Towards the end of the Cretaceous period, most sauropod dinosaurs were losing the survival game to duckbill dinosaurs such as Anatosaurus. However, rather like modern Australia, South America was an island continent and so life evolved rather differently there. Specifically, the duckbilled dinosaurs never gained a foothold and so sauropods continued on their own path of evolution.
Saltasaurus ("lizard from Salta") was one such highly-evolved sauropod, from 70 to 65 million years ago. When it was first discovered in 1980, it forced paleontologists to reconsider many of their assumptions about what was and was not a sauropod¹. For Saltasaurus, although clearly a sauropod, had armour plating! It had always been assumed that their size alone was enough defence for a sauropod. However, Saltasaurus had a body covered in bony knobs 10 or 12cm in diameter. For example, Titanosaurus, first named in 1893 as a saurapod, had been reclassified as an ankylosaur when pieces of armour were found with some later skeletons. However, Saltasaurus showed that a dinosaur could be armoured and retain its status as a sauropod - Titanosaurus was duly restored to "sauropodhood". Since then, paleontologists have investigated the possibility that other sauropods also had armour; for example, the Argentinian Laplatasaurus.
Saltasaurus was first described by José Bonaparte and Jaime Powell in 1980, and has an estimated length of 12 metres and mass of 7 tonnes. Like all saurapods, Saltasaurus was purely herbivorous, and like many diplodocids, it could rear up on its hind legs to reach higher branches. The name "Saltasaurus" is taken from the region of north-west Argentina where the first fossils were recovered, and other fossils have since been found in Uruguay. The three known species of Saltasaurus are S. australis, S. robustus and S. loricatus, of which S. australis is the best-known, with at least ten individual finds. The fossils we have of Saltasaurus include vertebrae, limb bones and several jaw bones - plus the various pieces of armour that have been found. Some of these appear to have been spiked also, but there is not enough evidence available to be sure as yet.
1: One source I read while researching this called it "putting the Ornitholestes amonst the pterosaurs"! I was tempted to lift the phrase but it's just too damn corny.