Sauropoda, the sauropods, are a suborder or infraorder of the saurischian ("lizard-hipped") dinosaurs. They were the largest animals ever to have lived on land. Well-known genera include Apatosaurus (formerly known as Brontosaurus), Brachiosaurus and Diplodocus. 'Sauropod' is derived from 'lizard foot' in Greek.
They were herbivorous (plant-eating), usually long-necked quadrupeds (four-legged), with spatulate (spatula-shaped: broad at the base, narrow at the neck) teeth. They had small heads, huge bodies, and tended to have long tails. At least some of them laid eggs, like the camarasaurs and titanosaurs. According to paleontologist Robert Bakker there is a possibility they had large prehensile lips, reminiscent of moose lips.
Sauropods' most defining characteristic was their size. Even the dwarf sauropods (perhaps 5 to 6 metres, or 20 feet long) were counted among the largest animals in their ecosystem. Their only real competitor in terms of size is the Balaenoptera musculus, or blue whale. But unlike whales, they all lived on land. Some, like the diplodocids, probably held their heads low, while others, like Camarasaurus, held them high.
Their body design did not vary as much as other dinosaurs, perhaps due to size constraints, but they still displayed ample variety. Some, like the diplodocids, were extremely long, with sails running down the back of their spines, and with tremendously long tails which they may have been able to crack like a whip to make sonic booms. Supersaurus, at 40 metres (130 ft), is probably the longest, but Seismosaurus and even the old record holder, Diplodocus, are still extremely long. Though a possible century-old hoax, Amphicoelias fragillimus, of which only a drawing of a single vertebra survives, at 55 to 60 metres (180 to 200 ft) would have a spine even longer than B. musculus. The longest terrestrial animal alive today, the reticulated python, can only reach lengths of up to 10 metres (35 ft).
Others, like the brachiosaurids, were extremely tall, with high shoulders and extremely long necks. Sauroposeidon is probably the tallest, reaching 18 metres (60 ft) into the air, with the previous record for longest neck being held by Mamenchisaurus. By comparison the giraffe, the tallest of all living animals, is only 4.8 to 5.5 metres (16 to 18 ft) tall.
Some were almost incredibly massive, like the armour-plated Saltasaurus. Argentinosaurus is probably the heaviest at 80 to 100 metric tonnes (90 to 110 tons), though Paralititan, Andesaurus, Antarctosaurus, and Argyrosaurus are of comparable sizes. There is some very poor evidence of an even more massive titanosaurian, Bruhathkayosaurus, which might have weighed between 175 to 220 tonnes (190 to 240 tons). The largest land animal alive today, the Savannah elephant, weighs up to no more than 10 tonnes (11 tons).
From early on there has been speculation by Osborn and others  that sauropods could reach up on hind legs, using their tail as the third 'leg' of a tripod (somewhat like kangaroos), and a famous restoration of a Barosaurus rearing up on hind legs in the American Museum of Natural History illustrates this hypothesis well. One interesting study has postulated that if sauropods had adopted a bipedal posture at times there would be evidence of stress fractures in the forelimb 'hands'. However, none were found after examining a large number of sauropod skeletons.
If a sauropod stood in the tripod posture, there would be a heavy weight load on the haemal spines on part of the tail. As the sauropod got heavier as it grew, when it reared, these haemal spines would have to carry more and more load, until some of them would break due to fatigue fracture, and that would make rearing painful and the sauropod would have to stay on four feet after that. That may have evolved as a safety measure to prevent rearing when it got too heavy for rearing to be safe. There are reports of such haemal spine fractures being found in sauropod tail vertebrae.
Old restorations had sauropods dragging their tails. However, more recently, they have been depicted as elevated off the ground. The evidence for this is provided by the stiffening structures between the tail vertebrae. Furthermore, fossilised sauropod tracks (ichnites) generally lack tail marks.
Incomplete Fossil Finds
Unfortunately, complete fossil finds are rare. Many species, especially the largest, are known only from isolated disarticulated bones. Many more near complete specimens lack heads, tail tips and limbs. Some palaeontologists have postulated that these bits are those most likely to be carried off by Mesozoic scavengers after the death of the animal and before it is covered by sediment and fossilised.
Sauropods first appeared in the Late Triassic Period, where they somewhat resembled the Prosauropoda. By the Late Jurassic (150 million years ago) sauropods where widespread (especially the diplodocids and brachiosaurids). By the Late Cretaceous, only the titanosaurians survived, though with a near-global distribution. However, as with all other non-avian dinosaurs, the titanosaurians died out in the Cretaceous-Tertiary extinction event. Fossilized remains have been found on every continent except Antarctica.
The following are two alternative recent classifications (showing supra-generic clades only in the second example). These are by no means an exhaustive list of recent sauropod classification schemes. In some cases, families like Vulcanodontidae, Cetiosauridae and Omeisauridae are not included because they are considered paraphyletic, or even (in the case of Camarasauridae) polyphyletic.
- Infraorder SAUROPODA
- Family Anchisauridae
- Family Melanorosauridae
- Family Blikanasauridae
- Family Vulcanodontidae
- Family Cetiosauridae *
- Family Omeisauridae
- Division Neosauropoda
From Upchurch et al. 2004: (genera omitted, see above)
Recently-discovered short-necked variety
In June 2005, a new genus of dicraeosaurid sauropod was discovered with a short, stubby neck. This new genus, Brachytrachelopan, measuring less than 10 metres (33 ft) long, is unusually small for a sauropod. Unlike other sauropods, whose necks could grow to up to four times the length of their backs, the neck of Brachytrachelopan was shorter than its backbone. Unable to lift its neck and head above horizontal, this shorter length was likely an adaptation to eating an abundant or nutritious low-growing plant. 
- Osborn HF."A skeleton of Diplodocus, recently mounted in the American Museum,". Science, new series, vol. 10 (1899), pp. 870-874
- Rothschild BM, Molnar RE (2005). “Sauropod Stress Fractures as Clues to Activity”, Carpenter, Kenneth and Tidswell, Virginia (ed.): Thunder Lizards: The Sauropodomorph Dinosaurs. Indiana University Press, 381–391. ISBN 0-253-34542-1.
- * Wilson, J. A. 2002. Sauropod dinosaur phylogeny: critique and cladistic analysis, Zoological Journal of the Linnean Society 136:217-276.
- Kristina Curry Rogers and Jeffrey A. Wilson, 2005, The Sauropods: Evolution and Paleobiology, University of California Press, Berkeley, ISBN 0-520-24623-3
- Upchurch, P., Barrett, P.M. and Dodson, P. 2004. Sauropoda. In The Dinosauria, 2nd edition. D. Weishampel, P. Dodson, and H. Osmólska (eds.). University of California Press, Berkeley. Pp. 259-322.