Robert T Bakker

by The Dinosaur Heresies (pdf)

  right. Protecting oneself from a predator was always a vital func-

  tion, and any anatomical device that could be wielded as a ram or

  club was useful in that connection. But in the evolutionary scheme

  of things, staying alive means nothing unless one's genes are passed

  on to the next generation. So, the ability to butt a sexual rival hard

  in the ribs might provide an edge in the great evolutionary dating

  game.

  Without doubt the most dangerous devices for active defense

  among the Dinosauria emerged in Triceratops. The scene has been

  portrayed in paintings, drawings, and illustrations hundreds of times,

  but it remains thrilling. Tyrannosaurus, the greatest dinosaur to-

  reador, confronts Triceratops, the greatest set of dinosaur horns.

  No matchup between predator and prey has ever been more dra-

  matic. It's somehow fitting that those two massive antagonists lived

  out their co-evolutionary belligerence through the very last days

  of the very last epoch in the Age of Dinosaurs. Tyrannosaurus stood

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

  over twenty feet tall when fully erect, and a large adult was as heavy

  as a small elephant—five tons. No predatory dinosaur, no preda-

  tory land animal of any sort, had more powerful jaws. Withstand-

  ing a Tyrannosaurus's attack required either tanklike armor—the

  approach taken by Ankylosaurus—or most powerful defensive

  weapons—the approach taken by Triceratops.

  Triceratops's body was designed for lunging and charging. The

  torso was very short, the chest broad, the hips wide and strong.

  Fore- and hind limbs were very thick for the body size—much

  thicker than an elephant's of the same weight—and the paws were

  wide and compact. No armor plate encased the hide of Tricera-

  tops, because its defense was active, head-first, and devastatingly

  effective. Triceratops and its kin carried far and away the largest

  and heaviest skull ever to evolve on a land creature—six, seven,

  even eight feet long, up to four feet wide, and of very solid con-

  struction. Where the neck muscles attached to the back of the skull,

  the cranial bones had expanded sideways and upward and were

  reinforced to support sudden twisting lunges of the great horns

  located on the brows.

  Triceratops's horns were wonderful examples of Mesozoic ar-

  mature. From the eye socket to tip, the horn cores could reach

  four feet in length and often had a graceful double curve like the

  horns of longhorn cattle from the Wild West. When the first Tri-

  ceratops horns were discovered in Colorado in 1880, Professor

  Marsh thought they had belonged to ancient buffalo. But although

  Triceratops horns were shaped like a buffalo's, they were located

  on the head in a far more dangerous orientation. Longhorn cattle

  and buffalo horns face sideways, and their horn thrusts can only

  be delivered by tossing the. head to the left or right. Triceratops

  thrusts could be far more precise. Its horns curved forward and

  slightly outward over the long snout. Although the head was mas-

  sive, it was nearly perfectly balanced on the ball-and-socket-type

  joint between the head and the neck. The heavy snout forward

  was counterbalanced by the broad head shield extending back-

  ward. The entire apparatus was a marvelous combination of deli-

  cate musculoskeletal poise and brute power, allowing Triceratops

  to lunge forward at its opponent with the entire set of horn tips.

  The neck is a vulnerable point in any vertebrate, and Tricer-

  atops protected its neck with a flaring collar of bone, fringed by

  MESOZOIC ARMS RACE I 241

  Two Triceratops confront a Tyrannosaurus.

  Triceratops drumsticks. Muscular power for quick charges came from the

  huge calf muscles that attached to the inner and outer sides of the big bony

  crest of the shin (cnemial crest). This same crest also gave the knee-

  straightening muscles great leverage.

  short, sharp, horn-covered spikes. Part of this frill was covered by

  an extension of the jaw muscles. But the wide periphery of the

  bony frill was pure armor, covered with tough horny skin. Below

  each eye and just above the jaw joint was a short horn-covered

  spike that protected the cheek. This defensive master-machine alive

  and in action must have been a sight to behold, its eight-foot skull

  pivoting easily left and right, its neck frill swinging in wide arcs.

  Triceratops was not the only giant horned dinosaur found on

  the Laramie Deltas. It was accompanied by the rare Torosaurus—

  the "bull lizard"—which sported an even longer neck frill. And in

  New Mexico, during Late Cretaceous days, a splendid long-horned

  relative of Torosaurus walked the floodplain— Pentaceratops, the "five-

  horned face," named for its combination of unusually long cheek

  horns, brow horns, and nasal horn.

  The finest display of horned dinosaur heads anywhere in the

  world is located in the Cretaceous Hall of the American Museum

  of Natural History in New York. The horned legions reached their

  greatest variety during Judithan times, a few million years earlier

  than the age of Triceratops. Judith River beds in Montana and Al-

  berta have been very generous to dinosaurophiles. The New York

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

  Perfect balance at the

  Triceratops skull pivot.

  Horned dinosaurs had a

  ball-and-socket joint

  connecting the massive head

  to the neck. Since the joint

  was placed under the eyes,

  just at the natural balancing

  point of the huge head, the

  neck muscles could toss the

  head in any direction with

  great precision of

  movement. Botton cutaway

  view shows the ball joint on

  the skull separated from the

  socket.

  museum displays Centrosaurus, a. short-frilled variety whose weak

  brow horns were compensated by an erect and very long horn over

  the snout. Its close kin Styracosaurus possessed the great nose horn

  plus a magnificent set of curved spikes over the frill, giving its head

  a monumentally prickly appearance. Monoclonius is there too, with

  its stout nose horn. In general, Judith River horned dinosaurs sort

  out into two systems of attack. The first includes the genera with

  huge nose horns and weaker brow horns. These animals probably

  thrust their powerfully armed snouts straight upward as they tried

  to gore the softer underparts of a tyrannosaur. The second system

  includes the Triceratops-like configurations—long brow horns

  curving forward. Such long brow horns are rare among the Judith

  fauna—but the species Chasmosaurus kaiseni is there to represent

  this second system.

  MESOZOIC ARMS RACE 245

  In the 1890s, horned dinosaurs confronted science with an

  evolutionary puzzle: These dinosaurs were so highly evolved for

  an aggressive defense that paleontologists were at a loss as to how

  such creatures could have descended from any other kin
d of beaked

  Dinosauria. Even the oldest horned dinosaur fossils from North

  America manifested the very complexly designed snout, horns, frill,

  and neck muscle attachments in a fully developed state. It was as

  though the horned dinosaurs had sprung directly from the mind

  of the Creator.

  Today the early fossil record of the horned dinosaurs is still

  imperfect, but we are two big steps closer to understanding their

  evolutionary origin, owing to discoveries in the Cretaceous sands

  of the Gobi Desert. The spectacular discoveries made in the Gobi

  came about through a colossal error of scientific theory. American

  scientists in the early 1900s wanted to explore the Mongolian des-

  ert because a theory popular at the time maintained that human

  evolution occurred fastest and most efficiently on a dry, invigorat-

  ing plateau such as the country of Central Asia—the "Roof of the

  World." Conversely, tropical lowlands were supposedly evolution-

  ary slums where stagnant water and fetid air suppressed the de-

  velopment of higher life forms. According to this theory, only where

  air was dry and thin—as on the Asian Plateau—could lively spe-

  cies evolve. These Asian plateaus were terra incognita at the time;

  no thorough scientific surveys of Gobi zoology or paleontology had

  been made, and no one knew what sort of beasts had evolved there.

  After the first World War, wealthy Americans contributed funds

  for a grand American Museum expedition to the Gobi. The trip

  was billed as the search for the missing link; the key to human

  evolution was to be found in the windswept desert. Roy Chapman

  Andrews, naturalist-explorer par excellence, was its leader.

  So far as missing links in evolution were concerned, the ex-

  pedition was a bust. No important protohuman fossils were found.

  We now know in fact that nearly all the steps in human evolution

  took place in warm tropical realms, not on high plateaus. But the

  Gobi expedition uncovered a boundless treasure trove of dino-

  saurs, whole new families of them. These Gobi Cretaceous dino-

  saur beds were totally different from the Judith and Laramie Deltas

  familiar to the American geologists. As we have already noted, Late

  Cretaceous habitats in America were mostly humid deltas, but

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

  Central Asian habitats of the time were dominated by desert and

  near-desert conditions. Red Gobi sandstones preserved the sedi-

  mentary work of Cretaceous winds that drove sand into dunes

  around shallow lakes. Over millions of years these Cretaceous dunes

  coalesced into dune fields, and these fields, in turn, piled upon one

  another to produce hundred-mile-wide layers of preserving sand.

  Andrews's field parties found innumerable white skeletons in the

  red rock—small, chunky-bodied dinosaurs with long hind legs,

  powerful beaks, and short frills protecting their necks. Andrews's

  men had found primitive horned dinosaurs, the ancient uncles of

  Centrosaurus and Triceratops.

  When the scientists in New York unpacked the first crate-

  loads of dune rock from the Gobi, it was clear a dinosaur missing

  link had been found. The new dinosaur's name was a tribute to

  Andrews's leadership: Protoceratops andrewsi, "Andrews's ancestral

  horned-face." Protoceratops's cranial structure was almost perfect as

  the ancestral state of the large American horned dinosaurs. The

  basic horned dinosaur design was proclaimed by the deep beak,

  solidly connected skull bones, and a well-braced neck frill. But

  Protoceratops displayed only the suggestion of horns. In the biggest

  skulls a roughened bump on the snout must have supported a low

  horny crest in life. And Protoceratops1 % legs, hips, and shoulders were

  delicate compared to the massive strength of the American horned

  dinosaurs.

  Protoceratops and its close relatives must have swarmed over

  Asia, because their bones and nests of eggs are the commonest

  dinosaur fossils found in the widespread dune beds. But not one

  Protoceratops has ever been reported from the rich beds of the

  American Judith and Laramie Deltas. Swampy meadows and broad

  humid floodplains were evidently not to Protoceratops1 % liking, though

  Canada and Montana did play host to relatives in Late Cretaceous

  times—the genera Leptoceratops ("diminutive horn-face") and Mon-

  tanoceratops. Leptoceratops probably was an immigrant from Asia.

  Roy Chapman Andrews's team also discovered a second miss-

  ing link in horned-dinosaur history: an earlier Mongolian family

  which at last revealed how the horned-dinosaur story began. At

  first the announcement created little stir—two skeletons and some

  odd bones from a very small beaked dinosaur out of the Early

  Cretaceous beds of Inner Mongolia. (One of the skeletons had

  MESOZOIC ARMS RACE I 249

  Protoceratops: male (left) and female (right)

  nicely preserved gizzard stones.) But before long dinosaur anato-

  mists began to discover what they had; these tiny beaked dino-

  saurs possessed a very deep, parrotlike snout that looked just like

  that of Protoceratops. It was this beak that suggested the name for

  the little animal— Psittacosaurus, "parrot lizard." However, not just

  the beak, but the rest of this parrot dinosaur's skull began to look

  suspiciously like that of an ancestor for Protoceratops. Parrot dino-

  saurs protected their cheeks with sharp, crestlike horns, and they

  showed just the beginnings of a neck frill. Especially striking to

  the anatomist's eye was the core of the upper beak; it was formed

  by a bone separate from the rest of the skull, a most unusual trait,

  found elsewhere only in true horned dinosaurs.

  Thanks to these Asian discoveries, and those made in recent

  decades by Polish, Russian, and Chinese expeditions, we now pos-

  sess an outline of horned-dinosaur history. Parrot dinosaurs must

  have been close to the original ancestral stock—they have the ex-

  tra-long hind legs and short forelimbs so common among primi-

  tive beaked dinosaurs of all sorts. In parrot dinosaurs we already

  find the trend toward an exceptionally strong head with a power-

  ful beak and strong bite. Parrot dinosaurs were leaf-eaters, it's clear

  from their teeth. But their beak served both as an herbivory and

  an antipredator device, equally snipping off branches and snap-

  ping menacingly at predators that threatened attack. The crest-ar-

  mored cheek bones would protect the parrot dinosaurs when they

  lunged to bite at an enemy. From such a beginning it was only a

  short evolutionary step to Protoceratops and Leptoceratops with their

  incipient horns.

  The horned-dinosaur story shows how paleontologists can trace

  the major evolutionary events. Rarely do fossils yield a complete

  evolutionary sequence from mother to daughter to granddaughter

  species. Evolution is too bushy to permit such a straightforward

  story, too full of side branches. As clans evolved, the ever-branch-

  ing species spread over the continents. Since fossils
come from a

  few small areas, it is impossible to follow every stage of an evo-

  lutionary line. But it's possible to make out an overall progression

  of uncles and nieces, even when the parent—daughter sequences

  cannot be found. Parrot dinosaurs probably weren't the direct

  ancestors of Protoceratops, since the parrot clan had already branched

  off in their own unique direction (parrot dinosaurs had evolved

  MESOZOIC ARMS RACE | 251

  Three Psittacosaurus,

  parrot dinosaurs,

  flee a predator.

  very narrow forepaws with fewer fingers than true horned dino-

  saurs). And Protoceratops probably wasn't the direct ancestor of

  Triceratops or Monoclonius. Protoceratops and its sibling genus Lep-

  toceratops had evolved into an evolutionary sideline where the tail

  had become very slender from side to side but quite tall from top

  to bottom. This tall-tailed condition was probably part of Protocer-

  atops's tactics of intimidation—broadside huff and bluff. The big

  North American horned dinosaurs had neither the narrow paws of

  parrot dinosaurs not the billboard-type tails of Protoceratops. To sum

  up, parrot dinosaurs were the granduncles of Protoceratops, which

  was the granduncle of Triceratops.

  Altogether the history of dinosaur arms and armor must rank

  as one of the most dramatic aspects in the pageant of evolution,

  but it poses a question. Was this parade of tanks and dread-

  noughts one natural unit of evolution, a single major branch of

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

  the dinosaur family tree, or not? In the early days of American

  dinosaurology, museum scientists believed that stegosaurs, nodo-

  saurs, and ankylosaurs were closely related and that horned dino-

  saurs were perhaps cousins. But this idea lost favor among

  Americans in the 1920s for no good reason. And for the last half

  century most books place the armored groups—stegosaurs, anky-

  losaurs, and horned dinosaurs—into separate suborders.

  On the other side of the Atlantic, a very different conception

  was adumbrated by a flamboyant but perceptive Hungarian pa-

  leontologist, the Baron von Nopsca. Nopsca was a bright spot