Robert T Bakker
Page 35
evolved by the duckbill dinosaurs. Some duckbills evolved large,
342 I THE WARM-BLOODED METRONOME OF EVOLUTION
Mammal head-butters—the uintathere Loxolophodon from the Eocene epoch
(above). Early uintatheres—like Bathyopsis—had small horns and big,
dangerous canines. Later species—like Uintatherium—evolved big, blunt
nonlethal horns (below).
How the trombone-duckbill hooter works.
The two nasal passages looped up and back
through the crest—arrows show pathway of
inhaled air in cutaway of skull.
Corythosaurus, the hollow-helmet
duckbill. Arrows on cutaway view
of skull show air pathway.
display tails, but in general, they reserved the most vigorous
expression of their evolutionary changes for their heads. The
duckbills actually divided into four different subclans evolving ever
greater cranial specialization. Perhaps the most primitive display
was Kritosaurus's Roman nose. This animal had enlarged compart-
ments around its nostrils and probably amplified its bellows and
snorts through resonating nasal chambers. A bit more complex was
Saurolopbus, which combined sight with sound: A solid spike of
bone jutted backward from its head and probably supported a wide
flap of skin; meanwhile its nasal compartments were huge, imply-
ing great resonance when it snorted. A strictly audio approach was
favored by Edmontosaurus. Its head was large and its nasal com-
partments comparatively huge. The most complex headgear of all
among the duckbills belonged to Parasaurolophus. Each nostril
started with a separate trombone-shaped tube leading from the nose
up to the top of the skull, then out and behind the very long crest,
a sharp U-turn and back down the crest, then down along the head,
and through to the windpipe. Since each nostril had a complete
tube of its own, a crest in section reveals four separate cham-
bers—two ingoing and two outgoing.
Hollow-crested duckbills are widely regarded—certainly with
good reason—as head-hooters, amplifying and modulating their cries
through their crests. All of the varied, hollow cranial ornaments
were specialized outgrowths of the normal air tract. In a primitive
duckbill, like Kritosaurus, as the animal inhaled, the air would pass
through the nostrils, then through a short passage in the snout, to
the rear of the throat into the windpipe at the base of the tongue.
A hollow-crested duckbill complicated the course the air had to
follow: in through its nostrils, up and back through special bony
tubes growing backward from the nose, up and above the eyes into
a huge bony compartment, then down and forward into the throat
and windpipe. With all their loops and extra chambers, the hol-
low-crested duckbills could reproduce in bone some of the quali-
ties instrument makers seek to design into brass and wood today.
Duckbill springtime choruses may well have been the loudest and
richest cacophony evolution has ever produced. Being large con-
ferred great lung power. A male Parasaurolophus would have
weighed three or four tons. In the fossils of the Judith Delta in
Alberta, six different duckbills were found within a small area, each
SEX AND INTIMIDATION: THE BODY LANGUAGE OF DINOSAURS | 345
with its unique nasal amplifier. If all started playing their sexual
overtures together, the din must have been thunderous.
The great emphasis dinosaurs placed on auditory messages
correspondingly demanded an efficient, sensitive hearing system.
And, as a group, the Dinosauria were indeed equipped with good
to excellent hearing machinery. All dinosaurs had the skull notches
to hold a taut eardrum. And all dinosaur middle-ear bones were
thin and delicate, like a bird's, for picking up higher frequencies.
In the fluid-filled canals of the brain, the dinosaur's ear was rather
like a crocodile's. And since crocodiles today have the most sen-
sitive hearing of any "reptiles," the dinosaurs were certainly tuned
in to a wide range of airborne sound.
Taken as a whole, the dinosaurs' adaptations for sex and in-
timidation simply don't seem to fit the orthodox definition of their
cold-bloodedness and lethargy. The abundance of head-butting
devices, the extraordinary exuberance of the cranial hooting and
snorting apparatuses, are powerful arguments for the idea that the
Dinosauria as a whole put a lot of evolutionary energy into mat-
ing. Modern lizards, crocodiles, snakes, and turtles simply do not
show so many strongly modified organs for high-energy aggres-
sion. Male alligators, for example, have deep loud voices but have
never invested in cranial remodeling to achieve a wider range of
tones. Male rhinoceros iguanas butt one another, but their skulls
display nothing to compare with the highly specialized ramming
devices of the dome-headed dinosaurs.
How warm-blooded were the habits of dinosaurs when they
mated and defended their territories? The cranial evidence strongly
points to high energy, to a Mesozoic world where grunting and
crashing alternated with hooting and bellowing to rend the tropi-
cal silence as multi-ton monsters vigorously hurled their muscular
bulk at one another in pain, victory, and frustration.
346 | THE WARM-BLOODED METRONOME OF EVOLUTION
16
THE WARM-BLOODED
TEMPO OF THE DINOSAURS'
GROWTH
n animal's metabolism is inextricably connected to many of
A its characteristics—one of the most important being its rate
of growth. Some of the very best evidence for warm-bloodedness
in dinosaurs is supplied by the study of their patterns of growth,
research begun fifty years ago but nearly totally overlooked by
professional paleontologists until very recently. In 1972 I stum-
bled upon some fairly old monographs about the warm-blooded
texture found in the bones of dinosaurs. I was in the process of
working on my hypothesis about warm-bloodedness in dinosaurs,
relying on my own data about predator-prey ratios and some spec-
ulative ideas about the posture of limbs. But I had been ignorant
up to that point of clues that came from evidence about bone
texture discovered in the 1930s. From 1930 till 1970 the warm-
blooded style of the dinosaurs' growth had stood as a potent
support for the nonreptileness of the Dinosauria. But since ortho-
doxy suffocated dissent, no one paid much attention to the data
derived from growth rates. In the early 1970s, however, Armand
de Ricqles attacked the problem of growth with such vigor that it
became impossible to ignore.
In today's ecosystems, warm-bloodedness leaves its unmistak-
able mark on the patterns of birth, adolescence, and adulthood.
Warm-blooded mammals grow quickly. A German shepherd pup
weighing five pounds will become a nearly full-sized adult of 120
THE WARM-BLOODED TEMPO OF THE DINOSAURS' GROWTH | 347
Six years from egg to five-ton adult? The giant eight-spiked stegosaur,
Stegosaurus ungulatus, grew very fast, judg
ing by the bone texture preserved
in juvenile specimens, as fast or faster than a warm-blooded rhino or water
buffalo.
pounds one year later. And birds grow even faster. Ostriches grow
at astonishing rates, from egg to 150-pound bird in as little as nine
months. But a young, reticulated python of five pounds in the zoo
requires ten to twenty years to reach 120 pounds. And a reptile
in the wild grows even more slowly. Box turtles reach sexual ma-
turity at a weight of about four pounds, the size of a small adult
cat. A cat reaches breeding weight within half a year after birth,
but a wild turtle usually needs five to ten years. Alligators too are
slow growers. In its native Florida habitat, the Mississippi alligator
requires ten to twenty years to reach two hundred pounds, a weight
a lioness can reach in two years.
Our own human species is not a good example of warm-
blooded patterns of growth—we are exceptionally slow-growing
compared to nearly all members of the Class Mammalia. We lin-
ger in drawn-out adolescence, using up fifteen or twenty years to
reach our final adult size. A four-year-old hyena, white-tailed deer,
or porpoise is already adult and weighs 120 pounds. A four-year-
old human weighs about thirty pounds and has just begun to pass
through the many stages on the path to full physical and social ma-
348 I THE WARM-BLOODED METRONOME OF EVOLUTION
turity. The explanation for slow growth in humans probably re-
lates to the bewildering complexity of our adult society. We have
to grow slowly in order to absorb the myriad dos and don'ts of
our parents' culture. It's much simpler for a hyena to be socially
mature because hyena society contains but a few rules and regu-
lations.
Even the most socially complex reptiles—probably the alli-
gators and crocodiles—are still far less subtle psychosocial^ than
the average bird or mammal. An alligator therefore can't blame its
overly long prepubescence on its need to accumulate the wisdom
and social nuance of 'gator culture. In fact, from an evolutionary
point of view, their slow growth is a mistake. Alligators would be
much better competitors if they could match the rate of growth of
mammals or birds. The primary Darwinian goal for each and every
species is to breed—breed early, breed often. In the swamp, there
is only a limited supply of food to eat or burrows to hide in or
logs to bask on. And the species that fills the swamp with off-
spring monopolizes the natural economy. Moreover, fast rates of
reproduction are powerful evolutionary weapons; they provide an
enormous advantage in coping with predators or surviving climatic
catastrophes.
The surest method of speeding up rates of breeding is to be-
come warm-blooded. Why do alligators and tortoises continue to
grow slowly if this is an inferior evolutionary tactic? There is no
defect in their biomechanical system. Turtles and alligators rely on
the same basic systems of enzymes employed by mammals. If those
systems were exploited at full capacity, an alligator would be able
to grow as fast as an ostrich. But reptiles cannot exploit their full
potential for growth, because their cold-blooded physiology makes
them less effective in gathering food in the wild than a warm-
blooded creature. Their fluctuating body temperature forces them
to operate their food procurement and growing processes at levels
far below maximum for much of their lives. Warm-blooded birds
and mammals, on the other hand, may be absorbing nourishment
into their digestive systems at rates very close to the biochemical
maximum.
A lot of direct evidence proves that present-day Reptilia in
the wild usually operate their growing apparatus far below capac-
ity. Wildlife biologists generally study the stomach contents of their
THE WARM-BLOODED TEMPO OF THE DINOSAURS' GROWTH I 349
specimens in order to study the animals' diet. What is found in
alligators is surprising—on average, big crocodilians are empty, or
nearly so. Compared to the average lion or hyena, a Nile croco-
dile spends most of its life fasting. Lizards tell the same story—on
average, lizard stomachs are less full of food than are mammals'.
The ultimate proof that reptilian growth usually works far below
maximum capacity comes from what happens when reptiles are kept
in cages warmed to their favorite temperature and are continu-
ously provided with food. This turns out to be the only way to
accelerate an alligator's rate of growth to the maximum: Keep it
warmed all day long, seven days a week, and keep forcing protein-
rich food into it. Most research scientists couldn't afford to per-
form such an experiment, but the private sector has come to the
rescue. Alligator and crocodile skins sell to a lucrative market for
shoes and handbags, and since conservationist measures restricted
hunting of wild specimens, enterprising businessmen started to farm
them. Others have even tried turtle farming, because giant sea
turtles produce highly esteemed meat. On all these farms, croco-
diles, alligators, and turtles grow almost as fast as warm-blooded
mammals. The only side effect the reptiles suffer is an occasional
attack of gout from the combination of rich diet and lack of ex-
ercise.
Did the dinosaurs have a fast-growth weapon in their adap-
tive arsenal? Did Tyrannosaurus rex grow to breeding weight in five
years? Was part of the reason dinosaurs enjoyed such unchal-
lenged dominance throughout the Mesozoic that they bred earlier
and bred faster? A most intriguing question. Genuine mammals
were present during that time and were potential ecological threats
as their later development demonstrates. But mammals never did
evolve to large size until after the dinosaurs had died out. Maybe
the dinosaurs were just too good at growing quickly?
How can the dinosaurs' growth be measured? An accurate es-
timate can be derived from the texture of fossil bone. A thin slice
can be cut from a fossil-bone chip and glued to a glass plate. It can
then be ground so thin that light shines through. The slice under
the microscope will allow an observer to see precisely how the bone
crystals were arranged as the bone grew. This transparent thin sec-
tion, as it is called, is standard today for analyzing the structure of
the widest variety of hard natural substances—rocks, metals, sin-
350 I THE WARM-BLOODED METRONOME OF EVOLUTION
Dinosaurian inefficiency. If dinosaurs were truly warm-blooded, then it would
take thirty tons of meat to raise a one-ton ceratosaur from egg to adult. But a
cold-blooded finback from the Permian Period would be much more
efficient—three hundred pounds of meat would raise a one-hundred-pound
finback. Still, the much higher metabolism would let the dinosaur grow much
faster.
How dinosaur-bone microtexture differs from the texture in primitive cold-
bloods.
gle crystals, and bone from living species. Geologists originated
>
the thin-section technique in the 1830s, and it wasn't long before
paleontologists took it over for fossils. Since bones grow by add-
ing crystals of mineral, the microtexture of bone indicates how fast
the body grew.
When nineteenth-century scientists examined slices from fos-
352 | THE WARM-BLOODED METRONOME OF EVOLUTION
sil bones and teeth, they found that dinosaur bone looked very
like mammal bone. Both dinosaurs and mammals possess many tiny
channels for blood vessels running through their bone, and both
have the curious structures known as Haversian canals—long cyl-
inders, pointed at both ends, where bone mineral had been dis-
solved and then redeposited in concentric layers. When cut in cross
section, Haversian systems look like tiny onions sliced across the
middle. Cut lengthwise, they resemble tiny, multi-layered electri-
cal cables.
Using this technique, early twentieth-century scientists as-
sembled an impressive body of histological data about the entire
400-million-year history of vertebrates from the earliest fish to
Neanderthal Man. And all the dinosaur bone slices looked more
like mammal bone than reptile. These studies were masterfully
summarized in a series of papers published in the early 1950s by
two histologists from Texas, Enlow and Brown. But their labors
had astoundingly little impact. The standard textbooks on dino-
saurs had hardened into "cold-blooded" orthodoxy. And so the work
done by these histologists remained in relative obscurity.
The material concerning the texture of the dinosaurs' bones
and their rates of growth burst upon the world in the 1970s thanks
to two independent rediscoveries of the old published work. By
purest chance I ran across some articles dealing with the texture
of dinosaur bones and subsequently was led to the wealth of in-
formation published by Enlow and Brown. They had cut samples
from dozens of dinosaurs and concluded that the animals may have
been warm-blooded. In 1972, I published a paper in the journal
Nature, calling attention to all this forgotten material. Meanwhile,
in Paris, Armand de Ricqles had also rediscovered the question of
bone texture and had inaugurated a massive research project in-