Robert T Bakker Read online

the gigantic Diplodocus, Brachiosaurus, and Brontosaurus.

  III. The Age of the Low Feeders. The Cretaceous, when all the

  terrestrial habitats were overrun by big beaked dinosaurs which

  fed close to the ground. Each of these types had its own unique

  approach to cropping the foliage, so each must have made a dis-

  tinctive impact on the co-evolutionary history of plants.

  Orthodox dinosaurology has muddied the conceptual waters

  here by relegating the herbivorous dinosaurs to the swamps, where

  they are supposed to have gummed nothing but water plants. Now

  that we have a corrected view of them as dry-land herbivores, it is

  possible to begin reconstructing a much more accurate context for

  the Mesozoic evolution of the flowering plants. The first clue to

  the interaction of dinosaurs and angiosperms can be found in the

  timing of extinctions. Flowering plants first appeared in the Early

  Cretaceous just after the extinctions which occurred at the end of

  the second grand period (the age of stegosaurs and brontosaurs),

  and as the replacements for the third grand period (the age of the

  low feeders) were taking place. This sequence is highly suggestive.

  When the coalition of stegosaurs and brontosaurs died out at the

  end of the Jurassic, the plant-eating dinosaurs changed so pro-

  foundly that the rules of co-evolution must have been reset. Could

  this dramatic shift from Jurassic-type to Cretaceous-type dinosaurs

  have opened the way for flowering plants? It's an exciting hy-

  WHEN DINOSAURS INVENTED FLOWERS | 185

  Dynasties of plants and plant-eaters

  pothesis—that the revolution in dinosaur plant-eaters caused the

  single most far-reaching development in the kingdom of land plants.

  And the evidence suggesting such a cause-and-effect relationship

  is very good.

  To understand how the dinosaurs' plant eating changed and

  how dinosaurs may have invented flowers, Stegosaurus and its strange

  adaptations for eating plants must be understood. Here again or-

  thodoxy has obfuscated some obvious truths about dinosaur bio-

  mechanics. Stegosaurus is often portrayed as something of a misfit,

  a quadruped endowed with two sets of mismatched legs—the front

  pair too short and the hind too long. Stegosaur skeletons mounted

  in museums pose the beast with a clumsy, shuffling gait, its hips

  towering above its low-slung shoulders and its nose nearly at ground

  level. Stegosaurus and its close kin were the only common large,

  beaked dinosaurs in the Late Jurassic. Therefore its feeding habits

  must have had a major influence on the evolution of plants. The

  orthodox restorations depict Stegosaurus as an ungainly low crop-

  per, plucking plants from within a few feet of the ground. That is

  the precise inverse of the truth— Stegosaurus wasn't a badly de-

  signed low feeder, it was a superbly designed high feeder.

  The point the usual reconstructions miss completely is that

  the plan of the stegosaur's body was carefully balanced so the beast

  could rear up on its hind legs and tail to feed upright, in a tripodal

  stance. The very tall bony spines at the hips become understand-

  able only in that position. Tall vertebral spines supplied great le-

  verage to the back's muscles and ligaments, so that the entire weight

  of the trunk could be supported by the hindquarters. The brilliant

  English biophysicist D'Arcy Thompson pointed out the mechani-

  cal significance of tall spines at the hip as early as 1924. He dem-

  onstrated that Stegosaurus's spine-and-ligament construction worked

  much like a single-span suspension bridge. Such bridges are based

  on a tall central tower (equivalent to the stegosaur's hind legs). The

  span's vertical steel supports are shorter and shorter the further

  they are from that tower for the sake of easier leverage (just as

  the stegosaur's vertebral spines become shorter the further they

  are fore and aft from the hips). Just as the thick cables of the sus-

  pension bridge based on the central tower hoist the weight of the

  span, so did the thick ligaments of the stegosaur's vertebral col-

  umn based on the hips hoist the weight of its body. Modern cranes

  WHEN DINOSAURS INVENTED FLOWERS I 187

  Diracodon laticeps, a three-ton Late Jurassic stegosaur, tilting up its body with

  a push of a hind foot

  and derricks work on the same principle—the jib works like the

  Stegosaur's backbone, the cables work like its back muscles.

  All the details of Stegosaurus % construction meshed perfectly

  to produce a body machine that could swing up easily from a four-

  footed stance into a hind-legs-plus-tail posture. Its shortened front

  legs reduced the dead weight forward, so the animal could hoist

  its front end up with less effort. The base of the stegosaur's tail

  was provided with huge bony flanges to anchor immensely strong

  tail muscles designed to brace its body against the ground. The

  stegosaur's ancestors had had stiff, brittle tails containing long bony

  rods, like those of the duckbills, which held the tail and back rigid.

  Clearly the stegosaurs had evolved away from the ancestral stiff-

  tailedness and had acquired strong, supple tail joints all the way

  from hip to tail tip, so the rear half of the tail could be flexed flush

  against the ground. A masterful final touch completed the stego-

  saur's tripodal equipment. In most dinosaurs, the lower bony prongs

  at the tail's end (chevrons) were usually simple, straight spines. But

  on stegosaurs those bones were expanded into a shape like the

  runners on a sled, to provide a superior brace for the body's weight.

  188 I THE HABITAT OF THE DINOSAURS

  Tall vertebral spines give back muscles and

  ligaments leverage for raising the body—just like a

  crane's jib is braced by a cable. Stegosaurs had

  much larger back-raising leverage than does an

  elephant of the same weight.

  Live stegosaurs might have been slow when moving on all

  fours, but the excellence of their design would appear when they

  walked into a grove of trees that appealed to their taste. With a

  slight upward push from the front paws, the entire body would

  pivot upward from the hips, raising the head up to twelve feet above

  the ground—and now the stegosaur could poke its snout into high

  shrubs and the low canopy of trees, selecting the choicest leaves

  and branches.

  As an undergraduate at Yale, I published several papers at-

  tacking the orthodox theories and arguing for the tripodal habits

  I have described here. I believed I had arrived at some quite new,

  revolutionary ideas, until I discovered some papers actually pub-

  lished in the last century. I had in fact merely resurrected a view

  WHEN DINOSAURS INVENTED FLOWERS I 189

  carefully worked out some eighty years earlier. Professor Marsh

  had dug the first stegosaur skeleton in 1878, and many descrip-

  tions published in the 1880s and 1890s portrayed the animals

  rearing on their hind legs and tail, exactly as their bony anatomy

  indicated. Somehow this totally logical interpretation was lost i
n

  the 1920s; in its place orthodoxy substituted the nonsensical view

  of stegosaurs as ill-designed quadrupeds.

  Stegosaurus was not the only animal whose bodily configura-

  tion was adaptively modeled for high feeding. The brontosaurian

  dinosaurs, Diplodocus and Brontosaurus, evolved exactly the same

  set of tripodal characteristics: short backs and short forelimbs to

  lessen the weight of the trunk; tall, "suspension-bridge" vertebral

  spines at the hips; huge and supple tail bones; gigantic muscles at

  the base of the tail; and sledlike lower tail bones as final support

  for the body's weight. When a Diplodocus raised itself into a tri-

  podal posture, its feeding height was phenomenal by today's stan-

  dards—its nose would have reached to forty feet or more above

  the ground. Brontosaurus s

  1

  reach would have been a little less, but

  Barosaurus, a close relative of Diplodocus, could have reached to

  forty-five or fifty feet.

  A modern giraffe at its full height can reach only up to eigh-

  teen feet. Just as was the case with stegosaurs, these tripodal ad-

  aptations of the diplodocines were clearly understood by the

  pioneering American paleontologists; Elmer Riggs of the Chicago

  Museum wrote a detailed explanation in 1904. But, as with the

  stegosaurs, the orthodoxy of the 1920s forgot all about this work,

  so that between 1930 and I960 the standard view likewise main-

  tained that Diplodocus was a quadruped with maladaptedly short

  front legs.

  Every single one of the giant Late Jurassic dinosaurs was in

  reality a high browser of some sort. Bracbiosaurus's tail was too

  weak for a tripodal posture, but it compensated with its spectac-

  ularly long neck, so that even on all fours it could reach up forty

  feet. Camarasaurus was the shortest-necked brontosaur found at

  Como. It could nevertheless stretch up to twenty-five feet with its

  forelegs on the ground, and much higher if it assumed a tripodal

  stance. Never before nor since the Late Jurassic has the world wit-

  nessed such a profusion of high-feeding plant-eaters. This was

  nothing less than a unique epoch in the history of herbivorous

  190 | THE HABITAT OF THE DINOSAURS

  Some plant-eating dinosaurs were designed for rearing up on hind legs and

  tail. Brontosaurus and Stegosaurus had tall vertebral spines over the hips, so that the back muscles and ligaments had strong leverage for raising the body.

  Other big herbivores had shorter vertebral spines and must have stayed on all

  fours. (Each of these skeletons is drawn to the same length, hip to shoulder

  socket, to show the proportions of the backbone for easy comparison.)

  How dinosaur feeding style changed. At Como in the Late Jurassic, there were six long-necked brontosaurs plus a stegosaur. The stegosaur and three brontosaurs could rear up and feed tripodally (Barosaurus, Diplodocus, and Brontosaurus). But in Late Cretaceous habitats in Utah, there was only one kind of brontosaur— Alamosaurus—

  and low-feeding beaked dinosaurs dominated the plant-eating role.

  habits. No plant or leaf was safe from a dinosaur's mouth unless

  it stood over fifty feet above the ground! Since no flowering plants

  of any sort existed yet, the principal trees were broad-needle con-

  ifers and tall, spiny-fronded cycadeoids. These plants had to be able

  to protect at least some of their growth through a combination of

  alkaloid poisons, heavy oils, or spiny branches. As conifers and cy-

  cads grow slowly compared to many angiosperms, Late Jurassic trees

  had to guard themselves carefully to obviate the destruction of en-

  tire breeding stands.

  During these Late Jurassic times, one danger was minimal.

  Since few dinosaurs were specialized for feeding at ground level,

  conifer seedlings and other sprouts wouldn't suffer the same de-

  gree of cropping we find at work in the modern ecosystem, where

  cattle, horses, sheep, goats, and many other mammals pluck their

  plant food from the surface of the soil.

  When the Late Jurassic curtain fell and the Early Cretaceous

  began, most of the old, established high croppers died out. The

  mysterious hand of worldwide disaster swept across the conti-

  nents, killing off whole families of herbivorous species and thin-

  ning the ranks of the surviving clans. Stegosaurs disappeared

  forever. Hardly a species survived into the Cretaceous. Diplodo-

  cus, Brontosaurus, and nearly all the other tripodal brontosaurs died

  out too. Some of Brachiosaurus's relatives survived, but that was

  an exception to the rule. This tremendous disaster destroyed the

  high-browsing system permanently; it never recovered. And no

  Cretaceous dinosaur evolved high-browsing adaptations to replace

  Stegosaurus. A few new brontosaurs evolved— Alamosaurus, for in-

  stance—but they never came close in number to the glorious days

  of the Late Jurassic.

  As the dust settled after these extinctions, the opportunists—

  evolutionary carpetbaggers—started to move into the devastated

  ecosystem. New herbivorous groups blossomed into clusters of new

  species. And nearly all these new Cretaceous herbivores were

  committed to cropping near ground level. Among the earliest of

  the newcomers were the big, spike-shouldered nodosaurs. Igua-

  nodons also evolved early in the Cretaceous, reaching weights of

  two to three tons and developing broad muzzles for close crop-

  ping. Parrot dinosaurs emerged shortly after, as did the queer-

  looking domeheads, both groups with their backbones curved to

  WHEN DINOSAURS INVENTED FLOWERS I 193

  bring their snout close to the ground. More and more low feeders

  appeared as the Cretaceous continued—the wide-snouted ankylo-

  saurs, the large and small horned dinosaurs, the myriads of duck-

  bills.

  This was an unprecedented alteration in the nature of plant

  eating. In place of the Late Jurassic's tall browsers, the Cretaceous

  concentrated on munching close to the ground. Low shrubs and

  seedlings now faced a threat from herbivores magnified many times

  compared to the conditions prevalent during the Jurassic. In this

  ecological context the very first flowering plants appeared on the

  earth's surface. How did the Ur-angiosperms react to all this

  munching close to the ground? One ideal adaptive strategy for a

  194 | THE HABITAT OF THE DINOSAURS

  low woody shrub or seedling would be to grow as fast as possible

  to achieve a height where the low browsers could not threaten it.

  Early angiosperms could do that—their basic reproductive equip-

  ment gave them a fast-growing edge over many contemporaneous

  plants. A second approach would be a "dicey" strategy—scatter

  sufficient seeds onto an overgrazed patch of bare soil to produce

  a clump of shrubs quickly, and thus spread another generation of

  seeds before the herbivores returned and once again mowed

  everything to the ground. Early angiosperms could do that too—

  their seeds and flowers allowed them to spread more quickly than

  conifers or other nonflowering plants.

  The low-level cropping brigade of the Cr
etaceous. On a typical Late

  Cretaceous meadow in Alberta, all the big plant-eaters were specialized for

  feeding close to ground level. Broad-snouted duckbills bit off wide mouthfuls

  of vegetation; delicate-snouted parksosaurs nipped precision bites of selected

  leaves. And the other dinosaurs added their cropping activity to the medley

  of ground-floor herbivory.

  WHEN DINOSAURS INVENTED FLOWERS | 195

  How Cretaceous dinosaurs invented flowers. Nodosaurs, iguanodonts, and

  the other plant-eaters that fed near the ground threatened meadows with

  overgrazing. Nonflowering plants (conifers, cycads, ferns) couldn't regenerate

  as quickly as angiosperms and they couldn't recolonize as rapidly. So after a

  heavy raid by the plant-eaters, flowering plants took over.

  Obviously intense low cropping placed a premium on any and

  all plant adaptations for fast spreading, fast growing, and fast re-

  production. And early angiosperms performed exactly these bio-

  logical functions especially well. From this, a quite plausible scenario

  emerges: Low-feeding Cretaceous dinosaurs opened the way for

  the initial waves of angiosperms. Conifers, cycadeoids, and other

  non-angiosperms were probably far less adaptive for handling the

  assaults of new Cretaceous herbivores. Anywhere the plant-eaters

  196 | THE HABITAT OF THE DINOSAURS

  thinned out the conifer groves and cycadeoid thickets, an oppor-

  tunity for species of flowering plants to win a foothold in the hab-

  itat was created. Early angiosperms were probably cropped just as

  severely as their neighbors, but their basic adaptations permitted

  them to continue growing and reproducing in the face of the in-

  tense mowing action of the dinosaurs.

  Footholds are crucial for major adaptive revolutions. Any new

  group finds it difficult to break into an ecosystem already full of

  old, established groups. Conifers were highly adaptive old-timers

  in Early Cretaceous times—the conifers had begun indeed long

  before the first dinosaur evolved in the Triassic. Cycadeoids were

  old-timers too, already diverse in the Early Jurassic epoch. When

  angiosperms first evolved, they were confronted with meadows,

  woodlands, and forests full of long-lived, highly refined plants. Early