Robert T Bakker Read online

  dry climate and each a timekeeper and recorder of past climate.

  Taken altogether, this irregular staircase of rock is a chronicle of

  the dinosaurs' success throughout a great age in the history of life.

  The nodules grew from tiny mineral seeds in the well-drained soil

  of the floodplain where dinosaurs browsed the leaves of conifers,

  and birds with teeth glided from one tree crown to the other. In

  the rainy season, floods covered the landscape with chocolate-

  colored water full of mud and grit so that each flood added yet

  another layer of sediment to the gradually accruing stratigraphic

  pile. The seasonal flux of the water table—up near the surface

  32 | THE CONQUERING COLD-BLOODS: A CONUNDRUM

  during the rainy season, down during the dry season—stained the

  layers of sediment in blotches of green and mauve, blotches known

  to soil scientists by the Welsh word "gley."

  Here and there the low places of the floodplain filled with

  black, stagnant water and putrefying leaves and branches, leaving

  a record of dark, carbon-rich mudstone and shale. Torrential spring

  rains cut channels into the plain and filled them with gravel, cross-

  bedded sand and mud. For five million years the floodplain here

  served as the arena for all these environmental agents, each per-

  forming its function in shaping the quantities of soil and leaving

  its own unique imprint on the sediments. And everywhere, in every

  habitat, there were dinosaurs—huge multi-ton brontosaurs swing-

  ing their long necks from treetop to treetop; predatory allosaurs,

  running on their enlarged hind legs, like some nightmarish bird;

  armored stegosaurs, ornamented with bony triangles along their

  back, brandishing a formidable set of spikes at the end of their

  muscular tail; little theropods, some no bulkier than a turkey,

  darting through the meadows and gallery forests along the stream

  courses, catching small prey.

  The record of the rocks speaks eloquently here, without hes-

  itation or ambiguity—this was an Age of Dinosaurs, a time when

  all the large ecological roles on the terrestrial stage were played

  by dinosaurs of one family or another. The domination of the di-

  nosaurs extended across all the categories of large flesh-eater, large

  leaf-eater, and large omnivore. And dinosaurs spread their ecolog-

  ical hegemony across a worldwide empire, devoid of geographical

  limits. Dinosaurs are the unchallenged majority in all the fossil

  samples of large vertebrates on every continent from Australia to

  Siberia, New Jersey to Calcutta during this time. Dinosaurs like

  these lying in my Wyoming pit are being excavated by Iberian pa-

  leontologists in Portugal, by Chinese geologists in Yunnan Prov-

  ince, and by Zimbabwan naturalists along the banks of the Zambesi

  River in East Africa. No corner of the Mesozoic world withstood

  colonization by dinosaurs.

  How much grander in scope the dinosaurs' history is than the

  cartoonists' view of prehistory, which consigns all extinct crea-

  tures to one Antediluvian Age. The day of the dinosaur was not

  merely one geological instant, played out by a single cast of spe-

  cies. Neither was it one dynasty of evolving dinosaur species. The

  VWOMING REVERIE: MEDITATION ON THE GEOLOGICAL TEXT I 33

  dinosaurs' history was an extraordinary series of dynasties, one age

  followed by another and another, each filled with a complete cast

  of dinosaurs, and the entire dynastic series running through 130

  million years.

  No single spot on earth preserves this history in its entirety.

  But southeastern Wyoming comes close. When I walk north from

  the quarry to visit the staff at Rock Creek Fish Hatchery, I pass

  through the first half of the history of the dinosaurs. It is recorded

  in a thousand-foot-thick layer cake of sandstone, shale, and lime.

  In the rock strata near the fish hatchery's holding ponds, the di-

  nosaurs make their debut. The sedimentary record here is a bi-

  zarre sandwich of thin-bedded maroon, pink, and brick red

  sandstone and mudstone, a formation that enjoys the delightful la-

  bel Chugwater, named after a tiny stream where it was first dis-

  covered. The gaudily colored beds began as saline lakes, like those

  of Death Valley today, fetid bodies of soda-choked water too salty

  for fish to survive. Meandering rivers spread layers of sand on top

  of the mineral-rich muds accumulated on the lifeless lake bot-

  toms. Dinosaurs were there. Small hunters from chicken to os-

  trich size prowled along the stream edges, hunting for their prey,

  leaving their unmistakable three-toed footprints and, very rarely,

  leaving behind their bony carcasses to be buried by Chugwater sand.

  In those days, the dinosaurs' empire was in its infancy. The evo-

  lutionary pioneers of the Dinosauria had to share this terrestrial

  realm with a host of short-legged and ugly reptiles, the beaked

  rhynchosaurs, the dog-faced cynodonts, and the dinosaurs' own

  ancestral stock, the big-headed thecodonts. This was the Triassic

  Period, the first of the three great Mesozoic ages.

  The next was the Jurassic, the Golden Age of Giants, when

  the dinosaur clans burst out of their Triassic limitations. Wave after

  wave of ever-larger species filled the land habitats—long-necked

  brontosaurs, grotesquely armored stegosaurs, and a complete ar-

  ray of bird-limbed predators from ten pounds to five tons in live

  weight.

  At Como Bluff, the land was covered with warm ocean water,

  the Sundance Sea, alive with stout-shelled squid and porpoiselike

  fish lizards for most of the late Jurassic's twenty million years. Above

  the Chugwater layers, the somber green-gray sandstones of the

  Sundance Formation enter this marine epoch in the stratigraphic

  34 | THE CONQUERING COLD-BLOODS: A CONUNDRUM

  Sea croc of the Jurassic—the fish-tailed Metriorhynchus, about

  ten feet long

  chronicle. Butch Cassidy's sidekick, the Sundance Kid, got his name

  from the same tiny Wyoming town that gave this Jurassic forma-

  tion its formal geological nomenclature.

  The waves of the Sundance Sea finally beat their last cadence

  140 million years ago. But even now the rhythmic sea sound seems

  fresh, preserved in the ripple-marked sandstone surface and in the

  wave-winnowed piles of clam shells and fossil oysters, growing on

  top of each other, and the squid-pens known to the locals as "stone

  sea-gars." Dinosaurs were not here. Their fossils rarely appear in

  ocean beds. But rocks the same age as Sundance in India and Aus-

  tralia provide skeletons, proof that the land ecosystem was firmly

  under the control of the dinosaurs all through Jurassic times.

  Sundance fossils and all the other rich Mesozoic marine beds

  underscore the one geographic limitation of the dinosaurs' world.

  Their empire was firmly landlocked. Unchallenged though they

  were on land, dinosaurs rarely went to sea, and so seemed to suf-

  fer that abhorrence of salt water which has limited many a human

  empire from Alexander to Napoleon. Rept
ilian leviathans are found

  in the Sundance outcrop—fish lizards, sea crocodiles, and the ser-

  pent-necked plesiosaurs. But these sea monsters are all from groups

  only distantly related to the Dinosauria proper.

  If we proceed from the Fish Hatchery back to camp, climbing

  through the last Sundance sandstone ledge, the rock changes color

  from green to blotches of red and maroon, signalling the shift in

  the ancient habitats from shallow tropical sea to floodplain and river.

  This next layer of rock is the most famous dinosaur graveyard in

  the world: the Morrison Formation, named for a tiny Colorado town

  south of Boulder. It was the outcrops of the Morrison Formation

  here at Como Bluff that made Brontosaurus a household word in

  the 1880s. Union Pacific Railway station managers found huge

  bones along their right-of-way and cabled this news to Othniel

  Charles Marsh, stuffy but sagacious Yale paleontologist. Marsh hired

  the railway men to excavate the bones, crate them, and send them

  by boxcar to New Haven. News of the spectacularly complete

  Como dinosaurs galvanized the international community of schol-

  ars, who had been frustrated by the poor fragments of Jurassic di-

  nosaurs available from French and English quarries.

  American geology had been viewed as a scholarly backwater

  by most European scholars, whose tradition of analytical earth sci-

  36 | THE CONQUERING COLD-BLOODS: A CONUNDRUM

  ence was fully a century older. Como changed all that. For the first

  time Europe had to look to America for the lead in the paleon-

  tology of a major geological period. Woodcuts and lithographs of

  Marsh's Brontosaurus from Como appeared in European text-

  books, travel guides, and popular nature studies. "Brontosaurus"

  was even transliterated into Russian, Chinese, and Japanese.

  The famous brontosaur quarry lies in the midsection of the

  Morrison, in a zone full of the grey-blotched floodplain mudstone,

  about a hundred feet higher in the sediment-layer sequence than

  the topmost sandstones of the Sundance. Dinosaurs were every-

  where here. In one afternoon's walk I counted seven immense

  carcasses eroding out of the mudstone along a four-mile transect.

  And not only bones. There are also trackways of the living giants,

  pressed into the limy mud of shallow lakes, and now hardened into

  creamy-gray calcareous mudstone.

  For me, trackways and ripple marks have a special intimacy.

  Both can be so fresh-looking that they seem to hold the sounds

  made by the Jurassic world, the sucking noise of viscous mud being

  pulled by the cushionlike foot pads of brontosaurs as they stepped

  through the Jurassic muck. The size of their footprints almost de-

  fies the imagination. The largest are over three feet long and two

  feet across, and deep enough to hold sixty gallons of water—more

  than enough to bathe a three-year-old child or serve as a full-

  immersion baptism for the diminutive first dinosaurs of the Triassic

  Period. Ten miles north of Como, at Sheep Creek, a freshwater

  lake bed in the Morrison exposes an entire field of brontosaurs'

  footprints, dozens of tracks churned into each other, rendering the

  whole limestone slab a twisted craterland. A veritable symphony

  of noises must have filled the air as herds of brontosaurs executed

  their ponderous choreography.

  We know that brontosaurs traveled in herds, sometimes. A

  rare glimpse into their social structure is provided down at Dav-

  enport Ranch, Texas. There the limestone records the passage of

  two dozen brontosaurs in a compact mass, the very largest prints

  at the front periphery, the very smallest in the middle of the group.

  So brontosaur bulls—or maybe senior cows—must have guarded

  their young against the attacks of the allosaurs. The footprints at

  Davenport Ranch contain a Mesozoic recording of just such a drama

  of attack and defense, for the three-toed trackways of a great al-

  WYOMING REVERIE: MEDITATION ON THE GEOLOGICAL TEXT I 37

  losaur reveal that it was prowling along the strandline near where

  the brontosaur had passed.

  No rock formation provides a richer repertoire of dinosaur

  stories than the Morrison. Its quarries have been dug from south-

  ern Montana to the Cimarron River in Oklahoma, yielding hundreds

  of skeletons from every level and every fossil habitat.

  The sudden extinction of dinosaurs is one of the most pop-

  ularized topics in paleontology. Why, after all, did the last dynas-

  ties finally end in total extinction? In reality, however, the dinosaurs'

  history contains the drama of much more than a single death. They

  suffered three or four major catastrophes during their long pre-

  dominance, each one thinning the ranks of the entire clan. And

  after each such fall, they recouped their evolutionary fortunes, ris-

  ing again to fill the terrestrial system with yet another wave of new

  species and families of species. The final complete extermination

  did not come until sixty-five million years ago, at what geologists

  label the "Time of Great Dying," the grandest evolutionary disas-

  ter of all time.

  At Como I can walk right through one of the earlier extinc-

  tions, a time when the Jurassic families, which seemed so secure

  after fifty million years of success, suffered sudden extinction.

  There's nothing dramatic about the spot marking the event—merely

  a one-foot-thick bed composed of gray mud laid down in a stream

  and green mudballs that the rainy season's floods had torn from

  the banks and deposited in the creek bed downstream. Beds like

  this are everywhere, scattered all through the sedimentary layers.

  This particular one records a sudden jolt in the fortunes of the

  dinosaurs at the end of the Jurassic Period. Below the level of this

  bed—it's called the Breakfast Bench Sandstone because it makes

  a convenient shelf for the Coleman stove in the morning—the

  record of stegosaurs, Diplodocus, Brontosaurus, and Allosaurus can

  be followed up through the Morrison Formation for three hundred

  feet, equivalent to five or ten million years.

  But at Breakfast Bench these Jurassic threads are broken; the

  familiar stars of the Morrison disappear, and in their place a new

  cast enters to play the dominant roles. This introduces the Creta-

  ceous Period, the third and last age that made up the Mesozoic.

  Instead of Stegosaurus, with its flamboyant triangular spikes, a dif-

  ferent kind of dinosaur, an armor-clad herbivore, the nodosaur, is

  38 I THE CONQUERING COLD-BLOODS: A CONUNDRUM

  found. It was far less spectacular, but thoroughly protected by its

  armor coat of big and little plates that formed a mosaic over its

  entire back and neck. Instead of Brontosaurus and its close kin Di-

  plodocus, there appeared the teeth and vertebrae of brachiosaurids.

  This family of long-necked giants had been rare in the Jurassic but

  seem to have taken advantage of the catastrophe that struck most

  Jurassic families by moving in to take their place at the opening

  of the Cretaceous. Opportunism
such as this is a commonplace

  during times of extinction. As the preexisting dynasty loses its hold,

  families of animals that had previously been mere bit players on

  the ecological stage seize the leading roles.

  The knife-toothed predators suffered too as the curtain fell

  on the Jurassic stage. Gone were the Allosaurus and horn-toting

  Ceratosaurus, replaced by those most famous of all dinosaurian

  hunters, the tyrannosaurs. Smaller roles changed hands also. From

  the mudstone of Breakfast Bench, one of the crew excavated a

  magnificently preserved turtle skull whose boxy shape and adap-

  tive equipment were totally different from any of the long, low

  heads carried by the water-loving Jurassic turtles. This turtle head,

  like the nodosaur's armor and the brachiosaur's tails, strike the an-

  atomist as a jarring discontinuity in the flow of adaptive forms

  through time. Anyone who cherishes notions that evolution is al-

  ways slow and continuous will be shaken out of his beliefs by

  Breakfast Bench and the other geological markers of cataclysm.

  Our view of evolution must take into account the profoundly dis-

  orienting blows struck by the environment during these world-

  wide extinctions.

  The white sandstone blocks studded with the rounded beach

  gravel of Pine Ridge, the sedimentary sign left by the Mid Creta-

  ceous ocean, look west out over the eroded blister of the anti-

  cline. From this vantage point, the entire sequence of strata, from

  the red Chugwater in the center to the thin dark line made by the

  outcrop at Breakfast Bench, is visible. Pine Ridge is composed of

  Dakota sandstone, named for the Dakota Territory in the 1870s,

  long before the Dakota Sioux had given their name to the two

  Western states. East of Rock Creek, these Dakota outcrops are

  covered by a black mass of carbon-rich shale, the Benton Forma-

  tion, created by an ancient sea and named after old Fort Benton,

  built in the 1860s as the Union Pacific spread into Wyoming. The

  WYOMING REVERIE: MEDITATION ON THE GEOLOGICAL TEXT I 39

  Benton tells its own story of revolution and overthrow in the or-

  ganic world. In sandstone layers laid down in this mid-Cretaceous

  sea were entombed reptilian sea serpents of a distinctly Creta-

  ceous cast—the long-bodied teleorhinids, sea crocodiles with heads