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.

Ornithischian dinosaurs

All ornithischians are thought to have been herbivorous and, rather
like modern-day mammals, they seem to be far more diverse, and
numerous, than their potential predators.
Thyreophorans (Figure 28) are a major group of ornithischians that are characterized by bearing bony plates in their body wall, clubs or
spikes adorning their tails, and for having an almost exclusively
quadrupedal method of locomotion. These types of dinosaur
include the stegosaurs, named after the iconic Stegosaurus (well
known for its tiny head, the rows of large bony plates on its back,
and its spiky tail (Figure 31)); and the heavily armoured
ankylosaurs including such creatures as Euoplocephalus. The latter
was a huge tank-like animal that was so heavily armour-plated that
even its eyelids were reinforced by bony shutters and its tail was
terminated in a huge, bony club that it presumably used to skittle
potential predators.

Cerapodans (Figure 28) were very different to thyreophorans. These
were typically lightly built, unarmoured bipeds, although a few did
revert to quadrupedal methods of locomotion. Ornithopods were
one major group of cerapodans. Many of these dinosaurs were
medium-sized (2–5 metres long) and quite abundant (probably
filling the ecological niches occupied by antelopes, deer, sheep, and
goats today). These animals, such as Hypsilophodon, were balanced
at the hip ( just like theropods), had slender legs for fast running,
grasping hands, and, most importantly, teeth, jaws, and cheeks
adapted for a diet of plants. Throughout the reign of the dinosaurs,
small to medium-sized ornithopods were quite abundant, but
through the Mesozoic a significant number of larger types evolved;
these are known as iguanodontians (because they include animals
such as Iguanodon). Most important of all the iguanodontians were
the extraordinarily numerous duck-billed, or hadrosaurian,
dinosaurs of the Late Cretaceous of North America and Asia. Some
(but not all) of these dinosaurs did indeed have rather duck-shaped
snouts, and others had a wide range of quite extravagant, hollowcrested
headgear (see Chapter 7); this headgear may well have been
used for social signalling, and more particularly for making loud,
honking sounds. Marginocephalians were the other major
cerapodan group and appeared in Cretaceous times. These included
the extraordinary pachycephalosaurs (‘thick-headed dinosaurs’);
they had bodies that were very similar in general appearance to the
ornithopods, but their heads were very odd-looking. The majority
had a high dome of bone on the top, which looked vaguely similar to
the headgear of hadrosaurians, except for the fact that
pachycephalosaur headgear was made of solid bone. It has been
suggested that these creatures were the ‘headbangers’ of the
Cretaceous world – perhaps using head clashing in similar fashion
to that seen among some cloven-hooved animals today.
Finally, there were the ceratopians, a group of dinosaurs that
included the fabled Protoceratops referred to in the Introduction,
as well as the well-known Triceratops (‘three-horned face’). All
had a singular narrow beak at the tip of the jaws and tended to
have a ruff-like collar of bone at the back edge of the skull. While
some of these dinosaurs, particularly the early ones, maintained
a bipedal way of life, a considerable number grew greatly in
body size, with an enlarged head, which was adorned with a
huge frill-like collar and large eyebrow and nose horns. Their
great bulk and heavy head led them to adopt a four-footed
stance, and their similarity to modern-day rhinoceros has
not gone unnoticed. Clearly, as this all too brief survey shows,
dinosaurs were many and varied, judging by the discoveries
made over the past 200 years. But even though to date
about 900 genera of dinosaurs are known, this is only a tiny
fraction of the dinosaurs that lived during the 160 million
years of their reign during the Mesozoic Era. Many of these
will, unfortunately, never be known: their fossils were never
preserved. Others will be discovered by intrepid dinosaur hunters
in years to come.