Prosauropoda

Prosauropods are a group of relatively primitive dinosaurs with small heads, long necks, large bodies, and long tails, known from the Late Triassic through early Jurassic, from all continents except Australia (see Figure 8.2). In general, the front limbs were somewhat shorter than the hindlimbs, and all had five digits. Prosauropod hands were equipped with a large, half-moon-shaped thumb claw (Figure 8.5). Whether for food procurement, defense, or some unspecified social activity, the function of this claw remains unknown.

Figure 8.5. Left hand of the prosauropod dinosaur Plateosaurus, showing its well-developed thumb claw: (a) reconstructed hand; (b) thumb showing amount of movement permitted by skeleton.

Prosauropod lives and lifestyles

Feeding. In the mood for food, sure, but which? The skulls show almost none of the design features associated with chewing (see introduction to Part III: Saurischia); however, the jaw joint is slightly lower than the tooth row (Figure 8.6). The teeth are generally separated, leaf-shaped (Figure 8.7), and reveal few grinding marks, suggesting puncturing as the dominant tooth function.

Although they have traditionally considered prosauropods to be herbivores, some paleontologists have suggested carnivory because the prosauropod teeth lack herbivore specializations. Yet, supporting herbivory, the skull is proportionately smaller than that seen in carnivores. Recent treatments of the group split the difference, calling them predominantly herbivores that might have enjoyed an occasional meaty snack.

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Figure 8.6. Left lateral view of the skull of (a) Anchsaurus, (b) Coloradisaurus, (c) Lufengosaurus, and (d) Yunnanosaurus.

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Figure 8.6. Left lateral view of the skull of (a) Anchsaurus, (b) Coloradisaurus, (c) Lufengosaurus, and (d) Yunnanosaurus.

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Once the food was past the mouth, grinding took place via gastroliths - which have been found in association with prosauropod skeletons - and by stomach fermention, to judge from their barrel-shaped torsos (see Chapter 5).

The history of prosauropods parallels the rise of gymnosperms - seed-bearing plants (see Figure 13.9). That is, as gymnosperms became an important component of the land plant biota, prosauropods became an important component of terrestrial vertebrate fauna. If primarily herbivores, prosauropods must have been the first land creatures ever to take advantage of tall-growing plants.

Need for speed? In the most primitive of prosauropods, the forelimbs are shorter than the hindlimbs and the trunk region is relatively short, suggesting that these animals walked principally on their hindlimbs rather than on all fours. However, the largest and most derived of prosauropods (among them Riojasaurus and Melanorosaurus; see Figure 8.19) appear to have become fully quadrupedal. The early history of locomotion in Sauropodomorpha is consistent with the primitive condition for all dinosaurs: bipedality (see Chapter 4).

For all that, undoubted prosauropod tracks all come from animals walking only quad-rupedally. The trackways are broad, with the oval prints of the hindfoot turned outward from the midline. In keeping with a rearward-positioned center of gravity, the imprints of the hands are

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Figure 8.7. Teeth in selected sauropodomorphs. (a) Leaf-shaped prosauropod tooth of Plateosaurus; (b) spatulate tooth of sauropod Camarasaurus; (c) pencil-like tooth of Diplodocus. The lower part of each tooth is the root.

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10 cm smaller and somewhat shallower than the feet. Interestingly, the large thumb claw appears to have been held high enough to clear the ground.

Prosauropods appear to have been quite slow. Calculations suggest speeds of no more than 5 km/h, about the average walking speed of humans.

Socializing. Very little is known of prosauropod social habits. The existence of the famous Plateosaurus bonebeds in Germany and Switzerland, as well as others elsewhere, however, implies that prosauropods moved in herds; indeed, herds of prosauropods migrating across the European continent were proposed as early as 1915.

Detailed analyses of Plateosaurus, Thecodontosaurus, and Melanorosaurus (prosauropods for which large numbers of individuals are known) reveal sexual dimorphism in skull dimensions and in thigh bone size. Sexual dimorphism tends to be pronounced in highly social animals, and thus there may be a connection between the likelihood of herding and sexual dimorphism.

Eggs, nests, and babies. Eggs and nests are known for the prosauropods Mussaurus (Argentina) and Massospondylus (South Africa). Clutches tended to be small by dinosaur standards (something like 10 eggs), and the hatchlings small sized. Adult prosauropods are roughly 500-1,000 times larger than the hatchlings. How this occurred metabolically is unclear, although rapid growth rates are surely indicated by the disparity in bone size (see Chapter 12)!

Prosauropods are not particularly common beasts, did not hang around on Earth for a very long time, and thus much about them is lost to antiquity. Still, as the first tall-browsing herbivores, they represent the first appearance on Earth of the modern ecosystem that is with us today. Our hope is that, with more attention and finds, the future will bring more insights into this enigmatic, yet fundamental group of dinosaurs.

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