until a new species suddenly appeared.
Most of the anatomical fieldwork for my study had already
been done. American paleontologists have published superbly il-
lustrated studies of the Como dinosaurs, comparing specimens from
the different quarries with loving attention to every detail of the
bones and muscle processes that covered them. My task was chiefly
to fill a few gaps in the data—about where the old quarries had
been dug, in what strata of the rock, and in what sort of sediment.
The Sheep Creek Brontosaurus stands in the museum at the
University of Wyoming. It is a splendid skeleton from the lake
limestones close to the very bottom of the Morrison Beds. I sur-
veyed every square inch of it for my notes so as to compare it
with Yale's Brontosaurus from a quarry high up on Como Bluff,
and with the New York Museum of Natural History's Brontosau-
rus from Nine-Mile-Crossing, a quarry in an in-between layer. My
final notes contained a record of Brontosaurus through hundreds
of thousands of breeding generations, spanning many major envi-
ronmental shifts and climatic changes. Therein was contained
absolutely no evidence for continuous evolutionary change. Bron-
tosaurus had remained fixed in its adaptation through a million
years.
Not only did Brontosaurus remain static in form for a very long
time, but when it did change, it seemed to jump forward with a
quick evolutionary spurt. Adult brontosaurs found at Como had
thigh bones of about six feet (1,750 millimeters) maximum length,
indicating a total length of body of just under seventy feet. The
Sheep Creek Brontosaurus was that size, as were the one at Yale,
PUNCTUATED EQUILIBRIUM: THE EVOLUTIONARY TIMEKEEPER | 399
How new species of brontosaur appear suddenly in rock layers (measurements in feet and meters show
the rock level in the Morrison Formation)
the one in New York, and many others. No change in size was
indicated in the entire million-year span recorded at Como. But
in Colorado, in beds laid down a bit later than those at the top of
Como Bluff, much larger brontosaurs are found: gigantic speci-
mens, with thigh bones about six and one-half feet (2,000 milli-
meters) long. If the story of the rocks is read literally, Brontosaurus's
development was punctuated. Long epochs had passed without
change, followed by the sudden appearance of a new, larger spe-
cies.
Brontosaurs were not the only dinosaurs from Como to sup-
port the concept of punctuated equilibrium. Allosaurus, the con-
temporary predator, remained fixed at one adult size—a thigh about
three feet (850 millimeters) long—through the entire span of strata
from the lower Morrison Beds right up to near the top of the for-
mation on the Bluff. But in Colorado, in those same beds that yield
giant brontosaurs, are also found giant allosaurs, with thighs nearly
1,200 millimeters long. Just as was the case with the brontosaurs,
Allosaurus had stayed fixed in equilibrium throughout the million
years recorded at Como, and then suddenly changed, producing a
new, much larger species when the Colorado beds were laid down.
Another brontosaur, Camarasaurus, seems to have followed the
same pattern.
The discovery that punctuated equilibrium was a valid con-
ception of how evolution works, and that it therefore applies to
the history of dinosaurs, generates some intriguing new methods
of measuring the pace of evolution. Since the original theory was
suggested, Eldredge and Gould, its proponents, and Steve Stanley
at Johns Hopkins, have concentrated on finding out how different
families of animals display their own particular rate of evolution-
ary change. As a general rule, most species change very little from
the time they first appear until their final extinction. But the total
length of time each species exists varies a great deal. Stanley points
out that some clams are stubborn evolutionary sluggards that seem
to abhor alteration excepting on rare occasions. Most types of clams
go on for many millions of years with hardly any adaptive shifts.
Mammals are quite the reverse. Their species appear almost fre-
netic in their haste to evolve into something new. The average
mammalian species therefore lasts nowhere near as long as the av-
erage species of clam.
PUNCTUATED EQUILIBRIUM: THE EVOLUTIONARY TIMEKEEPER I 401
A very well-preserved segment of fossil history, like the one
at Como, permits a computation of how long the average species
and genus of dinosaur lasted. And that can be compared with the
rates of change computed for warm-blooded animals on the one
hand and cold-blooded on the other. Rates of evolutionary change
must somehow be linked to the metabolic rate of any given orga-
nism. When metabolism is very high, the high physical costs of its
living must drive the animal to be an aggressive competitor or
predator. And the rapid reproduction typical of warm-blooded an-
imals tends to fill habitats to overcrowded levels much more quickly
than the more leisurely breeding schedules characteristic of cold-
bloods. An ecological community full of warm-blooded species is
therefore a tough environment where the resident species jostle
one another for food and water, breeding sites and burrows, all
year round. In this sort of environment, the average species will
not last long in terms of geological time before it is driven to ex-
tinction either by a new species, or by a combination of old spe-
cies, or by an adverse change of climate.
Clams, by contrast, with their low metabolism, move around
very little to accomplish the tasks of their adult lives. It is conse-
quently not surprising that species of clams last much longer than
warm-blooded species. Crocodiles and turtles are far more active
than clams, but are still sluggish compared to the fully warm-
blooded mammals. If dinosaurs resembled cold-blooded reptiles
metabolically, their rate of evolutionary change would very nearly
match that found in crocodiles or large turtles. But if the dino-
saurs' metabolism was heated, then the average life span of one of
their species or genera would have to be short, like a mammal's.
One of the best places to study the rate of evolutionary changes
in dinosaurs is in the Late Cretaceous deltas of Wyoming, Mon-
tana, and Alberta. There the changes through the last ten million
years of the dinosaurs' history can be followed. As might be ex-
pected, the turtles and crocodiles show very little change in these
strata. The genera representing these cold-bloods hold on through
formation after formation. But what about the dinosaurs here? Their
evolutionary pace stands out as quite different. New species and
genera kept appearing and eliminating older ones at quite a brisk
tempo, geologically speaking. The average genus of dinosaur lasted
for only a fraction of the time of that of the average crocodile.
402 | DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY
Fast evolu
tionary turnover—A warm-blooded trait. Thoroughly cold-blooded
creatures—crocodiles and turtles—have a spaced-out evolutionary style, and
genera last for thirty million years or more without much change. But
dinosaur genera had a much brisker replacement rate. The horned dinosaurs
evolved so fast that the average genus lasted only five or six million years.
A faster rate of evolutionary change manifests itself in the
higher levels of classification as well. On average, three or four
species of dinosaur composed a genus, and six to twelve genera a
family. Families of cold-blooded genera are almost indestructible
because their slow rate of change implies that only very rarely will
all the genera in one family become extinct. For example, the fam-
ily that includes all modern crocodilians is the Crocodylidae, whose
debut occurred way back in the Mid Cretaceous, nearly a hundred
million years ago. The snapping turtle's family is as old, and so is
the soft-shelled turtle's. For all the cold-blooded reptiles, the av-
erage endurance of a family is fifty-five million years. But families
of dinosaurs change much more quickly. Their average life span is
only twenty-five million years, almost exactly the same as for
mammals. In sum, the pace of the dinosaurs' evolution suggested
by punctuated equilibrium is both fast and furious—much too high
to be consistent with the concept of cold-blooded dinosaurs.
Another aspect of the evolutionary pace also places the dino-
saurs in the warm-blooded category: diversification of species. Truly
warm-blooded families of genera diversified themselves quickly,
constantly splitting into new genera and new species that fill un-
occupied niches of the ecology and bump older species from those
they fill. The great American paleontologist Henry Fairfield Os-
born called this proliferation "adaptive radiation." By contrast, cold-
blooded animals are slow to increase their share of the ecosystem,
and their "adaptive radiations" (with a few exceptions) are rather
lethargic affairs.
Giant tortoises are a good illustration. They represent a fam-
ily of terrestrial plant-eaters that first evolved in Eocene times, fifty
million years ago. But tortoises were almost unbelievably slow to
diversify, and even now some zoologists would place all the spe-
cies of the last five million years into one single genus, Geochelone.
Compare such a conservative pattern with that of the duckbill di-
nosaurs. The first of them appeared fifteen million years before
the end of the Cretaceous. Within the next ten million years they
had expanded so quickly that seven distinct genera can be found
in one small outcrop of the Judith River Formation. Horned di-
nosaurs also exhibited such aggressive expansion during the same
period and produced five or six genera in the Formation. These
are rates of expansion every bit as fast as those clocked by the big
mammalian families during the Age of Mammals.
404 | DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY
One part of the orthodox story does appear to be unassail-
able, an ineradicable fact safe from even the wildest heretic: Di-
nosaurs are indeed all extinct. The fact of their extinction is the
cornerstone underlying the orthodox belief that dinosaurs were
maladapted failures. Recently, after a lengthy and intense dispu-
tation with an Old Guard paleontologist, he summarized his ar-
gument with what for him was a rhetorical question, "If your
dinosaurs were so hot, how come they're all dead?"
Dinosaurs are incontrovertibly dead. But that does not prove
what orthodoxy believes about them. Paradoxically, the extinction
of the dinosaurs is strong evidence that their biology was heated
to levels far above those of typical reptiles. The basic principle is
simple: The higher the metabolic needs of a group of species, the
more vulnerable it is to sudden and catastrophic extinction. What
is the best natural design for avoiding extinction? The answer is
animals with a lethargic metabolism, like the alligator or the large
turtle. Each individual is greatly resistant to drought or famine. And
corporately, the species is therefore resistant to extinction. An en-
tire family of lethargic species would be most difficult to kill off
all at once, worldwide. Conversely, the most effective way to de-
sign species that are almost guaranteed to die off completely is to
endow them with the highest, most compulsive need for calories
and protein. Any major perturbation of the environment might
render each and every species extinct at one blow.
The Cretaceous extinctions were the most massive in the his-
tory of the terrestrial ecosystem. But some families of species sailed
through the crisis without suffering any noticeable effects. The
survivors included reptiles and amphibians, families with low met-
abolic needs and very sluggardly evolutionary rates—gill-breath-
ing salamanders, monitor lizards, alligators, crocodiles, soft-shelled
turtles, snapping turtles, and the long-snouted champsosaurs. But
dinosaur after dinosaur became extinct until none was left. If di-
nosaurs were so hot, how come they're all dead? This question an-
swers itself. Dinosaurs went extinct because they were so hot.
PUNCTUATED EQUILIBRIUM: THE EVOLUTIONARY TIMEKEEPER I 405
20
THE KAZANIAN REVOLUTION:
SETTING THE STAGE FOR
THE DINOSAURIA
inosaurs should never be taken out of their historical con-
D text! To appreciate the adaptive dexterity of the Dinosauria,
they must be viewed in their place within the succession of evo-
lutionary dynasties. One of the greatest flaws in the orthodox con-
ception of dinosaurs is that it ignores the evolutionary patterns prior
to their appearance. The evolutionary development and destiny of
the beasts that preceded the dinosaurs are essential to understand-
ing their real place in the history of life. Such a context renders
the argument for their warm-bloodedness logical, and nearly irre-
futable.
The land ecosystem has hosted four great megadynasties
throughout the entire history of life on earth. Megadynasty I con-
sisted of the very primitive reptiles and amphibians of the Coal
Age and the Early Permian. All the dinosaurs filled Megadynasty
III, and mammals fill Megadynasty IV. Orthodoxy maintains, re-
member, that dinosaurs were cold-blooded sluggards, so the pro-
gression from Megadynasty III to IV is made to appear like a great
advance in physiological sophistication. But there's an enormous
problem with this view of the sequence. Megadynasty II, the one
preceding the dinosaurs, belonged to the protomammals, gener-
ally known as the Order Therapsida. They definitely included the
immediate ancestors of genuine mammals—and the advanced pro-
tomammals showed many signs of mammal-style adaptations in their
406 | DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY
Tartarian saber-toothed protomammals ot the Late Permian. These five-
hundred-poun
d gorgonopsians had warm-blooded evolutionary style.
body and skull. Most paleontologists therefore are willing to be-
lieve these later protomammals had already developed some de-
gree of warm-bloodedness. Now, if they were warm-blooded, it is
very strange indeed that they subsequently lost their dominant
position to the supposedly cold-blooded dinosaurs. To clarify this
question, we shall have to investigate Megadynasty II further. How
did the Therapsida rule their world? How had they replaced the
archaic creatures of Megadynasty I? And why did they yield to the
dinosaurs?
Before the emergence of Megadynasty II, the world was ruled
by primitive reptiles and their amphibian neighbors. They had such
an archaic structure that nearly every scientist who has studied this
THE KAZAN IAN REVOLUTION: SETTING THE STAGE FOR THE DINOSAURIA | 407
The megadynasties of
large land herbivores
time has come away convinced that all the large land animals were
entirely cold-blooded. The newer, heretical thinkers concur. Mega-
dynasty I's top predator was Dimetrodon. Its predator-to-prey
ratios and bone texture clearly demonstrate that it and all its con-
temporaries were slow-growing creatures equipped with low me-
tabolism. Their fossil footprints record a very slow average pace.
And the limb joints of Dimetrodon and its contemporaries were
designed only for slow walking and crawling; the hip socket was
too shallow to contain a strong thrust of the thigh from running,
and the bony crest on the shin bone was too weak to provide much
leverage for the knee-opening muscles. Taken altogether, this ear-
liest Megadynasty must have created a world in slow motion, where
the fastest gait was a lumbering waddle.
What sort of evolutionary punctuation might be expected in
such an age? If the theory that metabolism controls the tempo of
evolution contains any truth, it yields three predictions: (1) Gen-
era would survive unchanged for immense periods of time; (2) The
rate of proliferation of species would be extremely slow; (3) Sud-
den, mass extinction would not be possible. All three of these
predictions prove out. Dimetrodon, its vegetarian cousin Edapbo-
saurus, and the other chief genera of the dynasty endured for about
Robert T Bakker Page 41