Robert T Bakker

by The Dinosaur Heresies (pdf)

  South Africa studying the ancient mammal-like reptiles of the

  Permian Period, a colleague from Johannesburg Museum took me

  for a weekend outing to one of their famous parks. There were

  camels—though the species isn't native to South Africa. They had

  been imported for use in crossing desert regions and are popular

  exotic displays in the outdoor parks. Camels have thick cushiony

  pads under their toes, and these pads spread out under their own-

  THE CASE OF THE DUCKBILL'S HAND | 157

  er's foot as they walk. In the course of our visit I happened upon

  a dead camel in an unkempt corner of the park. It lay like a desic-

  cated mummy, all its natural juices evaporated by the hot Trans-

  vaal sun. The camel's mummification is not uncommon in such dry

  climates. Beneath its outstretched feet, its cushions, plump and

  elastic in life, were now dried-out bags of skin, which had flat-

  tened against the dusty soil surface. A spark of recognition shot

  through my brain. If camels were extinct and this carcass were found

  covered by flood-borne sand, wouldn't paleontologists conclude that

  the camel had webbed toes? The flattened skin of its paws created

  the perfect imitation of a web.

  The skin of the duckbill's paws was not marked by calluses

  the way camels' paw skin is. But the way the duckbill mummies

  are preserved permits the hypothesis that in life those flattened

  hands were in reality plump, rounded cushions of connective tis-

  sue—elastic shock absorbers for the impact of the ground on the

  wrist when the animal moved fast over hard ground. Duckbill fore-

  paws were so narrow and compact that a paw cushion would do

  invaluable service by lessening the load of impact within the joints

  of the toes. Fossil duckbill trackways, just now being excavated in

  158 | THE HABITAT OF THE DINOSAURS

  Foreclaw cushions,

  inside view

  Canada, suggest that indeed this line of reasoning may be correct.

  The forepaw impressions resemble smooth crescents, as though the

  individual toes were all imbedded within a single, insulating mit-

  ten. There is definitely no sign of a spreading ducklike web.

  This may be the true solution to the century-old mystery of

  the mummy's hand. That brown withered paw may have misled

  four generations of paleontologists into believing in a series of

  nonexistent adaptations for swimming. The mummy's hand, when

  alive and full of healthy tissue, may have worn a shock-absorbing

  glove, an earth-mitten entirely designed for walking on dry ground.

  THE CASE OF THE DUCKBILL'S HAND

  159

  8

  DINOSAURS AT TABLE

  Orthodox paleontologists insist most of their dinosaurs ate

  mush. They condemn both of the great tribes of plant-eat-

  ers—the brontosaurs and the duckbills—to a way of life at the

  water's edge, forced to eat nothing but soft water plants. In its own

  way, this theory epitomizes the traditional view of most dinosaurs

  as swampland creatures, virtual dead ends in evolution's race to

  develop lively, active species. In 1915, William Diller Matthew, a

  very respected mammal paleontologist, wrote, a highly influential

  book, Climate and Evolution, which argues that evolution bogs down

  in the soggy lowlands. Matthew believed that only on the high,

  dry soil of plains and plateaus did evolutionary forces create the

  most vigorous, most advanced creatures. There's a lot of truth in

  Matthew's thesis. It has been ascertained, for example, that water-

  loving turtles and crocodiles evolve most slowly, changing so little

  on average through geological time that a single genus can be fol-

  lowed for thirty million years or more. So the orthodox concept

  of a mush diet is consistent with the overall theory of sluggish di-

  nosaurs: soft, plant food was all they required for their sluggish

  metabolic needs, and the consequent swampy habitats limited di-

  nosaurs to slow rates of evolution.

  There may be some ground for believing the brontosaurs ate

  such soft foods. If the possibility of gizzard stones is ignored, the

  brontosaurs' dentition does seem little equipped to deal with meals

  160 | THE HABITAT OF THE DINOSAURS

  The duckbill Kritosaurus:

  life portrait and skull

  of tougher plants. But there are no grounds whatsoever for be-

  lieving it of duckbills. The mouth of a duckbill dinosaur contained

  one of the most efficient cranial Cuisinarts in land-vertebrate his-

  tory. Duckbill teeth and jaws were incomparable grinders, de-

  signed to cope with foods right inside the duckbill's oral

  compartment.

  The myth of mushy foods for duckbills began with a single

  error by one of the great pioneering American dinosaur hunters.

  Edward Drinker Cope discovered a fragmentary duckbill jaw in

  1885. His specimen had cracks running through the row of teeth,

  so that individual teeth fell out of the fossil jaw when he exam-

  ined it. Cope mistakenly assumed this condition was natural and

  DINOSAURS AT TABLE I 161

  jumped to the conclusion that a duckbill's teeth would break off

  whenever the beast tried to chew tough food. This error should

  have been corrected by 1895, when complete skulls and jaws re-

  vealed that duckbill teeth were firmly packed together and no one

  tooth could possibly fall out before it was totally worn down. Even

  then, whenever a worn tooth dropped out, a new tooth already

  stood beneath it ready to take over chewing duties. Duckbills ap-

  parently never ran out of teeth. No one has ever discovered a se-

  nile duckbill mouth; not one specimen exists with all its teeth either

  The head of Edmontosaurus,

  a duckbill. Life portrait at

  top, skull in the center, and

  skull cut through the tooth

  rows at the bottom.

  162 | THE HABITAT OF THE DINOSAURS

  worn out or fallen out. To all appearances, from the day they

  hatched out of the egg to their last breath, the duckbills enjoyed

  the use of healthy dental machinery, continually renewed by young

  teeth growing in to replace the old.

  Not only were the duckbills' teeth never-ending, their ar-

  rangement was designed especially for powerful grinding. At any

  one moment many rows of young teeth were growing into the

  mouth, providing the animal with grinding surfaces made up of

  hundreds of closely packed teeth. Each tooth was built up from

  two different biological materials: a thick layer of very hard enamel

  and a central core of softer dentine. Since many rows of teeth were

  packed together in each jaw, and all the rows together partici-

  pated in chewing action, the chewing surface was a mosaic of enamel

  ridges and dentine. Enamel ridges always protruded a little higher

  than the dentine cores, because the enameled parts of the teeth

  got worn down a bit more slowly than the softer cores. This ar-

  rangement was very effective. No matter how hard the duckbill

  How duckbill teeth work

  DINOSAURS AT TABLE I 163

  chewed or how hard its food was, the
enamel stuck up further than

  the dentine, young teeth kept replacing the old, and the duckbill

  maintained a grinding surface that worked much like a self-sharp-

  ening vegetable grater.

  Although Professor Marsh of Yale clearly illustrated the real

  qualities of the duckbills' chewing equipment in 1896, most pa-

  leontologists retained the mistaken theory and ignored the ob-

  vious adaptations for tough food. It required yet another Yale

  professor to set matters straight. In 1961, John Ostrom published

  his heretical interpretation of duckbills. He defined them as land

  creatures and emphasized the mechanical—ecological implications

  of their dental Cuisinart. He pointed out that the teeth of duck-

  bills had a pattern that virtually necessitated tough food. Their

  characteristic bills were also consistent with a tough-food diet, de-

  spite a superficial resemblance to the bill of modern water-feeding

  ducks. Way back in the 1880s, Cope had already found fossil rem-

  nants of the horny edge that had lined the bony beak of duckbill

  dinosaurs while alive. This horny edge was sharp and deep from

  top to bottom, more like the edge of a cookie-cutter than the soft,

  sensitive rim of a mud-dabbling duck. After Cope's initial discov-

  ery, other horny fossils turned up, making it clear that all duck-

  bills possessed deep, sharp-cutting edges along the entire upper

  and lower beak. Such sharp edges were obviously for cropping

  tough plants—not for grazing on mush. So soft-beaked ducks were

  never good analogues for duckbill dinosaurs, but modern tortoises

  are; the tortoise's beak is tall and sharp-edged, and constantly used

  to cut through tough blades and stems.

  If duckbill dinosaurs were truly efficient shredders of tough

  fodder, they would also have required good tongue—cheek coor-

  dination. Consider what it takes to chew something as recalcitrant

  as a piece of celery—your tongue contributes by moving the fi-

  brous lump between palate and teeth. Your cheeks play their role

  by retaining the mass of celery and preventing it from slipping.

  Tongue-in-cheek skill is characteristic of the best shredders among

  today's Mammalia—horses, cows, elephants, rabbits, kangaroos. All

  these herbivores possess large, active tongues and strongly mus-

  cled cheeks. Incidentally, that lump of food while being chewed

  in the mouth has been dignified with a technical scientific label:

  "bolus."

  164 I THE HABITAT OF THE DINOSAURS

  All of today's Reptilia are cheekless. Their open mouthline

  extends all the way back to the joint of the jaw before the ears.

  There is no skin to hold any food being chewed. Consequently,

  herbivorous reptiles—tortoises and iguana lizards, for example—

  are sloppy eaters; when their jaws slice off a piece of leaf, the part

  sticking out of the mouth simply falls to the ground. Each time

  they chew, they lose nearly half their mouthful, quite a wasteful

  business. Primitive meat-eating dinosaurs had similar wide-open

  mouthlines.

  Traditionally, duckbill dinosaurs have been portrayed as

  cheekless, with the mouthline running from chin to ear like a liz-

  ard's. A dissenting voice was raised by Yale Professor Richard

  Swann Lull (Yale's tradition of duckbills seems to have been con-

  sistently heterodox). In 1942, Lull restored duckbills with cow-style

  cheeks walling the sides of the oral space. But most of Lull's col-

  leagues rejected the idea because everyone knew dinosaurs were

  reptiles, and reptiles, by definition, didn't have cheeks. Such ob-

  jections were specious. No living reptile has cheeks. But no living

  reptile has grinding teeth anything remotely resembling those of a

  duckbill. If the duckbills could have evolved such unreptilian teeth,

  why couldn't they have evolved unreptilian cheeks?

  The final Yale duckbill—cheek conclusion was joined in the

  late 1960s. Peter Galton, an English paleontologist resident at Yale

  as a research associate, reinvestigated the question of the duck-

  bill's oral tissue. He concluded that Lull's reconstruction of cheeky

  duckbills was almost certainly correct. All duckbills had deep re-

  cesses in their skull and jaw bones running parallel with their

  mouthlines above and below where their teeth came together. This

  recessed zone resembled the deep hollowed-out areas found in the

  jaws of gophers, chipmunks, and other rodents which have capa-

  cious cheeks for holding food while they chew. A slightly rough-

  ened ridge often marks the top and bottom of the duckbill recess,

  and some sort of skin or muscle or both must have attached to it.

  Peter Galton drew diagrams of the cheek—pouch recesses in mod-

  ern species such as pigs, horses, elephants, and rodents, which

  demonstrated how duckbill pouches must have been as well de-

  veloped as any of these.

  What, then, did duckbills eat? Considering their prodigious

  dental powers, the flip answer might be "anything they wanted."

  DINOSAURS AT TABLE I 165

  But in terms of serious theory, those powers expand the boundary

  conditions of any hypothesis about their diet very widely. The

  duckbills might have masticated extremely tough leaves, stems,

  twigs, pinecones, even roots and tubers. Some clues as to their ac-

  tual dining habits can be gleaned from their very curious body

  posture. All duckbills had much longer and stronger rear than fore-

  legs and probably moved semibipedally, striding on their hind legs

  and using their forepaws only to touch down lightly for balance.

  Old restorations showed duckbills standing in a tripodal posture,

  their hind legs and tail supporting their weight, with their back

  and neck nearly vertical. Such a posture would have permitted the

  duckbills to feed high in the pine trees of their habitat. Yet that

  upright body posture was wrongly conceived. The build of the

  duckbill was clearly designed for low, near-the-ground feeding, not

  for tree-browsing. If duckbills had specialized in high-level feed-

  ing, they would have had shoulders and necks designed for reach-

  ing upward. But that is not the case. Instead, in the region of the

  shoulder their backbone bends permanently downward. This sharp

  flexure locates the base of the neck and the head on a very low

  anatomical level. The downward bend in the chest area is so marked

  that even when a duckbill raised its neck as far as it could go, the

  head was still below the level of the topmost point of the shoulder.

  Some mammals today exhibit this same downward curve of

  the backbone. In the American buffalo, for example, the line of

  the vertebral column curves sharply downward as it passes from

  shoulder to neck. Thus they must always hold their heads low, with

  muzzles close to the ground. As the song says, buffalo roam where

  the deer and the antelope play, but deer and buffalo represent di-

  vergent tactics for eating plants. Deer can carry their heads much

  higher than can buffalo, and can reach up into the trees to nibble

  on twigs, leaves, and bark. Buffalo stick to grou
nd level and use

  their strong, wide snouts to pull up tough grass deer cannot deal

  with. Clearly, the duckbills were more like buffalo than like deer.

  And the entire tribe of duckbills must have spent most of their

  time feeding at or near the ground.

  These considerations dramatically narrow the boundary con-

  ditions for any hypothesis about their diet. The duckbills' pre-

  ferred food must have been low-growing herbs or shrubs (grasses

  had not yet evolved in Cretaceous times). These boundaries still

  166 I THE HABITAT OF THE DINOSAURS

  Permanent downflex in the American buffalo and a duckbill (dashed line

  shows line of backbone)

  allow for a wide selection of Mesozoic roughage and greens:

  horsetails, ground pine, ferns, low tree ferns, seedling evergreens

  (pines, cypress, etc.), cycads and other tough-frond types, low-

  growing palms, magnolialike shrubs, and so on.

  It's probably barking up the wrong herb to try to find the one

  duckbill food. Duckbills were so varied in snout design that it's

  unlikely all species fed on the same plant stuffs. Today the ante-

  lope family demonstrates how snouts can be custom-tailored to fit

  each species' method of feeding. Cape buffalo (cows and buffalo

  are members of the antelope family) have very wide muzzles, fine

  for biting a wide swath through the sward but much too clumsy

  for picking out individual succulent tidbits. Royal antelope have

  slender snouts which they can use to pick and choose. Among the

  duckbills, Edmontosaurus had a huge, blunt muzzle and must have

  cropped wide batches of leaves with each bite. Duckbills with hol-

  low head crests, Lambeosaurus and its kin, adopted a totally differ-

  ent approach; their muzzles were narrow, and allowed them to poke

  around for a more discriminating bite.

  Everywhere on the Late Cretaceous deltas the duckbills' con-

  stant companions were the great horned dinosaurs. Side by side,

  three-horned Triceratops and wide-mouthed Edmontosaurus cropped

  the greenery. Duckbills and horned dinosaurs were distant cous-

  DINOSAURS AT TABLE I 167

  ins—both had beaks and traced their ancestry to the same ancient

  little dinosaur of the Triassic Period. But what an extraordinary