Some 30 feet long, and weighing about three tons, this Argentinian sauropod from the early Cretaceous stood firmly on all four short legs with a long neck out in front, balanced by an equally long tail. Discovered in 1984, it has a unique claim to fame. Amargasaurus had a double row of long spines sticking up from its shoulders and back. Were they for defense, or display? The experts are still arguing.