Robert T Bakker

by The Dinosaur Heresies (pdf)

  marine propulsion. And its needle-sharp claws would have been

  perfect for snaring slippery aquatic prey. Maybe Archaeopteryx

  sometimes hunted like present-day fishbats, occasionally snagging

  fish with its hind claws as it swoops and glides over the surface of

  the sea.

  It must be said, restoring Archaeopteryx to its proper place in

  the dinosaur's family tree has been a great boost to the morale of

  dinosaurophiles. No open-minded observer of the fossil se-

  quence, from Coal Age reptiles with stubby legs to the birdlike

  dinosaurs of the Jurassic, can be other than convinced that our

  present glorious array of feathered creatures is truly the direct de-

  scendant of those primitive land creatures via the intermediary

  agency of the dinosaurs. There are over eight thousand species of

  birds alive in today's ecosystems, and each one, from the hum-

  mingbird to the ostrich, is incontrovertible evidence that the

  bloodlines of the dinosaurs are still full of evolutionary vigor.

  The story of Archaeopteryx is a boon to dinosaur-lovers in an-

  other way as well. According to the orthodox theory, remember,

  dinosaurs didn't have enough metabolic energy to walk fast, let

  alone fly. But both pterodactyls and birds had to evolve high-pres-

  sure hearts and lungs before flight could be achieved. Pterodactyls

  most probably were the descendants of very primitive dinosaurs,

  of the bunnycroc variety, while birds were surely products of the

  advanced dinosaurs. If both branches possessed a high-pressure,

  ARCHAEOPTERYX PATERNITY SUIT: THE DINOSAUR-BIRD CONNECTION

  321

  hot-blooded metabolism, then it's not impossible to suppose that

  the entire stock of primitive dinosaurs was already equipped for

  high metabolism before either aerialist tribe evolved. In other

  words, it's quite possible that flying dragons and birds inherited

  their high-capacity hearts and lungs from their dinosaur forebears

  and that powered flight was simply one application by evolution

  of the fundamental bioenergy of the dinosaurs.

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

  PART 4

  THE WARM-BLOODED

  METRONOME

  OF EVOLimON

  15

  SEX AND INTIMIDATION:

  THE BODY LANGUAGE OF

  DINOSAURS

  Ever since the first Mesozoic fossils came to light, there have

  been features of them that appear to defy explanation, at least

  in terms of the usually considered aspects of the Natural Econ-

  omy—eating, drinking, preying, avoiding predators. As each new

  species was excavated, the list of prehistoric anomalies grew: sail-

  backed reptiles from the Coal Age, horned amphibians from the

  ancient red beds, battering-ram skulls on the protomammals, ba-

  roque crests on the heads of duckbill dinosaurs. American paleon-

  tologists traditionally favored a strictly utilitarian interpretation of

  these things; bones should be shaped to perform a useful function

  for procurement of food or defense. Bones of nonutilitarian shape

  were therefore puzzling in the extreme. Faced with a bewildering

  variety of crests and cranial ornamentations, the older generation

  of American paleontologists sometimes advocated a moralistically

  motivated theory of racial decadence: As an evolutionary family

  approached its time of extinction, its species would indulge in

  nonadaptive decoration. Like biological ancient Romans, they had

  supposedly lost control of their adaptive sense and hastened to their

  doom amid orgies of useless ornamentation.

  Until the 1970s, few American scientists referred to sex when

  they analyzed dinosaur skeletons. But evolution is full of sex. And

  natural selection favors structures that produce results in winning

  or enticing mates and discouraging rivals. The beauty of nature is

  SEX AND INTIMIDATION: THE BODY LANGUAGE OF DINOSAURS I 325

  The great finback Dimetrodon attacks

  the amphibian Eryops.

  not spoiled by the great influence wielded by sex and intimida-

  tion. Nor does it lessen the fascination of fossils to suspect that

  much of the most extraordinary bony paraphernalia may have served

  as enticements to prospective mates.

  The early chapter of the sexual epic can be read back in the

  steamy days of the Coal Age, long before there were any dino-

  saurs. The primitive vertebrates with legs of that period would be

  classed as amphibians in the reproductive sense of that term, for

  they laid eggs, frog-fashion, in water. It is certain they reproduced

  in water, because aquatic hatchlings are common fossils in the dark,

  carbon-rich shales laid down in the lakes of the Coal Age. Often

  the skin of these ancient larvae is outlined in the stone where the

  slow decay distilled the living body tissue into an oily stain sur-

  rounding the skeleton. These larval amphibians are fossils of

  unexcelled loveliness. Dark organic outlines mark out each limb

  and, behind the head, the long branched filaments of their gills.

  326 | THE WARM-BLOODED METRONOME OF EVOLUTION

  Gills just like these, pulsing with oxygen-rich blood, can be found

  in the throats of modern salamander larvae. Holding a live sala-

  mander in a handful of pond water is like looking back into a past

  300 million years old, back to a time when the evolutionary tree

  of land vertebrates had just taken root.

  For the first fifty million years of life on land, all of the ver-

  tebrates with legs were amphibians of one tribe or another. How

  did they court each other? Since they reproduced in water, their

  pre-nuptial displays must have taken place in ponds and quiet

  backwaters or along the banks of ancient waterways. Living am-

  phibians feature some of the richest sonic symphonies in today's

  ecosystem—the chorus of mating frogs. But another amphibian

  family, the salamanders, far more primitive than frogs, is nearly

  mute. Some salamanders (the newts) substitute dance for song. The

  male newt waves his tall, bright red tail in a kind of underwater

  flutter-dance as he minces before his prospective lady love. The

  fossil records from the earliest Amphibia do turn up some eel-like

  tails that could have been used in this fashion. But what about

  sound? When did amphibians evolve that marvelous capacity for

  serenade so characteristic of modern frogdom?

  The early fishes did not hear airborne sounds, and their ears

  were used mostly to maintain body balance. Ears for hearing on

  land require a taut membrane in the skull to pick up airborne vi-

  brations. Living species of frog have such a membrane shaped like

  a tiny drumhead, constructed of special skin. A deep notch in the

  frog's skull holds the eardrum (known technically as a tympanum),

  and between it and the brain stands an air-filled chamber: the mid-

  dle ear. To transmit sound to the brain, a slender ear bone runs

  from the eardrum to the canals of the ear in the side of the brain-

  case. If it could be discovered when this type of ear first evolved,

  it would constitute an important
clue about when the sexual chorus

  first began.

  The eardrum doesn't preserve in fossils, but the notch for it

  in the skull does. Earliest of all amphibian fossils is the famous

  Ichtbyostega from the lake beds of Greenland (its name means "fish

  with a roof," a reference to its primitive fishlike structure and the

  thick roof of its skull). This Ur-amphibian has no definite notch

  for an ear, and couldn't have possessed any special auditory ad-

  aptations. Therefore, when Ichthyostega and its kind waddled over

  the primeval land, they must have marched into a silent world where

  SEX AND INTIMIDATION: THE BODY LANGUAGE OF DINOSAURS | 327

  the humid stillness was broken only by the rustling of ancient rushes

  in the wind and the near-silent footsteps of Ur-spiders hunting in

  the leaf mold.

  But it did not take long for the fledgling land vertebrates to

  evolve greater sensory complexity. Early in the Coal Age, quite

  large notches for eardrums appeared prominently in the skulls of

  the keyhole amphibians (loxommatids), a tribe of aggressive, sharply

  fanged predators with alligatorlike skulls. ("Keyhole" refers to the

  peculiar shape of their eye socket; an enlargement at the front of

  it may have housed a gland.) Keyhole amphibians clearly could hear

  airborne sounds, and therefore quite possibly used their voices to

  bluff and challenge and court. Since their heads reached a length

  of two and a half feet, they would surely have uttered a croak that

  would command respect. Other ear-equipped amphibians evolved

  Armor-plated bone-braced

  amphibian eardrum holder.

  The Early Permian Cacops

  shows how amphibians

  evolved a way to keep their

  eardrums tight—a large

  notch in the skull, just

  behind the eye, acted as a

  bony drum head. Cacops was

  a land-living hunter, about

  two feet long, and had bony

  armor above its backbone,

  but the same sort of

  eardrum holder was

  standard equipment in many

  water-living amphibians too.

  328 | THE WARM-BLOODED METRONOME OF EVOLUTION

  Quick history of butting, bluff, and intimidation. It all started with the

  evolution of eardrum skull notches in the Coal Age, proceded through the

  Age of Finbacks in the Early Permian, and then up to the head-butters of the

  Late Permian-Triassic, the tooting and butting Cretaceous-Jurassic dinosaurs,

  and the modern mammals of the Cenozoic.

  after them as the Coal Age continued, so the spring mating season

  probably witnessed a diversified range of timbre and tone.

  Reptile ears are built to the same general pattern as are am-

  phibian ears, but the details of how the nerves pass through the

  auditory apparatus are different. Most paleontologists presently

  believe that reptiles evolved their ear independently and did not

  simply inherit their auditory machinery from amphibian ancestors.

  Today, the ears of lizards work much like the ears of frogs, but

  the ears of Coal Age reptiles are biosonic puzzles. A good notch

  for the eardrum evolved in some reptile tribes very early, yet the

  bone of the middle ear was thick and ponderous, not the delicate,

  thin bone absolutely required for hearing mid and high frequen-

  cies. Massive ear bones wouldn't transmit most vibrations from the

  eardrum to the brain, and some of these early reptile ear bones

  are as big as a man's thumb. What could these ancient reptiles have

  heard, if anything? It remains a mystery. Some anatomists have

  suggested that the heavy ear bone was suspended by delicate lig-

  aments and acted as a kind of seismograph for detecting very low-

  frequency sound. This suggestion evokes visions of a mating dance

  in which the courting couple stomps about producing minor earth

  tremors to communicate their lust. Reptiles did not evolve ears of

  high sensitivity until late in the Permian Period, long after the Coal

  Age, and the Reptilia certainly weren't equipped to transmit and

  receive airborne melodies before then.

  Sex is not all melody. Pushing and shoving, intimidation, have

  their place too. Frog suitors often try to kick their rivals off the

  back of a female in the mating pond. The rhinoceros iguana lizard

  of Cuba indulges in male-male wrestling contests. Males push each

  other with their snouts, grab loose skin in their teeth, and may

  clamp on each other's mouths in what is technically known as jaw

  wrestling. Not many living amphibians have specialized organs for

  sexual wrestling, but among the Coal Age fossils there is one

  spectacular case.

  Most amphibian skulls are designed quite straightforwardly.

  With few exceptions, they can be explained in terms of jaw mus-

  cles, bracing for teeth, sucking in prey underwater, or other purely

  dietary needs. But diet can't explain the most grotesque amphib-

  ians skull ever evolved—the boomerang-shaped head of Diplocau-

  lus ("two-tailed," a reference to its double-spined tail bones).

  330 | THE WARM-BLOODED METRONOME OF EVOLUTION

  Battle of the boomerang-heads. Swoosh, clunk, and thud on a Texas stream

  bottom during the Early Permian Period. Diplocaulus, a three-feet-long flat-

  bodied amphibian, probably used its grotesque head horns for sideways

  slugging matches. Eyes faced directly upward, so the underwater head-

  bashing had to be done by touch.

  The jaws, teeth, and face of this animal were quite "normal";

  its snout was flat top to bottom and its eye sockets faced directly

  upward. The animal was probably a pond and stream predator, lying

  in ambush on the murky bottom, awaiting unwary prey. This was

  an ecological role that evolved many times in separate amphibian

  tribes. Probably all of Diplocaulus's life—including courtship and

  mating—was spent underwater. Its young exhibited standard cra-

  nial geometry for the role of bottom-predator: a generally wide,

  rounded skull without significant protuberances. But as it grew into

  adolescence, a transformation carried it into an exceedingly un-

  usual development of the skull. The rear corners grew outward at

  great speed. Well before it was fully adult, its skull had become

  twice as wide as it was long. And even faster grew the hornlike

  devices at the extreme ends, until at maturity the head was finally

  four to six times wider than long. Viewed from the top, these heads

  resembled nothing so much as organically grown boomerangs.

  Paleontologists tried to explain the boomerang shape as an

  adaptation for swimming; supposedly it worked like an underwa-

  ter wing, imparting hydrodynamic life as the beast swam at high

  speeds through the Coal Age streams. But Diplocaulus was not a

  strong swimmer. Its body and tail were too flat to have borne the

  muscles needed for fast underwater propulsion. So hypotheses based

  on locomotion just don't seem plausible for the boomerang shape.

  To my knowledge, no one has suggested an hypothesis based upon

  sex and intimidation as the biological function that might make sense

  o
f the grotesque cranial shape.

  Diplocaulus was at a disadvantage in evolving organs for in-

  timidating other members of its species. Its life was spent looking

  up, so it couldn't easily see its neighbors lying alongside. It was

  moreover an animal that lived in flat areas—hence its very low and

  wide skull and body, no doubt to help it hide in ambush. And the

  most vigorous movement available to it was rather awkward un-

  dulation along the pond bottom. So how was evolution to work to

  create a sexually impressive Diplocaulus} The evolution of intimi-

  dation devices usually operates by modifying preexisting patterns.

  Diplocaulus moved by wriggling across pond bottoms. During its

  evolution, males and females must often have bumped into each

  other in so doing. Genes that favored wider corners of the skull

  could therefore yield an advantage. The longer the hornlike ex-

  332 | THE WARM-BLOODED METRONOME OF EVOLUTION

  tension, the greater the range of the bumping action. Boomerang-

  heads couldn't see one another, but they could reach out and bump

  someone.

  While Diplocaulus was evolving in the waters of the Late Coal

  Age and the following epochs of the Early Permian, evolution on

  land was producing a spectacular show of its own. In water and on

  land, walking tall has very frequently served as an effective sexual

  advertisement. Several kinds of modern lizards grow exaggerated

  spines from the backbone to provide themselves with a dominant

  profile. The dorsal crests displayed by the Jesus lizards of Mexico,

  for example, are impressive indeed. But none of this display can

  compare with that of the long-spined clans that began in the Coal

  Age. And most dramatic of the Permian finbacks was the predator

  Dimetrodon, a genus that included species up to seven feet long.

  Dimetrodon means "two sizes of tooth"—its razor-sharp dentures

  varied from large fangs in front to short molars behind. Its jaws

  were designed like Tyrannosaurus's (long before that creature saw

  the light of day) and its lethal combination of jaws and teeth made

  it the king of the Permian deltas.

  However, Dimetrodon's formidable head was not its chief

  characteristic. Far more impressive was the unusually long row of

  spines rising from its neck, torso, and hips. Complete Dimetrodon