Robert T Bakker

by The Dinosaur Heresies (pdf)

  as a gazelle's or an ostrich's, and most large dinosaurs had a

  shank + ankle -5- thigh index of 1 or perhaps a little higher. There-

  fore, it's been concluded that short-shanked dinosaurs were lim-

  ited to low-gear locomotion. Triceratops had a quite stubby ankle

  index and was therefore allegedly incapable of any fast movement

  at all. But all horned dinosaurs had shanks that were actually much,

  much longer than a rhino of the same weight. These dinosaurs only

  seem to have relatively short ankles and shanks because the thigh

  is much larger than a rhino's.

  Triceratops was indeed shorter in the shank than a modern rhino

  is, but that doesn't prove Triceratops couldn't run as fast or faster.

  Triceratops had tremendously strong limb bones, and that strength

  must have evolved to withstand great forces. The unbelievers who

  scoff at the notion of a galloping Triceratops will have to explain

  why dinosaurs evolved such strong, thickly shafted limbs if they

  were going to do no exercise more strenuous than a shuffle through

  the swamps.

  A third argument has occasionally been advanced against the

  notion of fast speeds in quadrupedal dinosaurs. Mammals today

  use their shoulder blades as arm extenders, swinging each long blade

  fore and aft with every stride. Dinosaurs supposedly possessed rigid

  shoulder blades that had to remain in place against the ribcage. If

  THE TEUTONIC DIPLODOCUS: A LESSON IN GAIT AND CARRIAGE I 219

  this theory of the stiff shoulder is correct, Triceratops would have

  had considerable trouble locomoting because its forelimbs were

  much shorter than its hind limbs. If both fore- and hind limbs were

  working at full stride, the rear end would move faster than the

  front end and the five-ton monster would have the option either

  of turning circles or of flipping over altogether—a most maladap-

  tive model of locomotion!

  Working on my undergraduate thesis, I had toyed with the

  hypothesis that the dinosaurs' shoulder blades might have swung

  across the ribcage, but I was unable to build a reliable support for

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

  such heterodox mechanics. Later, at graduate school, I met a fel-

  low student, Jane Petersen, who had just completed a thesis about

  the shoulders of chameleons. She proved that chameleons could

  swing their long shoulder blades fore and aft more freely than other

  lizards, because the chameleon's blade was not locked onto the chest

  by a bulky collarbone. This impressed me because I had already

  noticed that chameleons were the only lizards that looked like di-

  nosaurs in the shoulders. Both dinosaurs and chameleons have very

  long, slender shoulder bones that work completely free of re-

  straint from the collarbone, which anchored the shoulder blades

  Triceratops—stronger than a

  bull elephant. A five-ton

  African bull elephant has

  legs that are much thinner,

  and much weaker, than

  were those of a five-ton

  horned dinosaur. And so

  the dinosaur was able to

  withstand much greater

  stresses during running.

  THE TEUTONIC DIPLODOCUS: A LESSON IN GAIT AND CARRIAGE | 221

  in all most primitive reptiles. Chameleons evolved from some

  "normal" lizard ancestor that possessed a thick, stiff collarbone

  which held the shoulder blade in place. But chameleons shed that

  collarbone along their evolutionary path to provide themselves with

  more participation from their shoulders in the strokes of their fore-

  limbs. Dinosaur evolution must have been the same—dinosaurs

  experienced the same reduction of the collarbone and must have

  developed a similar free-swinging shoulder. And the big quadru-

  pedal dinosaurs evolved the longest shoulder blades of any verte-

  brate, past or present. As its yard-long shoulder swung alongside

  The horned dinosaurs—longer, faster,

  stronger legs than rhinos. A two-ton

  centrosaur had legs that were thicker, longer,

  and more powerfully muscled than those of a

  two-ton black rhino.

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

  Triceratops % ribcage, the extra length added to its forelimb must

  have given the animal a grand propulsive boost. Both fore- and

  hind limbs were consistently designed for fast, maneuverable

  movement.

  Such outlandish heterodoxy proves doubly sweet when sup-

  ported by independent confirmation. Fossil footprints are the only

  direct evidence left by locomoting dinosaurs, so a set of tracks left

  by some speeding Tyrannosaurus would provide dramatic confir-

  Swinging shoulder blades—a

  modern horse, a modern

  chameleon, and the three-

  ton horned dinosaur

  Centrosaurus

  BELOW: Collarbone

  prevented shoulder-blade

  swinging. Primitive dinosaur

  ancestors—like this Early

  Triassic Chasmatosaurus—

  couldn't use their shoulder

  blades for long fore and aft

  swings because the

  collarbone held the

  shoulder blade tightly

  against the sides of the

  chest and the breastbone.

  THE TEUTONIC DIPLODOCUS: A LESSON IN GAIT AND CARRIAGE | 223

  Five tons of Triceratops'

  at full gallop

  mation. The English biologist McNeil Alexander has worked out

  a clever formula for computing speed from trackways: all that is

  necessary is the length of stride and the toe-to-hip measurement.

  When first applied to some samples of dinosaur prints, the for-

  mula yielded low speeds—two to four miles per hour. Some com-

  mentators immediately jumped to the conclusion that this

  conclusively proved the theory of slow dinosaurs. That is non-

  sense. Most fossil trackways represent slow cruising speeds, not

  top speed, because all species spend most of their time moving

  along in an unhurried fashion. Bursts of maximum velocity erupt

  only rarely, when a predator charges or a plant-eater scampers for

  its life. Most tracks left by gazelles and rhinos today are made at

  a slow speed when these animals are feeding or going to or from

  water holes. Rhinos don't live their entire lives at thirty-five miles

  per hour; a trackway that caught one of these rare moments when

  the rhino was galloping full tilt would be a most extraordinary find.

  Trackways from big quadrupedal dinosaurs are rare—there exist

  only four sites with good brontosaur tracks—so the sample is far

  too poor to argue any case about top speed.

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

  Bipedal dinosaurs are represented by more tracks—hundreds

  altogether—so a few tracks might conceivably capture a moment

  of high speed. And a few two-legged trackways do provide such

  proof. Several medium-sized, fifty-pound to half-a-ton bipedal

  predators have left long-striding tracks which compute to speeds

  of twenty, thirty, or even forty miles per hour.

  Narrow tracks,
swinging shoulders, stout-shafted limbs that

  bounced at every stroke—all these bits of modern evidence agree

  with the lively restorations drawn for Marsh and Cope way back

  in the 1890s. Cope had a painting made of Dryptosaurus, showing

  a pair of the giant meat-eaters excavated from the phosphate mines

  of New Jersey. Cope's dryptosaurs were portrayed in violent lo-

  comotor exercise. One was flung on its back, hind legs lashing out

  in claw-tipped defensive strokes; the other was painted in mid-leap,

  its great hind legs having propelled its body far above the ground.

  A good painting, far more faithful to the real structure of dinosaur

  locomotion than the shuffling reconstructions popular in most or-

  thodox textbooks today. Speed and vigor were the way of the

  dinosaurs, multi-ton monsters able and ready to break into a fast-

  paced charge whenever necessary. The Mesozoic was life in the

  behemoth fast lane.

  THE TEUTONIC DIPLODOCUS: A LESSON IN GAIT AND CARRIAGE | 225

  11

  MESOZOIC ARMS RACE

  Humans are one of the least armored products of evolution.

  Perhaps our own defenseless hide renders the apparently bi-

  zarre armor plate sported by three great clans of beaked dino-

  saurs—the Stegosauria, the Ankylosauria, and the horned

  dinosaurs—especially fascinating. The story of these armored di-

  nosaurs is a drama out of the Mesozoic arms race, the co-evolu-

  tionary link between ever deadlier meat-eaters and ever more

  formidably protected prey.

  Stegosaur tails were without question one of the most dan-

  gerous weapons ever evolved by a plant-eating animal. At the ex-

  treme end of the stegosaur's long tail sprouted a fearsome war club,

  composed of four or eight sharply pointed spokes between two

  and three feet long. Extra-thick connective tissue in the skin an-

  chored the bases of these bony spikes so that the points extended

  outward, and upward, and backward. And pits left by blood ves-

  sels on these spikes show that they were sheathed by a very thick

  horn cover in life, much like the outer sheath of longhorn cattle

  today. Horn constituted the ideal sheathing material for such sharply

  pointed weapons because it is more flexible and less brittle than

  bone and thus can be honed to a much sharper point.

  To drive all those pointed tail spikes deep into the body of

  its adversaries, Stegosaurus required a tail of great power and flex-

  ibility, and both qualities were in abundant supply. To acquire

  226 I DEFENSE, LOCOMOTION, AND THE CASE FOR WARM-BLOODED DINOSAURS

  flexibility in the tail, the stegosaurs' evolution had to dispose of a

  major feature of their ancestry, the system of stiff tendons. Most

  beaked dinosaurs featured a latticework of bony tendons running

  down either side of their backbones from torso to tail. And all the

  earliest, most primitive beaked dinosaurs possessed such equip-

  ment. As has already been discussed, this latticework—best seen

  in duckbills and horned dinosaurs—would have provided an ad-

  vantage for supporting the body weight without muscular effort.

  But such bony tendons would have stiffened the stegosaur's tail

  The big-plate stegosaur Diracodon

  battles a Ceratosaurus

  MESOZOIC ARMS RACE | 227

  too much for easy swinging. Evolution therefore eliminated the

  system of tendons and the stegosaurs were the only beaked dino-

  saurs to do away with bony tendons entirely. But merely elimi-

  nating bony tendons wouldn't have been enough to render the

  stegosaur's tail optimally dangerous. Since the spikes stood at the

  tail's extreme tip, the bones of the tail had to be both strong and

  flexible all the way to the end. In most dinosaurs the tail joints

  grew progressively stiffer toward the end, but not so in stego-

  saurs. The joints between the successive segments of the tail gave

  its entire length from rump to tip enough suppleness to flex in a

  graceful S-shaped curve, and the vertebrae were much stronger than

  usual near the end.

  To achieve the muscular strength necessary to swing its club,

  the stegosaur evolved enlarged shelves of bone for anchoring its

  muscles (similar shelves had evolved in the big-tailed brontosaurs,

  such as Diplodocus). A twenty-foot-long stegosaur would have had

  more strength in its tail muscles than a large modern elephant has

  in one of its hind legs. And when the mighty tail muscles con-

  tracted, the stegosaur's caudal club swung with irresistible authority.

  The eight-spiked Stegosaurus ungulatus

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

  Stegosaurs had need of such a war club because they faced

  predators nearly as large as elephants. Allosaurus and Ceratosaurus,

  the two most common Late Jurassic flesh-eaters, both grew to

  lengths of thirty feet and more and would have weighed between

  one and two tons. Even larger was Epanterias (possibly a very large

  species of Allosaurus), a forty-five-foot predator that must have

  reached four tons, six times heavier than a large lion. If such huge

  flesh-eaters attacked in groups (a tactic widely believed possible),

  only the most heavily armed plant-eaters could have survived.

  Imagine the potential of the stegosaur's tail spikes in such a con-

  frontation. If the three- to four-foot-long spikes were driven full

  force into the chest or belly of even the largest predator, the re-

  sult would have been devastating. Not even Epanterias would have

  survived a direct hit.

  But to fight well, Stegosaurus would have had to maneuver

  quickly, pivoting about to keep its tail club facing the attacker. Al-

  losaurus and Ceratosaurus were long-legged and nimble-footed, and

  could have danced around the stegosaur in order to lunge in for

  bites at the vulnerable neck or shoulders. How could evolution

  equip the stegosaur with the necessary maneuverability to employ

  its tail club to best advantage? The solution was found in its unique

  body proportions and its short but thickly muscled forelegs.

  Stegosaurs appear ungainly at first sight—their hind leg was much

  longer than the fore, the hips much taller than the shoulder. The

  combination of a heavy rump and tail with short forelimbs placed

  the point of balance of the stegosaur's body just forward of the

  hips, so that the beast could easily have pivoted around by push-

  ing sideways with its forepaw.

  The muscles employed to push sideways with the arms are

  known as the deltoids. In most dinosaurs the deltoids were mod-

  erately strong but not unusually so. But stegosaurs possessed prize-

  winning deltoids, and the site where they attached to the upper

  arms (humerus) was gigantic, larger than in any other vertebrate.

  Obviously then, when threatened by a predator, the stegosaur

  shifted its weight back onto its hind feet, then pushed with its fore-

  feet, to rotate right or left in order to keep its deadly tail facing

  the foe. Its huge deltoids provided sufficient power for pivoting

  its entire body mass with ease.

  Stegosau
rus is, however, best known not for its war club, but

  MESOZOIC ARMS RACE I 229

  Stegosaur muscles for quick turns. The deltoid muscle group had a huge

  sideways-facing crest on the upper arm (humerus) so that stegosaurs could

  push their bodies to one side or another. Powerful triceps muscles running

  from shoulder blade to elbow gave the stegosaur a forward-lunge capacity.

  for the spectacular triangles of bone that rose up to four feet above

  its backbone. Though tall and broad, they were thin in section and,

  like the tail spikes, were sheathed in life by an outer layer of horn.

  Roughened zones along the bases reveal that these bony plates were

  embedded in the skin along the top of the spine. Most restora-

  tions show these plates sticking straight up from the back. But that

  is a most puzzling orientation for them. What could have been the

  bioengineering purpose of these strange triangles? Some paleon-

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

  How stegosaurs flapped their plates. Stegosaur ancestors had bony armor

  plates shaped like those of gators—the plate base was very wide and firmly

  embedded in the outer layer of tight skin. But during stegosaur evolution the

  plate base became very narrow and a sheet of skin muscle attached to the

  sides of the plate to swing it from side to side.

  tologists have suggested that if the stegosaur's plates stood up ver-

  tically, they might have offered some defense against bites directed

  at the backbone. But the stegosaur's spinal cord was already well

  protected without the plates. It lay deep beneath the very tall ver-

  tebral spines and the ligaments, which together constituted a very

  tough hump over the torso and hips, much like the ridge on a

  modern razorback hog. Any Allosaurus unwise enough to bite into

  that ridge would have broken off its teeth without inflicting sig-

  MESOZOIC ARMS RACE I 231

  nificant damage. Moreover, the largest plates were located over the

  hips and base of the tail, where the spinal cord was already best

  protected by vertebral spines. The stegosaur's spinal cord was so

  well armored by the backbone that the triangular plates really

  wouldn't have added extra protection. And it appears like a re-