Robert T Bakker

by The Dinosaur Heresies (pdf)

  rhinos and other large modern mammals and from the "sprawling

  gait" of the ground-hugging lizards.

  Alligators sprawled at the elbow much less than Professor Lull's

  Centrosaurus, and yet the horned dinosaur was supposed to be a

  much more advanced evolutionary design than the 'gator. Some-

  thing was deeply wrong here. Why would an advanced dinosaur

  exhibit a more sprawled posture than its more primitive relative?

  I needed evidence from the shoulder-bone structure which I could

  use to evaluate dinosaur forequarters. Two pieces of evidence came

  immediately to hand: First, the shoulder socket's shape. An ele-

  phant or rhino's shoulder socket is shaped like an oval saucer. It

  is a hollowed-out joint surface, elongated fore to aft, which faces

  downward and backward to fit over the top of the upper arm bone.

  But lizards and crocs, whose elbows sprawl, have a saddle-shaped

  shoulder joint, concave from bottom to top and convex from the

  inside out. This saddle-shaped notch lets the upper arm swing out

  and back and twist around like an axle, a complicated set of move-

  ments required by the sprawling and semi-erect gaits. Now, what

  kind of shoulders did dinosaurs have?

  I spent a year digging into museum drawers, and covering

  myself with dust while I diagrammed the shoulder sockets of the

  Dinosauria. Almost all had rhino-type joints. When properly

  mounted, dinosaur shoulder joints were concave sockets facing

  downward and backward. Markings on the bones showed clearly

  that the joint didn't curve around to face sideways as it did in 'ga-

  tors or lizards. Professor Lull's Centrosaurus had a misaligned front

  end, as did the mounts of most other horned dinosaurs.

  The second piece of evidence reinforced the first. Crocodil-

  ians and chameleon lizards had a semi-erect gait, and when I mea-

  sured their shoulder joints oriented to a side view, I found that

  both of these reptiles displayed a joint which slanted so that it faced

  slightly downward as well as outward and the upper edge of the

  joint overhung the lower edge. Fully sprawling lizards didn't ex-

  hibit a trace of this downward slant. On the other hand, dinosaurs

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  Shoulders designed for sprawling. Lizard shoulder joints are doubly curved

  notches shaped like a saddle, and the normal walking posture is with the

  elbows stuck far out to the side. (The upper-arm bone—the humerus—is

  pulled out of the socket a bit in the diagram to show the fit.)

  Horned dinosaur

  shoulders were designed

  for fully upright posture.

  The upper edge of the

  shoulder socket overhung

  the lower edge a great

  deal, even more than in

  crocodilians. And, viewed

  from the rear, the

  shoulder socket faced

  mostly downward, not

  outward.

  Shoulders for a more upright gait. Alligator shoulder joints are saddle-shaped

  but face more strongly downward than do those of lizards, and so the gator

  can hold its body higher off the ground.

  all manifested very strong downward slants, so that their entire

  shoulder socket had been reoriented from the primitive arrange-

  ment. This strong downward orientation meant that the dinosaur's

  upper arm could swing fore to aft in an upright stride. And the

  upward force of this limb's stroke would be braced against the

  downward-facing shoulder socket.

  Finally, there was the acid test of fossil footprints. Quadru-

  pedal dinosaur footprints aren't as common as those left by bipe-

  dal types, but each and every set of four-legged footprints showed

  forepaws working on a very narrow track. Triceratops and the rest

  of the four-legged Dinosauria did not splay their forelimbs. Marsh

  had been right in the 1890s, Lull wrong in the 1930s.

  Lull's own account of why he mounted the Centrosaurus with

  wide-set forepaws was quite surprising. Lull wrote that he had

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  carefully studied the fossil footprints of big quadrupeds found in

  Canada as his guides for posture. Charles Sternberg had published

  illustrations of those prints in 1930, several years before Lull

  mounted his sprawl-elbowed beast. But Sternberg's diagrams

  showed right and left forepaws quite close to the centerline, and

  not spread widely apart. Lull simply ignored this, because he was

  so convinced, a priori, about splayed forelimbs that the obvious

  facts simply didn't register, as they still don't for some. Several large

  quadrupedal skeletons have been erected in various museums during

  the last decade, and some still faithfully cling to the traditional stance

  with the widely splayed forepaws, despite the publication of doz-

  ens of footprint diagrams.

  I was pretty proud of myself when I finished my undergrad

  thesis on posture evolution. I published a couple of articles argu-

  ing that the dinosaurian fully erect gait was superior to the sprawl-

  ing gait because erect posture didn't waste as much muscular effort.

  It seemed like a logical idea, and Al Romer had used it way back

  in the 1920s. For example, if you do push-ups on the floor, you

  can put your arms in the lizard-style posture by bending your el-

  bows at right angles and holding your body halfway off the floor.

  In this position, you feel very uncomfortable strain in your arm

  muscles. If you hold your arms straight up and down, in a fully erect

  Footprints don't lie.

  All dinosaur tracks

  show that the forepaws

  were put down right

  under the body with

  only a little space

  between the line of

  march of the left and

  the right set of prints.

  But many museum

  reconstructions still

  show dinosaurs with

  widespread forepaws

  that would have left a

  sprawling-style

  trackway. (This

  drawing is from a

  model in the National

  Museum of Canada.)

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  posture, you can keep your body off the floor with less effort.

  When I got to Harvard, I had fun chatting with Romer about

  how my theories agreed with his. But then I got my comeup-

  pance. As part of my Ph.D. work, I had to run lizards on minia-

  ture treadmills inside micro-environmental chambers to measure

  just how hard they had to breathe to run at different speeds. (Hot,

  boring work for me and the lizards—each run was thirty minutes

  and I needed twenty runs per lizard.) When the results came tick-

  ing out of the oxygen analyzer, I was devastated—and my theory

  was totally deflated. My sprawling lizards were more efficient than

  fully erect mammals and birds. All the lizards used less energy to

  run at any given speed than did birds or mammals of the same

  size. As the old laboratory saying goes "The theorist proposes,

  Nature disposes."

  I trotted into Romer's offi
ce the next day and sadly an-

  nounced, "Our theory is dead." Then I plopped the computer

  printout on his desk. Romer scrutinized it. Then with a twinkle in

  his eye and a mock inquisitorial tone in his voice he said, "Your

  data are probably correct. But they must be suppressed. Our

  beautiful theory has got to be preserved." I felt better. If Romer

  could chuckle, so could I.

  So what advantage is the fully erect gait? Probably it allows

  for much higher speeds even if efficiency is sacrificed. Having a

  Correct stance. Here's the proper

  reconstruction of a horned dinosaur

  (genus Chasmosaurus) made to fit

  the fossil trackways.

  THE TEUTONIC DIPLODOCUS: A LESSON IN GAIT AND CARRIAGE

  213

  vertical limb stroke means that you can exert more of a thrust

  downward onto the ground with your paws. And the speediest gaits

  require such thrust to propel the body when all feet are airborne.

  When I finally arrived at Harvard in 1972, I was still inter-

  ested in the gait of dinosaurs. All the anatomical footprint evi-

  dence vindicated Marsh's light-footed and lively postural restorations

  of the 1890s. The forelimbs of dinosaurs were aligned quite per-

  fectly to match with the stride of the hind limbs. I now asked

  myself, "How fast might the big dinosaurs have been?" Most

  twentieth-century paleontologists had been willing to concede lively

  locomotion to the small, long-legged ostrich dinosaurs and to the

  smaller predators, but the big two-ton-plus species were always

  reconstructed as slow shufflers. But large mammals can gallop.

  While in South Africa I observed three-ton white rhino bulls at a

  full gallop with all four huge feet off the ground simultaneously in

  mid-stride. In fact, rhinos can accelerate and turn faster than horses,

  though in the stretch a horse can outdistance the short-winded

  rhinos. Perhaps big quadrupedal dinosaurs could also quick-start

  off into their own clomping high-speed charge.

  A useful piece of evidence about the speed of dinosaurs can

  be extracted from the angles in their joints. Seen from the side, a

  running rhino always exhibits greater flexure at the elbow, knee,

  hip, and shoulder than does an elephant. Elephants run straight-

  legged, thigh lined up with shank and upper arm with lower arm,

  so their legs look rather like mobile Doric columns. Rhinos run

  with a more bent-legged stride and are consequently faster than

  elephants—top speeds are thirty-five miles per hour for the rhino,

  twenty-two for the elephant. The rhino owes its greater velocity

  precisely to the bounce it gets from the stretching tendons at its

  joints each time its feet plant down. Flexing joints provide more

  of this bounce, and all the big mammals that gallop are so jointed.

  Elephants can never get all their feet off the ground simultane-

  ously, even at top speed, and their fastest gait can best be labeled

  a running walk. If we could compare the angles in dinosaur joints

  to those in these living mammals, we would have an important clue

  to the bounciness of their gait and hence their speed.

  Brontosaurus has a reputation for being a relatively slow di-

  nosaur, and here orthodoxy is correct—all the brontosaurs had

  rather straight elephantine legs that didn't flex very much and must

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

  Swinging shoulders and bouncing knee joints. Big modern gallopers—like

  rhinos—have more flexure at their joints than do elephants. Brontosaurs,

  such as Camarasaurus, had little flexure and must have run like elephants.

  But horned dinosaurs had much more bend in each joint and must have been

  more rhinolike in gait. Both brontosaurs and horned dinosaurs had very long

  shoulder blades.

  Immense power at the dinosaur calf

  and knee. A Ceratosaurus set of hips

  and hind legs are shown in running

  configuration. The extraordinarily

  long upper-hip bone (ilium)

  supported a huge knee-opening

  muscle that attached to the enlarged

  crest on the shin. This crest also

  was the attachment site for birdstyle

  calf muscles bulging backward and

  sideways.

  Giant calf muscles of Triceratops as

  seen from the front.

  Triceratops shin,

  front view

  have limited them to a running walk. But the bipeds and the

  quadrupedal horned dinosaurs display much sharper joint flexures

  and probably bounced quite a bit as their thick tendons stretched

  out and snapped back with each stride. How strong, then, was the

  bouncing stroke of such a limb? Big gallopers today possess strong

  knee muscles that attach to the kneecap and shank in such a way

  that the knee joint opens and closes under tremendous muscular

  power. A bony ridge, the cnemial (silent c here: "nee-mee-al") crest,

  marks the point of attachment for the knee tendons, and one can

  directly gauge the muscle power of a knee from the size of a cne-

  mial crest. Elephants, turtles, and salamanders are all slowpokes in

  their body-size classes and all have puny knee muscles and low

  cnemial crests on the shank bone. Rhinos have big cnemial crests,

  as do other large-bodied gallopers, such as water buffalo, giraffe,

  bison, and gaur. Big crests would also mean big calf muscles.

  All dinosaurs had bigger cnemial crests than do elephants, even

  those groups with relatively straight hind legs—the giant horned

  dinosaurs, stegosaurs, and brontosaurs. When these systems of

  oversized knee muscles contracted, the power exerted on the hind

  THE TEUTONIC DIPLODOCUS: A LESSON IN GAIT AND CARRLAGE I 217

  paw would have had no equal today. The biggest meat-eater, three-

  ton-plus Tyrannosaurus, had an absolutely huge cnemial crest, even

  by dinosaurian standards. At full speed, a bull Tyrannosaurus could

  easily have overhauled a galloping white rhino—at speeds above

  forty miles per hour, for sure. The consistent pattern of huge cne-

  mial crests is documentary evidence of super-powerful knees and

  calves that gave fast top speeds in most big dinosaurs.

  A quite different approach to the question of dinosaur speed

  is provided by calculating the maximum strength of the bone shafts

  of the limbs. Legs do break in nature, and evolution usually outfits

  a species with bone shafts strong enough to withstand the highest

  strains imposed when muscles contract. Rhinos have relatively stout,

  thick-shafted legs. Elephants feature a more spindly design. To

  measure the shaft strength of dinosaur limbs, I constructed scale

  models in clay of the life appearance of various species. I then cal-

  culated the live weight by measuring the volume of the model (most

  land animals are a little less dense than water, so live weight is

  about 95 percent of the body's volume in water). Brontosaurs and

  stegosaurs were somewhat thin-thighed, and in cross section their

  bones are about as thick as we would expect in an elephant of sim-

  ilar size. But Triceratops, Tyrannosaurus, and the other predators />
  were much more massively shafted, far stronger in girth of bone,

  and these dinosaurs could exert positively prodigious force through

  their limbs without fear of fracture.

  Tyrannosaurus moving at forty-five miles per hour is a hor-

  rendously heretical concept, and when I began to publish recon-

  structions of galloping dinosaurs, the shrill voice of outraged

  orthodoxy rose to deafening heights. The advocates of slow di-

  nosaurs had two strong arguments. They pointed out that the di-

  nosaurs' joint surfaces usually weren't smooth and polished as are

  those in mammals, but were roughened and pitted. Those pits held

  cartilages. It was therefore alleged that dinosaurs had too much

  gristle in their knees to stand the strain of fast trots and gallops.

  But this argument is flawed.

  In point of fact, cartilage is excellent biological material for

  absorbing shocks—better than dense, brittle bone, because carti-

  lage will compress under load, building up hydrostatic pressure in

  its fluid-filled micropores and springing back when load is re-

  leased. Adult mammals and birds have only a thin film of cartilage

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

  over their joint surfaces, but their young often possess thicker pads

  of cartilage, which fill pits in their bones like the pits found in di-

  nosaurs'. And adolescent animals usually display greater locomo-

  tor vigor than adults, not less. The pitted limb bones of dinosaurs

  would be no handicap to high speeds.

  The other argument against galloping concerns the question

  of long shanks versus short shanks. Many fast mammals have long

  shank bones in comparison to the length of their thighs and even

  more elongated ankle bones (called metatarsals). Gazelles and most

  other fast-running antelope show bones and shanks that are very

  long relative to the thigh. Ostriches are fast runners and also have

  long shanks and ankles and short thighs. According to the tradi-

  tional theory of shanks, to estimate the top speed of an extinct

  creature, one measures the length of shank + ankle and divides by

  length of thigh. If the resulting number is over 1.5, the animal is

  moderately fast; if over 2, the animal is in the gazelle category.

  Very few dinosaurs possessed shanks and ankles as long and thin