Legs heads hearts and lungs

Dinosaurs place their feet vertically beneath the body on straight, pillar-like legs. The only living creatures that also adopt this M 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 0 activity levels, when cold, for example, and reduce the supply of o nutrients to the brain, which is consequentially less sophisticated S and closely integrated to overall body functions. §

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