Size and Weight:
Most sauropods looked pretty much like most other sauropods--long necks, squat trunks and long tails--but Amargasaurus was the exception that proved the rule. This relatively slim plant-eater ("only" about 30 feet long from head to tail and two tons) had a row of sharp spines lining its neck and back, the only sauropod known to have evolved such an imposing feature. (True, the later titanosaurs of the Cretaceous period were covered with scutes and spiny knobs, but they were nowhere near as ornate as Amargasaurus.)
Why did the South American Amargasaurus possess such prominent spines? As with similarly equipped dinosaurs (like Spinosaurus and Ouranosaurus), there are various possibilities: the spines may have deterred predators, they may have had a role in temperature regulation (that is, if they were covered by a thin flap of skin capable of dissipating heat), or they may simply have been a sexually selected characteristic (Amargasaurus males with more prominent spines being more easily able to mate with females).
As distinctive as it was, Amargasaurus appears to have been closely related to two other unusual sauropods: Dicraeosaurus, which was also equipped with (much shorter) spines emanating from its neck and upper back, and Brachytrachelopan, which was distinguished by its unusually short neck (probably an evolutionary adaptation to the food available in its South American habitat).