A new species of dinosaur found in Argentina was a 65-ton behemoth the length of a high-school basketball court, making it one of the largest animals to ever walk the Earth, researchers said Thursday.Weighing as much as a dozen elephants, the dinosaur stood two stories tall at the shoulder, the researchers said Thursday in Scientific Reports. As it ate, each swallow traversed a 37-foot-long neck. Its whiplike tail measured 29 feet—the length of the current world-record long jump.

When it died, the 85-foot-long creature apparently was still growing, they reported. Its well-preserved remains make up the most complete skeleton known of any species from the Titanosaur family of gigantic long-necked dinosaurs.

“I look at this dinosaur every day now and I still can’t believe it exists,” said researcher Kenneth Lacovara of Drexel University in Philadelphia. “We are seeing something that is pushing the envelope of how big you can get on this planet,” said Dr. Lacovara, who led a team of 17 scientists from a dozen laboratories that excavated and analyzed the new species.

The bones, on research loan to the U.S., are scheduled to be returned next year to Argentina. They are set to be displayed at a museum in Río Gallegos in Santa Cruz Province at the tip of South America, the region where they were discovered in 2005.

Dr. Lacovara recalled spotting the first glint of bone poking through the rocks. By the end of their first day of digging, Dr. Lacovara and his co-workers had exposed 10 massive bones. In four field seasons, they extracted 16 tons of bones belonging to two specimens of Dreadnoughtus schrani, as the new species has been formally named—in honor of the early-20th-century battleship and Adam Schran, a Philadelphia entrepreneur who funded part of the research.

Dreadnoughtus schrani was unearthed in Argentina by a team led by U.S. researcher Kenneth Lacovara, shown. Drexel University

The best geologic evidence suggests the creatures lived between 66 million and 83 million years ago, an era when the region resembled the coast of Maine, bristling with conifers and broad-leaved trees. The dinosaurs likely muscled their way into these dense forests, gulping ferns and stripping leaves from treetops with two-inch-long peg-like teeth.

They most likely died in the aftermath of a river flood, trapped in primordial mire. “They got sucked down into this quicksand,” Dr. Lacovara said.

All told, about 70% of the creature’s skeleton is represented—145 bones in all—the researchers said. They have found one tooth, but have yet to locate a skull.

As part of their analysis, they made a laser scan of the fossils and expect to post the searchable three-dimensional images online.

Because the skeleton is so complete, the scientists said they could accurately determine the creature’s weight in life—a technical first for these long-extinct Titanosaurs.

By their calculation, the Dreadnoughtus weighed more than a Boeing BA 737-900 passenger jet.

The skeleton of Dreadnoughtus schrani was found in 2005 in Argentina’s Santa Cruz Province, at the tip of South America. Kenneth Lacovara

To be sure, there are many claimants for the title of world’s largest dinosaur.

Among the contenders are Supersaurus, Giraffatitan, and Argentinosaurus—all Titanosaurs. One supersize Titanosaur species called Futalognkosaurus dukei may have been up to 105 feet long, with hips nearly 10 feet wide.

So far, though, researchers haven’t found enough bones belonging to these species to gauge their weight reliably, so nobody knows exactly how large these creatures might have been when alive, the scientists said.

“Can we say Dreadnoughtus is the biggest ever? No,” said Dr. Lacovara. “We can say that Dreadnoughtus has the largest calculable mass of any known land animal.”

Write to Robert Lee Hotz at sciencejournal@wsj.com