Robert T Bakker

by The Dinosaur Heresies (pdf)

  cause this limestone was ideal for making lithographic slabs, plates

  of stone used to print drawings of delicate and subtle tones. The

  salty Jurassic sea that laid down these limestone beds in shallow

  bottoms behind reefs preserved tiny skeletons with exquisite fi-

  delity, because the hypersalinity discouraged the activity of scav-

  engers. As the layers of lime accumulated, bodies of horseshoe

  crabs, shrimp, and insects settled to the bottom and were entered

  into the ever-growing record. Occasionally, a tiny leather-winged

  form would fall into these briny waters, and 150 million years later,

  during the nineteenth century, quarrymen could send a slab con-

  taining the raven-size bones to a nearby German scholar. Before

  long this skeleton reached Baron Cuvier in Paris.

  DINOSAURS TAKE TO THE AIR | 273

  Quetzalcoatlus buzzes two tyrannosaurs.

  Cuvier's felicitous name for this creature was Pterodactylus—

  literally translated, "wing-finger."

  With the stroke of his pen, the baron evoked a creature with-

  out precedent in human knowledge: a large-headed aerialist that

  supported its batlike wing membranes on a single, elongated fin-

  ger in each wing. This single finger alone was longer than the crea-

  ture's head and body. Its long jaws set with small teeth and the

  general cast of the skull proved that Cuvier's Pterodactylus was nei-

  ther bat nor bird but sui generis, a unique and totally extinct order

  of organic creation.

  Cuvier decided Pterodactylus was closer to the crocodiles than

  to any other living family and thus was born the term "flying rep-

  tile." No other creature resurrected from the rock by the baron's

  scholarship gripped the public imagination more than Pterodactylus

  and its leather-winged kin. Nineteenth-century engineers and in-

  ventors regarded flight as the highest form of locomotion in the

  Scala Naturae, and they wistfully scrutinized bats and birds as at-

  tempt after attempt to build a flying machine failed. But there in

  the Jurassic strata was a "reptile," a member of the lower verte-

  brate class, possessing a breastbone keeled for flight muscles and

  arms designed for powerful flapping. "Flying reptile" seemed a

  contradiction in terms—by definition reptiles were crawling things,

  condemned to slinking across the surface. But Cuvier's Pterodac-

  tylus tore a veil from the present, revealing the unexpected

  achievements of the Reptilia past.

  Early reconstructions of pterodactyls depicted them as dark-

  hued animals of nightmarish aspect. Even in this century ptero-

  dactyls have been cast as villains in prehistoric drama—the over-

  sized wing finger that tried to make off with Fay Wray in King Kong

  gave Kong his chance to show his chivalry in the rescue. But dark

  colors and darker character were entirely inappropriate for flying

  reptiles. With few exceptions—the Texas giant is one—the aerial

  dragons habitually flew over shallow regions of the sea. As hunt-

  ers of fish and squid, they were therefore the equivalent of

  shorebirds in today's ecosystem. And shorebirds are rarely som-

  ber in plumage. As a group, pterodactyls probably sported the

  camouflaging color scheme common among shorebirds, a dark

  topside to hide it from bigger pterosaurs attacking from above, a

  white bottomside to hide it from prey in the water below. A prob-

  DINOSAURS TAKE TO THE AIR

  275

  Pterodactyl us probing

  for a worm

  able color pattern for Cuvier's Pterodactylus would be puffinlike.

  When the flying reptiles are portrayed in seabird tones, these Meso-

  zoic fliers lose their malevolent aspect and become positively

  handsome.

  Pterodactyls should evoke awe. But in the most commonly

  used twentieth-century paleontology textbook, these noble crea-

  tures were described as failures in everything they did. They

  couldn't have flown because, it was asserted, their wing mem-

  branes weren't stiff enough and were too crudely controlled by the

  reptile's muscles. A single finger was deemed far less efficient as a

  support for the wings than the four fingers bats employ to stretch

  276 | DEFENSE, LOCOMOTION, AND THE CASE FOR WARM-BLOODED DINOSAURS

  out their flying surface. It was postulated that pterodactyls couldn't

  have flapped at all because their wing surface had been too flaccid.

  Furthermore, the experts decided that pterodactyls were accident-

  prone. Since there existed no stiff anatomical structure within the

  wing to prevent a tear from running right across the entire sur-

  face, pterodactyls were supposed to have been vulnerable to snag-

  ging on branches or rocky outcroppings. Even on the ground

  pterodactyls were portrayed as clumsy locomotor machines, inca-

  pable of walking normally either on two legs or four. All told, the

  mid-twentieth-century portrait of the pterosaur was wretched: a

  flying creature that managed to get into the air only when the wind

  was precisely right, permitting its underpowered, floppy wings to

  glide passively on updrafts. According to the orthodox theory, these

  flying reptiles survived only because there was no aerial competi-

  tion. And when flying birds finally did evolve in the Cretaceous,

  their elegantly designed feathered wings were so manifestly su-

  perior to those of pterodactyls that the avian tribes quickly re-

  placed the obsolete harpies of the Mesozoic.

  Nineteenth-century scholars had more confidence in ptero-

  dactyl's design. Baron Cuvier certainly believed his little wing-fin-

  gered beast could fly. Mid- and late nineteenth-century students

  of flying reptiles had faith in pterodactyl's landing gear as well.

  During the last century, many restorations were conceived, show-

  ing Pterodactylus running successfully about the land on all fours,

  its long fingers folded back from the wrist raising the wingtips back

  over the hips. Some modern tropical bats—the South American

  vampires especially—move in this way and can be quite mobile on

  the ground. Which view is closer to the truth, that of the nine-

  teenth century or that of the mid-twentieth?

  In the 1970s new fossils and fresh studies of the old speci-

  mens began to rehabilitate the pterodactyl's image. From the very

  same Bavarian quarries that yielded the first Pterodactylus came

  specimens with the wing membranes preserved in perfect detail—

  discoveries that suggested that the theory of the flaccid membrane

  was wrong. The newly cleaned specimens confirmed that the wing

  tissue had not been weak, unsupported skin at all. In life, long,

  stiff fibers of connective tissue had stretched across the wing, and

  were probably attached to muscles controlling the tension of the

  wing's surface. Pterodactyls thus were equipped to fine-tune the

  DINOSAURS TAKE TO THE AIR I 277

  Dimorphodon feeding on squid

  shape and camber of their wings. A Ph.D. thesis completed at Yale

  in 1979 argued forcefully that pterodactyls possessed the equip-

  ment for flight under their own power; the deep kee
l of the

  breastbone showed that the "white meat" muscles were as large

  relative to the body's size as are those of many flying birds today.

  It is true that the joints at the pterodactyl's shoulder, elbow, and

  wrist were not identical to the birds', but the flying reptile cer-

  tainly could have executed powerful up-and-down strokes with

  them, and the muscle processes along the arm bones were huge.

  In fact, as nineteenth-century anatomists had pointed out,

  pterodactyls were more fully committed to an active aerial way of

  life than any modern bird or bat, with the possible exception of

  swifts or hummingbirds. Every section of its anatomy evinced the

  drastic remodeling performed by evolution in order to transform

  the pterodactyl's terrestrial ancestor into a consummate aerialist.

  Shoulder bones had been reshaped so that the shoulder socket,

  which usually faced rearward, faced forward and outward like a

  bird's. Why should the joint have been so totally reorganized un-

  less pterodactyls were actively flapping? In addition, if a powerful

  rhythm of muscular contractions did propel the pterodactyl's wings

  up and down in the figure-eight pattern required for active flight,

  then we would expect the shoulder to be very firmly braced against

  the torso. And so it is. The pterodactyl's entire torso was highly

  compact from front to rear and the whole was reinforced by two

  rigid bony girders. Where the shoulder blade touched the back-

  bone, the shoulder abutted the anterior bony girder, composed of

  a half-dozen vertebrae firmly stuck together. This girder was a

  naturally evolved back brace that could support the enormous

  stresses of the beating wing. In front of the hip joint, the right and

  left hip bones were fused to another long set of vertebrae, consti-

  tuting a second back brace. Together, these shoulder and hip braces

  made the pterodactyl's torso a light but incredibly strong boxwork

  of bony struts, exceeding in strength the body of the most mod-

  ern birds. All this evolutionary modeling in the pterodactyl would

  have made no sense if these creatures had been merely passive

  gliders. Great strength in the bony frame evolves to resist great

  forces—in this case, the forces of active and strenuous flapping.

  Bird skeletons delighted medieval anatomists because of their

  lightness and economy. The bones of most flying birds are of a

  DINOSAURS TAKE TO THE AIR

  279

  tubular-strut design. All the major limbs are cast in a thin-walled,

  hollow construction. Just so were the pterodactyl's bones de-

  signed. Even the apparently massive upper arm bone (humerus) of

  the gigantic Texas pterodactyl had only an outer shell of very hard

  bone a few millimeters thick. And just as avian bones achieve their

  greatest lightness by being filled not by marrow but by a core of

  air sacs connected to the lungs, likewise the pterodactyl's bones

  are constructed to contain air-sac liners. Though lung tissue itself

  is never preserved in fossils, the presence of air sacs can be de-

  tected from the characteristic pores in the bony walls which pro-

  vided entrance for the air canals. Running one's finger over the

  smooth-edged pore in the arm bone of a great aerial dragon is like

  feeling the fossil breath of the giant, now long gone. Through these

  pores surged the oxygen-rich air each time the stout basketwork

  of the ribcage drew the Mesozoic atmosphere through the dragon's

  nostrils.

  Arm power was wing power in pterodactyls, as it is in birds

  and bats. The pterodactyl's upper arm bones were short compared

  to birds', but its forearm and wrist were longer—a difference that

  must surely reflect an as yet undiscovered contrast in the mechan-

  ics of upstroke and downstroke. Strong fliers among today's birds

  put greater stresses on the upper arm than on any other bony

  component. In these birds the humerus is the largest bone in the

  skeleton. And the distribution of stress in a pterodactyl's skeleton

  can be discerned by scanning the patterns of girth in the bones—

  the humerus is always the thickest, usually twice the girth of the

  thigh. There's no ambiguity here; pterodactyl's evolutionary trans-

  formation had concentrated nearly all the body's strength in the

  flight organ.

  Pterodactyl's wrists are evolutionary chimeras, combining me-

  chanical features found today in two or three different animal

  families. The long bones of the wrist were tightly bound into a

  single bundle, much like the wrist of the rabbit. This strong wrist

  structure certainly could have functioned for running and hopping

  on the ground. Anatomists number fingers from the thumb out-

  ward. Counting in this fashion, the pterodactyl's enormous wing

  finger was number four. But fingers one through three were spe-

  cialized for grasping, not flying. Each of these three inner fingers

  was short and flexible, ending in a sharp claw, which was deep from

  280 | DEFENSE, LOCOMOTION, AND THE CASE FOR WARM-BLOODED DINOSAURS

  Jurassic air piracy: A Scaphognathus

  attacks three Anurognatbus.

  top to bottom, thin from side to side. This type of claw proves

  that pterodactyls could cling to trees. Among living mammals, such

  a deep, narrow claw shape is the preferred device for clinging in

  species that roost in trees—the most specialized arboreal squirrels

  are so equipped. And even closer to pterodactyls in claw pattern

  is the strange Indonesian "flying lemur," a glider with a wide skin

  membrane between its front and rear limbs. Flying lemurs use their

  deep, sharp claws to cling firmly to the back of tropical tree trunks.

  Pterodactyls must similarly have gripped the trees of Mesozoic

  forests when they rested from the day's hunt.

  The construction of pterodactyl's hind limbs also proves that

  all its body mechanics—not the front limbs alone—were subser-

  vient to its flying organs. Pterodactyl's thighs and shins were long

  and slender. When hopping, the knee joint's geometry imparted a

  strong natural bow outward to the legs. If pterodactyls had to cling

  to trees and cliff faces to roost and breed, then they would also

  have required unusual mobility in their thighs for maneuvering

  around tree trunks, branches, and rocky crags. And, indeed, ptero-

  dactyl's hips and thighs were the most mobile of any Mesozoic

  vertebrate's. They mimic the extraordinary flexibility developed

  independently by modern bats. The hip socket of these animals

  was shaped as a nearly perfect ball-in-socket, circular in outline—

  a most unusual configuration. The surfaces of the thigh bone, which

  fit into that socket, permitted the leg to swing in all directions.

  Finally, the hind foot's claw matched the forefoot's in shape and

  gripping strength. So pterodactyls were both agile and strong as

  they scampered over their roosting sites above the Mesozoic ground

  surface.

  After reading Professor Seeley's Dragons of the Air, first pub-

  lished in 1901, I am at a loss to explain why the popular textbooks

  of the 1
940s and the 1950s portrayed pterodactyls as so faulty in

  design. Seeley summed up his thirty years of firsthand study by

  describing pterodactyls as precision-crafted flying machines. What

  could account for the later change of view? The best guess is

  probably that after 1920, few careful scholars interested them-

  selves in pterodactyls. Graduate students were steered away from

  the Mesozoic monsters of any sort and into more respectable

  fields—such as the horse's evolution, fossil beavers, and the tiny

  mammals of the Mesozoic. The textbook writers drifted into the

  282 | DEFENSE, LOCOMOTION, AND THE CASE FOR WARM-BLOODED DINOSAURS

  view of pterodactyls as inefficient, influenced by the generally anti-

  dinosaur atmosphere among English-speaking scientists. Nowa-

  days the flying dragons are enjoying a renaissance. Innovative and

  vigorous young scientists in China, Europe, and the Americas are

  discovering winged dragons of unexpected sizes and shapes. The

  reappraisal of the pterodactyl's flying prowess is returning to the

  nineteenth-century view.

  Several years ago I began my own investigations of pterodac-

  tyls' prey-catching devices. Most studies, naturally enough, have

  concentrated on their flight mechanics. But the pterodactyls' heads

  and necks spin an intriguing tale of flying dragons altering their

  hunting tactics through evolutionary time.

  Sharp fangs and large jaw muscles marked the head design of

  the most primitive pterodactyls. The muscle arrangement resem-

  bled those found in many of the primitive small birdlike dino-

  saurs: the jaw muscles of the skull compartment behind the eye

  were small, but those in front of the eye in the snout were large.

  The strongest bite belonging to any pterodactyls must have been

  owned by the big-snouted Dimorphodon, described with loving care

  in 1840 by the superb Victorian anatomist Sir Richard Owen. Un-

  til the discovery of this fierce-looking species from the black shales

  of Lyme Regis on the Dorset coast of England, all the pterodactyls

  had been found in France and Germany. But Owen's research lo-

  cated Dimorphodon (the name means "beast with two sizes of teeth")

  in the key position near the base of pterodactyl's family tree, a