Every breath you take

For prosauropods and especially sauropods, the trachea (wind-pipe) would have been exceptionally long, approximately the same length as the arteries carrying blood from the heart to the brain. The trachea brings oxygen into contact with the alveoli in the lungs, sites where oxygen is transmitted to the blood and where carbon dioxide is passed back to the air.

In animals that pass air bidirectional^ into and out of the lungs (that is, during inhalation and exhalation) like a bellows (mammals, lizards, crocodilians, and snakes), the trachea creates physiological dead space: some portion ofthe inhaled air never reaches the lungs. It is simply brought into the respiratory system and returned without being involved in oxygen-carbon dioxide exchange.

By contrast, birds have unidirectional air flow, in which when inhaled air passes into the lungs, nearly all the oxygen is absorbed into the blood stream, and the now oxygen-depleted air is run through a series of air sacs around the lungs and back into the trachea for exhalation. Obviously, the avian system wrings more oxygen out of the air than the bidirectional, bellows-style lungs found in mammals and other tetrapods (Figure B8.1.1).

In animals that have long necks, the problem of physiological dead space can be acute without unidirectional air flow. Long-necked bidirectional breathers such as giraffes, circumvent the problem of dead space by having an inordinately narrow trachea: dead space is reduced by limiting the surface area of the trachea. In fact, it is thought by some that giraffes may be the longest-necked animals capable of combining bellows-style lungs and a very long trachea. That being the case, sauropodomorphs may have had unidirectional, avian-style lungs in order to eliminate the problems associated with all of that physiological dead space engendered by the very long trachea. In this case, perhaps the development of pleurocoels may be interpreted as related to respiration, as in living birds. And given the elongation ofthe neck region in Saurischia as a whole, isn't it possible that unidirectional breathing should be described as saurischian and not "avian?"

Giraffes Trachea

Figure B8.1.1. Unidirectional respiration, shown diagrammatically. As the animal inhales (a), air enters the lungs and posterior air sacs (here represented by a single sac), which expand. Air that goes into the lungs is deoxygenated, and then stored in the anterior air sacs (here represented by a single sac), which expand and fill with deoxygenated air. As the animal exhales (b), the posterior air sac contracts, and its air - still oxygenated - is pumped through the lungs, where it is deoxygenated. The rest of the deoxygenated air, in the lungs and anterior air sac, is expelled via contraction out of the trachea.

Figure B8.1.1. Unidirectional respiration, shown diagrammatically. As the animal inhales (a), air enters the lungs and posterior air sacs (here represented by a single sac), which expand. Air that goes into the lungs is deoxygenated, and then stored in the anterior air sacs (here represented by a single sac), which expand and fill with deoxygenated air. As the animal exhales (b), the posterior air sac contracts, and its air - still oxygenated - is pumped through the lungs, where it is deoxygenated. The rest of the deoxygenated air, in the lungs and anterior air sac, is expelled via contraction out of the trachea.

well-developed thumb claw could have played a role, ripping vegetation off plants into bite-sized strips.

Swallowing sped the bolus of food down its long journey through the esophagous (tube leading to the stomach) whereupon it entered the abdomen, and in particular, the gizzard. This muscular chamber, sitting just ahead of the glandular part of the stomach, contained a collection of gastroliths (see Chapter 5). Contraction of the walls of the gizzard churned the gastroliths, grinding the food among them as it passed further along in the gut.

Figure 8.17. Front view (a) and left lateral view (b) of one of the back vertebrae of Brachiosaurus, with pleurocoels indicated in cross-section (c).

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Figure 8.17. Front view (a) and left lateral view (b) of one of the back vertebrae of Brachiosaurus, with pleurocoels indicated in cross-section (c).

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In all sauropodomorphs, the gut must have been capacious, even considering the forward projecting pubis (in contrast to all ornithischians, which rotated the pubis rearward to accommodate an enlarged gut; see introduction to Part III: Saurischia). Sauropods likely had an exceptionally large fermentation chamber (or chambers) that would have housed endosymbionts; that is, bacteria that lived within the gut of the dinosaur. The endosymbi-onts would have chemically broken down the cell walls of the plant food, thereby liberating whatever nutrition was to be had. Considering the size of the abdominal cavity in sauro-podomorphs, these animals probably fed on foliage with high fiber content (see Chapter 13); perhaps they also had low rates of passage of food through the gut in order to ensure a high level of nutrient extraction from such low-quality food. We can only conclude that these huge animals, with their comparatively small mouths, must have been constant feeders to acquire enough nutrition to maintain themselves. The digestive tract of a sauropod had to have been a non-stop, if low-speed, conveyor belt.

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