Sauropods

The sauropods were gigantic quadrupedal, herbivorous dinosaurs. Their large size may have been facilitated by increased oxygen content of the atmosphere, which also contributed to the evolution of aerial reptiles (Sect. 6.3). They appeared in the Lower Jurassic and survived until almost the end of the Cretaceous period (McIntosh et al. 1997). During most of this time, they were the dominant terrestrial herbivores of the earth. First thought to be amphibious, they are now recognised as having been facultative bipeds. They would have used a quadrupedal stance when moving slowly, but adopted a bipedal position for fast locomotion or when browsing on high vegetation. Even in the early stages of their evolution, they were enormous (Sect. 7.4). Most sauropods were well over 15 m in length. Among the largest was Paralititan (Fig. 107) from Bahariya oasis in the Sahara Desert of Egypt. With a humerus measuring 1.69 m in length, Paralititan must have been ca. 30 m long and could have weighed between 75 and 80 tonnes. Even heavier, Argentinosaurus is estimated to have been 7.5% longer still. When first discovered, Seismosaurus ('earth-shaking lizard'; Fig. 108) was claimed to have exceeded a length of 33 m (Gillette 1994). In general, however,the longest sauropods were not necessarily the heaviest. The graceful Diplodocus (Fig. 66) reached a length of ca. 25 m,but weighed only about 20 tonnes. In contrast, the massive body of Barosaurus

(Diplodocidae; Upper Jurassic; length ca. 30 m)

(Diplodocidae; Upper Jurassic; length ca. 30 m)

■ Fig. 108. Seismosau-rus (Diplodocidae; Upper Jurassic; length ca. 33 m). (Based on Gillette 1994)

('heavy lizard'; length ca. 27 m) from East Africa and North America probably weighed 30 tonnes - or even more. This huge dinosaur may have been only slightly longer than Diplodocus because, although its neck was extraordinarily elongated (ca. 9 m), its tail was relatively short.

The division Neosauropoda contains five families: Cetiosauridae, Brachio-sauridae, Camarasauridae, Titanosauridae (Fig. 103), Nemegtosauridae and Diplodocidae (Benton 2004). The largest dinosaurs of all were members of the Diplodocidae - Paralititan (Fig. 107), Argentinosaurus, Seismosaurus (Fig. 108), Apatosaurus (Fig. 109; see below) and so on - or of the Brachiosauridae. Brachio-saurus (Fig. 110) from Europe,Africa and North America is the largest of all land animals for which a complete skeleton exists. Mounted and displayed in the Palaeontological Museum of Humboldt University, Berlin, its bones were excavated in Tanzania in 1908-1912. Brachiosaurs differed from other sauropods in that their front legs were longer than their hind legs. Consequently, the body sloped backwards like that of a giraffe. The relatively short tail resulted in an overall length of only ca. 25 m. Calculations regarding their mass range between 50 and 80 tonnes. The Berlin specimen reached 22.5 m in length and its head was

■ Fig. 109. Brachiosaurus (Brachiosauridae; Upper Jurassic; length ca. 23 m). (Based on Spinar 1995)

held about 13 m above ground level. Gillette (1994) mentioned the bones discovered in western Colorado by J.A. Jensen and described in 1985 as representing other supergiant dinosaurs - Supersaurus, Ultrasaurus (based mostly on a single vertebra) and Dystylosaurus. Two of these, however, could have belonged to the same individual, while at least one of them might be part of the skeleton of a Brachiosaurus.

The long-necked browsing Jurassic sauropods were characterised by a uniform body plan as far as their post-cranial skeleton was concerned. Their skulls were small with the nostrils opening dorsally; they had straight limbs

■ Fig. 110. A herd of Apatosaurus (Diplodocidae; Upper Jurassic; length ca. 21 m). (Cloudsley-Thompson 1994)

and broad feet like those of elephants. Pads on these would have absorbed the shock of each footfall. Their long, supple necks were balanced by long tails. The teeth of Diplodocus and Apatosaurus were reduced in number and limited to the front of the jaws. Teeth were more numerous in Brachiosaurus and Ca-marosaurus, however, where they were present on the sides as well as at the front of the jaws. In the latter genus, they were relatively large and robust, suggesting that some mastication of the food may have taken place - although most of it must have been processed in the gut (Sect. 10.2; Colbert 1993). Lacking the ability to chew fodder, as their peg-like teeth did not occlude, it seems highly likely that the large sauropods closed their mouths around the branches of trees and pulled their heads back, raking the foliage off as they did so (Mcintosh et al. 1997; Tiffney 1997).

Fossils of many of the largest dinosaurs have been found in what are now South America and Africa. As explained above (Sect. 9.1; Fig 84), during much of the Mesozoic Era, Gondwanaland, the supercontinent in which they lived, was partially separated from Laurasia in the north by the Tethys Sea. Titanosauridae, for instance, the latest family of sauropods to evolve, survived until about the end of the Cretaceous period. Although a few genera have been found in southern parts of Laurasia, they were a truly Gondwanan group. Sauropods were nearly extinct in the rest of the world when titanosaurids were the dominant plant-eaters in Gondwanaland, occupying the ecological niche of the ornithopods in Laurasia. Some meat-eating dinosaurs, such as Carno-taurus and other members of the family Abelisauridae, were also typically Gondwanan. A few of them, however, must have been able to cross the Tethys Sea (Sect. 9.1) and radiate on a small scale in various parts of Laurasia. This subject has been accorded fascinating treatment by Benton (1996).

Evidently some dinosaurs not only aggregated for feeding and breeding pur-poses,but remained together throughout much of their lives (Ostrom 1972),so they were probably truly social animals (Figs. 109,110). The herds might well have had home ranges, but it seems unlikely that these would have been defended against conspecific herds. Nevertheless, hierarchical behaviour may have occurred - especially among the duck-billed hadrosaurs that had large crests but were without elongated nasal bulbs (Sect. 9.3.1). Probably most species, especially among the large sauropods, were not aggressive. Odours and colour may well have been exploited in sexual displays.

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