Four years ago, a rancher in the Patagonia region of Argentina came upon an old bone sticking out of his desert property near La Flecha.

With recent news of exciting dinosaur finds in that country in mind, he scratched around some more. Then he went to a local museum to ask paleontologists to come look for more fossils.

Many important dinosaur discoveries are made by nonexperts in just this casual way. The rancher’s find soon led to the exposure of skeletal remains of six of the biggest titanosaurs. These herbivores lived about 100 million years ago, in the Late Cretaceous Period, on all continents, including Antarctica. They seemed especially plentiful in southern lands.

Now, the most imposing one of these dinosaurs from the far south of South America, assembled from 84 fossil pieces excavated from the rancher’s land, is the newest eyeful of ancient life on display at the American Museum of Natural History in Manhattan. The hulking skeleton cast made its debut as a permanent attraction on Friday. Museum officials and scientists called it a must-see addition to the ranks of such popular icons as the institution’s great blue whale and the fierce Tyrannosaurus rex.