Sauropoda

Design

Getting really big takes some serious evolution, and sauropods were really big dinosaurs. Yet, the sophisticated sauropod design, once it appeared, remained unique and little changed during their 140 million years on Earth (Figure 8.8).

The skull itself was distinctive: the tooth row was not inset, as one sees in mammalian and ornithischian herbivores. The teeth, depending upon the sauropod, had simple crowns

Borophaginae Skeleton
(a)

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Figure 8.9. Dorsal view of the skull of (a) Brachiosaurus and (b) Diplodocus. Note the dorsally placed external nares, especially in Diplodocus (arrow).

and were triangular, spatulate, or slender and pencil-like (see Figure 8.7). There is even a tendency in the clade to limit the teeth to the front of the jaws. In most cases, there is not even a complete mouthful of teeth, let alone the dental batteries seen in other dinosaurian herbivores (see introduction to Part III: Saurischia, and Chapter 7). The obvious conclusion is that chewing was not a big part of life as a sauropod.

Sauropod skulls tended to be delicately built, with large openings. The skulls appear absurdly tiny - until you realize that only an idiot would design a large, heavy skull at the end of an extremely long neck. The external nares, instead of residing at the tip of the snout, had an as yet unexplained phylogenetic tendency to migrate upward, toward the top of the head (Figures 8.9 and 8.10).

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Figure 8.10. Left latéral view of the skull of (a) Shunosaurus, (b) Brachiosaurus, (c) Camarasaurus, (d) Diplodocus, and (e) Nemegtosaurus.

Diplodocus Migrate

The "extremely long neck" turns out to have been made up of a complex system of girders and air pockets that maximized lightness and strength. Distinctive in sauropods were the Y-shaped neural arches on the vertebrae. These held the nuchal ligament, an elastic rope of connective tissue that ran down the back of the animal and supported the head and neck, so that it was not held up exclusively by muscles (Figure 8.11).

Sauropods were quadrupeds, having secondarily evolved a quadrupedal stance from their bipedal ancestors. The limbs were pillar-like, and would have done a Greek temple proud. The bones are composed of denser material than that found in the upper parts of the skeleton, an adaptation locating the weight and strength in the skeleton where it was most needed. The hindlimbs articulated with an immense, robust pelvis.

Figure 8.11. Anterior neck vertebrae in Diplodocus. The neural spines are bifurcated, and are thought to have held a ligament supporting the neck, the nuchal ligament (shown in solid blue) running from the head, down the neck, and beyond.
(b)

Figure 8.12. Sauropod left (a) forelimb and (b) hindlimb. The "hand" is far more digitigrade than the foot, which is nearly plantigrade (as shown in the limb cross-sections).

The forefeet (the "hands," as it were, on the forelimb) were digitigrade, which means that the animal was standing on its finger tips. The fingers were arranged in a nearly symmetrical horseshoe-shaped semicircle, and the first digit (the thumb) carried a large claw (Figure 8.12a). By contrast, the hindfeet were semi-plantigrade, which means that the animal's weight was supported along the lengths of its toe bones.1 The foot was asymmetrical, and generally had three large claws (on digits I, II, and III; Figure 8.12b). Sometimes the trackways reveal the impression of a heel pad which nestled behind the claws of both the fore- and hindfeet, and supported the body as well. Sauropod footprints are immense, a single print not uncommonly spanning as much as 1 m!

1. For reference, humans are supported along the lengths of their toe and foot bones, and are thus fully plantigrade.

Despite the fact that the trunk of sauropods was relatively broad (although not as proportionately broad as that seen in ankylosaurs), most sauropod trackways tend to be quite narrow, with the feet aligned toward the midline of the body. Most significantly, relatively few trackways include a tail-drag mark, providing strong evidence that many sauropods carried their immense, whip-like tails - as long as 15 m in the longest cases - entirely off of the ground (Figure 8.13).

Trackway Morrison Formation
Figure 8.13. Five parallel trackways of Late Jurassic age, Morrison Formation, Colorado, USA. Tracks are thought to have been made by diplodocids walking alongside each other. Notice the absence of any mark made by the tail.

Thoughts of a sauropod

This section will necessarily be quite short because, on brain size alone, sauropods did not have obvious pretensions to deep thought. The fact is that sauropods had the smallest brains for their body size (and the lowest EQs; see Box 12.4) of any dinosaur. Yet their long, successful record of survival speaks volumes; as we shall see, their behavioral repertoire may have been more sophisticated than one might expect for an animal with proportionally so small a brain.

Lifestyles of the huge and ancient

A place to roam. When we find the remains of these magnificent animals, they come from a myriad different environments, from river floodplains to sandy deserts. Some environments, such as those of the Upper Jurassic Morrison Formation in the American West, required sauropods to cope with long dry seasons during the year. Annual droughts may have been severe enough to have forced sauropods to migrate, a point to which we will return when considering sauropod herding.

Yet, at Tendaguru (see Box 14.7) in southeastern Tanzania, as well as in the USA in northern Texas at the famous Glen Rose trackway sites and in Maryland where the remains of the brachiosaurid Astrodon have been uncovered, there is strong evidence that these environments were once close to the sea and quite humid. Perhaps these were some of the conditions that sauropods found most congenial.

Quagmired? For many years, reconstructions commonly showed sauropods as swamp-dwellers, their great bulk buoyed up by water. In this way, so the story went, they could have remained deeply submerged, breathing with only their high nostrils poking out of the water. But what evidence is there for this?

Paleontologists have examined the barometric consequences, that is the changes in atmospheric pressure, that would occur by submerging a sauropod. Because the thorax (in vertebrates, the part of the body between the neck and stomach) - and hence the lungs -would be under a column of water some six or more meters deep, the thorax would be under nearly double the pressure it would experience on land. This would tend to push whatever air was in the lungs out of the body. How the next breath might be taken is hard to say, since the lungs would have to be expanded against pressures well beyond those experienced in any vertebrate. Unless sauropods had exceedingly powerful chest muscles, they would have been unable to inhale.

In fact, close study of sauropod habitats and anatomy, gives no evidence that sauropods whiled away their palmy days buoyed up in Mesozoic swamps. Our best evidence, supported by biomechanical studies of sauropod limbs, is that the dense-boned, massive, and pillar-like limbs were designed for fully terrestrial locomotion.

Beating hearts and necking. Sauropod necks have been likened to those of giraffes, inviting the inference that they fed in tall trees. Recent reconstructions, however, indicate that the head in most sauropods was generally held at or near the height of the shoulder, and that a giraffelike, vertically oriented neck was not likely.

For Brachiosaurus, things may have been different. Not only was the neck very long, but the front limbs were longer than those in the rear (Figure 8.14). With this "extra boost,"

Prehistoric Human LifeSauropod And Human

Figure 8.15. Systolic blood pressures compared: (a) a sauropod (approximately 630 mm), (b) a giraffe (320 mm), and (c) a human (150 mm).

Figure 8.15. Systolic blood pressures compared: (a) a sauropod (approximately 630 mm), (b) a giraffe (320 mm), and (c) a human (150 mm).

its head could apparently be raised to a height of 13 m, providing the opportunity to feed on foliage to which virtually no one else had access. But animals like Brachiosaurus had to pay a price for such posture. Now that its head was perched so high, its brain (the relatively smallest among dinosaurs) must have towered about 8 m above its heart.

To push blood through the arteries up its 8.5 m long neck, the heart of a Brachiosaurus must have pumped with a pressure exceeding that known in any living animal - indeed, double that of a giraffe. It would indeed take a very muscular heart - some estimate one weighing as much as 400 kg - to do the pumping (Figure 8.15). How fine capillaries in the brain might have withstood such pressures is again a matter for speculation.

Diplodocus and other long-necked sauropods may have gained access to foliage at high levels in the trees by adopting a tripodal posture, rearing up on their hindlimbs and using their tails as a "third leg" (Figure 8.16). In tripodal posture, these dinosaurs would have had to pay the same price as Brachiosaurus (which itself was probably not able to rear up): elevated blood pressure and a large, powerful heart to produce it.

None of these considerations addresses yet another challenge posed by long-necked life: the extraordinary amount of unused, wasted, air contained in the neck if sau-ropods simply breathed in and out, bidirectionally, as do mammals and most tetrapods. If, however, sauropods used a unidirectional, avian style of respiration, in which the lungs are pumped by auxillary air sacs, more oxygen could be extracted from the inhaled air, and the problem of the amount of air contained within the neck would be diminished (Box 8.1).

Dinosaur Long Neck

Good idea . . . but is there any evidence that sauropods used unidirectional respiration? The auxillary air sacs used in unidirectional respiration in birds, at least, are partly accommodated in hollow cavities within the bones. Such bone is called pneumatic, and the cavities are accessed by small openings called pneumatic foramina (see Chapter 10). In birds, many parts of the vertebral column, pelvis, and ribs are pneumatic, leaving a diagnostic bony record of avian-style unidirectional breathing. Interestingly, some sauropods have cavities, called pleu-rocoels, in their backbones, suggesting at least the possibility of air sacs and avian-style unidirectional breathing (Figure 8.17).

Considerations of blood pressure, heart size, lung capacity, and breathing style leave us unsure of how sauropods really functioned, but remind us that, in these respects at least, sau-ropods were a highly evolved, very specialized group of animals.

Feeding

Tooth form and especially tooth wear indicate that sauropods nipped and stripped foliage, unceremoniously delivering a succulent bolus to the gullet, largely unchewed. Here, too, the

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