Robert T Bakker

by The Dinosaur Heresies (pdf)

  dundant use of bone to place the tallest plates over the strongest

  segments of the back.

  American paleontologists of the last four generations have

  puzzled over the apparent incomprehensibility of the arrangement

  of the stegosaur's plates. Several prominent museum scientists even

  concluded there simply wasn't any mechanical function at all for

  the plates—they were purely ornamental devices to make the

  stegosaur look more intimidating to enemies and sexier to poten-

  tial mates. A sexy look and an intimidating profile are worthy evo-

  lutionary results. But in fact if the evolution of the stegosaur's plates

  is carefully considered, it becomes possible to see how they could

  have functioned as a very effective addition to defensive arma-

  ment.

  Armor plate was a long-standing characteristic in the Dino-

  sauria. Crocodiles sported armor on their neck, torso, and tail when

  they first appeared during Triassic times, 220 million years ago.

  And all living croc species retain a flexible body shield of horn-

  covered bony plates joined together by sheets of ligaments within

  the deep skin layer. No croc possesses stegosaur-style triangles,

  but most crocodilians do display big oval plates of armor on the

  back of the neck, and these usually have raised, pointed ridges.

  Similar arrangements of armor protected other dinosaur relatives

  (the Thecodontia) from the Triassic Period.

  The process whereby oval, keeled plates evolved into the up-

  right, thin triangles of stegosaurs was fairly simple: by reducing the

  bony bases and enlarging the ridges, the oval plates quickly be-

  came thin stegosaur triangles. However, the key question about

  stegosaur armor is why would the animals evolve such weak nar-

  row bases for their plates? A broad base firmly embedded in tough

  skin would have prevented the armor from bending when a pred-

  ator struck. But stegosaur plates were so tall, and their bases so

  narrow, it was most unlikely they could remain stiff or upright.

  Maybe the stegosaur's approach to armor design was dynamic.

  Muscles in the skin might have moved the plates so they could

  232 | DEFENSE, LOCOMOTION, AND THE CASE FOR WARM-BLOODED DINOSAURS

  point upward or downward, depending on the point of origin of

  the attack. When erect, the points would have warded off attacks

  from above; when flexed downward, they could have provided the

  stegosaur with a very effective flank defense. Held horizontally,

  the tallest plates would have stuck out sideways three to four feet,

  so that no predator could approach near the stegosaur's hide. Since

  the muscles and flanks of the hind limb were especially vulnerable

  zones, the plates over the rump had to be the largest.

  A flexible armor defense would at last explain all the most

  peculiar features of how stegosaurs were designed. The pitted tex-

  ture of the plates' basal surface would have provided purchase for

  some sort of tissue—ligament or muscle—to embrace the plates at

  the base for about half a foot. Thin bases embedded in the skin

  would have been necessary to permit the hingelike movement of

  the plates. The thin, triangular shape conveyed the greatest strength

  to the pointed tip with the least concomitant amount of weight.

  Muscles in the skin have evolved several times in different

  vertebrate groups, and might easily have evolved in stegosaurs. Skin

  muscles have evolved in mammals to move hair as when a horse

  twitches the skin on its back to flip off a fly. (Human evolution

  took a U-turn here. We had skin muscles once but lost most of

  them when we evolved our naked skin, and now possess few ex-

  cept on our faces.) Most living reptiles bunch skin muscles thickly

  around the throat, and they are what slips the neck frill forward

  on the Australian frilled lizard, for example. The skin muscles of

  birds are used to control the orientation of their feathers. If skin

  muscle could evolve to flip frills in lizards and feathers in birds,

  stegosaurs might have evolved them to flip their armor plates.

  Stegosaurus must have been a grand performer under attack—

  a five-ton ballet dancer with an armor-plated tutu of flipping bony

  triangles and a swinging war club. Browsing peacefully on the tops

  of bushes, perched upon its hind legs and tail, its keen eye quickly

  catches the movement of two huge Allosaurus, hunting au pair, along

  the floodplain. The pair of allosaurs stride quickly to the attack,

  one from either side. The Stegosaurus alights defensively on all fours.

  The first Allosaurus darts quickly for the stegosaur's neck. In-

  stantly the intended victim pivots and lowers its armored trian-

  gles. The allosaur suffers a cut across its snout for its efforts. The

  second allosaur lunges at the other flank, but the stegosaur's tail

  MESOZOIC ARMS RACE I 233

  slashes out to meet the charge. With an audible "whoosh," the four-

  spiked tail barely misses the allosaur. That's too much for the would-

  be predators. They back off from the stegosaur still capably bran-

  dishing its weapons. And off they go to find easier pickings.

  Sudden extinction interrupted the evolution of dinosaur ar-

  mor at the end of the Jurassic when the true stegosaurs died out

  totally or at least became very rare. But as the Cretaceous Period

  dawned, new dinosaur dreadnoughts appeared: the nodosaurs.

  Nodosaurs shared some characteristics with the stegosaurs—their

  hips were high and their deltoid muscles were strongly developed

  for sideways maneuver. But on the whole, the nodosaur's ap-

  proach to defense was much more massive. A complete flexible

  Four tons of

  charging armor—

  the nodosaurian

  Edmontonia

  234 | DEFENSE, LOCOMOTION, AND THE CASE FOR WARM-BLOODED DINOSAURS

  pavement of small bony plates armored the entire upper surface

  of nodosaurs—head, neck, torso, and tail. None of these plates were

  anything like the tail triangles of the stegosaurs, but some nodo-

  saur species had broad-based plates topped by tall conical spines.

  Nodosaur armor plating was therefore much more continuous than

  that of stegosaurs, and their hip bones had expanded into an im-

  mense solid roof protecting the upper surface of the abdomen.

  Some paleontologists believe nodosaurs defended themselves

  passively. According to this view, these massively armored mon-

  sters employed their carpet of plates much like a mobile bomb

  shelter. Under attack, they would merely hunker down on the

  Cretaceous forest floor, legs folded under their body, to wait out

  the tyrannosaur's attack. But there is ample reason to believe no-

  dosaurs could become actively dangerous antagonists and turn the

  tables on their attacker. What made some nodosaurs dangerous was

  their sharp shoulder spikes. Mounted on broad bony bases

  embedded in the skin of the shoulder stood long, sharply pointed

  horn cores that curved forward. In life, an outer covering of horn

  made these shoulder spikes as long and deadly as the tail spikes

  of stegosaurs
had been. Like stegosaurs, nodosaurs had very strong

  elbow muscles (the triceps muscle group) perfect for quick, for-

  ward lunges. Altogether, the forward end of a nodosaur resem-

  bled the gigantic, short-legged warhorse of medieval times, coated

  with armor, and ready to charge with wickedly sharp lances jutting

  forward from either side of its head.

  Wrestlers and short-legged fullbacks know the advantage of

  short, strong legs—a long-limbed wide receiver can run faster, but

  the shorter legs provide a greater initial acceleration. The nodo-

  saur's enemies were the very long-limbed tyrannosaurs, fast enough

  to catch any nodosaur very quickly. But once the combatants were

  close, the advantage of speed disappeared. A tyrannosaur could

  stretch downward to snap at the nodosaur's tail or back, but the

  predator would only succeed in breaking its teeth against the im-

  pregnable carapace. The tyrannosaur's only hope would have been

  to flip the nodosaur in order to attack its unprotected belly. But

  nodosaurs were very wide across the hips and had a low center of

  gravity. A fully grown nodosaur would have been as easy to flip as

  a modern wide-track station wagon. And then the tyrannosaurs

  would have had to face counterattack. The nodosaur could have

  MESOZOIC ARMS RACE I 235

  Weakest teeth in the Dinosauria—the nodosaurs. Edmontonia was typical of

  the strangely constructed nodosaurids. Thick armor plates covered the snout

  and forehead, and a sharp-edged beak was built around the muzzle, but the

  teeth were absurdly tiny. Did this dental decrepitude condemn Edmontonia to

  a diet of soft-water plants?

  kept pivoting to face its long-limbed attacker, awaiting its op-

  portunity. If the tyrannosaur allowed any opening at all, the no-

  dosaur's powerful elbows and knees could instantly drive its

  armor-plated body forward. And its murderous shoulder spikes

  might catch the predator's calf or leg, tripping the tyrannosaur or

  ripping a nasty wound. Before such a lethal charge, any tyranno-

  saur might have beaten a well-advised retreat.

  Late Cretaceous days were the high point of armor develop-

  ment for the Dinosauria. Nodosaurs of several species stalked the

  meadows of the Cretaceous deltas of North America. And they

  were joined by a new family of dreadnoughts, the ankylosaurids.

  At first glance the ankylosaurids seem less dangerous than the no-

  dosaurs: they were shorter at the hips and weaker in the shoulder,

  couldn't pivot as quickly, and lacked the lethal shoulder spikes. As

  compensation, ankylosaurs had better head protection than no-

  dosaurs—overhanging plates protected their eyes and cheeks.

  But the tail functioned as the cornerstone of the ankylosaur's

  defensive tactics. Like the nodosaur's, the root of the ankylosaurs'

  tail was powerful and supple. Unlike nodosaurs, the last half of

  the ankylosaurs' tail was stiffened by a series of bony tendons that

  converted this end into a prolonged handle for a bone-crushing

  war club. At the very end of the tail, three large masses of bone

  fused together to create a sort of monster cloverleaf-shaped club

  head. By contracting its tail muscles, the animal could quickly swing

  the stiff handle from side to side and powerfully flail its three-leafed

  club head.

  The ankylosaurs' war club was less precise but quicker than

  the stegosaurs'. A stegosaur's tail joints were supple right down to

  the very tip of its tail, so the animal could control the movement

  of the spiked club carefully. Such care was necessary for effective

  defense because the sharp spikes required accurate aim. Ankylo-

  saurids had less finesse, but the massiveness of their club guaran-

  teed damage no matter how the blow landed.

  In nodosaurs, ankylosaurids, and stegosaurs, the dangerous

  weapons were carried on shoulders or tail. But one group of ar-

  mored dinosaurs, the boneheads, used their skulls as their princi-

  pal offensive device. "Greatest Dinosaurian Bonehead!"—touted

  the label beneath the skull in the New York museum. In the glass

  case stood Pachycephalosaurus ("heavy-headed lizard"), a bonehead

  MESOZOIC ARMS RACE I 237

  indeed, with a two-foot-long skull topped by a dome of solid bone

  eight inches thick covering the forehead and crown.

  Bone-headed dinosaurs at first provoked the same combina-

  tion of awe and ridicule that originally greeted the stegosaurs. Here

  was yet another case of outlandish dinosaur construction without

  any conceivable explanation from body mechanics. Back in the early

  1900s, the theory of racial senescence would have served as a re-

  spectable hypothesis to explain strange dinosaur adaptations such

  as bonehead skulls. Flamboyant and senseless crests, plates, and

  spines were supposedly signs that all Dinosauria had gone senile

  in their evolutionary old age. Like old ladies wearing out-of-fash-

  ion headgear and mismatched gowns and coats, the dinosaurs had

  allegedly lost their adaptive vigor and had become incapable of

  evolving anything but nonfunctional ornaments and maladaptive

  excrescences. As a theory, racial senescence was bankrupt by 1920,

  but it still pops up here and there in bad books about dinosaurs.

  The bodies of boneheads were nothing unusual: a pair of long

  hind legs, long, stiff tail, short arms, a barrel-shaped body mass to

  accommodate masses of vegetation. This general configuration

  wasn't different from that of a host of other bipedal beaked di-

  nosaurs. And even bonehead skulls weren't noticeably deviant in

  the snout, jaws, and teeth. The strangeness of boneheads was con-

  centrated entirely in the domelike swelling over the top of the

  braincase. Some species had only a slightly thickened skull roof of

  otherwise normal construction. But the fully developed bone-

  heads, like Pachycephalosaurus, had giant domes that suggested great

  intellect. As the original discoverers of bonehead dinosaurs were

  quick to point out, the supposedly brainy appearance was a sham—

  the brain itself was tiny and occupied only a small volume deep

  inside the bony dome. Most of that dome was in fact filled with

  bone cells arranged in a radiating pattern like the fibers in a cross

  section of grapefruit. Bone cells usually grow in the direction of

  greatest stress, so the bonehead's pattern of growth is a clue to the

  head's function. The radiating pattern strongly suggests that the

  dome was subjected to enormous outside pressures. But of what

  sort?

  Dome-headed dinosaurs can probably best be understood as

  wearing NFL-style football helmets over their minuscule brain-

  case. Modern football helmets weren't designed for merely pas-

  238 | DEFENSE, LOCOMOTION, AND THE CASE FOR WARM-BLOODED DINOSAURS

  Stegoceras—two individuals ramming

  sive protection; they were built so the wearer could ram his head

  into the unfortunate player opposite him. Old-fashioned leather

  helmets weren't as good for the head-first block, but coaches soon

  discovered that the human head could serve as a weapon, so
the

  helmets were redesigned. A domed, impact-resistant helmet was

  invented to provide the head-ramming linebacker with the ability

  to strike blows without damaging his own skull. Dome-headed di-

  nosaurs had evolved the physical equivalent of a ramming helmet

  millennia before, a configuration that strongly suggests a head-first

  mode of attack.

  Peter Galton, a leading expert on beaked dinosaurs (he's the

  man who studied the cheeks and digestive tracts in duckbills), first

  worked out the head-butting theory in 1971 and won nearly

  worldwide acceptance of his basic idea. Galton pointed out that

  dome-headed dinosaurs had exceptionally strong muscles holding

  the head at a right angle to the neck, so that the dome would face

  forward when the beast lowered its head and charged. The animal

  possessed all the qualities for an optimal butting attack—a bull neck,

  MESOZOIC ARMS RACE I 239

  a low head position in the charge, and a thick skull covering a small

  brain.

  Galton believed sex was the ultimate motivation behind the

  head-butting behavior. He argued that male domeheads would have

  banged their crania against one another in ritualized combat, much

  like bighorn sheep. On this point, I would disagree a bit. A big-

  horn sheep's horns are wide and flat, so when two males clash, their

  horns meet across a wide surface. The resulting collision is a true

  test of strength, because the full force of the sheeps' bodies is de-

  livered. But the rounded shape of bonehead dinosaur domes made

  a precise head-to-head blow nearly impossible. If two boneheads

  did clash, their heads would probably have struck only glancing

  blows. Domed heads, therefore, like football helmets, were prob-

  ably for butting an adversary in the body, not in the head.

  Polish expeditions to Mongolia in the 1960s found a marvel-

  ous bunch of bonehead dinosaurs. The Poles wondered whether

  the head-butting equipment really was for sex-related contests—

  the head seemed too dangerous a weapon for such encounters. The

  Polish hypothesis argued that the bonehead was essentially an an-

  tipredator weapon. It is difficult to judge between the alternatives.

  Did the boneheads ram one another, or did they ram meat-eating

  dinosaurs that threatened attack? Probably both hypotheses are