FIGURE 10.5 Diplodocus, the Late Jurassic sauropod that inspired a pub song.
(A) Skeletal reconstruction; Denver Museum of Science and Nature. (B) Left pes and ankle of Diplodocus, showing the large ungual on digit I.
Diplodocoids can be distinguished from other sauropod clades by:
1 relatively more cervical and caudal vertebrae than other sauropods, adding up to some considerable body lengths;
2 teeth restricted to the anterior portion of the skull; and
The caudal vertebrae were so numerous (70-80) and small distally that the tails tapered into a whip-like structure. The proximal caudal vertebrae had prominent, horizontally-oriented chevrons developed on the ventral parts of the vertebrae, presumably for added support and protection of blood vessels. With their cervical, dorsal, and caudal vertebrae added together and projected into a possible total length for each genus, Seismosaurus and Supersaurus may have been the longest dinosaurs. By some conservative estimates, these sauropods were about 27 meters long, but lengths of more than 30 meters have been also hypothesized.
Because other sauropods had similarly long necks and tails, such as the Middle Jurassic Omeisaurus and Late Jurassic Mamenchisaurus of China, they were originally interpreted as sharing ancestry with diplodocids. Omeisaurus and Mamenchisaurus had 17 and 19 cervical vertebrae, respectively, making them animals with necks that were about half of their total body lengths. However, cladistic analyses show that Omeisaurus and Mamenchisaurus are in clades outside of Neosauropoda, although they are in Eusauropoda.
Of the clade Camarasauromorpha, probably the most common sauropod species, and accordingly one of the best studied, is the Late Jurassic Camarasaurus of the western USA. Camarasaurus (= "chambered lizard"), so named because of its relatively spacious skull, is known from numerous well-preserved and articulated specimens,
from juveniles to adults. These were all discovered in the Morrison Formation of Utah, Colorado, and Wyoming (Fig. 10.7). Camarasaurus has a more rounded skull than diplodocids and most other sauropods, as well as spoon-like teeth. The cervical, dorsal, and caudal vertebrae were also more conservative in number, resulting in body proportions somewhat in between the extremes presented by diplodocids in length and brachiosaurids in height. Two sauropod genera that were once considered as close relatives of Camarasaurus were Euhelopus from the Late Jurassic of China and Opisthocoelicaudia from the Late Cretaceous of Mongolia. However, Euhelopus is now considered too primitive and Opisthocoelicaudia too derived; the former is outside of Neosauropoda, and the latter is in Saltasauridae. Regardless, Camarasauromorpha has a geologic range from the Late Jurassic through to the Late Cretaceous on the basis of Camarasaurus and Opisthocoelicaudia, among others.
Titanosauriformes includes another well-known sauropod genus, Brachiosaurus, the representative genus of brachiosaurids. Brachiosaurus (= "arm lizard"), named with respect to its long forelimbs, occurs in Upper Jurassic strata of the western USA and Tanzania. Cedarsaurus and Sauroposeidon are also brachiosaurids, both coming from the Lower Cretaceous of the western USA. Like Camarasaurus, brachiosaurids had more rounded skulls than diplodocids and their nares were positioned more anteriorly and below the orbits. Brachiosaurid necks also had long cervical and dorsal vertebrae with well-developed pleurocoels (Fig. 10.7). Furthermore, they had a reduced digit I in the manus, which may have been an adaptation to more weight bearing on the fore limbs. Brachiosaurus was perhaps the most massive and tallest of all dinosaurs, weighing as much as 50 tonnes. Less conservative estimates have placed it at 80 metric tons, a difference of nearly 60%. It also had a neck that, when extended fully vertical, was close to 13 meters high. In part, the height of Brachiosaurus was related to its lengthened fore limbs, where its humerus exceeded the femur length. A sort of trade-off between height and length in diplodocids and
■i brachiosaurids is evident. For example, Brachiosaurus had shortened caudal vertebrae and was consequently shorter overall as compared to the diplodocoid Apatosaurus, which it outweighed by at least 2:1.
Titanosauriformes contains members of the last of the great sauropods, represented within the stem-based clade Titanosauria. Titanosaurs had several distinguishing features, but one of the most surprising was in the skin of some genera: dermal armor and dorsal spines. Before the discovery of titanosaurs, sauropods were regarded as defenseless animals with no natural protection against predation other than size or staying in herds. Because titanosaurs are the only sauropods to have developed osteoderms and most lived in the Cretaceous, some paleontologists have proposed that natural selection from predation pressures led to a "Red Queen" type of coevo-lution (Chapter 6). In other words, their large size would have helped to counter the increasingly larger and bigger-toothed theropods, such as Giganotosaurus and Carcharodontosaurus, that lived in the same areas as these sauropods (Chapter 9). Procoelous caudal vertebrae also help to identify titanosaurs. Fortunately, this is a unique trait because no complete skeleton of a titanosaur, let alone a complete skull, has ever been found in association with their vertebrae or limb bones. Some examples of titanosaurs are: Titanosaurus, Antarctosaurus, Argyrosaurus, Neuquensaurus, and Saltasaurus of Argentina; Janenschia, Malawisaurus, and Paralititan of eastern Africa; Phuwiangosaurus of Thailand; and Austrosaurus of Australia. These sauropods collectively hail from the former southern continent of Gondwana, indicating that titanosaurs mostly proliferated and diversified there. However, one titanosaur did make it from part of Gondwana (South America) into North America - the Late Cretaceous Alamosaurus of the western USA. Lastly, no description of titanosaurs is complete without mentioning what likely was one of the largest sauropods that ever lived, the Early Cretaceous titanosaur Argentinosaurus. This gigantic sauropod was known initially through just one prodigious dorsal vertebra and a few other fragments, but the discovery of more bones has helped to reconstruct it (Fig. 10.8).
In summary, sauropods are represented by a large number of clades that were only recently well-defined by detailed cladistic analyses. The results of these analyses caused considerable changes to how sauropods were classified, which means that many genera were reassigned to clades different from their traditional taxonomic groupings. The intention of such revisions is to better understand
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