Robert T Bakker

by The Dinosaur Heresies (pdf)

  twenty million years—far longer than the genera of dinosaurs or

  mammals lasted in Megadynasty III and IV. Moreover, in Mega-

  dynasty I the ecological niches werf undersaturated, because new

  types did not develop quickly. On the dry floodplains there was

  only one genus of large vegetarian, Diadectes, and only the one big

  predator, Dimetrodon. Such an impoverished system appears very

  underpopulated compared to most dinosaur habitats. And unlike

  the dinosaurs, which suffered several mass extinctions, nothing

  similar ever struck the ecological community ruled by Dimetrodon.

  Genera went extinct one by one, with new genera entering ac-

  cording to a rather leisurely schedule.

  When the first protomammals appeared, Dimetrodon and the

  entire somnolent world of Megadynasty I passed away. This was

  an extraordinary upheaval, which I call "the Kazanian Revolu-

  tion"—eventually it will set the stage for the appearance of the

  dinosaurs. The Kazanian Epoch is named from the old Russian

  province of Kazan, west of the Ural Mountains. There, the most

  ancient protomammals are found in the red-stained sediment and

  THE KAZANIAN REVOLUTION: SETTING THE STAGE FOR THE DINOSAURIA | 409

  yield their bones to the careful spadework of Soviet paleontolo-

  gists. The Kazanian families burst into the evolutionary drama with

  unprecedented energy. They started out as slender-limbed, wolf-

  sized predators, and rapidly expanded their empire into nearly every

  role in the ecology, sweeping away the ancien cold-blooded regime.

  Within a few million years Kazanian protomammals had taken over

  all the carnivorous roles—large, medium, and small—nearly all the

  herbivorous roles, and produced dozens of small, insect-eating

  species as well. Never before had the ecosystem witnessed such a

  spectacular proliferation of new species from a single family. The

  Kazanian Revolution was the first terrestrial example of explosive

  "adaptive radiation."

  Right from the beginning, the Kazanian protomammals stuffed

  whole clusters of species into each role. The best preserved of these

  are found in the red beds of the South African Karoo and display

  a richness far greater than anything recorded before. Protomam-

  mals produced four different families of predators, with eight or

  ten different species, to prowl through the floodplains and forests

  of the Karoo. Biggest of them were the dome-headed anteosaurs,

  the size of polar bears, with thick, bony buttresses over their eyes

  for head-butting in the mating season. Anteosaurs were armed with

  a great row of long teeth that meshed together to clamp down on

  prey. Predators from other families, the size of wolves and jag-

  uars, displayed a wide variety of lethal devices for dealing with their

  prey. Plant-eaters were numerous, too. Five families and a score

  of species munched their way through the greenery of the ancient

  Karoo.

  Why did these Kazanian protomammals evolve so quickly?

  They produced new species at very heated rates, and adapted them

  very speedily, so that most lasted only a few million years before

  they were replaced. The evolutionary tempo of the Kazanian ap-

  pears as fast as that of our own Class Mammalia during the Age

  of Mammals. Yet they suffer from the same bias maintained against

  the dinosaurs. They included some large species—up to one or two

  tons—but paleontologists dismiss those as behemoths of low

  metabolism that had to use their bulk to keep them warm. As a

  student at Harvard, I became interested in these Kazanian proto-

  mammals because they displayed the same evolutionary vigor I

  discerned in the dinosaurs. And I began to suspect that both they

  and the dinosaurs had been warm-blooded.

  410 | DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY

  First explosion of the warm-bloods! Before the Kazanian Epoch, the

  evolutionary style was slow and spaced out. Species lasted for many millions

  of years and there was only one common large predator family and only one

  common large plant-eating family in most habitats on the land ecosystem. But

  suddenly the Kazanian protomammals burst upon the evolutionary stage and

  in a few million years branched out into five separate meat-eating families

  and four separate plant-eating families. Habitats were filled to overflowing

  with fast-evolving species. And soon after this great evolutionary boom, there

  was the first gigantic crash—a mass extinction that wiped out most of the

  Kazanians. A few protomammals survived, and a new evolutionary bloom

  followed—the Tartarian radiation. (Each head portrait represents one family

  in this chart.)

  A Kazanian scene: Trochosaurus, a protomammal, attacks an herbivorous

  pareiasaur.

  My desire to study the Kazanians firsthand took me to Cape

  Town, South Africa, where 90 percent of the specimens are housed.

  My hypothesis was that if the Kazanians had been the first warm-

  blooded animals, then they would have required a great deal of

  food per year, and therefore the predators would be rare. Large

  samples of fossils are needed to prove such an hypothesis. And

  fortunately for me, one man, Liewe Dirk Boonstra, had poured an

  entire lifetime into excavating the Karoo and carefully sorting the

  species. Almost singlehandedly, he had built a detailed picture of

  the Kazanian world. His storehouse of fossils left little room for

  doubt: Although the Kazanian predators had been diverse and fast-

  evolving, they had been rare compared to the plant-eaters, very

  412 | DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY

  rare. The total body weight of all the Kazanian predators repre-

  sented only 7 percent of the total for the entire fauna. This was a

  predator-to-prey ratio as low as in some mammal faunas, strong

  support for the case for warm-bloodedness.

  I did not wish to rely on that argument alone, so I sought cor-

  roboration from a totally separate line of evidence—the microtex-

  ture of bone. At the very same time I was examining the

  protomammals in Boonstra's collection, Armand de Ricqles was

  cutting bone samples from them in Paris. Each of us was doing his

  work unknown to the other, but both of us suspected something

  special was to be learned from the Kazanians. De Ricqles pub-

  lished his results in Annates de Paleontologie; I published mine in

  Scientific American. Our separate lines of detective work con-

  verged on the same conclusion: The Kazanians had been a new

  phenomenon in the history of life, the first vertebrates whose bone

  microtexture indicated fast growth, the first ecosystem whose

  predators were rare, the first with a warmed-up metabolism.

  Consistent with a warm-blooded metabolism, the Kazanian

  therapsids would have developed a more sophisticated design in

  the mechanics of their limbs. The older style limbs of the Coal

  Age wouldn't have been adequate for warmed-up metabolic needs.

  If de Ricqles and I were correct about Kazanian metabolism, ma-

  jor adaptive remodeling should have been manifest in limb joints,r />
  adaptions for faster speeds. On this point, Boonstra once again

  supplied most of the preliminary material; his excavations had re-

  covered literally dozens of good skeletons, and he had published

  precise diagrams of every limb from shoulder to wrist, hip to an-

  kle. It turned out that both the shoulder and hip sockets of the

  Kazanians were much deeper than any found in the Coal Age. They

  had obviously been built to withstand much more powerful pres-

  sures from the muscles of the limbs. The knee joints indicated the

  Kazanians were designed for fast, bouncy gaits—the crests for

  supporting the extensive muscles of the knee were massively de-

  veloped. When those muscles contracted on a one-ton protomam-

  mal of the Kazanian, the great beast would surely have bounded

  forward into a lively run. Clearly, the shuffling age of the Carbon-

  iferous was over; the Age of Trots had begun.

  All the pieces of the Kazanian puzzle seemed to fall into place

  with unusual ease. Warm-blooded metabolism and fast-growing rates

  THE KAZANIAN REVOLUTION: SETTING THE STAGE FOR THE DINOSAURIA | 413

  were consistent with the overall picture of fast evolutionary rates.

  They were also corroborated by the remodeling of limbs for faster

  moving speeds. And finally, they were consistent with the new

  sexual vigor implied by the head-butting armament commonly

  found. All of these things implied the Kazanian protomammals had

  extra energy to burn.

  In Kazanian times, the Karoo experienced cool winters be-

  cause it was closer to the South Pole than it is now. To remain

  active all year round, the smaller animals might well have needed

  some kind of insulation. Orthodox paleontology insists that hair is

  a uniquely mammalian invention, the adaptive badge of our own

  Class Mammalia. Several dissenters have however suggested that

  perhaps hair evolved long before the first true mammal appeared

  in the Late Triassic. Perhaps hair did evolve at the very beginning

  of warm-bloodedness, back in the Kazanian, fully forty million years

  before the earliest true mammals. Maybe our picture of the Ka-

  zanians should include shaggy protomammals stalking their prey

  through a winter snowstorm, hot breath steaming from their nos-

  trils.

  A last, quite important, piece of the evolutionary puzzle also

  falls easily into place concerning the Kazanian: mass extinction.

  Warm-blooded protomammals would of course have been vulner-

  able to catastrophic die-offs. And exactly such a disaster did cut

  most of them down. After a reign of five to ten million years, nearly

  all of them went extinct, leaving only a few surviving groups. This

  disaster was the earliest truly mass extinction in the history of land

  ecosystems. The Kazanian protomammals paid the price of their

  warmed-up metabolism.

  In the badlands of the Karoo Basin and the outcrops north-

  west of the Urals, the end of the Kazanian and the beginning of

  the Tartarian Epoch are clearly marked. At the dawn of the Tar-

  tarian, the surviving protomammals rebounded into ecological

  dominance, exploding into a riot of new species, genera, and fam-

  ilies. Dicynodonts ("two-tuskers") replaced the extinct domeheads

  as the big plant-eaters. Saber-toothed gorgons replaced anteosaurs

  as giant meat-eaters. This second wave of protomammals pro-

  duced a very rich array of species, just as the first had. And they

  evolved quickly.

  Were the Tartarians also warm-blooded? Almost certainly. De

  414 | DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY

  Ricqles found a mammal-type bone texture in all of the Tartarian

  protomammals. And I found very low predator quantities. Sam-

  ples from both the Karoo and from Russia revealed that predators

  made up only 5 to 12 percent of the total preserved fauna.

  The high metabolism of the Tartarian protomammals is also

  attested by the sudden collapse of their system. Their dominance

  lasted only a few million years before it crashed into a mass die-

  off corresponding with the very end of the Permian Period.

  During the next epoch, the Scythian, yet another wave of proto-

  mammals evolved to replace the vanished Tartarians. This third

  wave also exhibited all the signs of warm-bloodedness. Appar-

  ently, once it had evolved, the genie was out of the bottle, never

  again to be imprisoned. After the first takeover by warm-blooded

  families in the Kazanian, each new wave of animals of Megady-

  nasty II displayed all the marks of high metabolism—fast-growth bone

  structure, ecosystems where predators were rare, and vulnerabil-

  ity to mass extinction. And the most important thing to remember

  World geography in the

  Kazanian Epoch. All the

  continents were jammed

  together in one mass, and

  the climate in the far north

  and far south had cold

  winters. The Karoo

  protomammals must have

  had some sort of warm-

  blooded adaptation to cope

  with the weather.

  THE KAZANIAN REVOLUTION: SETTING THE STAGE FOR THE DINOSAURIA | 415

  about those three waves of protomammals is that they existed long

  before the appearance of the first dinosaur.

  The disaster that wiped out most of the Tartarian groups at

  the end of the Permian Period was also an opportunity for new

  families to expand their ecological roles. One such group con-

  sisted of protomammals that survived from the Tartarian. Several

  dicynodonts survived to produce a new adaptive radiation in the

  Scythian and flourished tremendously right down through the rest

  of the Triassic Period. Two new groups entered the role of large

  predator and herbivore. One of these was the cynodonts, with their

  doglike faces. This group had been limited to a small body size in

  Tartarian times. But extinction of all the Tartarian top predators

  took the lid off their evolution. The Scythian cynodonts were able

  to evolve into wolf-sized predators. These cynodonts were the

  protomammals closest to the heart of Harvard's Al Romer be-

  cause they included the direct ancestors of all true mammals, from

  platypuses and 'possums to monkeys, apes, and ourselves.

  The other group that took advantage of the Tartarian extinc-

  tions to expand their roles during the Scythian were the Archo-

  sauria, the group that would evolve crocodiles, pterodactyls, and

  dinosaurs. The earliest archosaurs of the Scythian Epoch were those

  big predators the "crimson crocodiles" (Family Erythrosuchidae),

  named after the red stain on the bones of the first specimens dis-

  covered. All through the Triassic, from the Scythian till the close

  of the Period, a titanic ecological battle was waged between the

  advanced protomammals, led by the dicynodonts and cynodonts

  and the Erythrosuchidae and their descendants. On the outcome

  of this conflict balanced this history of the Mesozoic Era. Had the

  protomammals won, they and their descendants would have dom-

  inated the ecosystem during the Jurassic and the Cretaceous. If the

>   crimson crocodiles and their descendants won, a totally new evo-

  lutionary line would gain control.

  If the clash of mighty empires is your favorite historical fare,

  the Triassic is irresistible. Two mighty evolutionary dynasties col-

  lided in direct competition: the advanced protomammals against

  the early Archosauria. At first, the protomammals appeared to re-

  capture most of their lost glory. The two-tuskers regained their

  dominant position as the big herbivore. And they evolved very

  advanced limb muscles, arranged nearly exactly like those of prim-

  itive mammals. Mammal loyalists can be proud of these Triassic

  416 I DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY

  Archosaurs displaced the protomammal dynasties. As the Triassic wore on,

  more and more archosaur families—shown here in black—invaded the

  ecological roles of large predator. By Late Triassic times, all the large

  predator roles were taken over and the archosaurs had begun to invade the

  herbivore guilds. And finally, in Early Jurassic times, the dinosaurs secured

  complete control of both plant-eater and meat-eater components of the land

  vertebrate system.

  A

  two-tuskers because they demonstrate how mammalian design can

  win great ecological success. They can be even prouder of the cy-

  nodonts, for these animals ascended the evolutionary Scala Na-

  turae even more rapidly. Unlike the rather tubby two-tuskers, the

  predatory cynodonts evolved sleek profiles, elongated bodies, and

  slender limbs designed for rapid movement. They also produced

  the most advanced, most mammal-like faces, teeth, and jaws. When

  a Triassic cynodont snarled, it bared teeth that strongly resembled

  a wolf's—the large canines were located far forward in the doglike

  snout, the mouth front was lined with short nipping teeth (inci-

  sors), and behind the canines ran a long row of teeth with multiple

  cusps for slicing and chewing. The muscles of their jaws were bio-

  mechanical marvels. The two muscles involved pulled across each

  other, an arrangement allowing the cynodonts to bite hard with-

  out placing excessive strain on the joint of the jaw bones. We hu-

  mans today enjoy the advantages of such a jaw joint inherited from

  our cynodont ancestors of the Triassic. Our outer jaw muscles (the

  masseters) pull upward and forward while our upper set of mus-