Robert T Bakker

by The Dinosaur Heresies (pdf)

  skeletons are rare, but there is some suggestion that one gender's

  (presumed to be female) spines were shorter than the other's. Even

  at its shortest, however, Dimetrodon's spiny back-sail made a splen-

  did spectacle as it strutted slowly, puffed itself up, and displayed

  itself broadside to potential mates and sexual rivals. Dimetrodon

  couldn't waggle its crest, because each supporting spine was rig-

  idly anchored to a vertebra. But the skin stretching between those

  spines might well have contained some message indicated by the

  pattern of scales. Maybe, like many amphibians and reptiles today,

  Dimetrodon's scales changed color during the mating season.

  Dimetrodon s> spines generated a hundred years of debate within

  the paleontological community—a great deal of it unnecessary, in

  my opinion. The learned Edward D. Cope of Philadelphia made a

  tongue-in-cheek suggestion: the fin on Dimetrodon's back was a sail

  to allow it to scud across Permian ponds like a scale-covered rac-

  ing yacht. Al Romer, who spent a lifetime studying Dimetrodon and

  related clans, believed the sail worked well as a radiator. In the

  SEX AND INTIMIDATION: THE BODY LANGUAGE OF DINOSAURS I 333

  morning, when the creature was cold from the night air, its sail

  would be turned toward the sun to soak up the warming rays. When

  the noonday Permian sun became too hot, and Dimetrodon was in

  danger of overheating, the sail could be turned into the breeze for

  a cooling effect. Two quantitative paleontologists developed ele-

  gant mathematical models to show how blood could flow to and

  from the skin of Dimetrodon s sail to provide both solar heating

  and wind cooling.

  This heating-cooling hypothesis is widely accepted, but it has

  weaknesses. The chief problem is that Dimetrodon had a close rel-

  ative, Spbenacodon, that didn't have a dorsal sail. Sphenacodon was

  identical to Dimetrodon in all the details of its anatomy. Only the

  spines of its back differed. Sphenacodon's spines were only very

  slightly elongated. If we accept the heating-cooling theory, it would

  have to be concluded that Sphenacodon was very different from Di-

  metrodon in its thermoregulatory adaptation. That implies a most

  unusual evolutionary development. In today's ecosystem, closely

  related species usually exhibit far greater differences in their

  courtship behavior than in the way they use heat. In other words,

  evolution usually works faster in changing display behavior than

  in changing thermoregulation. Several genera of lizards alive to-

  day include some species with backbone crests and others with no

  elongation of the spines. Except for the spines, these clusters of

  closely related species are adaptively very similar. On balance,

  therefore, it's far more reasonable to interpret Dimetrodon's sail as

  a display for sex and intimidation. It might indeed have been a

  radiator—anything sticking out from the body might be. But the

  overall pattern of evolution implies that display organs evolve more

  rapidly into grotesque shapes than do such utilitarian devices as

  radiators.

  At least four other early Permian creatures carried equally

  extraordinary display sails on their backs. A distant relative of

  Dimetrodon was Edaphosaurus ("earth lizard"), a small-headed,

  barrel-bellied reptile, up to eight feet long, that preferred swampy

  habitats. Edaphosaurus evolved its fin totally independently of

  Dimetrodon and even featured extra ornamental devices—knobby

  crosspieces sticking out sideways from the long spines. Edapho-

  saurus itself had a close relative with simple spines.

  Permian amphibians were hardly upstaged by the skeletal

  334 | THE WARM-BLOODED METRONOME OF EVOLUTION

  Body billboards in the armadillo toad. Platyhystrix, a close relative of Cacops, strutted around the Early Permian landscape with a sexual billboard

  constructed from armor-plated vertebral spines. The three-feet-long

  Platyhystrix and its smaller kin Cacops both belonged to a very successful

  family of strong-legged amphibians that sported armor plate over their spinal

  columns, a trend that gave them the nickname "armadillo toads." (Platyhystrix

  went even further by evolving armor over some of the ribs.)

  theatrics evolved among the finback reptiles. The amphibians

  evolved an outstanding finback of their own. A strong-legged, three-

  foot-long amphibian, Platyhystrix ("flat-spine"), evolved a dorsal

  display piece every bit as baroque as the edaphosaur's. Just as with

  Dimetrodon, Platyhystrix had close relatives that hadn't evolved such

  a crest. A single quarry at Rattlesnake Canyon, Texas, has pro-

  SEX AND INTIMIDATION: THE BODY LANGUAGE OF DINOSAURS | 335

  duced specimens of Platyhystrix, Edaphosaurus, and Dimetrodon.

  What was so special about Early Permian times that they should

  have produced so many giant dorsal displays in such profusion?

  No one knows.

  The Golden Age of the finbacks is, however, a sobering dis-

  covery for scientists who believe in the principle of extreme uni-

  formitarianism. This theory insists that evolutionary processes work

  the same way at all times. But there has never been another age

  of finbacks to compete with the Early Permian. And why this ep-

  isode in the evolution of body organization should have remained

  unique is one of the great unsolved mysteries in the history of life.

  The Age of Finbacks ended suddenly about halfway through

  the Permian Period, and the Age of Head-Butting began. Proto-

  mammals (generally called mammal-like reptiles) took over the

  leading roles in the land ecosystem, and they generally did not in-

  dulge in extravagant visual displays—a curious state of affairs be-

  cause protomammals descended from some close relative of

  Dimetrodon. With them, courtship and intimidation evolved along

  very different lines. From their very beginning, mammals evolved

  hornlike growths, thickened skull roofs, or knobs and bumps to

  cover their faces. Earlier paleontologists were at a loss to explain

  these cranial excesses (of course the theory of racial decadence was

  invoked). But in recent years Herb Barghusen, a Chicago anato-

  mist, has developed a strong case for head-to-head butting matches

  during the mating season as the most likely explanation.

  Some of the head-butters had wide, flat snouts, enabling two

  males to indulge in a pre-mating shoving match. The most ex-

  traordinary protomammals were the dinocephalians, the "terrible

  heads." These animals evolved skulls that, at a distance, looked a

  lot like a bowling ball with a snout attached. All the skull bones

  around the forehead, cheeks, and eyes were enormously thick-

  ened, endowing the beast with a bony puffiness all over its face.

  There can be no doubt that such heads were designed for butting.

  Their necks entered their skulls from a right angle, so a charging

  male could lower its head and bash its opponent with the mass of

  its forehead facing forward. When two charging males collided, at

  full speed, the Late Permian air must have resounded with lo
ud,

  clunking crashes.

  Dome-headed protomammals raise an interesting question

  336 I THE WARM-BLOODED METRONOME OF EVOLUTION

  concerning the evolutionary connection between sex and warm-

  bloodedness. Recently, popular health and diet magazines have

  discovered what physiologists have known for a century—sex can

  be a strenuous exercise and often an important way to burn up

  calories. Sexual practices embrace not only the physical act of cop-

  ulation, but all the pre-mating ritual, strutting, dancing, brawling,

  and the rest of it. They can consume enormous amounts of en-

  ergy. Successful bull elephant seals are tattered, scarred, and ex-

  hausted after the long mating season of wrestling matches against

  rivals. A male moose may lose weight during the rut, because of

  the exertions expended when running into other males. Mammals

  can afford to squander vast amounts of energy during courtship

  and mating since their warm-blooded system produces a huge

  amount of energy. In comparison, a totally "cold-blooded" animal

  produces a tiny amount of energy and therefore cannot expend as

  much effort in sexual athletics. When a five-hundred-pound moose

  spends 50 percent of its total energy in mating contests, the total

  calories burned are ten times greater than those burned when a

  five-hundred-pound tortoise uses 50 percent of its energy for sex,

  because the tortoise starts out with much less.

  Protomammal head-butters of the Late Permian: the one-thousand-pound

  Tapinocephalus

  Knobby-snouted protomammal of the Late Permian. Aulacephalodon was a

  plant-eating two-tusker of the Tartarian Epoch and may have had a warm-

  blooded pre-mating style, complete with vigorous competitive butting and

  pushing. It was about four feet long and two hundred pounds adult size.

  Judging from a consideration of the evolution of available en-

  ergy, the contrast between the Age of Finbacks and the Age of

  Head-Butting is intriguing. The amphibians and primitive reptiles

  of the Early Permian adopted a low-energy approach to sex and

  display; the tall sails on their backs were visual signals that really

  didn't require violent body language to be effective. Head-butting

  was something else again. The robust construction of its head and

  neck testifies to the vigor of the protomammal's physical effort.

  There may be an important clue here to a major increase in avail-

  able energy that had occurred during the evolution of the first

  protomammals. Quite possibly they expended much larger totals

  338 I THE WARM-BLOODED METRONOME OF EVOLUTION

  of calories than do typical reptiles today. As we'll see in a subse-

  quent chapter, there is excellent evidence from other sources that

  suggests the Late Permian head-butters were indeed the first warm-

  blooded animals ever to evolve. Whatever their metabolic rate, in

  any case, it must be recognized that the Late Permian head-butters

  were certainly the first land vertebrates to escalate sexual gymnas-

  tics into a high energy level.

  Protomammals continued to butt their heads until the end of

  the Triassic Period, when dinosaurs took over the roles of large

  herbivore and carnivore on land. The dinosaurs' approaches to sex

  and intimidation ran the entire gamut from elaborate dorsal dis-

  plays to head-butting and perisexual symphonies. Largest of the

  dinosaurs resorting to display was the appropriately named Spino-

  saurus, the "spine lizard," a forty-foot predator probably related

  distantly to Allosaurus. All specimens of Spinosaurus are frustrat-

  ingly fragmentary, but it's clear a tall sail decorated its back, rising

  six to eight feet above the backbone. A strutting Spinosaurus must

  have been a singular sight—striding on its long hind legs, its head

  twenty feet above the ground, turning broadside to dare its rival

  to test its potency. Sex also probably explains the tails of duckbill

  dinosaurs. Those tails were very deep from top to bottom and well

  suited for conveying messages. Some duckbills even evolved true

  sails constructed from vertebral spines over the base of their tail.

  Torso and tail were not the only sites of sexual adornment.

  The Early Jurassic carnivore Dilophosaurus evolved a striking cra-

  nial profile: two tall crests, very thin from side to side, rose from

  the edges of its skull from snout to forehead. Lower, thicker crests

  in the same location decorated the heads of the Late Jurassic Al-

  losaurus and Ceratosaurus and the tyrannosaurs of the Cretaceous.

  Dilophosaur crests were so thin that they could have been only

  for visual effect. But the bony crests of the later meat-eaters were

  heavy and covered by stout layers of horny skin. Allosaurus and its

  relatives probably butted heads during confrontations on the field

  of sexual valor. To be sure, a pair of male allosaurs, driven by their

  hormones, could have bitten each other to death. But I suspect

  such terminal contests were relatively rare. Evolution tends to fa-

  vor the sexual soldier who can win multiple contests and who

  therefore, by implication, does not run the risk of being dismem-

  bered in his first bout. Less than lethal horns would confirm this

  •

  SEX AND INTIMIDATION: THE BODY LANGUAGE OF DINOSAURS I 339

  Wagtail fintail Montanoceratops. Like

  its close relative Protoceratops,

  Montanoceratops had incredibly tall

  tail spines that made the tail a

  billboard for social messages. The

  tail was highly flexible from side to

  side and so the entire tail could be

  wiggled. Total length about four

  feet long.

  general theory. The allosaurs' ancestors possessed little in the way

  of crests or horns but they did have dangerously sharp teeth and

  quick-biting jaws. These earlier carnivores could have resorted to

  biting to settle mating contests, but they were probably restrained

  by genes that programmed for less dangerous encounters. And the

  success of genetic changes that increased the disposition toward

  head-butting among the later, larger carnivores indicates that but-

  ting, not biting, was the best strategy for maximizing success in

  mating. Big mammals show the same pattern—clans with danger-

  ous teeth often evolve nonlethal horns.

  Dinosaurs as a class must have owned large quantities of en-

  ergy to pour into the rigors of courtship and mating, because head-

  butting evolved several times in different families. Tyrannosaurus

  rex's massively thick skull edges, covered with horn, represent the

  acme of the evolutionary trend toward head-ramming among car-

  nivores. Pachycephalosaurs, thick-headed dinosaurs described in a

  previous chapter, evolved huge, bowling ball—shaped skulls very

  much like the ones carried by the dome-headed protomammals of

  the Late Permian. Some Polish scientists have suggested that head-

  ramming was too powerful to be used against sexual rivals and that

  the head-down charge at full tilt must have been employed against

  predators. However, the domes both on dome-headed p
rotomam-

  mals and on dome-headed dinosaurs really resembled bony box-

  ing gloves built atop the skull. The boxing glove, even one of bone,

  delivers a blunt, stunning blow. If evolution had really been work-

  ing to produce a deadly antipredator weapon, a spearlike point on

  the head would have been much more effective than a blunt dome.

  It seems far more feasible therefore to envisage the domes as ideal

  weapons for sexual contests without incurring the danger of esca-

  lating the match to the point where even the winner goes away

  mortally wounded. A mortally wounded winner doesn't win in the

  game of evolution because dead heroes can't mate.

  Quite the reverse, however, must hold true for the headgear

  of Triceratops and the other long-horned dinosaurs. Recently, a pair

  of American paleontologists, apparently caught up in the rush to

  reinterpret dinosaur features as organs for sexual display, sug-

  gested the horns and frill on Triceratops were not for defending

  against Tyrannosaurus but for display to other Triceratops. But the

  head ornaments of Triceratops were simply much too deadly to serve

  a sexual purpose. Those long, sharply pointed horns were for kill-

  SEX AND INTIMIDATION: THE BODY LANGUAGE OF DINOSAURS | 341

  Nonlethal butting crests of the

  meat-eating dinosaurs.

  Tyrannosaurus had thick, low

  butting ridges, Allosaurus had

  shorter, sharper crests, and

  Dilophosaurus had tall, thin snout

  crests.

  ing. The wide frill, sometimes edged with horn-covered spikes,

  served for protection against dangerous bites. The little horned

  dinosaurs, Protoceratops and its relatives, probably had started out

  designed for nonlethal jousting. Their snouts were strong, but only

  the slight suggestion of a blunt horn grew above their nostrils. Large

  suites of Protoceratops skeletons from the red Mongolian sand dunes

  indicate that males had larger heads and stronger nose horns than

  females. So springtime probably did bring thoughts of sex and

  snout-butting into the minds of the little horned dinosaurs. But

  that could no longer have been true of their larger, far more le-

  thally endowed descendants.

  By far the most spectacular devices for sex intimidation were