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-
Robert T Bakker Page 45