Robert T Bakker Read online

storage drawers in the United States and Canada. The effort to do

  so would be incomparably rewarding for both professionals and

  the public, if for nothing other than the opportunity of at last

  viewing one of the most formidable gastrointestinal systems in the

  Dinosauria. From the side, the ankylosaurs and domeheads pre-

  sent a tubby appearance—deep ribs arching out from the chest and

  belly. Looking from above straight down on the ribs and hips, the

  entire hind region from belly to tail was enormously expanded,

  nearly beyond the anatomically credible. Ribs became longer from

  mid-torso to hips, until the rearmost ribs arched out so far that

  the afterbelly must have been wider from side to side than it was

  deep from top to bottom. This extra-wide fermenting compart-

  ment continued beneath the upper hip bones (the ilia), where the

  normally narrow pelvic architecture was transformed into an im-

  mensely broad horizontal roof. The ensemble was a dinosaurian

  body broadened to twice the usual width through the compart-

  ments housing the intestines and colon.

  No other dinosaur's gastrointestinal system was nearly so en-

  176 I THE HABITAT OF THE DINOSAURS

  Guts of a nodosaur

  larged relative to the body mass. No mammal or bird possesses

  comparable skeletal architecture today. The exact layout of stom-

  ach, intestines, and colon in ankylosaurs will never be certainly

  known. It is certain however that every leafy bolus received an

  extraordinarily thorough biochemical treatment in a long series of

  enzyme baths and fermentation vats. The ankylosaur's teeth were

  indeed weak, but its beak was strong and sharp-edged. So the an-

  DINOSAURS AT TABLE I 177

  kylosaur began the process of feeding by stuffing broad mouthfuls

  of leaves into its capacious cheeks. Then with its simple row of

  teeth it cut the longer leaves and stems a few times, and wadded

  up the pieces between its cheeks and tongue into a coarse bolus.

  The entire ball passed down to the superenlarged gastrointestinal

  chambers. Now, the coarsely chopped wad was broken down by

  successive biochemical assaults. The huge compartment for the

  colon at the base of the tail provided room for the enormous af-

  terburner, so a final posterior appendix exposed the fodder to one

  last digestive procedure.

  The ankylosaur's rearward digestive system with its special af-

  terburner surely was big enough to make up for its weak teeth.

  Even quite tough vegetation could have been handled in large

  volumes. My colleague and friend Ken Carpenter has evidence in-

  dicating some species had gizzard stones as well. Could gastro-

  chemical treatment have supplied the ankylosaurs with enough food

  energy to be warm-blooded? Absolutely—at least the boundary

  conditions from the dietary perspective must include this possibil-

  ity. And at the very least the ankylosaurs too are rescued from the

  category of soft-food-eating, low-energy semi-invalids.

  Besides the major families of herbivorous dinosaurs dis-

  cussed so far, there were a dozen smaller groups all outfitted with

  plant-eating equipment of the sort already described for the major

  families. Iguanodon was a relative of the duckbill. It won interna-

  tional fame as the first dinosaur made known to science, when it

  was dug from road-gravel quarries in Sussex, England, in 1822. The

  iguanodont's adaptations were styled after the duckbill's—closely

  packed chopping shredding teeth (although iguanodont's weren't

  as complex as duckbill's). Dryosaurs must have been very selec-

  tive eaters, using their narrow muzzles to crop carefully chosen

  fodder. The fabrosaurs, the most primitive beaked dinosaurs, were

  bipeds with small, loosely packed teeth like those of the much later

  ankylosaurs.

  Altogether, each dinosaur dynasty, from Early Jurassic to Late

  Cretaceous, was equipped for a comprehensive attack upon fo-

  liage, buds, bark, tubers, and fruit. Not one plant-eating dinosaur

  has been found to subsist on aquatic plant mush. Every herbivo-

  rous clan could have harvested land plants at rates and quantities

  sufficient for high metabolism.

  178 I THE HABITAT OF THE DINOSAURS

  9

  WHEN DINOSAURS

  INVENTED FLOWERS

  Darwin and his followers regarded the ecological drama as a

  complex, choreographed struggle among competitors, pred-

  ator, and prey. "Nature red in tooth and claw" expressed the vio-

  lent aspect of natural selection, the killing and bloody rending of

  flesh by predators' fangs, the maiming of sexual rivals during the

  vicious combats between dominant males during the mating sea-

  son. But Darwin was clever and observant; for all the violence of

  nature, he knew that most evolutionary dramas were played to a

  subtler script, the day-to-day interaction between the antelope and

  the grass, the squirrel and the acorn. Plants and plant-eaters co-

  evolved. And plants aren't the passive partners in the chain of ter-

  restrial life. Hence today's Pop Ecology movement is quite wrong

  in believing that plants are happy to fill their role as fodder for

  herbivores in a harmonious and perfectly balanced ecosystem. A

  birch tree doesn't feel cosmic fulfillment when a moose munches

  its leaves; the tree species, in fact, evolves to fight the moose, to

  keep the animal's munching lips away from vulnerable young leaves

  and twigs. In the final analysis, the merciless hand of natural se-

  lection will favor the birch genes that make the tree less and less

  palatable to the moose in generation after generation. No plant

  species could survive for long by offering itself as unprotected

  fodder.

  Plants evolve all sorts of devices to foil plant-eaters: They

  WHEN DINOSAURS INVENTED FLOWERS I 179

  The pygmy dinosaur, Nanosaurus, in the Late Jurassic underbrush.

  Nanosaurus was a four-feet-long omnivore and a very primitive beaked

  dinosaur. The understory plants are: a gingko (upper left), two cycadeoids

  (on both sides, with diamond-sculpture trunks and big fronds), a fern (lower

  left), and ground pine (foreground creepers).

  poison them with deadly alkaloids; they keep them away with thorns

  and spines; they render plant tissue unchewable by incorporating

  rock-hard phytoliths into the plant cells or by toughening plant fi-

  bers with cellulose; they avoid being eaten by producing new leaves

  in early spring when plant-eating populations are low. Of course

  the plant-eaters fight back. The evolution of herbivores leads

  inexorably to better teeth for crushing the toughest leaves, to more

  complex digestive systems where enzymes can detoxify plant poi-

  180 | THE HABITAT OF THE DINOSAURS

  sons, to taller shoulders and longer necks to reach higher into the

  trees, or to lower heads and square muzzles perfect for cropping

  ground-hugging leaves.

  The warfare between plants and herbivores began on land 400

  million years ago, when the first algae colonized the bare ground

  during the Silurian Period and the herbivoro
us arthropods evolved

  to follow them. Vertebrate plant-eaters on land appeared much

  later, during the last epochs of the Coal Age, 270 million years

  ago. Dinosaurs captured the herbivorous niches on land during the

  Triassic, 200 million years ago, and subsequently maintained their

  dominance through the entire Jurassic and Cretaceous. But how

  did dinosaurs co-evolve in relation to the plants of their world?

  Dinosaurs held the roles of large land herbivores for longer than

  any other vertebrate group, so there must have been a rich history

  of adaptive attack and counterattack between plant-eater and plant.

  Moreover, herbivorous dinosaurs suffered several episodes of ex-

  tinction and adaptive revolution that must also have been re-

  flected in contemporary plant systems. And there was a momentous

  development in the plants during the Mesozoic, for the Jurassic

  and Cretaceous witnessed the single greatest event in the evolu-

  tion of the modern system of plants—a turning point that must

  have changed the life of every plant-munching dinosaur—the ap-

  pearance of the flowering plants.

  Today flowering plants, known collectively as angiosperms, are

  by far the most numerous of land foilage, literally thousands of

  species, including nearly all the plants that feed mankind and our

  mammalian relatives. So numerous are angiosperms that to the av-

  erage person, the term "plant" is synonymous with "flowering plant."

  Oaks, birches, maples, and all the other broad-leafed trees are an-

  giosperms, as are nearly all the berry-producing bushes and shrubs.

  Palms, grasses, sedges, and dandelions also belong to the angio-

  sperms, as do tulips and all the other species with showy flowers:

  squash, beans, coconuts, lilies-of-the-valley, peaches, apples, or-

  anges, rhubarb, tomatoes, cucumbers, onions, garlic, potatoes,

  scallions, leeks, lettuce, spinach, broccoli, and thousands more. All

  angiosperms are members of one natural group, descended from

  a common ancestor that first appeared at the midpoint of the di-

  nosaurs' reign.

  The anatomy of the angiosperms is the key to their success.

  WHEN DINOSAURS INVENTED FLOWERS | 181

  They have distinctively complex reproductive organs—flower and

  fruit—and most woody species additionally possess highly ad-

  vanced conduction tubes in their roots, stems, and leaves, which

  give them enormous advantages over other plants. Angiosperms

  use their brightly petaled flowers to attract animal pollinators (in-

  sects, bats, birds), and many use large fruit containing tough seeds

  to attract animals as agents of dispersal. (Some modern angio-

  sperms are wind-pollinated, but this is an evolutionary reversal. The

  earliest flowering plants probably exploited animal vectors exclu-

  sively.) Different flower shapes attract different species of insects,

  bats, and birds, and thus each angiosperm creates the opportunity

  of spreading its pollen efficiently without the wholesale waste in-

  evitable in pollination by wind. The same is true for angiosperm

  seeds and fruit, which are far more diverse and distinctive than

  those of non-angiosperms.

  So overwhelming is the advantage of the angiosperms today

  that non-angiosperms are forced to play subordinate roles in the

  flora of most areas. Today, the most conspicuous non-angiosperms

  are conifers, cycads, ferns, ground pine, and horsetails. None of

  these non-angiosperms produce flowers, and most rely upon the

  wind to spread their spores, pollen, and seeds. Conifers—the

  needle-leafed trees—are important in temperate forests, but they

  are outnumbered by angiosperms ten to one on a worldwide av-

  erage. Cycads with their spiny fronds are always a tiny minority in

  every flora. Ferns, ground pine, and horsetails, very ancient relics

  of Coal Age flora, make important contributions to the forest un-

  dergrowth and to swampy herbiage. But these living Coal Age fossils

  are outnumbered thirty to one by angiosperm species in nearly all

  habitats.

  How did flowering plants begin to win this unchallenged he-

  gemony? Whatever the story, dinosaurs must have had a hand in

  it because the earliest angiosperms sprouted up in a landscape

  dominated by dinosaur plant-eaters. And they remained the major

  outside factor for plants all through the first forty million years of

  the angiosperms' evolution. But, for no apparent reason, modern

  science has ignored the dinosaurs' role in plant evolution nearly

  completely. Paleobotanists theorize about new insect groups which

  might have co-evolved with the flowers in Late Cretaceous times.

  Mammal paleontologists assert that Cretaceous mammals, no mat-

  182 | THE HABITAT OF THE DINOSAURS

  Iguanodon browses among the broadleaf saplings. Flowering plants began their

  spectacular evolutionary career during the Early Cretaceous, when big-beaked

  dinosaurs like Iguanodon fed close to the ground. Early angiosperm leaves

  included some sassafraslike species (upper left), the broadly rounded

  Proteaephyllum (lower left and in Iguanodon % mouth), and the oaklike

  Vitiphyllum (right).

  ter how tiny and unimportant, made a major impact on the evo-

  lution of angiosperm fruits, nuts, and leaves. But hardly anyone

  has argued for the interaction of Cretaceous dinosaurs with the

  plants that fed them—an extraordinary oversight, considering the

  dinosaurs were the only herbivores large enough to gobble an en-

  tire flowering shrub in one gulp or strong enough to push an an-

  giosperm tree so as to get at the tender young leaves at the top.

  The consistent neglect of the dinosaurs' potential role in the

  evolution of plants is one of the most pernicious examples of the

  orthodoxy that relegates the dinosaurs to what amounts to an evo-

  WHEN DINOSAURS INVENTED FLOWERS I 183

  lutionary sideshow, a menagerie of irrelevant dead ends that can

  be ignored so far as any large implications are concerned. Today,

  large herbivores can change the structure of the flora overnight.

  Rhinos and elephants can level acacia groves and rapidly crop down

  thickets, converting dense African bushland into open woodland.

  In the early nineteenth century, the American buffalo kept push-

  ing back the boundary between prairie and forest by its intensive

  grazing on seedlings. Surely four-ton nodosaurs and three-ton

  iguanodonts did the same in the Early Cretaceous system.

  Another bias also works against herbivorous dinosaurs, how-

  ever. Paleobotanists are a bit chauvinistic about their objects of

  study. They tend to regard plants as the movers and shakers in

  evolution, and the plant-eaters are consigned to the role of reac-

  tors and followers. As one paleobotanist expressed it, "The sun

  gives energy to plants, and plants give energy to the animals.

  Therefore, the plants evolve and the animals must co-evolve." Stated

  thus, the assertion is understandable, but it's misleading. Co-evo-

  lution works both ways. When plant-eating dinosaurs evolved more

  effective teeth or fermenting chambers, the plant
species had to

  adjust to the new weaponry or die. Whichever evolved faster, plant

  or animal, had the evolutionary initiative. And plant-eating dino-

  saurs evolved fast, faster than the plants. On average, a species of

  dinosaur endured two or three million years before becoming ex-

  tinct and being replaced by a new species. That's a brisk rate of

  evolutionary turnover, as fast as the mammals'. Such rapid re-

  placement of old adaptive models by new ones guaranteed that the

  dinosaur plant-eaters were always coming up with novel ways to

  bite, chew, ferment, and digest plant tissue. Mesozoic plants, on

  the other hand, usually evolved more slowly—the average species

  of plant lasted eight million years before being replaced by a new

  one. Since the turnover wasn't as fast, the plants must have been

  lagging behind the dinosaurs in the evolutionary race.

  Herbivorous dinosaurs in fact were the fastest-evolving part

  of the entire Mesozoic land ecosystem, even faster at adaptive re-

  modeling than their meat-eating relatives. Tyrannosaurus rex, the

  fifty-foot-long Cretaceous killer with seven-inch teeth, was really

  just a sophisticated variation on the basic predator plan first evolved

  a hundred million years earlier in the Late Triassic. Bone by bone,

  Tyrannosaurus rex was fundamentally little different from its an-

  184 I THE HABITAT OF THE DINOSAURS

  cient Triassic ancestors. But the Cretaceous plant-eaters—three-

  horned Triceratops, club-tailed Ankylosaurus, broad-beaked Edmon-

  tosaurus—carried skull and jaw developments totally unknown in

  the Triassic.

  To follow the pattern of co-evolution between dinosaurs and

  plants, the major turning points in the development of each must

  be defined, then laid side by side. Among the herbivorous dino-

  saurs, three grand periods of development are clearly marked:

  I. The Age of Anchisaurs. The Late Triassic and Earliest Juras-

  sic, when the long-necked anchisaurs ruled. Anchisaurs were

  primitive, crude plant-eaters by Cretaceous standards. They had

  simple, iguanalike teeth, suitable for soft leaves only, and their

  digestive system wasn't much expanded.

  II. The Age of the High Feeders (stegosaurs and brontosaurs).

  The Mid and Late Jurassic, when the spike-tailed stegosaurs joined