Sauropoda is supported by more than a dozen unique features, many of which relate to the attainment of great size and weight on land (Figure 8.20).
Evolution within Sauropoda has only recently been evaluated using cladistic approaches. As currently understood, sauropods consist of several primitive taxa (among them Blikanasaurus, Vulcanodon, and Kotasaurus) on the one hand, and the more derived clade Eusauropoda on the other. Eusauropods are diagnosed by many features (Figure 8.20). The most primitive known member of the group, Shunosaurus, was a 9 m long sauropod from the Middle Jurassic of China (Figure 8.21). Its skull is relatively long and low, and vaguely reminiscent of the primitive sauropodomorph condition, with nostrils near the front of the snout and a mouth filled with many small and spatulate teeth.
As sauropod evolution proceeded, various aspects of jaw mechanics and body form appear to have been linked. Early in their evolution, sauropods developed a fully quadrupedal stance. At the same time, there occurred a significant increase in body size.
Figure 8.20. Cladogram of Sauropoda, with more distant relationships with Prosauropoda and Theropoda. Derived characters include: at 1, special laminar system on forward cervical vertebrae, forelimb length greater than 60% hindlimb length, triradi-ate proximal end of ulna, subrectangular distal end of radius, length of metacarpal V greater than 90% that of metacarpal III, compressed distal end of ischial shaft, reduced anterior trochanter on femur, femoral shaft elliptical in horizontal cross-section, tibia length less than 70% femur length, metatarsal III length less than 40% tibia length, proximal end surfaces of metatarsals I and V are larger than those of metatarsals II, III and IV, metatarsal III length is >85% metatarsal V length, ratio is 0.85 or higher; at 2, broadly rounded snout, caudal margin of external nares that extends behind the posterior margin of the antorbital fenestra, lateral plate on premaxillae, maxillae, and dentaries, loss of the anterior process of the prefrontal, frontals wider than length, wrinkled tooth crown enamel, most posterior tooth positioned beneath antorbital fenestra, at least 12 cervical vertebrae, neural spines of the cervical vertebrae that slope strongly forward, dorsal surface of sacral plate at the level of dorsal margin of ilium, block-like carpals, metacarpals arranged in U-shaped colonnade, manual phalanges wider transversely than proximodistally, two or fewer phalanges for manual digits II—IV, strongly convex dorsal margin of ilium, loss of the anterior trochanter of femur, lateral muscle scar at mid length of fibula, distally divergent metatarsals II—IV, three phalanges on pedal digit IV, ungual length greater than 100% metatarsal length for pedal digit I; at 3, subnarial foramen on premaxilla-maxilla suture, preantorbital fenestra in base of ascending process of maxilla, quadratojugal in contact with maxilla, pedal digit IV with two or fewer phalanges; at 4, subrectangular snout, fully retracted external nares, elongate subnarial foramen, reduction of angle between midline and premaxilla-maxilla suture to 20° or less, most posterior tooth rostral to antorbital fenestra; at 5, greatest diameter of external nares greater than that of orbit, subnarial foramen found within the external narial fossa; at 6, nearly vertical dorsal premaxil-lary process, splenial extending to mandibular symphysis, acute posterior ends of pleurocoels in anterior dorsal vertebrae, metacarpal I longer than metacarpal IV; at 7, prominent expansion of rear end of sternal plate, very robust radius and ulna.
In time, the snout broadened, the lower jaw strengthened, and wear indicating front and rearward movement of the jaws is found on the teeth. Within Neosauropoda, that great clade of sauropods that includes camarasaurs, brachiosaurs, and titanosauroids (Macronaria) on the one hand, and Diplodocoidea on the other (see Figure 8.20), the skull shows additional strengthening (closure of the antorbital fenestra). Macronarians generally show a shortening and elevation of the skull, indicating a more powerful biting force. Among the most distinctive characters uniting macronarians (see Figure 8.20) are the modified hollow spaces, the
pleurocoels, in the back region (see Figure 8.17). Within Macronaria are the smaller, stouter camarasauromorphs (in the shape of camarasaurs) including the very familiar Camarasaurus (Figure 8.22).
Our visit to Macronaria would not be complete without a very brief mention of titano-saurians (see Figure 8.20). Among the most famous is Alamosaurus, an enigmatic sauropod from the Late Cretaceous of the western USA (Figure 8.23). The backs of Saltasaurus, from the Late Cretaceous of Argentina, and Malawisaurus were evidently covered with a pavement of osteoderms (Figure 8.24).
By contrast, diplodocoids restricted a series of highly evolved, peg-like teeth to the front of the jaws. Tooth wear is apparent at the apexes of the teeth (instead of along the tooth), and elongation of the snout in diplodocoids suggests that the group abandoned the front-back jaw movement.
Some remarkable dinosaurs are among the diplodocoids, including Amargosaurus, with its extraordinary neural arches, from the Early Cretaceous of Argentina (Figure 8.25). The 21 m long Apatosaurus (Late Jurassic, Western Interior, USA) is best known by its incorrect name "Brontosaurus" (Box 8.2). Diplodocus, renown for its long neck and tail, may also have carried dragon-like osteoderms along the length of its back (Figure 8.26).
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