Another posture-related observation can be linked to the efficiency b 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 M 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.

Lung tissue

Lung tissue

33. Bird air sacs provide for a highly efficient respiratory system

This observation certainly supports the contention that some dinosaurs (theropods and sauropodomorphs) had the ability to 0

maintain high aerobic activity levels. However, it also highlights o the fact that all dinosaurs (saurischians and ornithischians) S

should not be presumed to have been the same in all aspects of d their physiology, because ornithischians show no trace of an |

air-sac system. b

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