The titanosaur—the American Museum of Natural History’s newest, largest dinosaur—was unveiled on Thursday. Photograph by DON EMMERT / AFP / Getty

A black curtain hung in the American Museum of Natural History on Thursday morning. The opening chords of a brass fanfare played on repeat. A scrum of more than a hundred journalists stood waiting, as still and silent as the Gomphotherium skeleton (a ten-million-year-old-relative of the elephant, from Texas) that was displayed behind them in the Hall of Advanced Mammals. “I’m going to count down from ten,” a man called out. “The drape is going to drop on one.” Smartphones were held aloft.

“This is the biggest thing happening in New York right now,” one reporter whispered. “I’m really surprised the Mayor isn’t here.” Countdown commenced. The crowd chanted along. The curtain fell. A toothy, grinning titanosaur appeared, poking its skull out through the entrance of its cavernous new home. The beast seemed to be blinking in the dazzle of flashes. Jack Nicholson in “The Shining” (“He-e-e-re’s Johnny!”) came to mind.

The titanosaur, which stomped across Serengeti-like plains in present-day Argentina about a hundred million years ago, is the museum’s newest, largest dinosaur, and this was its international début. The hall was dimly lit, and the spotlights beneath the cast of the skeleton gave it a blushing complexion. It resembled a Brontosaurus afflicted with gigantism. (Titanosaurs and brontosaurs are both sauropods, a suborder of dinosaurs that were herbivorous, with petite heads and monolithic legs.) Its spiny neck was as long as a city bus, and its tail stretched into the shadowy recesses of the hall like an endless rack of barbecue ribs. The creature seemed so long and gangly that it was hard not to feel sorry for it. From its snout to the tip of its tail, it measures a hundred and twenty-two feet, which is twenty-eight feet longer than a regulation N.B.A. basketball court. The museum’s other celebrity monsters, like the crusty old ninety-four-foot blue whale, which has been hanging in the Hall of Ocean Life since 1969, were now ho-hum.

Rubén Cúneo, the director of the Museo Paleontológico Egidio Feruglio, in Patagonia, which owns the new fossils, was standing beside the titanosaur’s pelvis. He had flown to New York from Argentina for the unveiling. He explained that, three years ago, the titanosaur was unknown, entombed in sandstone under scrubby Patagonian sheep fields. “So one day this rancher was out riding his horse, tending his animals, and he said, ‘What the hell is this?’ ” Cúneo told me, throwing his arms out to mime surprise. “These people, they know their land. They know a llama bone, an ostrich bone, a horse bone. When they see something different, they know it.” What the rancher had spotted was a saucer-sized portion of the titanosaur’s femur, crowning out from some hillside rocks. He reported the strange bone to the ranch owner, who stopped by the museum. (In Argentina, Cúneo explained, anything underground, including fossils, belongs to the state.) A month later, a team of paleontologists drove out to the site with shovels, picks, and brushes.

“They started to dig—a little more, then a little more—and at two metres, they said, ‘O.K.! Stop! This is something huge.’ ” The paleontologists Diego Pol and José Luis Carballido spent the next three weeks exhuming what turned out to be an eight-foot-long femur, or thighbone. A major excavation began; it lasted eighteen months. The team built a road to the site and dug a hundred-metre-wide pit with bulldozers and backhoes. They discovered more than two hundred bones from six different animals of the same species. “It was a graveyard!” Cúneo said. “A once-in-a-lifetime find.” Pol, who had also flown to town for the occasion and looked less comfortable than Cúneo in a suit and tie, said that they had no idea why the animals had died in that spot. By analyzing the fossils’ bone tissue, his team had determined that they were young adults when they died, likely between fifteen and twenty years old. “The site is close to a water hole, so perhaps they would go there when they were stressed,” he theorized, shrugging.

Other members of the group Titanosauria have been discovered in South America, and on other continents—genuses like Argentinosaurus and Dreadnoughtus—but the femur that Pol and Carballido found has a unique shape and size, leading them to conclude that it belonged to a new, previously unknown species. They will soon publish a paper presenting their findings. Until then, it’s a titanosaur with no name. Pol said that they calculated its mass to have been roughly seventy tons, the equivalent of ten adult elephants. His team believes that it was the largest dinosaur ever to walk the earth. And yet their method for arriving at this estimate—extrapolating mass from the circumference of the femur and the humerus (the upper arm)—is imprecise. Other paleontologists have already challenged the Argentines’ claim.

“We discovered the largest dinosaur in the world,” Cúneo went on. The choice to exhibit it first in New York was a practical one. “We had no place to store it,” he said. Back home, his museum has started planning an extension that will house a replica of the titanosaur in New York, as well as the original fossils, five of which were hanging illuminated on the wall behind him, on temporary view. Other museums had been interested in exhibiting the titanosaur, but the American Museum of Natural History had been the fastest, Cúneo said, at making decisions.

Another reason for New York’s lucky get was more personal. Pol did his graduate studies under Mark Norell, the chair of the Museum of Natural History’s paleontology division. “It’s nice to see our son come home,” Norell said. He was fielding questions next to the titanosaur’s back left foot, which, like all the bones in the mount, was a lightweight 3-D print made of fibreglass. Eighty-four of the skeleton’s bones are exact replicas cast from the fossils, while the rest were modelled from close relatives of the species.

Norell noted that the position of the long neck, straight out from the body, reflected the current scientific thinking. Unlike the sneezing Brachiosaurus in “Jurassic Park,” this sauropod’s neck would not have risen more than eighteen degrees. It did not feed off the treetops. “They were big lawnmowers,” Norell said. “They would take a step, then eat everything on the ground in a giant semicircle in front of them” (ferns and conifers, mostly). Paleontologists believe that their tails also stuck straight out, above the ground, rather than dragging crocodile-style, since the fossil record shows no sign of furrows trailing titanosaur footprints.

A class of fourth-graders from P.S. 87 had entered the hall for the sneak preview. Lucas Ruano-Lumpris, a boy with metal-framed rectangular glasses, said that he wouldn’t be that scared if he were to encounter the titanosaur alive in the wild. “It’s a herbivore,” he explained, gazing up. “I’ll be completely honest: it’s smaller than I thought it’d be.”