the advantage of being well-enough armed to attack successfully
even the largest plant-eater. Today's lions and hyenas are not big
enough to kill healthy adult rhinoceroses, elephants, or water buf-
falo. But that is not typical of the situation during the entire Age
of Mammals. Most of the time in the past, the carnivores were
large and strong enough to assault the biggest prey; for example,
the giant wolf-bear Pliocyon found in Nebraska eight million years
ago averaged five to six hundred pounds. If it hunted in packs,
Pliocyon could have killed elephant-sized prey with ease. In gen-
EATERS AND EATEN AS THE TEST OF WARM-BLOODEDNESS | 389
eral, the typical mammalian fauna of the past included far more
formidable top predators than do any of today's ecosystems. Since
fewer plant-eaters were immune to attack, the top predators would
have been more efficient at culling them and building up the pred-
ator-to-prey ratio. On balance, these considerations suggest a so-
lution to the paradox presented by the difference between ancient
and modern ratios. Warm-blooded predators could achieve ratios
as high as 5 percent when climate was favorable, when they were
strong enough, and when there were no humans to befoul the
sample.
Would this general picture also have applied to the dino-
saurs? In other words, were some of their habitats sufficiently fa-
vorable so that as warm-blooded predators they could attain ratios
as high as 4 or 5 percent? The habitat least favorable to predatory
dinosaurs would certainly have been the barren sand dunes of Outer
Mongolia during the Cretaceous Period. This environment of-
fered little cover, water was scarce, and droughts must have been
severe. Plant-eaters are commonly found in these red sands, but
meat-eaters are rare and mostly of small size. The total mass of
predators to prey was far below 1 percent. But across the North
Pacific, the Late Cretaceous habitats in Alberta were ideal for
predators. Forested deltas and floodplains provided ample cover
for attacking, and the predators were very large, powerful, and
nimble—adult tyrannosaurs grew to two tons, enough to attack the
rhinoceros-sized duckbills and horned dinosaurs. In Alberta, the
ratio of predator to prey averaged 4 percent, much higher than in
Mongolia. These percentages clearly mean that tyrannosaurs were
probably as warm-blooded as the saber-toothed cats and wolf-bears
that took over the top predator roles many millions of years later.
Quite interesting as a test case was the situation during the
Late Jurassic in Wyoming. Here the conditions for predators were
not as trying as in the barren dunes of Mongolia nor as easy as in
the densely forested deltas of Alberta. The conditions were inter-
mediate—Morrison Formation sediments show that these habitats
contained more trees than the dune fields but suffered longer dry
seasons than the Alberta deltas. Allosaurus was the most common
predator of the Morrison, and it didn't enjoy the same advantages
tyrannosaurs would later have. Allosaurs averaged one ton in adult
weight, large by modern standards, but tiny compared to the twenty-
390 | THE WARM-BLOODED METRONOME OF EVOLUTION
and thirty-ton brontosaurs. Logically, then, the rules of predator
to-prey relationships derived from warm-blooded animals predict
that Allosaurus would not have been able to reach as high a level
as tyrannosaurs could later because they wouldn't have been able
to cull their prey as effectively. And this prediction is justified by
the evidence. The predator-to-prey ratios of the allosaurs aver-
aged 1.5 percent, lower than for tyrannosaurs.
Orthodox paleontologists greet these arguments from preda-
tor-to-prey ratios with incredulity. But they make ecological sense.
Predatory dinosaurs exhibited very low ratios (1 percent or less)
in the same sorts of difficult habitats where warm-blooded mam-
mals obtained low ratios. And they achieved higher ratios in more
favorable situations, exactly as did the extinct mammals. Most im-
portant point of all was that both dinosaur predator-to-prey ratios
and fossil and living mammal predator-to-prey ratios averaged far,
far lower than those of certifiably cold-blooded reptiles.
There is also important independent confirmation to be de-
rived from the evidence from the microstructure of bones dis-
cussed in a previous chapter. All the extinct groups whose bone
texture indicated fast growth—dinosaurs, protomammals, and
mammals—also had low predator-to-prey ratios. Moreover, my ar-
guments from predator-to-prey ratios caused such controversy that
I sought additional supports as well. In 1982, it occurred to me
that footprints could serve as further proof.
It seemed logical that warm-blooded animals would be forced
to move around for food at much higher average speeds than their
cold-blooded cousins because high metabolism demands a more
or less continuous supply of calories, hence a more or less contin-
uous search for them. If correct, this notion implies that fossil
footprints would be good indicators of the number of required
calories. Since they usually record unhurried, and not maximum,
speeds, fossil footprints should be reliable indicators of the aver-
age intensity of foraging.
I tested this idea by calculating the speeds indicated by foot-
prints made by living species. As expected, average walking speed
today is much higher in mammals than in cold-blooded amphib-
ians and reptiles. I then calculated walking speeds from the foot-
prints of fossil mammals. They fell into the range of their modern,
living relatives, confirming that extinct mammals had as high a me-
EATERS AND EATEN AS THE TEST OF WARM-BLOODEDNESS I 391
tabolism as modern. Next, I turned to testing this idea against the
footprints of the very primitive reptiles and the amphibians of the
Coal Age—species more archaic in bone structure than living liz-
ards. Once again, the hypothesis proved out. Average speed in the
ancient Coal Age and Early Permian animals was exceptionally low,
only one to two miles per hour. This was dramatic proof that a
leisurely mode of foraging was in fact a concomitant of cold-
bloodedness.
Having established the reliability of this approach, I finally
turned to calculating the average walking speeds for the dinosaurs
and for their ancestors of the Triassic, the thecodonts. There was
absolutely no ambiguity in the figures: Dinosaurs and the theco-
donts before them moved through their Mesozoic world at aver-
age speeds as high as those of warm-blooded mammals and much
higher than those of the cold-bloods. Such speeds make evolu-
tionary sense only if metabolic rates were constantly demanding
ingestion of calories at a thoroughly warm-blooded rate.
There are, then, three ways of accomplishing the apparently
impossible, of counting calories in species long extinct. And the
conclusion to be drawn from all three methods coincides pre
-
cisely. The fossil animals that have low predator-to-prey ratios are
the ones that also have bone texture indicating fast growth, and
also had high average walking speeds. The picture of dinosaurs
painted by these three sources of information is one where the
metabolic levels were set on high, where the drama of growing to
adulthood was acted out speedily, and where the cast of characters
was in constant motion about the Mesozoic landscape, engulfing
prodigious quantities of indispensable food.
392 | THE WARM-BLOODED METRONOME OF EVOLUTION
PART 5
DYNASTIC FRAILTY
AND THE PULSES
OF ANIMAL HISTORY
19
PUNCTUATED EQUILIBRIUM:
THE EVOLUTIONARY
TIMEKEEPER
s cientific thinking often benefits from the throwing of "bombs"—
the publication of ideas so revolutionary that one half of the
profession is scandalized, while the other half is captivated by the
prospect of daring new solutions to old problems. Even when the
revolutionary idea finally proves not entirely correct, the natural
tendency to accept orthodoxy unchallenged is beneficially shaken.
And certainly the vigorous reexamination of facts and conclusions
provokes the creation both of new methodologies and new ideas.
Darwin's Origin of Species, published in 1859, was such a "bomb."
European scientists had been excavating fossils for sixty years, and
were anchored to the conviction that the species they were finding
were units of the Creator, fixed and unchanging through time. Such
a conception of species corresponded perfectly with the prevailing
theological conviction that man was a special creation entirely apart
from all other organic beings. Darwin's "bomb" was the first well-
researched argument in favor of the idea that one species could
indeed evolve into another, and then another, and so on until the
disorganized primordial slime had been transformed into some-
thing as complex and elegant as a Guernsey cow or an Anglican
bishop.
The intellectual upheaval that followed upon Darwin's publi-
cation of his ideas is well known. Religion and religious thinking
have never recovered in Western society from the vast defeat they
PUNCTUATED EQUILIBRIUM: THE EVOLUTIONARY TIMEKEEPER I 395
Fast evolutionary turnover.
Most dinosaur genera were
very short-lived, lasting only
six million years or less.
Arrhinoceratops and
Diceratops are two
examples—these horned
dinosaurs had a geological
life-span of only four
million years.
suffered when they tried to resist the conclusions about nature that
he stimulated. The contest was very like a war. And, as in most
wars, the winners wrote the history. In the long run, Darwin won.
The evidence for the evolutionary transmutation of one species into
another became so strong that by 1900, nearly all scientists had
converted to "Darwinism" of one form or another. Darwin be-
came an heroic figure, the champion of rational thought. His op-
ponents became stubborn obscurantists, superstitious defenders of
organized religion despite massive amounts of scientific evidence.
396 | DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY
Not all of Darwin's opponents, however, were in that camp.
Many honest, soundly trained paleontologists, hardworking schol-
ars who weren't religious bigots at all, saw in the rocks abundant
evidence to suggest that species were fixed and largely unchang-
ing. Darwin himself was not a professional paleontologist. As the
naturalist on H.M.S. Beagle, he gathered the fossil bones of giant
ground sloths he found on the Argentine pampas, and sent them
on to Sir Richard Owen for study, his own anatomical expertise
being too weak. So he was open to the charge of being a mere
amateur when it came to fossils. Perhaps he did not realize how
good the evidence for long-lasting and unchanging species was. By
the 1850s, paleontology was already a mature science. English,
French, German, and Russian scientists had thoroughly studied the
rock strata, and had gathered many thousands of fossil specimens.
These experts had no trouble recognizing the same species of el-
ephant, for example, across vast expanses of geography and through
enormous thicknesses of strata. The same was true for dinosaurs.
The conclusion seemed inescapable: Species were fixed units that
did not change much through time and across space.
In 1859, Hugh Falconer, a prominent English paleontologist
with a wide reputation as an expert on fossil elephants, did sup-
port Darwin. But he urged Darwin to modify his belief that most
of evolution was a slow and continuous process. Falconer was con-
vinced that the evolution of one species into another was a sudden
event, and that most of the time species remained unchanged dur-
ing the long intervals between the sudden transmutations. Unfor-
tunately, Falconer died in 1866, before his modified view of
Darwinism could become widely known. When science jumped
onto the Darwinian bandwagon around 1900, all these quite legit-
imate objections were lost sight of. Primitive reptiles, protomam-
mals, dinosaurs, pterodactyls, mammoths, and saber-toothed cats
were all supposed to have evolved gradually, one species imper-
ceptibly into another, all through the History of Life. The fossil
facts did not read that way, but everyone assumed the "missing"
intermediates between species had existed. The fossil record was
just too incomplete to preserve many of the transitional stages.
Darwin had argued exactly this line to counter his critics.
Shortly before I entered graduate school at Harvard in 1972,
the biggest paleontological "bomb" of the century had been dropped
PUNCTUATED EQUILIBRIUM: THE EVOLUTIONARY TIMEKEEPER I 397
by Niles Eldredge of the American Museum in New York and
Stephen Jay Gould of Harvard. They published an article entitled
"Punctuated Equilibria." This rather short paper produced enor-
mous repercussions. Eldredge and Gould had rediscovered the idea
that species might not change continuously through time. They
combined this idea with another developed by twentieth-century
naturalists, that new species usually form in small breeding popu-
lations isolated from the main body of the old parent population.
The "punctuated" part of their idea was the notion that most evo-
lutionary change happens suddenly, like a punctuation mark in a
sentence. The "equilibrium" part was that most of the time, most
species are not evolving at all because conditions in large, stable
populations do not permit changes to occur. Eldredge and Gould
insisted that the fossil record was not nearly so poor as Darwin
believed. The rarity of missing links between species, so bother-
some to a century of Darwinists, was in fact a faithful representa-
tion of evolutionary history. If most new species form suddenly,
in isolated populations, then
only rarely would fossils preserve that
fleeting evolutionary moment. And if typical, widespread species
endure millions of generations without changing, then the fossil
record will consist mostly of long-lived invariant species. It can be
said that, in their own terms, Eldredge and Gould had rediscovered
Hugh Falconer's position.
Two things convinced me that punctuated equilibrium was an
idea worth considering. First of all was Professor Bernie Kum-
mel's endorsement. His own work on fossil armored squid made
him enormously well informed about how fossil species changed
through strata. And his habit of mind was extremely rigorous, very
little given to unsound flights of speculative fancy. I expected only
the most qualified discussion of the theory from him. I was there-
fore all the more impressed by his unequivocal endorsement. I
questioned Bernie one evening at a party for us students, "Is Steve
right?" "Yup," said Bernie. He pointed out that it had been known
all along that really good fossil strata always showed species lasting
through millions of years. Only misguided geneticists insisted that
species evolved all the time.
The second thing to convince me was my own studies the fol-
lowing summer. With Bernie Kummel's endorsement of punc-
tuated equilibrium deeply impressed upon me, I went off to Como
Bluff to test the idea in the field among the brontosaur habitats.
398 | DYNASTIC FRAILTY AND THE PULSES OF ANIMAL HISTORY
Como Bluff and Sheep Creek nearby were nearly perfect for test-
ing punctuated equilibrium with fossil dinosaurs. Good skeletons
could be found from the bottom to the top of the Morrison For-
mation, strata 250 feet thick representing approximately a million
years. And the habitats represented in these strata clearly changed
dramatically throughout this time—from shallow, seasonally dry
lakes and swamps, to well-drained floodplains crossed by large,
shifting streams, to poorly drained floodplains dotted by carbon-
rich ponds. If orthodox genetic theory were correct, then the Como
dinosaurs should have evolved continuously throughout this time
to match the changing demands of their environment. If punc-
tuated equilibrium were correct, the fossils should reveal species
of dinosaurs that remained static, unresponsive to local conditions
Robert T Bakker Page 40