Robert T Bakker

by The Dinosaur Heresies (pdf)

during the dreary days of the 1920s when English and American

  dinosaurology was settling into a muddle-headed and lackluster

  orthodoxy. Nopsca was a genuine Transylvanian aristocrat with a

  fondness for dressing in Balkan native dress and for reconstruct-

  ing the sex lives of dinosaurs. He was also gay, a life style quite at

  odds with the macho frontier style of American bone diggers.

  Nopsca was universally recognized as a creative thinker—at his

  worst no less thoughtful and no less accurate than muddling or-

  thodoxy, at his best head and shoulders above his contemporaries

  in discerning evolutionary patterns.

  The hallmark of the

  Thyreophoroids. All

  the armored dinosaurs

  were specialized in

  losing the obturator

  prong, a short flange of

  bone that connected

  pubis and ischium. The

  nonarmored dinosaurs

  retained the prong.

  MESOZOIC ARMS RACE | 253

  Nopsca argued the case for an evolutionary unity among all

  armor-clad Dinosauria. Testing his evidence in Geologica Hungar-

  ica in 1928, he coined the term "Thyreophoroidea"—shield car-

  riers—for all armored and spiked dinosaurs. Americans more or

  less ignored his hypothesis, but since 1975 there has been a Thy-

  reophoroid revival. Walter Coombs, an expert on ankylosaurs,

  pointed out that boneheads, stegosaurs, and ankylosaurs all shared

  a most unusual feature, armored eyelids. All three groups had stiff

  plates of bone embedded in their upper eyelid to protect the eye-

  ball from attacks delivered from above (only the accessory eyelid

  was armored; the inner eyelid was soft skin and could close over

  the eye). Such armored eyelids support the baron's theory. The

  most primitive beaked dinosaurs lacked any such wide, bony eye-

  lids, so all dinosaurs with them could have inherited their armored

  blinkers from one common ancestor.

  If all the shield carriers, domeheads, ankylosaurs, horned di-

  nosaurs, and stegosaurs were related, then evidence of this pedi-

  gree should be found in body architecture. And we do find clues.

  All the primitive beaked dinosaurs' lower hip bones (pubis and is-

  chium) were joined together by a short shelf of bone, called the

  obturator prong. But all the later shield carriers lacked this telltale

  shelf—evidence that perhaps one common ancestor had done away

  with the obturator prong when it diverged from the primitive

  beaked condition. Other clues of common pedigree can be found

  in the skull. In the most primitive beaked dinosaurs, the bones of

  the roof of the mouth (palate) were loosely connected. But in all

  the shield carriers, the skulls were far more rigid and the palate

  bones firmly connected to one another.

  Altogether, the baron's hypothesis now seems a happy sug-

  gestion indeed. The armor-clad "suborders" probably were evo-

  lutionary cousins, descendants of one branch of dinosaurs that

  embarked on the adaptive path leading toward armored resis-

  tance, passive and active, against the threats of the meat-eaters. The

  Early Jurassic Scelidosaurus may be close to the Thyreophoroid stem.

  And so the baron's term, "Thyreophoroidea," should be resur-

  rected as the appropriate label for this grand tribe of armored di-

  nosaurs.

  254

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

  12

  DEFENSE WITHOUT ARMOR

  T hroughout their entire history, dinosaurs and their prey co-

  I evolved in a mutually stimulating arms race. A new defense plan

  among the plant-eaters would give rise to a new mode of attack

  among the meat-eaters. In the previous chapter we met the ar-

  mor-clad tribes. Here we shall review the parade of unarmored

  plant-eaters and the evolution of their defensive equipment. These

  dinosaurs with naked hides defended their vulnerable bodies with

  slashing claws and lashing tails against wave after wave of meat-

  eating species.

  The earliest wave of big herbivores evolved during the late

  epochs of the Triassic and Early Jurassic periods. These were the

  long-necked anchisaurs, distant uncles of the brontosaurs. Anchi-

  saurs displayed no body armor, but they wielded huge curved claws

  on their powerfully muscled thumbs and long pointed claws on their

  stout hind feet. These plant-eaters therefore had defensive claws

  both front and rear, a combination unusual today. So wrestling with

  an anchisaur was a dangerous business. Modern anteaters have

  hooklike claws on their forefeet, while the most dangerous mod-

  ern ground bird, the cassowary (a two-hundred-pound flightless

  creature from New Guinea), has a big hind claw. Together, ant-

  eaters and cassowaries demonstrate how anchisaurs fought. Living

  species of anteaters grow only up to 150 pounds maximum weight,

  but their hooked foreclaws are potent weapons of defense against

  DEFENSE WITHOUT ARMOR

  255

  The first dinosaur panzer— Scelidosaurus. Early in the Jurassic Period, the

  scelidosaur clan evolved top and side armor composed of stout bony cones.

  The twenty-foot-long predator Dilophosaurus would have found it hard to

  deliver an effective bite against this defense (dilophosaurs had two thin bony

  crests running down their snout—probably a sexual advertising device).

  jaguars. When angry and cornered, the anteater stands erect on its

  hind feet and tail and lashes out with left and right swings of its

  foreclaws. Knowledgeable zoologists take great care in the face of

  this attack, for if the anteater strikes full force in a vulnerable area,

  such as the stomach, its great claws can effect a full disembowel-

  ment.

  Anchisaurs' tails were stoutly muscled and they could easily

  have reared up, foreclaws at the ready, to face their enemies. An-

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

  chisaur hind claws, especially the one located on the large inner

  toe, could lash out with even more powerful blows than the

  foreclaws. Cassowaries jump to strike with the full force of their

  massive thighs behind their long inner toe. Zoo keepers always

  treat cassowaries with the utmost respect—these birds are much

  more dangerous than their bigger cousin, the ostrich. And they

  are just plain mean, often attacking humans without provocation.

  Yet anchisaurs grew to much larger sizes than do cassowaries: a

  half-ton anchisaur could have unleashed a kick five times more

  powerful than can any cassowary.

  The predators that threatened to attack throughout the Juras-

  sic and Cretaceous Periods were the long-legged theropod dino-

  Long-snouted Coelophysis

  attacks an anchisaur.

  DEFENSE WITHOUT ARMOR | 257

  saurs. This clan is best known for Allosaurus of the late Jurassic

  Period and for Tyrannosaurus belonging to the last days of the

  Cretaceous. The very earliest theropod meat-eaters had appeared

  side by side with the anchisaurs and the other early species of di-

  nosaurs during the Late Triassic.


  From the earliest days of dinosaur hunting in the mid-1800s,

  these predatory dinosaurs, especially those from the Triassic, have

  constituted the most cherished discoveries of any field expedition.

  The reason is simple—they are quite rare. Over six seasons in the

  field digging for dinosaurs, I have personally seen only one pred-

  ator skull, one battered predator backbone, and one predator claw.

  On average, in the Jurassic beds, one can't expect more than one

  Allosaurus skeleton at most per ten brontosaurs. This scarcity of

  predator remains is especially acute for the dawn of the Age of

  Dinosaurs, the end of the Triassic and beginning of the Jurassic.

  For over a hundred years paleontologists sought predator skele-

  tons from this earliest epoch with disappointing results.

  But two great discoveries during the last thirty years have

  provided us with a wonderful glimpse of the first predatory dino-

  saurs. The first was the grandest of all: not just one perfect skull,

  nor one complete skeleton, but a whole quarry filled with the

  complete and partial skeletons of one Late Triassic species, all

  preserved in the red mudstone of Ghost Ranch, New Mexico. Ned

  Colbert of the American Museum made this discovery, and under

  his direction the museum technicians have erected quite beautiful

  displays of these predators and have sent excellent casts of them

  to dozens of institutions throughout the international community

  of scholars. Colbert had stumbled upon a most unusual prize: a

  predator trap, a pocket of mudstone that formed in a peculiar lo-

  cale where predators had huddled together in death. Predator traps

  constitute one of the most puzzling enigmas in paleontology. What

  would have attracted meat-eaters to one small spot a few hundred

  yards wide, and what had killed and buried them there?

  The Tar Pits at La Brea, California, dating from a time late in

  the Age of Mammals, are the best-known predator traps and the

  best-studied. And they shed some light on Colbert's Triassic pred-

  ator trap. La Brea is filled with saber-tooth cats and huge wolves,

  all jumbled together in tar-soaked sand which dates from about

  twenty thousand years ago. A few plant-eaters—mammoths, cam-

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

  els, horses, and others—have been quarried out of La Brea, but

  the overwhelming majority of bones are those of the big meat-

  eaters. La Brea seems to have been a deathtrap for wolves and big

  cats, acting much like a giant sticky flypaper surface whose tar-

  soaked sand entrapped the meat-eating mammals' paws in viscous

  asphalt, miring them until the exhausted beasts sank down and died.

  Dead and dying animals would attract more predators to the tar

  sweeps, unwary meat-eaters who thought they could get a meal

  with little effort. And each new victim would add to the lure.

  Could Ghost Ranch have been such a flypaper trap? The site

  hasn't yet been analyzed sufficiently to yield any conclusions. The

  mudstone at Ghost Ranch did not yield the slightest trace of as-

  phalt. But it's not impossible that sticky mud might have served

  to have the same effect, trapping dinosaur feet in a viscous, ines-

  capable mire.

  Colbert's splendid skeletons seem to belong to the same ge-

  nus Cope had named from fragments in 1880: Coelophysis, roughly

  translated as "hollow-boned beast." Hollow it indeed was—all of

  the major limb bones and vertebrae were constructed like those

  of birds, with an outer shell of dense bone rind surrounding an

  empty core. So perfect are Colbert's skeletons that no guesswork

  is required to reconstruct these bodies. Coelophysis was small com-

  pared to its Jurassic nephews Allosaurus and Ceratosaurus; the fully

  adult length was only six feet, half of which was tail. Compared to

  those Jurassic predators, Coelophysis was long and slender in the

  torso and very long in the neck—the neck, body, and tail all seem

  to flow into one another to create an unusually smooth profile. Al-

  though it appeared early in dinosaur history, Coelophysis was al-

  ready a birdlike biped with wide upper hip bones and deep lower

  hip bones, the whole design providing for ample thigh muscles and

  quick thrusts of the hind leg. The vertebrae in the neck were an-

  gled, producing a natural S-shaped curve, so the head was carried

  high above the shoulders as a bird's would be.

  How did Coelophysis hunt? Its graceful yet strong neck could

  lunge forward for a quick snap at a small prey or for hit-and-run

  attacks against large prey. Teeth always provide the best biome-

  chanical clues to the killing tactics. Coelophysis's dental pattern was

  totally different from the killing apparatus we find in mammalian

  predators—wolves, leopards, and lions. When a wolf or cat bites,

  DEFENSE WITHOUT ARMOR | 259

  the four fanglike front teeth (canines) penetrate deeply into the

  prey. This bite is precise. The lower canine pair bites just in front

  of the upper, and the two pairs together lock the prey in a killing

  grip. Cats and foxes kill rabbits with one bite through the nape of

  the neck. Lions kill big prey—zebras and buffalo—by clamping their

  canines down on the throat and holding on until the prey suffo-

  cates. Coelophysis's killing teeth were organized for a very different

  technique. Instead of two pairs of canine fangs, Coelophysis had a

  long row of small, curved, daggerlike teeth, each with the sharp,

  serrated edge both fore and aft characteristic of nearly all hunting

  dinosaurs. A bite from such an assemblage of teeth would have

  left a long, shallow wound across the prey's flesh.

  Coelophysis's teeth were designed to slash through flesh, not

  to hold it. A cross section of this dinosaur's tooth shows a tear-

  drop outline, with a blunt, rounded front edge and a tapered,

  sharply chiseled rear. When Coelophysis bit through its prey's hide,

  the blunt front edge prevented the prey from slipping away while

  Elegantly designed meat

  slicer—the skull of

  Coelophysis. This lively Early

  Jurassic hunter epitomizes

  the light, flexible

  construction of paper-thin

  bony sheets and slender

  struts.

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

  the sharp rear edge slashed through the flesh. The backwardly

  curved tips of the teeth assisted in driving the whole tooth row

  backward through the wound. Saw-toothed from top to bottom,

  the serrated rear edges were designed to assist this backwardly di-

  rected slash, since they would allow the entire tooth to saw back-

  ward through hide and muscle. But on the front edge the serration

  was only at the tip. The blunt base along most of the front of the

  tooth was smooth, and so would hold the prey as it struggled to

  free itself. All of the structural details were cunningly calculated

  to permit the tooth to act as both knife and fork, cutting and

  holding.

  Some living species of monitor lizards have teeth like thoser />
  of Coelophysis, and these lizards inflict long, jagged wounds when

  they bite. Komodo dragons, the biggest monitor lizards alive to-

  day, can even kill cows and people with the wounds they inflict.

  Since both monitor and dinosaur teeth curve backward, the jaw

  muscles must be arranged to pull the teeth rearward as the jaws

  Coelophysis teeth worked like a combination fork

  and steak knife. All the teeth were backwardly

  curved blades with saw edges running along the

  entire trailing edge and the tip of the leading edge.

  In cross section (shown in black) the leading edge

  was blunt but the trailing edge was very sharp.

  Upper teeth were much larger and much sharper

  along their trailing edges than lower teeth. But

  lower teeth had stronger, blunter leading edges. So

  when the biting muscles contracted, the lower teeth

  held the prey and prevented it from slipping out of

  the mouth while the sharp upper teeth slashed

  backward, making a long, nasty wound.

  DEFENSE WITHOUT ARMOR

  261

  Dinosaurian answer to the electric

  carving knife— Coelophysis biting

  mechanism. The huge hole in the

  snout bones was for the big muscle

  that powered the bite. A much

  smaller biting muscle was in the

  hole just behind the eye socket, and

  the jaw-opening muscle was strung

  from prongs sticking backward from

  the head and jaws. The big snout

  muscle was arranged to pull the

  skull down and backward (line of

  pull shown by the black arrow). So

  when the muscle contracted, the big

  upper teeth slashed back and

  downward toward the lower teeth.

  close. The skull and jaws of the lizards feature extra joints rather

  like a snake's to permit this. Yet Coelophysis's killing bite must have

  been different from the lizards' in one fundamental way. The up-

  per and lower teeth of Komodo dragons are the same size, but in

  Coelophysis the upper teeth were much larger than the lower. Con-

  sequently, more muscle power was required to pull the upper teeth

  back through prey than was necessary for the lower. The upper

  teeth were also more sharply edged, so they must have produced

  more of the cutting action, while the lowers did more of the hold-