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.

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