A rancher came across an old bone sticking out of his desert property near La Flecha in the Patagonia region of Argentina four years ago. With the 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 to look for more fossils.

Many important dinosaur discoveries are made by non-experts 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. 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.

The new research is expected to yield insights into the physiology of dinosaurs and how they were able to grow and function as such large creatures.“Paleontology has become less geological and more biological in the last 20 years or so,” said Mark A. Norell, chairman of the paleontology division at the museum and a leading dinosaur researcher.

He cited the field’s new “geochemical tools” for determining diet, growth patterns, and locomotion. “All of us are simply biologists who work on fossils,” he added.

The exhibit is not only a centerpiece for the museum’s fossil collections but also the start of a wide range of dinosaur programs for the year, including symposiums and another exhibition, “Dinosaurs Among Us,” The Patagonian skeleton was not an easy fit in its New York home. At 122 feet in length, it was a bit too long for the gallery.

Part of its 39-foot-long neck extends through an opening in a wall toward the elevator banks as if to welcome visitors to the fossil floors.

This titanosaur was a young adult, gender undetermined. Its appetite for all kinds of vegetation must have been prodigious. Based on bone sizes, researchers estimated that this individual weighed 70 tons — as much as 10 African elephants, the heaviest land animals today. Think of its possible heft if it were fully grown.

Think of it satisfying its huge appetite by stretching its long neck to graze far and wide. With only a few shifts in position, it might have mowed the equivalent of all the grass in Yankee Stadium in a morning.

Weight was also a factor in preparing the skeleton cast for display, a task undertaken by Research Casting International in Canada. The actual mineralized fossils were too heavy to mount. Instead, all “bones” are made from lightweight fiberglass based on digital copies of the original fossils.

Much of the grueling excavation leading to the discovery was done by teams led by José Luis Carballido and Diego Pol, paleontologists at Paleontological Museum Egidio Feruglio in Argentina. They began excavating for months at a time after the rancher’s visit. Sometimes it took a week of digging to isolate a single femur or a forelimb.

Thighs and upper arms are critical to judging the size and weight of a dinosaur.

Pol said the excavations revealed that at least six of these giant individuals, all young adults, had died at the site of what had been a flood plain near a river. Their deaths had happened at three distinct times, anywhere from a few years to centuries apart. Like many herding animals, they may have become isolated from the group and died of stress and hunger near their watering hole.

“That’s when we realized this was a once-in-a-lifetime discovery,” Dr. Pol said. Dinosaurs are the big game to fossil hunters, and these were some of the biggest plant-eating dinosaurs ever found.

The size and distinctive shape of an eight-foot femur of one specimen astonished scientists. This appeared to be a previously unknown titanosaur species, yet unnamed. Dr. Pol said a report that is being prepared may soon propose a formal species name.In a bravura moment, Dr. Pol had his picture taken stretched out on the ground beside the femur, about the size of a living room couch.

The photograph caught the attention of paleontologists at the natural history museum in New York, where Dr. Pol had done his Ph.D. research. “Maybe we can get that thing,” one said. “That would look great for a renovated dinosaur gallery,” another said.

Early last year, Dr. Novacek signed the deal with the Argentine museum to build the full-size skeleton cast for permanent display in New York. On a visit a few days before the titanosaur’s unveiling, workers were applying finishing touches as Dr.

Norell paused at the entrance, under the watchful eye and toothy jaw of the star attraction.“I guarantee you are going to remember this first impression,” Dr. Norell said. “Seeing something like this, you don’t quite have anything to compare its size and aspect with.”

He was right. The dinosaur was inexpressibly strange and big. Once again, we are reminded of what we know: Dinosaurs mostly were big, an engaging mystery and a challenge. The titanosaur’s arrival at the museum may inspire a new understanding of these incredible creatures. It is exciting enough to walk a corridor to the fossil galleries in anticipation of meeting it and spending some time with old friends of fond memory.

Dr. Novacek is not bothered by some skepticism that the specimens are from the biggest dinosaur discovered so far and that these may soon be eclipsed in size by new excavations. “Every time we find the biggest dinosaur,” he said, “we soon find a bigger one in the next dig.”