Robert T Bakker

by The Dinosaur Heresies (pdf)

  rare. Simpson's index for this time falls to 1.4, a low score. Some-

  thing was happening—new species were not evolving adaptations

  fast enough to permit them to take a more even share of the eco-

  system. And similar low evenness continued through the next for-

  mation, the Edmonton—Hell Creek, for which Simpson's index is

  only 1.3. One genus, Triceratops, made up 70 to 80 percent of the

  finds of large dinosaurs. This unbalanced situation endured for two

  million years before the final extinction.

  I first became aware of the pattern of evenness in the evolu-

  tion of the dinosaurs when I was a graduate student. It was clear

  that, even if a cosmic collision had killed off the very last dino-

  saurs, all the dynasties had already been badly weakened from some

  other cause. I turned to checking the other great extinctions to

  determine whether there had been a disturbance of evenness be-

  fore the final collapse on those occasions as well. I invested three

  years collecting evidence from the museums of Africa, Europe, and

  the United States. After counting and measuring thousands of skulls

  THE TWILIGHT OF THE DINOSAURS

  437

  Judith River dinosaurs enjoyed high evenness—no one species dominated.

  But Lance faunas were unhealthily uneven— Triceratops made up 80 percent

  or so of all the big dinosaur sample.

  and skeletons, the killing agents' mode of operating came into sharp

  focus. Every well-recorded mass extinction fit the pattern of the

  Late Cretaceous. Long before each final extinction, a decay in

  evenness had occurred. The saber-toothed gorgons of the Tartar-

  ian Epoch are a perfect case in point. Their ecosystem precisely

  displayed the typical three stages: a time of faunal richness; a sub-

  sequent decay of evenness; and the final collapse, when the gor-

  gons disappeared entirely.

  The historical pattern followed by mass extinctions simply does

  not support the theory of a Death Star's killing off faunas sud-

  denly, within a few years' time. What, then, does the iridium layer

  mean? I am not certain. Sediment-depositing processes had slowed

  down at the very end of the Cretaceous when the iridium-rich layer

  was laid down. Some geologists have therefore suggested that ter-

  restrial volcanoes might have produced the iridium, which became

  438 I DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY

  concentrated because it was mixed with unusually small amounts

  of sediment. On the other hand, Erie Kauffman, a Boulder pa-

  leontologist specializing in the Cretaceous, believes a celestial body

  did strike the earth. But he views the crash as the coup de grace

  to a dying ecosystem already suffering from massive problems.

  According to this hypothesis, the dinosaurs were diminishing long

  before the great collision, but the final celestial blow put the fin-

  ishing touch to the moribund system.

  In any event, history proves celestial collisions cannot be the

  chief culprits in the collapse of ecosystems. At best they are ac-

  cessories. But that leaves the more important question unan-

  swered. What is it that attacks the evenness of an ecosystem? For

  an answer, I am surprised at how little attention is paid to the old,

  well-thought-out theory of the shallow seas. The best answer for

  the extinction of the great sea animals is that their favorite haunts

  disappeared when the warm, shallow seas drained off the conti-

  nents. And the best answer for the extinction of the open-water,

  deep-sea creatures is that surface water becomes colder and more

  thoroughly mixed with deep water when the shallow seas drain off.

  These changes in the ocean are well documented. There is abso-

  lutely no need for an extraterrestrial hypothesis for those extinc-

  tions when there is a perfectly good explanation on earth.

  The well-established drain-mix-and-cool theory of extinction

  for the ocean, however, leaves it hard to determine how those dis-

  turbances could affect large, active land animals like the Tartarian

  gorgons or the Cretaceous dinosaurs. Climates do cool a bit on

  land when shallow seas drain off, because shallow bodies of water

  act as thermal buffers. But large, active animals are usually more

  resistant to cold than smaller ones; yet the mass extinctions struck

  at them harder. There must therefore be something more than a

  cooling trend of the temperature contributing to killing off the

  vigorous giants.

  The probable culprit was a natural agent so ordinary and

  earthbound it seems totally devoid of glamour compared to the

  hypotheses of death-dealing cosmic collisions. And that culprit was

  clearly described as long ago as 1925 by the great paleontologist

  Henry Fairfield Osborn.

  Let us observe the historical sequence that unfolds on the land

  during the mass extinctions. Shallow seas drain off, so that land

  THE TWILIGHT OF THE DINOSAURS | 439

  Evolutionary good times—when the seas spread over the continents. Shallow

  water from the oceans covered huge areas of the continents during the

  middle of the Cretaceous Period (inundated area shown here in black). All

  this water made the climate warm and humid on the lowland landscapes along

  the shores, but when the seas drained off the continents, the winters got

  cooler and the summers got drier.

  areas once underwater become dry and regions that had been sep-

  arated from each other become connected by land bridges or is-

  land chains. At the same time, mountain-building forces weaken

  so that there are fewer barriers dividing the terrestrial regions. Such

  changes of course require millions of years. But the net result is a

  more homogenized ecosystem where species can pass more easily

  from one end of a continent to another, and from one continent

  to another. Such easy intercontinental exchange can be found pre-

  cisely at the end of the Cretaceous. Until late in the Cretaceous,

  Mongolia had supported a quite different fauna from that of North

  America. There were many advanced mammals and protoceratop-

  sid dinosaurs in the Central Asiatic Highlands not found in Al-

  berta, Montana, and Wyoming. But very late in the last epoch of

  440 | DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY

  How Asia exports its species to the U.S. All through the history of life,

  Eurasia has exported hordes of species across the Bering Strait whenever sea

  level was low enough and Arctic climate mild enough. Seventy million years

  ago, protoceratopsid dinosaurs and some early mammals (multituberculates)

  came over; twenty million years ago came big bear dogs and saber-toothed

  cats; and a hundred thousand years ago came mammoths, tundra antelope

  (saiga), and pandas. And American species passed eastward in the opposite

  direction at the same time.

  the Cretaceous Period, the Asiatic mammals and dinosaurs began

  appearing in North America. These immigrants could only have

  passed over the Bering Land Bridge where the northeastern tip of

  Asia met Alaska.

  Other well-studied times of crisis show the same symptom,
a

  stepped-up exchange of species across continents. Large land

  THE TWILIGHT OF THE DINOSAURS I 441

  mammals were extremely hard hit by extinctions at the beginning

  of the Eocene Epoch, a time of extraordinarily brisk exchange of

  species all across Europe, Asia, and North America. Could such

  interchanges over the continents cause extinction?

  There can be no question about the answer here. It certainly

  could. One of the unshakable tenets of animal geography is that

  the most extreme consequences are possible when foreign species

  move into a new region. Every species of reptile, bird, and mam-

  mal carries its own unique load of parasites and disease organisms.

  And many foreign organisms will find no native enemy to hold them

  in check, so they will run amok. All the worst outbreaks of dis-

  ease that have swept through mankind or its domestic stock have

  ultimately come from the introduction of foreign species. The Black

  Plague came from somewhere in Asia before it swept through Eu-

  rope. Rinderpest, an Asian cattle disease, got into Africa when Lord

  Kitchener's army used Indian cattle to haul cannon up the Nile to

  fight the Mahdi in the late 1800s. Released among the native Af-

  rican hooved stock, Rinderpest became the Black Death of the

  antelopes, massacring millions of ruminants from the Sudan to the

  Cape. Even now, after a century of attempts to control it, game

  wardens worry more about Rinderpest than any other threat to the

  continent's wildlife.

  Osborn was aware of the Rinderpest's history, and he made a

  special study of the international exchange of species. It was clear

  to him that if just one disease, from just one foreign species, could

  wreak such unprecedented havoc across a whole continent, then

  the most appalling catastrophes could occur when entire faunas—

  scores of species from previously separated regions—mixed to-

  gether. Disaster would be inevitable. As long as species remain in

  one biological region, they adapt to their predators, competitors,

  and parasites. This is often referred to as the law of co-evolution.

  On a large time scale, co-evolution over millions of years will usu-

  ally allow an entire ecosystem to adjust itself in literally innumer-

  able ways. But when two continents mix their faunas, each group

  will be challenged by enemies for which there has been no co-

  evolutionary preparation.

  Germs and bacteria are not the only death-dealing tourists.

  Larger animals can function in the same fashion. A well-inten-

  tioned New Yorker imported European starlings into Central Park

  442 | DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY

  a while ago to brighten Manhattan with the birds of English liter-

  ature. Starlings are not pests in their native Old World habitats.

  But here in North America they are spreading like feathered lo-

  custs; no native species can stop them. Rabbits are minor nui-

  sances in England. But released into Australia, which had evolved

  its fauna separately from Europe throughout the Age of Mam-

  mals, they exploded unchecked across the land. During most of

  the Age of Mammals, South America was an island continent. South

  American mammals and birds evolved into all manner of species

  found nowhere else (giant ground sloths twenty feet tall, saber-

  toothed pouched mammals, flightless killer birds larger than a lion).

  North American mammals crossed into South America only two

  million years ago when the isthmus formed at Panama. Among the

  immigrants were representatives of the elephants, jaguars, deer,

  tapirs, and wolves, to name only a few. These North American

  immigrants devastated the native fauna. Most of the big South

  American species went extinct, victims of predation and competi-

  tion from the northerners, as well as of their diseases.

  The Late Cretaceous world contained all the prerequisites for

  this kind of disaster. The shallow oceans drained off and a series

  of extinctions ran through the saltwater world. A monumental im-

  migration of Asian dinosaurs streamed into North America, while

  an equally grand migration of North American fauna moved into

  Asia. In every region touched by this global intermixture, disas-

  ters large and small would occur. A foreign predator might sud-

  denly thrive unchecked, slaughtering virtually defenseless prey as

  its populations multiplied beyond anything possible in its home

  habitat. But then the predator might suddenly disappear, victim of

  a disease for which it had no immunity. As species intermixed from

  all corners of the globe, the result could only have been global

  biogeographical chaos.

  Such a scenario is hardly hypothetical, and it hardly requires

  an extraterrestrial hypothesis. Global disaster was simply the in-

  evitable result of unleashing pests and pestilence on natives and

  foreigners alike. The worst effects would fall on the most widely

  traveled. Large land animals crossed geographical barriers easily,

  so they spread more havoc and suffered more. Small species can-

  not migrate as easily, because even a small river can block their

  progress. Therefore extinction caused by faunal mixing would al-

  THE TWILIGHT OF THE DINOSAURS I 443

  ways be hardest on the biggest, most active animal—exactly fitting

  the picture for all the great extinctions in geological history.

  How would warm-bloodedness fit into this explanation? Would

  dinosaurs have been more at risk if they possessed high metabo-

  lism than if they had been merely good reptiles? Again the answer

  is certainly yes. Cold-blooded creatures with their very low me-

  tabolism do not travel well. The relatively small amount of their

  energy confines them to small home territories and very slow rates

  of geographical expansion. Only big energy spenders require large

  territories and constantly push at their geographical limits. The

  fastest-spreading land vertebrates should also be the largest, most

  metabolically active. And warm-bloodedness adds a further vul-

  nerability as well. The most effective way to nurture a large crop

  of germs is to keep them constantly warm. A rattlesnake in the

  desert discourages such incubation because its temperature fluc-

  tuates from near freezing to 90°F within the space of a single day.

  But tissue kept warm by high metabolism would be ideal from a

  parasite's point of view. Today, animals with high, constant body

  temperatures (mammals and birds) have a much longer list of dis-

  eases carried than do reptiles and amphibians. Dinosaurs with high

  metabolism would have been at much greater risk of mass extinc-

  tion during intercontinental exchange than would the giant, low-

  metabolism reptiles.

  Such a scenario explains all the details of the mass extinction

  without resorting to extraterrestrial agents of any kind, the mun-

  daneness of Osborn's theory is stunning to those fascinated by the

  hypothesis of a celestial collision. Perhaps a meteor or a large as-

  teroid did strike the last populations of the dinosaurs. Maybe there


  is a place for an occasional bolt out of the heavens to kill off the

  remnants of a weakened ecosystem. But the overwhelming share

  of the credit (or blame) for the grand rhythm of extinction and

  reflowering of species on land and in the sea must surely go to the

  earth's own pulse and its natural biogeographical consequences.

  444 | DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY

  22

  DINOSAURS HAVE CLASS

  A public lecture delivered on March 10, 1969, at Yale's Pea-

  body Museum stated authoritatively the advantages we en-

  joy today over old-fashioned paleontologists. They believed the

  Dinosauria constituted a natural group, but we knew they were a

  miscellany of unrelated reptiles. As such, they did not deserve the

  recognition of a formal zoological label. Only natural groups—

  species descended from one common ancestor—merited such a la-

  bel. The term "Dinosauria" ought therefore to be expunged from

  the lexicon, banished from our speech.

  I feel especially bad about that lecture because it was part of

  a program given to teachers of high school science—but most of

  all, because I wrote it. At the time, I was serving as Docent in

  charge of Special Programs. It was my responsibility to ensure that

  each lecture contained only the most up-to-date material. Since how

  you defined a dinosaur was one of the most frequently asked

  questions, I made sure all the lectures reported the most modern

  theory: Dinosaurs were an unnatural group.

  Ever since Darwin, most zoologists have insisted that "real"

  groups must have phylogenetic integrity, a unity of descent. To

  qualify as real and natural, a group would have to prove that all of

  its species traced their evolution back to one common ancestor. If

  that could be done, the group of species was entitled to a formal

  zoological label.

  DINOSAURS HAVE CLASS

  445

  The fanged beaked dinosaur, Heterodontosaurus, about three feet long, from

  the Early Jurassic of South Africa. Heterodontosaurs had a twist-thumb claw,

  built like that of anchisaurs.

  As science improved at reconstructing evolutionary trees dur-

  ing the 1880s and 1890s, under those criteria many zoological

  groups were stripped of their labels. The Pachydermata are an ex-