

  arrangement allows them easy balance on their hindlegs. In the air,

  all the force of the wingbeat passes through the upper shoulder

  joint, and the design of the intestines lets birds have very short,

  very strongly braced torsos to anchor the stresses and strains of

  flying.

  Some beaked dinosaurs had every possible plant-digesting

  device—the parrot dinosaurs, for example, had (1) strong, deep

  beaks, (2) closely packed teeth, (3) large jaw muscles, (4) a fore-

  stomach gastric mill with stomach stones, and (5) a long intestine.

  These parrot dinosaurs were the ultimate in dietary adaptation. They

  were, however, exceptions to the rule. Most advanced beaked di-

  nosaurs heavily developed one type of digestive device or an-

  other, not all at once. Duckbills possessed cranial Cuisinarts par

&
nbsp; excellence. They had developed the best teeth for shredding leaves

  and twigs into tiny bits. Large horned dinosaurs were the long-

  slicers, the best at cutting tough vegetables into digestible chunks.

  Yet neither duckbills nor horned dinosaurs have ever been found

  with gastroliths, even though dozens of good skeletons have been

  hewn out of the Late Cretaceous rocks. Probably these strong-jawed

  dinosaurs substituted the power of their teeth for stone power in

  the stomach. And the size of their digestive system varied consid-

  erably in these species: wide-beaked duckbills had broad tail bones

  that supported enlarged colons and intestinal appendices; hollow-

  crested duckbills and horned dinosaurs were less enlarged in the

  rump.

  The successful development of rearward fermenting vats pro-

  vides the solution to one of the biggest puzzles about plant-eating

  dinosaurs—the weak-gummed giants. Although brontosaurs,

  DINOSAURS AT TABLE | 175

  duckbills, and horned dinosaurs were all apparently well provided

  with grinders of one anatomical sort or another (teeth or gizzard

  stones or both), two groups of big plant-eating, beaked dinosaurs

  seem to have been totally unprepared for grinding plants: the ar-

  mored ankylosaurs and the dome-headed dinosaurs. Both groups

  seem to have followed the wrong evolutionary path—their teeth

  are smaller, weaker, and less tightly packed than in the ancestral

  early beaked dinosaurs. Ankylosaur and domehead had tiny crowns

  on their teeth, and the teeth were loosely spaced in the huge bony

  jaws. Traditional paleontologists looked at those mouths and con-

  cluded they couldn't have chewed anything tough with teeth like

  those. These dinosaurs must have had restricted diets of soft food

  and therefore low metabolic rates. As usual with such interpreta-

  tions, they immediately arouse suspicion. After all, ankylosaurs and

  domeheads were advanced dinosaurs bristling with specialized neck,

  skull, limbs, vertebrae, and armor. So the very logical question is:

  Were there any equally advanced gastrointestinal adaptations that

  might have compensated for this chewing apparatus? There was

  indeed: giant afterburners.

  No American museum presently displays an entire ankylo-

  saur or domehead, even though a dozen good skeletons lie in