Robert T Bakker Read online

that resemble frying pans. The long snout looks like the handle,

  the squarish cranium the pan. These Meerkrokodiliers, as our Ger-

  man colleagues call them, are not the lineal descendants of the sea

  crocodiles of the Jurassic, but are a new oceangoing group de-

  scended from Jurassic freshwater crocodiles.

  Opportunism again. When the Jurassic sea crocodiles were

  exterminated by the Early Cretaceous disturbance, an ecological

  opportunity presented itself to any reptile that could swim and catch

  fish and that could adapt to fully oceanic conditions. And so the

  rivers gave to the sea a new player for the reorganized Cretaceous

  marine systems. All through vertebrate evolution the flow has been

  mainly in this direction—from the fresh waters to the ocean shore

  to the high seas. Just so did the rivers give us the first whales some

  fifty million years ago, descendants of some river-haunting preda-

  tory mammal, one of the many mammalian lines that were rushing

  in to fill the empty niches left by the final extinction of the dino-

  saurs and sea reptiles.

  The Benton Sea supported a wonderful menagerie of Creta-

  ceous oceanic reptiles. In Colorado, Benton-age shales produced

  a nearly complete elasmosaur skeleton, a fast-cruising type of ple-

  siosaur that slipped through the tropical Late Mesozoic water with

  the propulsive power of four narrow, tapered flippers, snatching

  prey with its snakelike neck.

  At Como, Othniel Charles Marsh's men found an armor-plated

  nodosaur lying on its back embedded in the now hardened depos-

  its left by the mud on the sea floor. Finds like this were excep-

  tions to the rule that dinosaurs did not go to sea. Was the nodosaur

  swimming in the Cretaceous shallows before it met its end? Or is

  it the remains of some terrestrial individual that died a death on

  dry land and then, in the form of a dried-up carcass, was washed

  out to a final oceanic resting place by flood-swollen river waters?

  The problem of oceangoing nodosaurs is especially perplexing be-

  cause the Como carcass, upside down at the bottom of the Benton

  Sea, is not an isolated instance. Nodosaur carcasses lying on their

  backs cropped up in marine beds in Kansas in 1909 and several

  times since in similar sedimentary circumstances.

  40 I THE CONQUERING COLD-BLOODS: A CONUNDRUM

  There are no duckbill dinosaurs at Como, because the Ben-

  ton Formation is too old. The duckbill dynasty began past the

  midway point of the Cretaceous. A short trip east through the

  Laramie Mountains to Red Bird places us in duckbill country, the

  Lance Formation of the later Cretaceous, a mass of pale brown river

  sandstones with interbedded chocolate-colored mudstones, some-

  times faintly discolored by the pink of oxidized iron. Nearby is

  Lance Creek, supposedly named for the cavalry lance carried by

  mounted Sioux warriors.

  Lance outcrops give their name in turn to the final terrestrial

  epoch of the Cretaceous world, the Lancian Faunal Age—a time

  that witnessed the adaptive deployment of the most exotic and bi-

  zarre skulls of all the panoply of dinosaurs. Triceratops was here,

  the scientific etymology "three-horned face" being, in this case, an

  excellent shorthand description of this formidably armed herbi-

  vore. Over each reinforced eye socket grew a horn of such size as

  to threaten even the largest Tyrannosaurus rex. In life these weap-

  ons were long, sharp, and deadly because the underlying bone was

  covered with a horny sheath like that surrounding the cores of cattle

  and buffalo horns today. Out on the snout was a third, midline

  horn, and below it a toothless beak, deep and powerful like that

  of a multi-ton snapping turtle. This too was clothed in life by a

  shiny hornlike substance, giving the beak an ever-growing, self-

  sharpening edge. Plant-eater though it was, Triceratops could turn

  the branch-cutting apparatus of its beak into a defensive set of

  nippers strong enough to inflict wounds on even the largest an-

  tagonist.

  Truculence, nippers, and horns seem to go together. Today,

  the great Indian one-horn rhino can turn into the terror of the

  mahouts as it charges domestic elephants. The largest Triceratops

  weighed nearly ten tons, bearing horns that, fully sheathed, were

  four feet long. No species that has ever evolved on land could

  withstand the full charge of such an animal.

  Duckbill dinosaurs did not display the deadly cranial arma-

  ture worn by Triceratops. Nonetheless the duckbill group enjoyed

  an extraordinary evolutionary flourish of head ornaments and ad-

  aptations in the final days of the Cretaceous. The term "duckbill"

  is a biomechanical misnomer. True, the duckbill dinosaurs did have

  wide, flattened beaks, which at a distance vaguely resembled that

  of a mallard. However, the edges of their beak were turned down

  WYOMING REVERIE: MEDITATION ON THE GEOLOGICAL TEXT I 41

  into a sharp, cookie-cutter edge, sheathed in life by a self-sharp-

  ening horn. The entire apparatus was a leaf-cropping adaptation

  for slicing off mouthfuls of tough fodder in a single bite. Duckbill

  teeth were one of the true marvels of mastication, cited every-

  where in texts on dental evolution. Instead of one single row of

  teeth along each jawline, the duckbill had multiple rows, which

  combined to make a leaf-shredding surface equivalent in function

  to an ever-sharp carrot grater. No evolutionary device has ever

  evolved to masticate tough plant fiber more effectively than the

  dental shredder of the duckbills.

  Although the feeding devices of the duckbills have provoked

  no end of wonder among paleontologists since the first duckbill

  was excavated in the phosphate fertilizer mines of New Jersey in

  the 1850s, it is the array of duckbill head ornamentation that stirs

  up the most puzzlement and debate. The common Lance Creek

  duckbill, Edmontosaurus, seems built to a no-nonsense, practical

  design. Its skull houses the beak, teeth, jaw muscles, and sense

  organs. But close relatives from Alberta and New Mexico show

  no such restraint in their headgear: Parasaurolophus carried a dou-

  ble-hollow bony tube like a trombone slide on the back of its skull;

  Saurolophus had a solid bony spike in the same position; Hypacro-

  saurus sported a thin-shelled bony crest rising high above the full

  length of its forehead and skull table.

  This cranial exuberance at first glance reminds one of all the

  head appendages some families of birds employ to show off in pre-

  mating rituals, such as the combs of roosters, the domed fore-

  heads of some species of geese, the crests of cassowaries. And

  perhaps here the first impression is the correct one. Dinosaurs had

  to have sex, although one would never guess so from the scrubbed

  Sunday school versions of dinosaur biology presented in the chil-

  dren's books. Sex and pre-mating ritual are parts of the basic evo-

  lutionary game: genes that produce adaptations which succeed in

  increasing their representation in the next generation are the ge
nes

  that survive. The genes of the dinosaurs must have played by the

  same statistical rule. If a garish head crest and some accompanying

  behavior, such as a strutting head-bobbing walk, made the male

  Parasaurolophus more attractive to the female and more intimidat-

  ing to his rivals, then eventually the genes responsible for this

  equipment and its use would be fixed in the species. For most of

  42 | THE CONQUERING COLD-BLOODS: A CONUNDRUM

  this century, American paleontologists avoided sexual interpreta-

  tions of dinosaur structures.

  The European contemporaries of American scientists weren't

  so prudish. The Swedish paleontologist Carl Wiman hired an

  American dinosaur hunter, Charles Sternberg, to quarry and crate

  duckbills from New Mexico and horned dinosaurs for the Swed-

  ish Museum at Uppsala. Sternberg sent a magnificent Parasauro-

  lophus to Wiman, who noticed that the double-hollow tube of the

  crest was simply a U-shaped elaboration of the air tract from its

  nostril to its windpipe. Wiman was a broadly educated naturalist,

  well aware of the multitudinous ways in which modern species of

  bird, frog, and mammal make love by making noise—hooting,

  gurgling, chirping, and bellowing. So what was Wiman to think of

  the U-tube in the duckbill's air passages? It looked like a trom-

  bone, it was a trombone! If the duckbill inhaled or exhaled with

  force, the U-tube would be a resonating chamber, enriching the

  tone and amplifying the noise. Hollow crests in other duckbills also

  The trombone duckbill, Parasaurolophus

  WYOMING REVERIE: MEDITATION ON THE GEOLOGICAL TEXT I 43

  connected the throat to nostrils, and the variety of crest shapes

  from species to species would certainly produce a variety of hoots,

  wheezes, and amplified snorts specific to that species.

  Even crestless duckbills like the Lancian Edmontosaurus had

  highly arched palates, and the vaulted roofs of their mouths could

  be used to modulate tones and increase decibel levels. And the

  crestless duckbill probably had additional sound equipment in its

  nasal compartment. The bone around the outer surface of the ed-

  montosaur must have housed nasal diverticula, pouches of skin

  opening into the main nostril channel. Horses have similar diver-

  ticula, though of modest size compared to the edmontosaur's. Watch

  a stallion snort: The diverticulum shudders with pulses of forced

  air from the lungs, the sound controlled by sphincter muscles in

  lip and nose. The Late Cretaceous evenings in southeastern Wy-

  oming must have been punctuated by reverberating snorts as the

  duckbills, driven by their genes, strove to impress each other.

  The final hours of the Cretaceous are not to be found at Como

  or Lance Creek. This most profound of land extinctions may be

  witnessed if we go north, through Wyoming to northern Montana,

  to Hell Creek. Here, better than anywhere else in the world, the

  stratigraphic pile records in detail the events surrounding the ex-

  tinction of the ultimate Great Dying.

  Any attempt to analyze the events of the extinction of the di-

  nosaurs runs into the fundamental difficulties that hinder the in-

  vestigation of any of these mass murders of species. Most fossil

  bones owe their preservation to quick burial by sediment right after

  the death of their owner. But generally most spots in the terres-

  trial biosphere suffer erosion, not deposition. Only in slowly sink-

  ing basins, pieces of real estate hundreds of miles across, can we

  hope to see a long interval of time recorded by the preservation

  of fossils. If a broad, basin-like valley was near sea level, its rivers

  and estuaries could blanket the landscape with layers of mud and

  sand every flood season. The very weight of these blankets of mud

  and sand tended to push the land surface as if the basin itself were

  a sagging rubber bowl. If the sinking of the valley's surface kept

  up with the rate of buildup in the blankets' thickness, then the

  pile of sediment grew thicker and thicker, even though the aver-

  age height of the land above sea level remained the same. The re-

  sult, after ten or twenty million years, was a thick sandwich of

  sediment that might reach a vertical height of five miles.

  44 I THE CONQUERING COLD-BLOODS: A CONUNDRUM

  Sinking basins don't sink forever. If they did, it would be

  possible to read the entire fossil record of life from bottom to top

  in one mine shaft sunk into a single valley. Instead, to understand

  the changing habitats of the end of the Cretaceous, it is necessary

  to hop from state to state, basin to basin, in order to piece to-

  gether the disjointed narrative in the sediment, much as silent-movie

  buffs might try to reconstruct an entire lost feature by splicing

  fragments of film found in a dozen different studio storage vaults.

  The fragment of the story recorded at Lance Creek carries us

  late into the Cretaceous, but not to the very end. In Hell Creek,

  Montana, and nearby Bug Creek, however, there is a sedimentary

  section, rich in fossils, that passes right through the last moments

  of the Cretaceous and continues into the next epoch, the Paleo-

  cene. Even at Bug Creek the strata do not record a year-by-year

  surveillance of the scene of the crime that would allow us to catch

  the perpetrator in the very act of extinction. In the best of basins,

  fossils weren't preserved every year, or even every hundred years.

  Big bones, such as those of dinosaurs, required big floods of mud

  to cover them, and these events didn't happen except at long in-

  tervals, perhaps hundreds or thousands of years apart. Even when

  buried, bones weren't necessarily safe. Acid groundwater might

  percolate through the sand, dissolve the bone mineral, and leave

  nothing behind but a gross, misshapen carbon stain where a duck-

  bill's skull once lay. Or a sudden shift in a river's course could erode

  part of the sedimentary layer it had deposited years before, and

  all the entombed bones would go tumbling down the new chan-

  nel, breaking into irretrievable fragments. Paleontologists are

  grateful to streams for their blanketing of bones, but most streams

  also cannibalize. In one century they lay down deposits over the

  valley floor, in the next they might chew through their own sedi-

  mentary handiwork, churning and cracking buried bones and eras-

  ing the very fossil record they have previously preserved.

  The movie-film analogy allows us to visualize the frustrating

  process of investigating the Cretaceous. Instead of a continuous

  film, one frame a year for each of the last million years of the

  Mesozoic, only short bursts of film remain intact, each a few dozen

  frames together, separated by hundreds of feet of totally missing

  footage. If something important, like the final extinction of dino-

  saurs, happened suddenly, within a few years, we wouldn't have a

  prayer of catching the deed in the film clips of sediment.

  WYOMING REVERIE: MEDITATION ON THE GEOLOGICAL TEXT I 45

  The best detective stories are those that comma
nd our rapt

  attention to every scrap of clue, so that we can solve the crime in

  the final chapter, just before the sleuth announces the identity of

  the murderer. Dinosaur extinction attracts the best of paleonto-

  logical detectives. Up to Bug Creek and Hell Creek they go, dig-

  ging quarries, running sediment through fine sieves to sift for the

  tiniest of bones and teeth. But much of the mystery remains. Only

  a few facts are clear. The final dying was sudden, compared to the

  immense length of the history of the dinosaurs: It took no more

  The Mammalia take over. Four million years after the dinosaurs died, the

  mammalian hordes evolved into big tuber-digging herbivores, like

  Psittacotherium (at left), and big predators, like Ancalagon (at right).

  46 I THE CONQUERING COLD-BLOODS: A CONUNDRUM

  than two million years—maybe much less—to exterminate all the

  Cretaceous dynasties. And there were opportunists waiting around

  for the dinosaurs to die: small, furry, insect-eating, berry-chewing

  mammals scurrying around the underbrush, fidgeting about,

  grooming their whiskers. As the dinosaurian clans were thinned

  out, with the extinction rate exceeding the production rate of new

  species, these Late Cretaceous furballs expanded their ecological

  sphere of influence. The fossils show new types of small, mam-

  malian plant-eaters and insectivores blossoming in Montana at the

  very time the evolutionary fortune of the dinosaurs was sinking

  into its final, irrevocable decline. Passing upward through the sed-

  imentary pile in Montana, exposed now in dry gulches, we can see

  the shifting census of evolutionary success. The mammals were di-

  versifying rapidly near the very end of the Cretaceous, and dino-

  saurs dwindled until a level is reached in the layer of mud and sand

  through which no species of dinosaur passed. This layer marks the

  end of the Lancian Epoch, the end of the Cretaceous, the end of

  the Mesozoic. This time the dinosaurs would not recover.

  WYOMING REVERIE: MEDITATION ON THE GEOLOGICAL TEXT | 47

  3

  MESOZOIC CLASS WARFARE:

  COLD-BLOODS VERSUS

  THE FABULOUS FURBALLS

  henever I read Kipling's "Rikki-Tikki-Tavi," I root for the

  w

  V snake. There's something very irritating about the story's