Robert T Bakker Read online

floodplain mud preserves mostly landlubbers. What is needed, then,

  beyond the single Cam Bench Camarasaurus, to solve the mystery

  of the brontosaur's habitat is a broad statistical survey. Thanks to

  a National Geographic research grant, a group of brontosauro-

  THE CASE OF THE BRONTOSAURUS: FINDING THE BODY I 115

  philes called the "Morrison Dinosaur Habit Research Group" was

  able to make just this type of survey between 1974 and 1977.

  A study trench was dug up through the whole three-hundred-

  foot thickness of the Morrison Formation at Como. It laid bare

  cycle after cycle of life, death, burial, and new life. Twenty differ-

  ent kunkar layers were exposed, each marking a time when a

  floodplain was dry and green and its vegetation nourished dino-

  saur life. And just above each kunkar layer were zones of fossils

  —chewed dinosaurs—whose bodies had been written into rock his-

  tory by entombing layers of mud. When the statistics were tabu-

  lated, eighty percent of the brontosaur graves were in floodplains,

  twenty percent in river channels and zero percent in lakes or

  swamps.

  But the quarries at Sheep Creek, Wyoming, twenty miles north

  of Como, posed a special mystery. Here was a big quarry full of

  brontosaurs—and their carcasses had been entombed in lake-bot-

  tom limestone. Had lake-dwelling brontosaurs been found at last?

  Did the Sheep Creek skeletons represent bottom-walking behe-

  moths that fed on soft lake plants just as brontosaur orthodoxy

  had preached for eighty years? If the Sheep Creek limestone was

  deposited in a deep lake, then perhaps the brontosaurs found here

  were the classic dwellers in aquatic habitats.

  A careful investigation of the quarry made it clear that some-

  thing was wrong with the deep-lake theory. As a carcass sleuth looks

  for clues, he or she must be alert for negative evidence; some-

  times what's missing reveals more than what is present. And neg-

  ative clues were everywhere at Sheep Creek. No fish bones or

  crocodile bones and almost no turtle remains (only one fragment

  of shell) were ever found in the Sheep Creek limestone. What sort

  of lake had no fish or crocodiles or aquatic turtles? Nearly all

  tropical lakes today are quite full of these swimming creatures. Snail

  shells were found, but only of the type usually present in ponds

  and along lake margins.

  More perplexing clues turned up. Kay Behrensmeyer, among

  the best young American taphonomists, found giant dinosaur

  footprints in the same limestone which contained the dinosaur

  bones. Bottom-walkers can make shallow prints in the mud under

  deep water. But these prints were far too deep to have been made

  by a brontosaur buoyed by twelve feet of lake.

  116 | THE HABITAT OF THE DINOSAURS

  Finally, a key piece of the puzzle fell into place. "Fossil sun-

  light," in the form of mud cracks in the lake bottom, turned up.

  When mudflats are exposed to air, the mud surface dries and con-

  tracts, cracking itself into a mosaic of hexagons separated by fis-

  sures. In a sunny, dry climate, mud cracks can grow to depths of

  a foot or more quite quickly. And such cracks can fossilize when

  a subsequent flood washes a layer of sand over the cracked mud-

  flat, filling the fissures with sediment. Usually the soil filling the

  fissures has a different texture from that of the mudflat. So when

  a fossil mudflat and its crack-filling are exposed by erosion, the

  ancient dried surface faithfully preserves the record of sunrays

  millions of years old.

  Mud cracks in the lake-bottom limestone proved that the lake

  had dried up repeatedly. Such sun cracks were found on several

  layers piled on top of one another on the ancient lake bottom. If

  the Sheep Creek dinosaurs had died in a shallow lake, then the

  strange negative clues could be easily explained. Shallow lakes are

  subject to cycles of drying up and wetness; some lake beds in to-

  day's tropical Africa are dry most of the time and fill up only dur-

  ing exceptional floods. Small snails can live in such shallow water,

  but bigger swimming creatures—fish, turtles, and crocodiles—ob-

  viously cannot. The best explanation for the peculiarities of the

  Sheep Creek fossils produced an unexpected twist to the theory

  of swamp-living brontosaurs. These brontosaurs probably did die

  on a lake bottom, or at least near it. But there probably was not

  enough water in the lake to float a small crocodile, let alone a multi-

  ton dinosaur.

  The ultimate irony at Sheep Creek is that the dinosaurs may

  have died there during a drought. When dead bodies lie on dry

  land for awhile, the air and sun desiccate the muscles and back

  ligaments, contorting the entire body so that the neck and tail are

  twisted up above the level of the back. Did drought kill the bron-

  tosaurs? There was strong evidence for dry seasons during the

  brontosaur's heyday in the late Jurassic. Beds of lime pellets show

  up in dozens of layers, and each kunkar zone recorded a time of

  repeating dry seasons. But the contorted state of the brontosaur

  bodies provided the strongest proof of terrible drought. At Sheep

  Creek, the eighty-foot body of a brontosaur was twisted precisely

  in the manner of a drought victim. And at Dinosaur National

  THE CASE OF THE BRONTOSAURUS: FINDING THE BODY | 117

  Monument in Utah, a four-hundred-foot-wide stream bed pre-

  serves dozens of gigantic dinosaur bodies, all twisted with their huge

  necks and tails over their backs. Similar scenes of Jurassic death

  by drying are repeated in quarry after quarry.

  Ordinarily, carcasses are pulled apart by scavengers right after

  death. If the weather is mild, lions, hyenas, and jackals can reduce

  a dead elephant to a scattered mass of disjointed bones in a few

  days. But drought kills animals faster than the scavengers can dis-

  member them, and severe drought kills the scavengers too. So the

  landscape becomes full of bodies drying up under the sun. Scores

  of brontosaur bodies found in the Morrison beds show the telltale

  signs of mummification under the merciless sun of Jurassic droughts.

  Our scenario of what occurred at Sheep Creek contains the

  script for a Jurassic tragedy. Around the shores of a drying lake

  bed gather the beleaguered giants— Diplodocus, Brontosaurus, Steg-

  osaurus. With their huge elephantine feet they dig into the mud,

  trying to reach the water table fast receding under the desiccating

  Mesozoic sun. For a few weeks the giants survive, drinking from

  the muddy water which seeps into the deep holes excavated by

  their feet. Smaller dinosaurs sneak in to drink when the giants are

  unaware, though the biggest brontosaurs angrily defend their

  dwindling water stores. Finally even the strongest cannot survive.

  Twenty-ton bodies collapse on the lake shore and twist into sun-

  dried mummies. After six months or a year or two, the monsoon

  rains return. The level of the lake rises, spreading a soft blanket

  of limy mud over the sun-dried bodies lying on the parched clay.
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  The final box score tabulated for the survey of brontosaurs

  was emphatically in favor of land habits. Few brontosaurs were

  buried in deep lakes. Many died and became entombed on flood-

  plains where the water creatures—crocs and turtles—were rare or

  absent. Altogether, the brontosaurs showed the same burial pat-

  tern Kay Behrensmeyer has observed for land-living elephants in

  East African sediments.

  We then reconstructed the overall environment from the

  patchwork of different individual habitats found throughout the

  landscape during Morrison times. This broader level of sleuthing

  yielded a picture of the Jurassic world in the Western United States.

  It consisted of a system of broad, flat floodplains, small rivers,

  shallow ponds, and occasionally deep lakes, all subjected to cycles

  of killing droughts.

  118 | THE HABITAT OF THE DINOSAURS

  After the results of our carcass-sleuthing in the Morrison, a

  nagging question remained. Why did the brontosaurs die out in

  Wyoming and Colorado at the end of the Jurassic Period?

  After achieving extraordinary success right up into the last

  Late Cretaceous brontosaurs avoided swampy forests. The Alberta delta was

  wet year-round most years, and brontosaurs weren't there. But in North

  Horn, Utah, there was a distinct dry season (producing kunkar) and the

  brontosaur Alamosaurus enjoyed the climate.

  THE CASE OF THE BRONTOSAURUS: FINDING THE BODY

  119

  zones of the Jurassic, the brontosaur clan declined suddenly in

  Wyoming and Colorado before the waves of new dinosaur fami-

  lies flooding the land ecosystems at the beginning of the Creta-

  ceous. These new dinosaurs of the Cretaceous were all members

  of the beaked Dinosauria. And the entire Cretaceous Period re-

  cords the proliferation of beaked dinosaurs into several rich fam-

  ilies of duckbill dinosaurs, armored dinosaurs, and horned dinosaurs.

  This transition from the Jurassic to the Cretaceous marks the most

  profound shake-up in the history of dinosaurian families.

  Why did brontosaurs lose their ecological preeminence in these

  areas? According to most books on dinosaurs written for children,

  the swamp-loving brontosaurs would have died out as their marshy

  haunts drained away. Our comprehensive survey of brontosaur

  quarries has already shown that this view simply won't hold—Ju-

  rassic brontosaurs were living and breeding for millions of years

  in habitats with a distinct dry season.

  There is, however, another good explanation for the absence

  of the brontosaurs from the areas they had formerly dominated,

  an explanation that takes dinosaur orthodoxy and stands it on

  its head.

  To understand this disappearance of the brontosaurs prop-

  erly, we must turn to the great deltas of the Cretaceous. Deltas

  are formed where rivers meet the sea. Where the modern Missis-

  sippi flows into the Gulf of Mexico, for example, the mud-laden

  river water dumps millions of tons of sediment every year along

  the boundary between land and ocean. This influx of sediment is

  building a huge mud platform out into the Gulf. In Late Creta-

  ceous times, a series of short rivers flowed eastward from the ris-

  ing young mass of the Rocky Mountains into a broad and shallow

  sea that covered most of what is now the High Plains country of

  eastern Alberta, Montana, and Wyoming. Together, these Creta-

  ceous rivers built a continuous sequence of overlapping deltas from

  Canada south to New Mexico, and these deltas are full of dino-

  saur skeletons.

  These Cretaceous deltas are among the best-studied dinosaur

  habitats of the entire Mesozoic, because compelling economic in-

  terests drive geological exploration—the deltaic sands are often full

  of oil and the deltaic swamp deposits are full of coal. Thanks to

  the oil and coal companies, which have poured millions into re-

  120 I THE HABITAT OF THE DINOSAURS

  search, and to American and Canadian museums, which have con-

  sequently excavated hundreds of dinosaurs, the climate and

  landscape of the Cretaceous deltas can be reconstructed in minute

  detail. Conditions had clearly changed from Jurassic days. Gone

  were the dry meadows and kunkar-layered soils. In their place stood

  stagnant bayous, estuaries, and cypress swamps like those of pres-

  ent-day Louisiana. Beneath these broad bodies of delta water, or-

  ganic residue from innumerable rotting leaves and branches was

  continuously pressed into a compact, carbon-rich coal.

  These deltas began yielding dinosaurs in the 1880s. The rock

  layers they contained were named the Laramie Beds after the Wy-

  oming frontier town. Now, if the orthodox story of brontosaur

  habits is to be believed, then the Laramie Deltas should have been

  prime brontosaur country. But the brontosaurs aren't there. Di-

  nosaurs in profusion are found in the Laramie Deltas—beautifully

  preserved skeletons, with every joint in place, of duckbill dino-

  saurs, horned dinosaurs, armored dinosaurs. But after a century of

  thorough exploration, not one brontosaur has turned up.

  Yet brontosaurs did still exist during this time. Late Creta-

  ceous beds in Brazil, Argentina, India, Mongolia, and even in close-

  by New Mexico have yielded dinosaurs. Why then did these

  latter-day brontosaurs avoid the Laramie Deltas?

  Perhaps a consummately heterodox suggestion answers that

  question: Maybe the brontosaurs did not need soggy terrain, maybe

  they positively hated it. Some of today's large animals—zebra, wil-

  debeest, and lions, for example—dislike swamps intensely. Bron-

  tosaurs may have disliked the mushy soil so much that they avoided

  the swampy deltas entirely.

  This idea could be tested in the quarries of the North Horn

  Mountains of Utah, the northernmost locales that contain Late

  Cretaceous brontosaurs. If the North Horn quarries contain evi-

  dence for dry, well-drained floodplains—the type of habitat miss-

  ing further north in the area of the Laramie Deltas—then a strong

  case for hydrophobic dinosaurs can be made, and their decline in

  Wyoming and Colorado in Cretaceous times consequently ex-

  plained.

  Alamosaurus, "lizard of the Alamo," is the name given to the

  Cretaceous brontosaur found in Utah. "Alamo" in this case refers

  to the Ojo Alamo Mountains of New Mexico where the beast was

  THE CASE OF THE BRONTOSAURUS: FINDING THE BODY I 121

  The Alamo brontosaur in its dry Utah home. Two Alamosaurus (at left) watch

  as a meat-eating Albertosaurus tries to attack the spiny-frilled Styracosaurus (at right). A trombone-crested duckbill (Parasaurolophus) flees the scene. During

  these Late Cretaceous times, the brontosaur clan avoided the humid habitats

  of the northern deltas in Wyoming and Montana and Alberta, but farther

  south, where summers were dry and hot, Alamosaurus reigned supreme as the

  biggest plant-eater.

  first found in 1922. When that first Alamosaurus was found, it jolted

  American paleontology; until then, no one had found a single scrap
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br />   of Late Cretaceous brontosaur in the United States. And it had

  been assumed that all the North American brontosaurs had died

  out long before Late Cretaceous times.

  The best specimen of Alamosaurus was excavated from a

  beautiful cliff face within the North Horn Mountains of Utah. Di-

  nosaurs from the North Horn constitute an intriguing mix. Sci-

  entists from the Smithsonian Institution have disinterred horned

  dinosaurs, duckbills, and flesh-eating tyrannosaurs—all three groups

  that dominate in the Cretaceous deltas of Wyoming-Alberta. But

  they have also discovered several gigantic brontosaurs, all belong-

  ing to Alamosaurus. And the North Horn quarry did not at all re-

  semble a typical Cretaceous delta. Instead, it looked precisely like

  a Jurassic quarry from Como. A walk up the cliff face yielded a

  count of seven distinct layers of lime pellets. So the North Horn

  habitat in Late Cretaceous times was much less soggy, on average,

  than the contemporaneous locales on the deltas in Wyoming. Like

  its Jurassic forebears, therefore, Alamosaurus lived and died in a

  landscape of dry, well-drained floodplains.

  The ultimate in heterodox thinking seems justified. And

  brontosaur orthodoxy had it completely incorrect. Brontosaurs

  didn't require deep swamps to buoy their bulk; they didn't even

  like to be near swamps. Brontosaurus and its kin of the Jurassic

  Age favored truly terrestrial haunts with dry soils. And when great

  swamps did spread across vast areas of the Cretaceous world, as in

  Wyoming and Colorado, the brontosaur clans simply eschewed this

  soggy terrain and moved their evolutionary centers elsewhere, to

  locales where the brontosaurs could feel the reassuring texture of

  dry floodplains beneath their feet.

  124 | THE HABITAT OF THE DINOSAURS

  6

  GIZZARD STONES AND

  BRONTOSAUR MENUS

  Seen from a distance, a live Brontosaurus would appear not to

  have any head at all. Both neck and tail would just seem to

  taper gradually to a point both fore and aft. Up close the head

  would appear, of course—about the size of an average horse's. Less

  than two feet of brontosaur head to go with seventy feet of neck,

  body, and tail. A two-foot horse's head, with a mouthful of big

  molar teeth, can feed an eight-hundred-pound horse body. A two-