One other dinosaur in the scientific literature - also discovered in Argentina, in 1987, dubbed Argentinosaurus - has been estimated at well over 70 tons, but no upper arm bone (humerus) was found for that one. Still another contender in the same range was announced in May, again from Argentina (was there something in the water?), but that one has yet to be formally vetted by the scientific community.
So for now Dreadnoughtus, described in the journal Scientific Reports, holds a special place.
And it was not done growing, as evidenced by shoulder bones that had yet to fuse together, said one of Lacovara's coauthors, Matthew C. Lamanna, assistant curator of vertebrate paleontology at Pittsburgh's Carnegie Museum of Natural History.
Kingpin or no, scientists are not necessarily concerned with superlatives, but with what such fossils can tell us about how the big animals moved and lived.
This one is likely to reveal a lot, said David C. Evans, curator of vertebrate paleontology at the Royal Ontario Museum in Toronto.
Many of the biggest known dinosaurs are represented by just a few bones. This specimen was unusually well-preserved, with the Drexel-led team able to recover close to half of its 250-odd bones.
"This is a special look into a part of the dinosaur family tree that typically doesn't get preserved," said Evans, who was not involved with the discovery. "This is not the last you'll hear about this dinosaur."
Lacovara named the animal after the dreadnought class of battleships from the early 20th century, so nicknamed because they feared nothing - dreaded naught. Dreadnoughtus schrani was so big that few predators would have dared to attack it, Lacovara said. But if one of them did, the dinosaur could have responded with a smack of its muscular, 29-foot tail.
"It essentially had a weaponized tail," Lacovara said.
The schrani portion of the name is a tribute to Philadelphia tech entrepreneur Adam Schran, who helped fund the work. The research paper lists an international team of 17 authors, more than half with current or former ties to Drexel.
They probably were able to find so many of the bones because the animal was buried rapidly in sediment during a flood nearly 80 million years ago, and thus scavengers had little chance to pick it apart, Lacovara said.
He imagines that the big dino sustained itself by parking its large body in the forest for hours at a time, gorging on tens of thousands of calories' worth of leaves and plant matter.
Among modern animals, blue whales are even bigger, at up to 200 tons. But nothing on land today approaches Dreadnoughtus and others in the same dinosaur group, known as titanosaurs. Scientists are not sure why.
One possibility is that after most dinosaurs went extinct 65 million years ago, their remaining cousins, the birds, found themselves increasingly in competition with rising numbers of mammals, said P. Martin Sander, professor of vertebrate paleontology at the University of Bonn in Germany.
In any era, being big poses special challenges. Among other issues, larger living things have a higher volume relative to their amount of surface area. This is why ice melts more rapidly when you break it into little chunks: More surface area is exposed to the warm, surrounding air.
So a giant animal burning all those calories would need a way to dissipate the extra heat. Scientists think the titanosaurs did so in part with long necks and tails.
Another boost may have come from the presence of air cavities in vertebrae and ribs, said Sander, who was not part of the Dreadnoughtus team. If these cavities and pores were part of a complex internal respiratory system, they could have helped get rid of excess warmth.
Indeed, this "pneumatic" trait can be seen inside one of Dreadnoughtus' giant neck bones that has partly eroded, said the Carnegie Museum's Lamanna.
"It looks like a sponge," he said.
Lacovara found the first few bones in Argentina's southern Patagonia region in February 2005. He and colleagues came back year after year, a total of four expeditions, digging up the bones and encasing them in plaster "jackets."
They came to Philadelphia in 2009 via shipping container; the fossil preparation and cleaning work was then split among Drexel, the Carnegie Museum and the Academy of Natural Sciences, which later became part of Drexel.
The bones are owned by Santa Cruz province in Argentina and are headed to a museum there next year. But their likenesses will remain here, as Lacovara's team has made 3-D laser scans of all the bones.
Engineers already are using the scans to model the animal's muscles and cartilage, and plan to calculate such characteristics as its stride length and speed.
Meanwhile, Lacovara plans a return early next year to Argentina, where, he hopes, more big bones await.
Kenneth Lacovara, an associate professor of paleontology and geology at Drexel, discusses the discovery. See the video at www.inquirer.com/dino