The implications of these new discoveries are truly fascinating. It
has already been argued, with logic and some force, that small
theropod dinosaurs were highly active, fast-moving, and
biologically ‘sophisticated’ animals. On this basis, they seemed
reasonable candidates as potential endotherms; in a sense, our
inferences about their way of life suggested that they had most to
benefit from being endothermic. The Liaoning discoveries confirm
that many of these highly active, bird-like dinosaurs were small
animals. This is a crucial point, as small size puts greatest
physiological stress on endotherms because a large percentage of
internally generated body heat can be lost through the skin surface;
so small, active endotherms would be expected to insulate their
bodies to reduce heat loss. Small theropod dinosaurs, therefore,
evolved insulation to prevent heat loss because they were
endotherms – not because they ‘wanted’ to become birds!
Liaoning discoveries indicate that various types of insulatory
covering developed, most probably by subtle modifications to the
growth patterns of normal skin scales; these ranged from hair-like
filaments to full-blown feathers. It may well be that genuinely
bird-like flight feathers did not evolve for the purposes of flight,
but had a far more prosaic origin. Several of the ‘dinobirds’ from
Liaoning seem to have tufts of feathers on the end of the tail (rather
like a geisha’s fan) and fringes of feathers along the arms, on the
head, or running down the spine. Clearly preservational biases may
also play a part in how and on which parts of the body these may
be preserved. But for the present, it seems at least possible that
feathers evolved as structures linked to the behaviour of these
animals: providing recognition signals, perhaps, as in living birds,
or being used as part of their mating rituals, long before any
genuine flight function had developed.
In this context, gliding and flight, rather than being the sine qua
non of avian origins, become later, ‘add-on’ benefits. Obviously,
feathers have the potential for aerodynamic uses; just as with
modern birds, the ability to jump and flutter may well have
embellished ‘dinobird’ mating displays. For example, in the case of
the small creature Microraptor, a combination of fringes of feathers
along the arms, legs, and tail would have provided it with the ability
to launch itself into the air from branches or equivalent vantage
points. From just this sort of starting point, gliding and true
flapping flight seem a comparatively short ‘step’ indeed.
We should not, however, get too carried away with the scenario
outlined above. Although the Liaoning discoveries are indeed
incredibly important, offering, as they do, a richly detailed window
on dinosaurian and avian evolution in the Cretaceous, they do not
necessarily provide all the answers. One crucial point that must be
remembered is that the quarries of Liaoning are Early Cretaceous in
age, and their fossils are therefore considerably younger (by some
30 Ma at least) than the earliest well-preserved feathered dinosaur
with highly developed and complex wings, Archaeopteryx.
Whatever the path that led to the evolution of the first flying
dinosaurs, and ultimately to birds, it was emphatically not via the
extraordinary feathered dinosaurs from Liaoning. What we see at
Liaoning is a snap shot of the evolutionary diversification of avian
theropods (and some true birds), not the origin of birds: bird
origins are still shrouded by sediments of Middle or possibly even
Early Jurassic age – before Archaeopteryx ever fluttered to Earth.
Everything that we know to date points to a very close relationship
between theropod dinosaurs and early birds, but those crucial Early
or Middle Jurassic theropods that were ancestral to Archaeopteryx
are yet to be discovered. It is to be hoped that in future years some
spectacular discoveries will be made that fill in this part of the
story.
Chapter 5 concluded with the view that dinosaurs lived at a time in
Earth history that favoured large-bodied, highly active creatures
that were able to maintain a stable, high body temperature without
most of the costs of being genuinely endothermic. The ‘dinobirds’
from Liaoning suggest that this view is wrong – small, insulated
theropods simply had to be endothermic and their close
relationship to birds, which we know are endothermic, simply
reinforces the point.
My response to this is: well, yes and no. There is now little doubt
that bird-like theropod dinosaurs were endotherms in a true sense.
However, I do think that the arguments suggesting that the
majority of more traditional dinosaurs were inertial homeotherms
(their large body size enabled stable internal temperature) still hold.
There is some evidence in support of my view to be found among
living endotherms. Elephants, for example, have a much lower
metabolic rate than mice – for exactly these reasons. Mice are small,
lose heat rapidly to the environment, and have to maintain a high
metabolic rate to replenish the heat loss. Elephants are large
(generally dinosaur-sized) and have a stable internal body
temperature due to their size, not just because they are
endothermic. Indeed, being a large endotherm is, in part at least, a
physiological challenge. For example, elephants suffer problems if
they move around too quickly: their postural and leg muscles create
a great deal of extra chemical heat, and they need to use their large,
‘flappy’ ears to help them to radiate heat rapidly to prevent fatal
overheating.
Dinosaurs were on the whole super-large and their bodies would
have been capable of maintaining a constant internal temperature;
extrapolating from the elephant, it would not have been in
dinosaurs’ interests to be genuine endotherms, in a world that
was in any case very warm. Having evolved physiologically as
mass-homeotherms (having a stable internal body temperature
that was made possible by large body size), the only group of
dinosaurs that bucked the general dinosaurian trend toward large
size and evolved into a small-bodied group were the
dromaeosaurian theropods.
It is clear, from their anatomy alone, that dromaeosaurians were
highly active and would have benefited from homeothermy, and
their relatively large brains would have demanded a constant supply
of oxygen and nutrients. Paradoxically, homeothermy cannot be
maintained at small body size without an insulatory covering
because of the unsustainable heat loss through the skin. The choice
was stark and simple: small theropods had to either abandon their
high-activity lifestyle and become conventionally reptilian, or boost
internal heat production and become properly endothermic,
avoiding heat loss by developing skin insulation. So, I propose that
it is not a case of ‘all or nothing’; most dinosaurs were basically
mass-homeotherms that were able to sustain high activity levels
without the full costs of mammalian or avian styles of endothermy;
however, the small, and in particular the dromaeosaurian,
theropods (and their descendants, the true birds) were obliged to
develop full-blown endothermy and the associated insulatory
covering.
Wednesday, June 27, 2007
Dromaeosaurian theropods
These bird-like dinosaurs exhibit a number of interesting
anatomical changes to the basic theropod body plan. Some changes
are quite subtle, but others are less so.
One notable feature is the ‘thinning’ of the tail: the tail becomes
very narrow and stiffened by bundles of long, thin bones, the only
flexible part being close to the hips (Figure 16, top). As argued
earlier, this thin, pole-like tail may well have been valuable as a
dynamic stabilizer to assist with the capture of fast-moving and
elusive prey. However, this type of tail dramatically changed the
pose of these animals because it was no longer a heavy, muscular
cantilever for the front half of the body. If no other changes had
been made to its posture, such a dinosaur would have been
unbalanced and constantly pitch forward on to its nose!
To compensate for the loss of the heavy tail, the bodies of these
theropods were subtly altered: the pubic bone, which marks the
rearmost part of the gut and normally points forward and
downward from each hip socket in theropods, was rotated
backwards so that it lay parallel to the ischium (the other lower
hip bone). Because of this change in orientation, the gut and
associated organs could be swung backwards to lie beneath the
hips. This change shifted the weight of the body backwards, and
compensated for the loss of the heavy counterbalancing tail. This
layout of hip bones, with the pubis rotated backward, is seen in
living and fossil birds as well as maniraptoran theropods.
Another equally subtle way of compensating for the loss of the
counterbalancing tail would be to shorten the chest in front of the
hips, and this is also seen in these bird-like theropods. The chest
also shows signs of being stiffened, and this probably reflects the
predatory habits of these animals. The long arms and three-clawed
hands were important for catching and subduing their prey and
needed to be very powerful. The chest region was no doubt
strengthened to help securely anchor the arms and shoulders to
withstand the large forces associated with grappling and subduing
prey. Birds also have a short, and greatly stiffened, chest region to
withstand the forces associated with anchoring the powerful flight
muscles.
At the front of the chest, between the shoulder joints, there is a
V-shaped bone (which is in fact the fused clavicles, or collar bones –
Figure 17) that acts as a spring-like spacer separating the shoulders,
it also helped to anchor the shoulders in place while these animals
were wrestling their prey. Birds also exhibit fused collar bones; they
form the elongate ‘wish bone’, or furcula, that similarly acts as a
mechanical spring that separates the shoulder joints during
flapping flight.
The joints between the bones of the arm and hand were also
modified so that they could be swung outward and downward with
considerable speed and force to strike at prey in what has been
called a ‘raking’ action. When not in use, the arms could be folded
neatly against the body. The leverage for this system was also of
considerable advantage to these creatures, because the arm muscles
that powered this mechanism were located close to the chest and
operated long tendons that ran down the arm to the hand (rather
than having muscles positioned further out along the arm); this
remote control system kept the weight of the body closer to the hips
and helped to minimize the delicate problem of balance in these
theropods. The arm-striking and arm-folding mechanism is closely
similar to that employed by birds when opening and closing their
wings during and after flight.
Archaeopteryx
The early bird-like fossil Archaeopteryx (Figure 16, bottom) exhibits
many maniraptoran theropod features: the tail is a long and very
thin set of vertebrae that anchored the tail feathers on either side;
the hip bones are arranged with the pubis pointing backward and
downward; at the front of the chest there is a boomerang-like
furcula; the jaws are lined with small, spiky teeth, rather than a
more typical bird-like horny beak; the arms are long, jointed so that
they can be extended and folded just as in theropods, and the hands
are equipped with three sharply clawed fingers that in their
arrangement and proportions are identical to those seen in
maniraptoran theropods.
Chinese wonders
During the 1990s, explorations in quarries in Liaoning Province
in north-eastern China began to yield some extraordinary, and
extraordinarily well preserved, fossils of Early Cretaceous age. At
first, these comprised beautifully preserved early birds such as
Confuciusornis, and the skeletons included impressions of feathers,
34. Restoration of the living Archaeopteryx
beaks, and claws. Then in 1996, a complete skeleton of a small
theropod dinosaur, very similar in anatomy and proportions to
the well known theropod Compsognathus (Figure 14), was
described by Ji Qiang and Ji Shu’an. They named the dinosaur
Sinosauropteryx. This dinosaur was remarkable because there was
a fringe of filamentous structures along its backbone and across its
body, suggesting some sort of covering to the skin that was akin to
the ‘pile’ on a roughly made carpet; there was also evidence of soft
tissues in the eye socket and in the region of the gut. It was clear
that some small theropods had some type of body covering. These
discoveries led to concerted efforts to find more such fossils at
Liaoning; they began to appear with increasing regularity and
ushered in some truly breathtaking revelations.
Shortly after Sinosauropteryx was discovered, another skeleton
was revealed. This animal, named Protoarchaeopteryx, was the
first to show the presence of true bird-like feathers attached to its
tail and along the sides of its body, and its anatomy was much
more similar to that of dromaeosaurians than Sinosauropteryx.
Another discovery revealed an animal that was extremely similar
to Velociraptor, but this time named Sinornithosaurus (again,
apparently covered in a ‘pile’ of short filaments). Newer discoveries
have included Caudipteryx, a large (turkey-sized), rather
short-armed creature noted for a pronounced tuft of tail feathers
and shorter fringes of feathers along its arms; smaller, heavily
feathered dromaeosaurians; and in the spring of 2003 a quite
remarkable ‘four-winged’ dromaeosaurian, Microraptor, was
unveiled to the world. This latter creature was small and classically
dromaeosaur-like, with the typically long, narrow tail, bird-like
pelvis, long, grasping arms, and sharp rows of teeth lining its
jaws. The tail was fringed by primary feathers and its body
covered in downy ones. However, what was singularly impressive
was the preservation along the arms of flight feathers forming
Archaeopteryx-like wings and, very unexpectedly, similar wing-like
fringes of feathers attached to the lower parts of the legs – hence the
name ‘four-wing’.
anatomical changes to the basic theropod body plan. Some changes
are quite subtle, but others are less so.
One notable feature is the ‘thinning’ of the tail: the tail becomes
very narrow and stiffened by bundles of long, thin bones, the only
flexible part being close to the hips (Figure 16, top). As argued
earlier, this thin, pole-like tail may well have been valuable as a
dynamic stabilizer to assist with the capture of fast-moving and
elusive prey. However, this type of tail dramatically changed the
pose of these animals because it was no longer a heavy, muscular
cantilever for the front half of the body. If no other changes had
been made to its posture, such a dinosaur would have been
unbalanced and constantly pitch forward on to its nose!
To compensate for the loss of the heavy tail, the bodies of these
theropods were subtly altered: the pubic bone, which marks the
rearmost part of the gut and normally points forward and
downward from each hip socket in theropods, was rotated
backwards so that it lay parallel to the ischium (the other lower
hip bone). Because of this change in orientation, the gut and
associated organs could be swung backwards to lie beneath the
hips. This change shifted the weight of the body backwards, and
compensated for the loss of the heavy counterbalancing tail. This
layout of hip bones, with the pubis rotated backward, is seen in
living and fossil birds as well as maniraptoran theropods.
Another equally subtle way of compensating for the loss of the
counterbalancing tail would be to shorten the chest in front of the
hips, and this is also seen in these bird-like theropods. The chest
also shows signs of being stiffened, and this probably reflects the
predatory habits of these animals. The long arms and three-clawed
hands were important for catching and subduing their prey and
needed to be very powerful. The chest region was no doubt
strengthened to help securely anchor the arms and shoulders to
withstand the large forces associated with grappling and subduing
prey. Birds also have a short, and greatly stiffened, chest region to
withstand the forces associated with anchoring the powerful flight
muscles.
At the front of the chest, between the shoulder joints, there is a
V-shaped bone (which is in fact the fused clavicles, or collar bones –
Figure 17) that acts as a spring-like spacer separating the shoulders,
it also helped to anchor the shoulders in place while these animals
were wrestling their prey. Birds also exhibit fused collar bones; they
form the elongate ‘wish bone’, or furcula, that similarly acts as a
mechanical spring that separates the shoulder joints during
flapping flight.
The joints between the bones of the arm and hand were also
modified so that they could be swung outward and downward with
considerable speed and force to strike at prey in what has been
called a ‘raking’ action. When not in use, the arms could be folded
neatly against the body. The leverage for this system was also of
considerable advantage to these creatures, because the arm muscles
that powered this mechanism were located close to the chest and
operated long tendons that ran down the arm to the hand (rather
than having muscles positioned further out along the arm); this
remote control system kept the weight of the body closer to the hips
and helped to minimize the delicate problem of balance in these
theropods. The arm-striking and arm-folding mechanism is closely
similar to that employed by birds when opening and closing their
wings during and after flight.
Archaeopteryx
The early bird-like fossil Archaeopteryx (Figure 16, bottom) exhibits
many maniraptoran theropod features: the tail is a long and very
thin set of vertebrae that anchored the tail feathers on either side;
the hip bones are arranged with the pubis pointing backward and
downward; at the front of the chest there is a boomerang-like
furcula; the jaws are lined with small, spiky teeth, rather than a
more typical bird-like horny beak; the arms are long, jointed so that
they can be extended and folded just as in theropods, and the hands
are equipped with three sharply clawed fingers that in their
arrangement and proportions are identical to those seen in
maniraptoran theropods.
Chinese wonders
During the 1990s, explorations in quarries in Liaoning Province
in north-eastern China began to yield some extraordinary, and
extraordinarily well preserved, fossils of Early Cretaceous age. At
first, these comprised beautifully preserved early birds such as
Confuciusornis, and the skeletons included impressions of feathers,
34. Restoration of the living Archaeopteryx
beaks, and claws. Then in 1996, a complete skeleton of a small
theropod dinosaur, very similar in anatomy and proportions to
the well known theropod Compsognathus (Figure 14), was
described by Ji Qiang and Ji Shu’an. They named the dinosaur
Sinosauropteryx. This dinosaur was remarkable because there was
a fringe of filamentous structures along its backbone and across its
body, suggesting some sort of covering to the skin that was akin to
the ‘pile’ on a roughly made carpet; there was also evidence of soft
tissues in the eye socket and in the region of the gut. It was clear
that some small theropods had some type of body covering. These
discoveries led to concerted efforts to find more such fossils at
Liaoning; they began to appear with increasing regularity and
ushered in some truly breathtaking revelations.
Shortly after Sinosauropteryx was discovered, another skeleton
was revealed. This animal, named Protoarchaeopteryx, was the
first to show the presence of true bird-like feathers attached to its
tail and along the sides of its body, and its anatomy was much
more similar to that of dromaeosaurians than Sinosauropteryx.
Another discovery revealed an animal that was extremely similar
to Velociraptor, but this time named Sinornithosaurus (again,
apparently covered in a ‘pile’ of short filaments). Newer discoveries
have included Caudipteryx, a large (turkey-sized), rather
short-armed creature noted for a pronounced tuft of tail feathers
and shorter fringes of feathers along its arms; smaller, heavily
feathered dromaeosaurians; and in the spring of 2003 a quite
remarkable ‘four-winged’ dromaeosaurian, Microraptor, was
unveiled to the world. This latter creature was small and classically
dromaeosaur-like, with the typically long, narrow tail, bird-like
pelvis, long, grasping arms, and sharp rows of teeth lining its
jaws. The tail was fringed by primary feathers and its body
covered in downy ones. However, what was singularly impressive
was the preservation along the arms of flight feathers forming
Archaeopteryx-like wings and, very unexpectedly, similar wing-like
fringes of feathers attached to the lower parts of the legs – hence the
name ‘four-wing’.
Tuesday, June 26, 2007
Legs, heads, hearts, and lungs of Dinasaurs
Dinosaurs place their feet vertically beneath the body on straight,
pillar-like legs. The only living creatures that also adopt this
posture are birds and mammals; all the rest ‘sprawl’ with their
legs directed sideways from the body. Many dinosaurs were also
slender-limbed and apparently built for moving quickly; this
line of argument reflects the fact that Nature does not tend to do
things unnecessarily. If an animal is built as if it could run fast, it
probably did so; it might therefore seem reasonable to expect such a
creature to have an energetic ‘motor’, or endothermic physiology,
to allow it to move quickly. We do, however, need to be careful,
because it is also the case that ectotherms can move very quickly
indeed – crocodiles and Komodo dragons can outrun and catch
unwary humans! The crucial thing is that crocodiles and Komodo
dragons cannot sustain fast running – their muscles build up a large
oxygen debt very quickly and the animals then have to rest so their
muscles can recover. Endotherms, by contrast, can move quickly for
much longer periods of time because their high-pressure blood
system and efficient lungs replenish the oxygen in their muscles
very quickly.
A further refinement of this argument is the suggestion that the
ability to walk bipedally is linked exclusively to endothermy; many
mammals, all birds, and many dinosaurs are bipedal. This
argument relates not only to posture, but also to how that posture is
maintained. A quadruped has the advantage of considerable
stability when it walks. A biped is inherently unstable, and to walk
successfully a sophisticated system of sensors monitoring balance,
as well as a rapid coordinating system (the brain and central
nervous system), and rapid-response muscles to correct and
maintain balance, are essential.
The brain is central to this whole dynamic ‘problem’ and must
have a constant capacity to work quickly and efficiently. This
implies that the body is able to provide constant supplies of oxygen,
food, and heat to allow the chemistry of the brain to work optimally
all the time. The prerequisite for this type of stability is a ‘steady’
endothermic physiology. Ectotherms periodically shut down their
activity levels, when cold, for example, and reduce the supply of
nutrients to the brain, which is consequentially less sophisticated
and closely integrated to overall body functions.
Another posture-related observation can be linked to the efficiency
of the heart and its potential to sustain high activity levels. Many
birds, mammals, and dinosaurs adopt an upright body posture in
which the head is normally held at levels appreciably higher than
the position of the heart. This difference in head-heart level has
important hydrostatic consequences. Because the head is above the
heart, it has to be capable of pumping blood at high pressure ‘up’
to the brain. But the blood that is pumped at the same time with
each heartbeat from the heart to the lungs must circulate at low
pressure, otherwise it would burst the delicate capillaries that line
the lungs. To permit this pressure difference, the heart in mammals
and birds is physically divided down the middle, so that the left
side of the heart (the systemic, or head and body, circuit) can
run at a higher pressure than the right side (the pulmonary, or
lung, circuit).
All living reptiles carry their head at roughly the same level as their
heart. Their hearts are not divided down the middle like those of
mammals and birds because there is no need to differentiate
between the systemic and pulmonary circuits. Curiously, the
reptilian heart and circulation offers advantages for these creatures;
they can shunt blood around the body in ways that mammals
cannot. For example, ectotherms spend a lot of time basking in the
sun to warm their bodies. While basking, they can preferentially
shunt blood to the skin, where it can be used to absorb heat (rather
like the water in solar panel central heating pipes). The major
disadvantage of this system is that the blood cannot be circulated
under high pressure – a feature that is essential in any animal that
is behaving very actively and must bring food and oxygen to its
hard-working muscles.
The implication from all these considerations is that dinosaurs,
because of their posture, had a high-pressure blood circulation
system that was compatible with high and sustained activity levels
that are only found in living endotherms. This more comprehensive
and elaborate set of considerations resoundingly supports Richard
Owen’s provocative speculation.
Intimately associated with the efficiency of the heart and
circulatory system must be the ability to supply sufficient oxygen
to muscles to allow high levels of aerobic activity. In some groups of
dinosaurs, notably the theropods and the giant sauropodomorphs,
there are some tantalizing anatomical hints concerning lung
structure and function. In both these groups of saurischian
dinosaurs (but not the ornithischians), there are traces of distinct
pouches or cavities (called pleurocoels) in the sides of the vertebrae
of the backbone. In isolation, these might not have attracted
particular attention; however, living birds show similar features
that equate with the presence of extensive air sacs. Air sacs are
part of a bellows-like mechanism that permits birds to breathe
with remarkable efficiency. It is highly probable that saurischian
dinosaurs had bird-like, and therefore extremely efficient,
lungs.
This observation certainly supports the contention that some
dinosaurs (theropods and sauropodomorphs) had the ability to
maintain high aerobic activity levels. However, it also highlights
the fact that all dinosaurs (saurischians and ornithischians)
should not be presumed to have been the same in all aspects of
their physiology, because ornithischians show no trace of an
air-sac system.
pillar-like legs. The only living creatures that also adopt this
posture are birds and mammals; all the rest ‘sprawl’ with their
legs directed sideways from the body. Many dinosaurs were also
slender-limbed and apparently built for moving quickly; this
line of argument reflects the fact that Nature does not tend to do
things unnecessarily. If an animal is built as if it could run fast, it
probably did so; it might therefore seem reasonable to expect such a
creature to have an energetic ‘motor’, or endothermic physiology,
to allow it to move quickly. We do, however, need to be careful,
because it is also the case that ectotherms can move very quickly
indeed – crocodiles and Komodo dragons can outrun and catch
unwary humans! The crucial thing is that crocodiles and Komodo
dragons cannot sustain fast running – their muscles build up a large
oxygen debt very quickly and the animals then have to rest so their
muscles can recover. Endotherms, by contrast, can move quickly for
much longer periods of time because their high-pressure blood
system and efficient lungs replenish the oxygen in their muscles
very quickly.
A further refinement of this argument is the suggestion that the
ability to walk bipedally is linked exclusively to endothermy; many
mammals, all birds, and many dinosaurs are bipedal. This
argument relates not only to posture, but also to how that posture is
maintained. A quadruped has the advantage of considerable
stability when it walks. A biped is inherently unstable, and to walk
successfully a sophisticated system of sensors monitoring balance,
as well as a rapid coordinating system (the brain and central
nervous system), and rapid-response muscles to correct and
maintain balance, are essential.
The brain is central to this whole dynamic ‘problem’ and must
have a constant capacity to work quickly and efficiently. This
implies that the body is able to provide constant supplies of oxygen,
food, and heat to allow the chemistry of the brain to work optimally
all the time. The prerequisite for this type of stability is a ‘steady’
endothermic physiology. Ectotherms periodically shut down their
activity levels, when cold, for example, and reduce the supply of
nutrients to the brain, which is consequentially less sophisticated
and closely integrated to overall body functions.
Another posture-related observation can be linked to the efficiency
of the heart and its potential to sustain high activity levels. Many
birds, mammals, and dinosaurs adopt an upright body posture in
which the head is normally held at levels appreciably higher than
the position of the heart. This difference in head-heart level has
important hydrostatic consequences. Because the head is above the
heart, it has to be capable of pumping blood at high pressure ‘up’
to the brain. But the blood that is pumped at the same time with
each heartbeat from the heart to the lungs must circulate at low
pressure, otherwise it would burst the delicate capillaries that line
the lungs. To permit this pressure difference, the heart in mammals
and birds is physically divided down the middle, so that the left
side of the heart (the systemic, or head and body, circuit) can
run at a higher pressure than the right side (the pulmonary, or
lung, circuit).
All living reptiles carry their head at roughly the same level as their
heart. Their hearts are not divided down the middle like those of
mammals and birds because there is no need to differentiate
between the systemic and pulmonary circuits. Curiously, the
reptilian heart and circulation offers advantages for these creatures;
they can shunt blood around the body in ways that mammals
cannot. For example, ectotherms spend a lot of time basking in the
sun to warm their bodies. While basking, they can preferentially
shunt blood to the skin, where it can be used to absorb heat (rather
like the water in solar panel central heating pipes). The major
disadvantage of this system is that the blood cannot be circulated
under high pressure – a feature that is essential in any animal that
is behaving very actively and must bring food and oxygen to its
hard-working muscles.
The implication from all these considerations is that dinosaurs,
because of their posture, had a high-pressure blood circulation
system that was compatible with high and sustained activity levels
that are only found in living endotherms. This more comprehensive
and elaborate set of considerations resoundingly supports Richard
Owen’s provocative speculation.
Intimately associated with the efficiency of the heart and
circulatory system must be the ability to supply sufficient oxygen
to muscles to allow high levels of aerobic activity. In some groups of
dinosaurs, notably the theropods and the giant sauropodomorphs,
there are some tantalizing anatomical hints concerning lung
structure and function. In both these groups of saurischian
dinosaurs (but not the ornithischians), there are traces of distinct
pouches or cavities (called pleurocoels) in the sides of the vertebrae
of the backbone. In isolation, these might not have attracted
particular attention; however, living birds show similar features
that equate with the presence of extensive air sacs. Air sacs are
part of a bellows-like mechanism that permits birds to breathe
with remarkable efficiency. It is highly probable that saurischian
dinosaurs had bird-like, and therefore extremely efficient,
lungs.
This observation certainly supports the contention that some
dinosaurs (theropods and sauropodomorphs) had the ability to
maintain high aerobic activity levels. However, it also highlights
the fact that all dinosaurs (saurischians and ornithischians)
should not be presumed to have been the same in all aspects of
their physiology, because ornithischians show no trace of an
air-sac system.
Dinosaurs and warm blood
A number of areas of research on dinosaurs have attracted attention
far beyond the realm of those who take a purely academic interest in
these creatures. This common interest appears to arise because
dinosaurs capture the public imagination in a way that few other
subjects do. The following chapters focus on these topics in order
to illustrate the extraordinary variety of approaches and types of
information that are used in our attempts to unravel the mystery of
dinosaurs and their biology.
Dinosaurs: hot-, cold-, or luke-warm-blooded?
As we have seen in Chapter 1, Richard Owen, at the time of his
invention of the word ‘dinosaur’, speculated about the physiology
of dinosaurs. Extracting meaning from the rather long-winded final
sentence of his scientific report:
The Dinosaurs . . . may be concluded to have . . . [a] superior
adaptation to terrestrial life . . . approaching that which now
characterizes the warm-blooded Vertebrata. [i.e. living mammals
and birds]
(Owen 1842: 204)
Although the ‘mammaloid’ reconstructions of dinosaurs that he
created for the Crystal Palace Park clearly echo his sentiments, the
biological implications he was hinting at were never grasped by
other workers at the time. In a sense, Owen’s visionary approach
was tempered by rational Aristotelian logic: dinosaurs were
structurally reptilian, it therefore followed that they had scaly
skins, laid shelled eggs, and, like all other known reptiles, were
‘cold-blooded’ (ectothermic).
In a similar vein to Owen, Thomas Huxley proposed, almost
50 years later, that birds and dinosaurs should be considered close
relatives because of the anatomical similarities that could be
demonstrated between living birds, the earliest known fossil bird
Archaeopteryx, and the newly discovered small theropod
Compsognathus. He concluded that:
. . . it is by no means difficult to imagine a creature completely
intermediate between Dromaeus [an emu] and Compsognathus [a
dinosaur] . . . and the hypothesis that the . . . class Aves has its root
in the Dinosaurian reptiles; . . .
(Huxley 1868: 365)
If Huxley was correct, it should have been possible to ask:
were dinosaurs then conventionally reptilian (physiologically)
or were they closer to the ‘warm-blooded’ (endothermic) birds?
There appeared to be no obvious way of answering such
questions.
Despite such intellectual ‘nudges’, it was close to a century after
Huxley’s paper that palaeontologists began to search with greater
determination for data that might have a bearing on this central
question. The spur to renewed interest in the topic finds an echo in
the adoption of the broader and more integrated agenda for the
interpretation of the fossil record: the rise of palaeobiology, as
outlined in Chapter 2. We saw there how some wide-ranging
observations were strung together by Robert Bakker into a case for
endothermy in dinosaurs. Let’s now consider these and other
arguments in greater detail.
New approaches: dinosaurs as climatic proxies?
Attempts were being made to investigate the degree to which fossils
could be used to reconstruct climates in the ancient world. It is
widely recognized that endotherms (basically mammals and birds)
are not particularly good indicators of climate because they are
found everywhere, from equatorial to polar regions. Their
endothermic physiology (and clever use of body insulation) allows
them to operate more or less independently of prevailing climatic
conditions. By contrast, ectotherms, such as lizards, snakes, and
crocodiles, are reliant on ambient climatic conditions, and as a
result they tend to be found mainly in warmer climatic zones.
Using this approach to examine the geographic distribution of
obvious ectotherms and endotherms in the fossil record proved
useful, but then threw up several interesting questions. For
example, what about the immediate evolutionary ancestors of
endothermic mammals in Permian and Triassic times? Were they
also able to control their internal body temperatures? If they did,
how would it have affected their geographic distribution? And more
pointedly in this context, dinosaurs seemed to have a wide
geographic spread, so did this mean that they were capable of
controlling their body temperature rather like endotherms?
Patterns in the fossil record
The foundation of Bakker’s approach to endothermy in dinosaurs
was the pattern in the succession of animal types in the early
Mesozoic. During the time leading up to the end of the Triassic
Period synapsid reptiles were by far the most abundant and diverse
animals on land.
Right at the close of the Triassic and the beginning of the Jurassic
Period (205 Ma) the very first true mammals appeared on Earth
and were represented by small, shrew-like creatures. In complete
contrast, the latter part of the Triassic Period also marks the
appearance of the first dinosaurs (225 Ma), and across the
Triassic/Jurassic divide the dinosaurs become widespread, very
diverse, and clearly dominant members of the land fauna. This
ecological balance – rare, small, very probably nocturnal mammals
and abundant, large, and increasingly diverse dinosaurs – was then
maintained for the next 160 million years, until the close of the
Cretaceous Period (65 Ma).
As animals living in the present day, we are comfortable with the
notion that mammals are, along with birds, the most conspicuous
and diverse of land-living vertebrates. Mammals are self-evidently
fast-moving, intelligent, generally highly adaptable creatures, and
much of this present-day ‘success’ we attribute to their physiological
status: their high basal metabolic rate, which permits the
maintenance of a high and constant body temperature, complex
body chemistry, comparatively large brains, and consequently high
activity levels, and their status as endotherms. In contrast, we
generally observe that reptiles are considerably less diverse and
quite sharply climatically restricted; this is largely explained by the
fact that they have a much lower metabolic rate, rely on external
sources of heat to keep the body warm and therefore chemically
active, and have much lower and more intermittent levels of
activity: the ectothermic condition.
These, admittedly very general, observations permit us to have
expectations that can be superimposed on the fossil record. All
things being equal, we would predict that the first appearance
of true mammals at the Triassic/Jurassic boundary, in a world
otherwise dominated by reptiles, would spark the former’s rapid
evolutionary rise and diversification at the expense of the latter.
So the fossil record of mammals would be expected to show a rapid
rise in abundance and diversity in Early Jurassic times, until they
completely dominated the ecosystems of the Mesozoic Era.
However, the fossil record reveals exactly the opposite pattern: the
(reptilian) dinosaurs rose to dominance in the Late Triassic
(220 Ma) and the mammals only began to increase in size and
diversity after the dinosaurs had become extinct at the end of the
Cretaceous period (65 Ma).
Bakker’s explanation for this counterintuitive set of events was that
dinosaurs could have succeeded, evolutionarily, in the face of true
mammals only if they too had endotherm-like high basal metabolic
rates and could be as active and resourceful as contemporary
mammals. Dinosaurs quite simply had to be active endotherms – it
was to Bakker a self-evident truth. While the pattern revealed by the
fossil record was indeed clear, the scientific proof necessary to
support his ‘truth’ needed to be assembled and tested.
far beyond the realm of those who take a purely academic interest in
these creatures. This common interest appears to arise because
dinosaurs capture the public imagination in a way that few other
subjects do. The following chapters focus on these topics in order
to illustrate the extraordinary variety of approaches and types of
information that are used in our attempts to unravel the mystery of
dinosaurs and their biology.
Dinosaurs: hot-, cold-, or luke-warm-blooded?
As we have seen in Chapter 1, Richard Owen, at the time of his
invention of the word ‘dinosaur’, speculated about the physiology
of dinosaurs. Extracting meaning from the rather long-winded final
sentence of his scientific report:
The Dinosaurs . . . may be concluded to have . . . [a] superior
adaptation to terrestrial life . . . approaching that which now
characterizes the warm-blooded Vertebrata. [i.e. living mammals
and birds]
(Owen 1842: 204)
Although the ‘mammaloid’ reconstructions of dinosaurs that he
created for the Crystal Palace Park clearly echo his sentiments, the
biological implications he was hinting at were never grasped by
other workers at the time. In a sense, Owen’s visionary approach
was tempered by rational Aristotelian logic: dinosaurs were
structurally reptilian, it therefore followed that they had scaly
skins, laid shelled eggs, and, like all other known reptiles, were
‘cold-blooded’ (ectothermic).
In a similar vein to Owen, Thomas Huxley proposed, almost
50 years later, that birds and dinosaurs should be considered close
relatives because of the anatomical similarities that could be
demonstrated between living birds, the earliest known fossil bird
Archaeopteryx, and the newly discovered small theropod
Compsognathus. He concluded that:
. . . it is by no means difficult to imagine a creature completely
intermediate between Dromaeus [an emu] and Compsognathus [a
dinosaur] . . . and the hypothesis that the . . . class Aves has its root
in the Dinosaurian reptiles; . . .
(Huxley 1868: 365)
If Huxley was correct, it should have been possible to ask:
were dinosaurs then conventionally reptilian (physiologically)
or were they closer to the ‘warm-blooded’ (endothermic) birds?
There appeared to be no obvious way of answering such
questions.
Despite such intellectual ‘nudges’, it was close to a century after
Huxley’s paper that palaeontologists began to search with greater
determination for data that might have a bearing on this central
question. The spur to renewed interest in the topic finds an echo in
the adoption of the broader and more integrated agenda for the
interpretation of the fossil record: the rise of palaeobiology, as
outlined in Chapter 2. We saw there how some wide-ranging
observations were strung together by Robert Bakker into a case for
endothermy in dinosaurs. Let’s now consider these and other
arguments in greater detail.
New approaches: dinosaurs as climatic proxies?
Attempts were being made to investigate the degree to which fossils
could be used to reconstruct climates in the ancient world. It is
widely recognized that endotherms (basically mammals and birds)
are not particularly good indicators of climate because they are
found everywhere, from equatorial to polar regions. Their
endothermic physiology (and clever use of body insulation) allows
them to operate more or less independently of prevailing climatic
conditions. By contrast, ectotherms, such as lizards, snakes, and
crocodiles, are reliant on ambient climatic conditions, and as a
result they tend to be found mainly in warmer climatic zones.
Using this approach to examine the geographic distribution of
obvious ectotherms and endotherms in the fossil record proved
useful, but then threw up several interesting questions. For
example, what about the immediate evolutionary ancestors of
endothermic mammals in Permian and Triassic times? Were they
also able to control their internal body temperatures? If they did,
how would it have affected their geographic distribution? And more
pointedly in this context, dinosaurs seemed to have a wide
geographic spread, so did this mean that they were capable of
controlling their body temperature rather like endotherms?
Patterns in the fossil record
The foundation of Bakker’s approach to endothermy in dinosaurs
was the pattern in the succession of animal types in the early
Mesozoic. During the time leading up to the end of the Triassic
Period synapsid reptiles were by far the most abundant and diverse
animals on land.
Right at the close of the Triassic and the beginning of the Jurassic
Period (205 Ma) the very first true mammals appeared on Earth
and were represented by small, shrew-like creatures. In complete
contrast, the latter part of the Triassic Period also marks the
appearance of the first dinosaurs (225 Ma), and across the
Triassic/Jurassic divide the dinosaurs become widespread, very
diverse, and clearly dominant members of the land fauna. This
ecological balance – rare, small, very probably nocturnal mammals
and abundant, large, and increasingly diverse dinosaurs – was then
maintained for the next 160 million years, until the close of the
Cretaceous Period (65 Ma).
As animals living in the present day, we are comfortable with the
notion that mammals are, along with birds, the most conspicuous
and diverse of land-living vertebrates. Mammals are self-evidently
fast-moving, intelligent, generally highly adaptable creatures, and
much of this present-day ‘success’ we attribute to their physiological
status: their high basal metabolic rate, which permits the
maintenance of a high and constant body temperature, complex
body chemistry, comparatively large brains, and consequently high
activity levels, and their status as endotherms. In contrast, we
generally observe that reptiles are considerably less diverse and
quite sharply climatically restricted; this is largely explained by the
fact that they have a much lower metabolic rate, rely on external
sources of heat to keep the body warm and therefore chemically
active, and have much lower and more intermittent levels of
activity: the ectothermic condition.
These, admittedly very general, observations permit us to have
expectations that can be superimposed on the fossil record. All
things being equal, we would predict that the first appearance
of true mammals at the Triassic/Jurassic boundary, in a world
otherwise dominated by reptiles, would spark the former’s rapid
evolutionary rise and diversification at the expense of the latter.
So the fossil record of mammals would be expected to show a rapid
rise in abundance and diversity in Early Jurassic times, until they
completely dominated the ecosystems of the Mesozoic Era.
However, the fossil record reveals exactly the opposite pattern: the
(reptilian) dinosaurs rose to dominance in the Late Triassic
(220 Ma) and the mammals only began to increase in size and
diversity after the dinosaurs had become extinct at the end of the
Cretaceous period (65 Ma).
Bakker’s explanation for this counterintuitive set of events was that
dinosaurs could have succeeded, evolutionarily, in the face of true
mammals only if they too had endotherm-like high basal metabolic
rates and could be as active and resourceful as contemporary
mammals. Dinosaurs quite simply had to be active endotherms – it
was to Bakker a self-evident truth. While the pattern revealed by the
fossil record was indeed clear, the scientific proof necessary to
support his ‘truth’ needed to be assembled and tested.
Dinosaurs: a global perspective
In more recent times, this approach has been applied much more
broadly and in a much more ambitious way. Paul Upchurch of
University College London and Craig Hunn at Cambridge hoped to
explore the entire family tree of the Dinosauria for evidence of
similarities in patterns of stratigraphic ranges and cladistic
patterns by looking at large numbers of dinosaurs. These were
compared to the currently established distributions of the
continents at intervals through the entire Mesozoic Era. An
attempt was being made to find out whether an overall signal did
emerge that was suggestive of a tectonic influence on the
evolutionary history of all dinosaurs.
Despite the inevitable ‘noise’ in the system resulting largely from
the incompleteness of the fossil record of dinosaurs, it was
heartening to note that statistically significant coincident patterns
emerged within the Middle Jurassic, the Late Jurassic, and the
Early Cretaceous intervals. This indicates that tectonic events do, as
expected, play some role in determining where and when particular
groups of dinosaurs flourished. What is more, this effect has also
been preserved in the stratigraphic and geographic distributions of
other fossil organisms, so the evolutionary history of great swathes
of organisms was effected by tectonic events and the imprint is still
with us today. In a way, this is not new. I need only point to the
unusual distribution of marsupial mammals (found only in the
Americas and Australasia today), and the fact that distinct areas of
the modern world have their own characteristic fauna and flora.
What this new research suggests is that we may well be able to trace
the historical reasons for these distributions far more accurately
than we had supposed possible.
broadly and in a much more ambitious way. Paul Upchurch of
University College London and Craig Hunn at Cambridge hoped to
explore the entire family tree of the Dinosauria for evidence of
similarities in patterns of stratigraphic ranges and cladistic
patterns by looking at large numbers of dinosaurs. These were
compared to the currently established distributions of the
continents at intervals through the entire Mesozoic Era. An
attempt was being made to find out whether an overall signal did
emerge that was suggestive of a tectonic influence on the
evolutionary history of all dinosaurs.
Despite the inevitable ‘noise’ in the system resulting largely from
the incompleteness of the fossil record of dinosaurs, it was
heartening to note that statistically significant coincident patterns
emerged within the Middle Jurassic, the Late Jurassic, and the
Early Cretaceous intervals. This indicates that tectonic events do, as
expected, play some role in determining where and when particular
groups of dinosaurs flourished. What is more, this effect has also
been preserved in the stratigraphic and geographic distributions of
other fossil organisms, so the evolutionary history of great swathes
of organisms was effected by tectonic events and the imprint is still
with us today. In a way, this is not new. I need only point to the
unusual distribution of marsupial mammals (found only in the
Americas and Australasia today), and the fact that distinct areas of
the modern world have their own characteristic fauna and flora.
What this new research suggests is that we may well be able to trace
the historical reasons for these distributions far more accurately
than we had supposed possible.
Ornithopod evolution
The earliest work in this field of research, carried out in 1984,
concerned a group of dinosaurs that are quite closely related to
the familiar Iguanodon. Generally, these types of dinosaur are
known as ornithopods (‘bird feet’ – this comes from a passing,
trivial resemblance in the structure of the feet of these dinosaurs
to those of modern birds). Comparing in some detail the anatomy
of a number of the then known ornithopods, a cladogram was
constructed. To convert this into a genuine phylogeny it was
necessary to chart on to the cladogam the known distribution of this
group through time and their geographic distributions.
Some surprising patterns in the history of these ornithopod
dinosaurs emerged from this analysis. First it seemed to
demonstrate that the forms most closely related to Iguanodon
(that is to say, members of the group known as iguanodonts) and
their closest relatives (members of the hadrosaur family) probably
originated as a result of continental separation during Late Jurassic
times. The ancestral population from which both groups may have
evolved became subdivided by a seaway at this time. Following this
isolation, one population evolved into the hadrosaurs in Asia, while
iguanodonts evolved elsewhere. These two groups appear to have
evolved distinct from one another through the Late Jurassic and
Early Cretaceous period. However, during the latter half of the
Cretaceous, Asia became reconnected to the rest of the northern
hemisphere continents and its hadrosaurs were apparently able to
spread across the northern hemisphere pretty much unhindered
and replaced iguanodonts wherever they came into contact.
While the pattern of replacement of iguanodonts by hadrosaurs in
Late Cretaceous times appeared to be reasonably uniform, there
were one or two puzzling anomalies that needed to be investigated.
There were reports, written at the turn of the 20th century, of
iguanodonts from Europe (primarily France and Romania) in
rocks of very latest Cretaceous age. From the analysis above, these
would not have been expected to have survived into Late Cretaceous
times because everywhere else the pattern was one of hadrosaurs
replacing iguanodonts. In the early 1990s, the best-preserved
material came from Transylvania, a region of Romania. However,
the phylogenetic analysis prompted expeditions to reinvestigate
these discoveries. Fresh study proved that this dinosaur was not a
close relative of Iguanodon, but represented an unusually
long-lasting (relict) member of a more primitive group of
ornithopods. An entirely new name was created for this dinosaur:
Zalmoxes. So, one of the outcomes of the preliminary analysis was a
great deal of new information about an old, but apparently not so
well understood, dinosaur.
A report published in the 1950s suggested that a very Iguanodonlike
dinosaur lived in Mongolia in Early Cretaceous times. This
tantalizing report also needed to be investigated further to check
whether its anomalous geographic range – in Asia in Early
Cretaceous times – was real or, as in the Romanian example,
another case of mistaken identity. The material, though
fragmentary, was stored in the Russian Palaeontological Museum
in Moscow, and had to be re-examined. What emerged was again
not as expected. This time the earlier reports proved correct, the
genus Iguanodon itself seemed to be present in Mongolia in Early
Cretaceous times, and the pieces recovered were indistinguishable
from the very well known European Iguanodon.
This second discovery did not fit at all comfortably with the
evolutionary and geographic hypothesis that had been created in
the 1984 analysis. Indeed, in more recent years a suite of very
interesting Iguanodon-like ornithopods have emerged in Asia,
as well as North America, in what can best be described as
‘middle’ Cretaceous times. Much of this very recent, and steadily
accumulating, evidence suggests that the original evolutionary and
geographic model had a number of fundamental flaws that
continued investigation and new discoveries were able to expose.
concerned a group of dinosaurs that are quite closely related to
the familiar Iguanodon. Generally, these types of dinosaur are
known as ornithopods (‘bird feet’ – this comes from a passing,
trivial resemblance in the structure of the feet of these dinosaurs
to those of modern birds). Comparing in some detail the anatomy
of a number of the then known ornithopods, a cladogram was
constructed. To convert this into a genuine phylogeny it was
necessary to chart on to the cladogam the known distribution of this
group through time and their geographic distributions.
Some surprising patterns in the history of these ornithopod
dinosaurs emerged from this analysis. First it seemed to
demonstrate that the forms most closely related to Iguanodon
(that is to say, members of the group known as iguanodonts) and
their closest relatives (members of the hadrosaur family) probably
originated as a result of continental separation during Late Jurassic
times. The ancestral population from which both groups may have
evolved became subdivided by a seaway at this time. Following this
isolation, one population evolved into the hadrosaurs in Asia, while
iguanodonts evolved elsewhere. These two groups appear to have
evolved distinct from one another through the Late Jurassic and
Early Cretaceous period. However, during the latter half of the
Cretaceous, Asia became reconnected to the rest of the northern
hemisphere continents and its hadrosaurs were apparently able to
spread across the northern hemisphere pretty much unhindered
and replaced iguanodonts wherever they came into contact.
While the pattern of replacement of iguanodonts by hadrosaurs in
Late Cretaceous times appeared to be reasonably uniform, there
were one or two puzzling anomalies that needed to be investigated.
There were reports, written at the turn of the 20th century, of
iguanodonts from Europe (primarily France and Romania) in
rocks of very latest Cretaceous age. From the analysis above, these
would not have been expected to have survived into Late Cretaceous
times because everywhere else the pattern was one of hadrosaurs
replacing iguanodonts. In the early 1990s, the best-preserved
material came from Transylvania, a region of Romania. However,
the phylogenetic analysis prompted expeditions to reinvestigate
these discoveries. Fresh study proved that this dinosaur was not a
close relative of Iguanodon, but represented an unusually
long-lasting (relict) member of a more primitive group of
ornithopods. An entirely new name was created for this dinosaur:
Zalmoxes. So, one of the outcomes of the preliminary analysis was a
great deal of new information about an old, but apparently not so
well understood, dinosaur.
A report published in the 1950s suggested that a very Iguanodonlike
dinosaur lived in Mongolia in Early Cretaceous times. This
tantalizing report also needed to be investigated further to check
whether its anomalous geographic range – in Asia in Early
Cretaceous times – was real or, as in the Romanian example,
another case of mistaken identity. The material, though
fragmentary, was stored in the Russian Palaeontological Museum
in Moscow, and had to be re-examined. What emerged was again
not as expected. This time the earlier reports proved correct, the
genus Iguanodon itself seemed to be present in Mongolia in Early
Cretaceous times, and the pieces recovered were indistinguishable
from the very well known European Iguanodon.
This second discovery did not fit at all comfortably with the
evolutionary and geographic hypothesis that had been created in
the 1984 analysis. Indeed, in more recent years a suite of very
interesting Iguanodon-like ornithopods have emerged in Asia,
as well as North America, in what can best be described as
‘middle’ Cretaceous times. Much of this very recent, and steadily
accumulating, evidence suggests that the original evolutionary and
geographic model had a number of fundamental flaws that
continued investigation and new discoveries were able to expose.
Dinosaur systematics and ancient biogeography
This type of research can have interesting, if slightly unexpected,
spin-offs. One spin-off that will be considered here links
phylogenetics with the geographic history of the Earth. The Earth
may in fact have exerted a profound influence on the overall pattern
of life.
Unravelling the genealogy of dinosaurs
The geological timescale of the Earth was pieced together through
painstaking analysis of the relative ages of sequences of rocks
exposed at various places on Earth. One important component
that assisted this process was the evidence of the fossils that they
contained: if rocks from different places contained fossils of exactly
the same type, then it could be assumed with reasonable confidence
that the rocks were of the same relative age.
In broadly similar fashion, evidence of the similarity of fossils from
different parts of the world began to suggest that the continents
might not have been as fixed in their positions as they appear to be
today. For example, it had been noted that rocks and the fossils that
they contained seemed to be remarkably similar on either side of
the southern Atlantic Ocean. A small aquatic reptile Mesosaurus
was known to exist in remarkably similar-looking Permian rocks in
Brazil and in South Africa. As long ago as 1620, Francis Bacon had
pointed out that the coastlines of the Americas and Europe and
Africa seemed remarkably similar, (see Figure 32d) to the extent
that it seemed as if they could have fitted together as a pair of
gigantic jigsaw pieces. On the basis of evidence from fossils, rocks,
and general shape correspondence, Alfred Wegener, a German
meteorologist, suggested in 1912 that at times in the past the
continents of the Earth must have occupied different positions to
the ones they are in today, with, for example, the Americas and
Eur-Africa nestled together in the Permian Period. Because he was
not a trained geologist, Wegener’s views were ignored, or dismissed
as irrelevant and unprovable speculations. For all its self-evident
persuasiveness, Wegener’s theory lacked a mechanism: common
sense dictated that it was impossible to move things the size of
continents across the solid surface of the Earth.
However, common sense proved to be deceptive. In the 1950s
and 1960s, a series of observations accumulated that supported
Wegener’s views. Firstly, very detailed models of all the major
continents showed that they did indeed fit together remarkably
neatly and with a correspondence that could not be accounted for
by chance. Secondly, major geological features on separate
continents became continuous when continents were reassembled
jigsaw-like. And finally, palaeomagnetic evidence demonstrated the
phenomenon of sea-floor spreading – that the ocean floors were
moving like huge conveyor belts carrying the continents – and the
historical remnants of magnetism in continental rocks confirmed
that the continents had moved over time. The ‘motor’ that was
driving this motion was in effect the heat at the core and the fluidity
of rocks in the mantle layer inside the Earth. The theory of plate
tectonics that accounts for the movement of continents over the
surface of the Earth over time is now well established and
corroborated.
From a dinosaur evolutionary perspective, the implications of
plate tectonics are extremely interesting. Reconstructions of past
configurations of the continents, largely based on palaeomagnetics
and detailed stratigraphy, indicate that at the time of their origin all
the continents were lying clustered together in a single gigantic
landmass, known as Pangaea (‘all Earth’) (Figure 32a). Dinosaurs at
this time were quite literally capable of walking all over the Earth,
and in reflection of this it appears to be the case that the fossil
remains of rather similar types (theropods and prosauropods) have
been found on nearly all continents.
During subsequent Periods, the Jurassic (Figure 32b) and
Cretaceous (Figure 32c), it is evident that the supercontinent began
to fragment as the immensely powerful tectonic conveyor belts
imperceptibly, but remorselessly, wrenched Pangaea apart. The end
product of this process at the close of the Cretaceous was a world
that, though still different geographically (note particularly the
position of India in Figure 32c), has some very familiar-looking
continents.
The earliest dinosaurs seem to have been able to disperse across
much of Pangaea, judging by their fossils. However, during the
Jurassic and subsequent Cretaceous Periods it was clearly the case that the unified supercontinent became gradually subdivided by
intervening seaways as continent-sized fragments gradually drifted
apart.
An inevitable biological consequence of this intrinsic (Earth-bound)
process of continental sundering is that the once cosmopolitan
population of dinosaurs became progressively subdivided and
isolated. The phenomenon of isolation is one of the keystones of
organismal evolution – once isolated, populations of organisms
tend to undergo evolutionary change in response to local changes to
their immediate environment. In this instance, although we are
dealing with comparatively huge (continent-sized) areas, each of
the continental fragments carried its own population of dinosaurs
(and associated fauna and flora); each of which, with the passing
time, had the opportunity to evolve independently in response to
local changes in environment, stimulated by, for example,
progressive changes in latitude, longitude, adjacent oceanic
currents, and prevailing atmospheric conditions.
Logic dictates that it must clearly have been the case that tectonic
events during the Mesozoic affected the scope and overall pattern of
32(d). The continents as they are today. Close the Atlantic Ocean and
the Americas fit neatly against West Africa.
the evolutionary history of dinosaurs. Indeed, it seems perfectly
reasonable to suppose that the progressive fragmentation of
ancestral populations over time must have done much to accelerate
the diversification of the group as a whole. Just as we can
represent the phylogeny of dinosaurs using cladograms, we could
also represent the geographic history of the Earth through
the Mesozoic Era as a series of branching events as
continental areas separated from the ‘ancestral’ Pangaean Earth.
Of course, this general approach is a simplification of true
Earth history because, on occasion, continental fragments have
coalesced, welding together previously isolated populations.
But at least as a first approximation, this provides a fertile area
for investigating some of the larger-scale events in Earth
history.
If this model of the natural history of dinosaurs were generally true,
we might expect to be able to detect some evidence in its support by
probing the details of the fossil record of dinosaur species, and the
tectonic models of continental distribution through the Mesozoic.
This type of approach has been developed in recent years to probe
for coincident patterns in the evolutionary history of dinosaurs and
whether their evolutionary history is echoed in their geographic
distribution.
spin-offs. One spin-off that will be considered here links
phylogenetics with the geographic history of the Earth. The Earth
may in fact have exerted a profound influence on the overall pattern
of life.
Unravelling the genealogy of dinosaurs
The geological timescale of the Earth was pieced together through
painstaking analysis of the relative ages of sequences of rocks
exposed at various places on Earth. One important component
that assisted this process was the evidence of the fossils that they
contained: if rocks from different places contained fossils of exactly
the same type, then it could be assumed with reasonable confidence
that the rocks were of the same relative age.
In broadly similar fashion, evidence of the similarity of fossils from
different parts of the world began to suggest that the continents
might not have been as fixed in their positions as they appear to be
today. For example, it had been noted that rocks and the fossils that
they contained seemed to be remarkably similar on either side of
the southern Atlantic Ocean. A small aquatic reptile Mesosaurus
was known to exist in remarkably similar-looking Permian rocks in
Brazil and in South Africa. As long ago as 1620, Francis Bacon had
pointed out that the coastlines of the Americas and Europe and
Africa seemed remarkably similar, (see Figure 32d) to the extent
that it seemed as if they could have fitted together as a pair of
gigantic jigsaw pieces. On the basis of evidence from fossils, rocks,
and general shape correspondence, Alfred Wegener, a German
meteorologist, suggested in 1912 that at times in the past the
continents of the Earth must have occupied different positions to
the ones they are in today, with, for example, the Americas and
Eur-Africa nestled together in the Permian Period. Because he was
not a trained geologist, Wegener’s views were ignored, or dismissed
as irrelevant and unprovable speculations. For all its self-evident
persuasiveness, Wegener’s theory lacked a mechanism: common
sense dictated that it was impossible to move things the size of
continents across the solid surface of the Earth.
However, common sense proved to be deceptive. In the 1950s
and 1960s, a series of observations accumulated that supported
Wegener’s views. Firstly, very detailed models of all the major
continents showed that they did indeed fit together remarkably
neatly and with a correspondence that could not be accounted for
by chance. Secondly, major geological features on separate
continents became continuous when continents were reassembled
jigsaw-like. And finally, palaeomagnetic evidence demonstrated the
phenomenon of sea-floor spreading – that the ocean floors were
moving like huge conveyor belts carrying the continents – and the
historical remnants of magnetism in continental rocks confirmed
that the continents had moved over time. The ‘motor’ that was
driving this motion was in effect the heat at the core and the fluidity
of rocks in the mantle layer inside the Earth. The theory of plate
tectonics that accounts for the movement of continents over the
surface of the Earth over time is now well established and
corroborated.
From a dinosaur evolutionary perspective, the implications of
plate tectonics are extremely interesting. Reconstructions of past
configurations of the continents, largely based on palaeomagnetics
and detailed stratigraphy, indicate that at the time of their origin all
the continents were lying clustered together in a single gigantic
landmass, known as Pangaea (‘all Earth’) (Figure 32a). Dinosaurs at
this time were quite literally capable of walking all over the Earth,
and in reflection of this it appears to be the case that the fossil
remains of rather similar types (theropods and prosauropods) have
been found on nearly all continents.
During subsequent Periods, the Jurassic (Figure 32b) and
Cretaceous (Figure 32c), it is evident that the supercontinent began
to fragment as the immensely powerful tectonic conveyor belts
imperceptibly, but remorselessly, wrenched Pangaea apart. The end
product of this process at the close of the Cretaceous was a world
that, though still different geographically (note particularly the
position of India in Figure 32c), has some very familiar-looking
continents.
The earliest dinosaurs seem to have been able to disperse across
much of Pangaea, judging by their fossils. However, during the
Jurassic and subsequent Cretaceous Periods it was clearly the case that the unified supercontinent became gradually subdivided by
intervening seaways as continent-sized fragments gradually drifted
apart.
An inevitable biological consequence of this intrinsic (Earth-bound)
process of continental sundering is that the once cosmopolitan
population of dinosaurs became progressively subdivided and
isolated. The phenomenon of isolation is one of the keystones of
organismal evolution – once isolated, populations of organisms
tend to undergo evolutionary change in response to local changes to
their immediate environment. In this instance, although we are
dealing with comparatively huge (continent-sized) areas, each of
the continental fragments carried its own population of dinosaurs
(and associated fauna and flora); each of which, with the passing
time, had the opportunity to evolve independently in response to
local changes in environment, stimulated by, for example,
progressive changes in latitude, longitude, adjacent oceanic
currents, and prevailing atmospheric conditions.
Logic dictates that it must clearly have been the case that tectonic
events during the Mesozoic affected the scope and overall pattern of
32(d). The continents as they are today. Close the Atlantic Ocean and
the Americas fit neatly against West Africa.
the evolutionary history of dinosaurs. Indeed, it seems perfectly
reasonable to suppose that the progressive fragmentation of
ancestral populations over time must have done much to accelerate
the diversification of the group as a whole. Just as we can
represent the phylogeny of dinosaurs using cladograms, we could
also represent the geographic history of the Earth through
the Mesozoic Era as a series of branching events as
continental areas separated from the ‘ancestral’ Pangaean Earth.
Of course, this general approach is a simplification of true
Earth history because, on occasion, continental fragments have
coalesced, welding together previously isolated populations.
But at least as a first approximation, this provides a fertile area
for investigating some of the larger-scale events in Earth
history.
If this model of the natural history of dinosaurs were generally true,
we might expect to be able to detect some evidence in its support by
probing the details of the fossil record of dinosaur species, and the
tectonic models of continental distribution through the Mesozoic.
This type of approach has been developed in recent years to probe
for coincident patterns in the evolutionary history of dinosaurs and
whether their evolutionary history is echoed in their geographic
distribution.
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