In 2013, an old Argentinian shepherd named Aureliano Hernandez found a fossilized bone protruding from a rock at the farm where he worked. The remains of giant dinosaurs festoon Argentina’s landscape, and knowing the strict rules that govern such fossils, the farm’s owners—the Mayo family—contacted local paleontologists. By the time the team arrived, Hernandez had passed away. He never knew that as one of his final acts, he had discovered the largest dinosaur ever known.

The fossil he had found was so big that it took two weeks to unearth it. It was a thigh bone, and the largest ever found—eight feet from end to end. There are photos of paleontologists lying next to it for scale, and they look like bemused pixies, their bodies and imaginations dwarfed by what they had found.

A. Otero

The bone clearly belonged to a sauropod—a long-necked dinosaur like Brontosaurus, Diplodocus, and Brachiosaurus. Specifically, it was one of the titanosaurs—the last-surviving group of sauropods, and likely the biggest of them. But even known titanosaurs didn’t have thighs that big. Team leaders Luis Carballido and Diego Pol from the Egidio Feruglio Paleontology Museum (MEF) estimated that the bone’s owner would have stood 130 feet from nose to tail, and tipped the scales at 77 tons. Speaking to journalists, they compared it to 14 elephants, a seven-story building, and two trucks (with trailers) parked end to end.

News of the new titan spread quickly. Last January, a cast of the sauropod went on display at the American Museum of Natural History, while legendary naturalist David Attenborough released a documentary about its discovery. And now, Carballido and his team have finally published an official scientific description of the dinosaur. They’ve also given it a name—Patagotitan mayorum. The first half refers to the Patagonian region where the dinosaur was found. The second half honors the Mayo family who kindly welcomed the scientists onto their land and into their kitchen.

After careful analysis, the team think that Patagotitan is slightly smaller than they previously thought—69 tons or so. But even after that downsizing, it’s still twice as heavy as more familiar giants like Brachiosaurus and Apatosaurus, and 10 percent bigger than the previous record-holder—another Argentinian titanosaur called Argentinosaurus. For now, it’s the biggest dinosaur for which we have accurate measurements, and perhaps the biggest land animal that ever lived.

“We have a decent idea how well the various methods for estimating dinosaur body mass work, and they tend to agree well enough most of the time,” says John Hutchinson from the Royal Veterinary College. “Even taking into account the uncertainty of those methods, Patagotitan comes out as a 60- to 80-ton behemoth. And nothing else we know of yet comes very close.”

Patagotitan cast, in a hangar. D. Pol.

Patagotitan lived during the Cretaceous period around 101 million years ago. And for some reason, it frequented the area that eventually became the Mayo family’s farm. Carballido and Pol’s team returned to the site more than a dozen times, disinterring every fossil they could find. In the process, they built a road and partially removed a hill. Eventually, they recovered bones from at least six Patagotitan individuals. And their bones reveal that they were in their prime—young, still growing, and not yet at their full adult size.

Carballido thinks that these individuals all died at different times, but he has no idea why they all died in this one place. He found the teeth of many meat-eating dinosaurs around the site, but he doubts any predator could have tackled such gargantuan prey. “They were too strong,” he says. “It would have been too risky for a carnivore.” Whatever their reasons, their attraction to this one place meant that Carballido’s team eventually uncovered more than 200 Patagotitan bones, covering most of the animal’s skeleton. “The most amazing moment for us was realizing that the dinosaur is not only large, but also more complete than any other titanosaur,” Carballido says.

It’s ironic that scientists have found very little of the largest animals to have walked the land. We only know that Argentinosaurus existed based on a few handfuls of bones—some vertebrae, ribs, and leg bones, most of which are incomplete. Puertasaurus, another contender for the record books, is known from just four vertebrae. There are near-complete skeletons for some titanosaurs like Futalognkosaurus, but these were smaller species that weighed in at just 50 tons or so. The true colossi of the dinosaurs are hard to come by.

As science writer Brian Switek once wrote, this means that “prehistoric creatures ballyhooed as ‘the biggest ever’ upon discovery have a tendency to shrink by time of publication.” Ultrasaurus, which was supposedly the biggest dinosaur during my childhood, turned out to be a mish-mash of other species. Bruhathkayosaurus, a possible Indian titanosaur, was claimed to be bigger than Argentinosaurus, but based on some poorly described fossils that were lost in a monsoon flood. And Amphicoelias is the most ludicrous supergiant of all—a sauropod that’s “known” from a fragment of a single vertebra, which was either lost or destroyed a century ago.

Even when you have a good skeleton, it can be hard to estimate the mass of a sauropod, says Charlotte Brassey from Manchester Metropolitan University. “They were so unusual, with long, slender necks and tails, and an air-filled skeleton, that we have no convincing analogues in the modern animal kingdom,” she says. “And by virtue of being extremely large, we need to extrapolate our understanding of how animals function far beyond the upper limits of living land animals, such as elephants. The more we need to extrapolate, the less confidence we have in our reconstructions.”

Jorge Gonzalez

To deal with these uncertainties, Carballido and Pol’s team used two separate methods to work out how heavy Paragotitan was. First, they used an equation that estimates an animal’s mass based on the circumference of its femur (thigh bone) and humerus (upper arm bone). “These are the two most important bones for moving the animal,” and the most likely to reflect its actual weight, says Carballido. The equation gave an estimate of 69 tons, with a range of 52 to 86.

Second, the team fleshed out a three-dimensional model of Patagotitan’s skeleton, and calculated its volume to then estimate its mass. As it often does, that technique gave a similar but slightly smaller range—45 to 77 tons.

These are wide ranges. But crucially, they’re bigger than similar ranges, estimated for other dinosaurs using the same methods. There are some exceptions, but Carballido doesn’t think much of them. For example, one team estimated that Argentinosaurus weighed somewhere between 66 and 97 tons, based on one badly preserved femur. “It’s difficult to measure the circumference of that bone because most of it was reconstructed with plaster,” says Carballido, “so we can’t accurately know its weight.”

By comparing their new discovery to the bones of other sauropod species, the Argentinian team could reconstruct the group’s evolutionary history. They think that sauropods were stable in size for much of their history, with most species weighing between 12 and 20 tons. But at some point, one group of titanosaurs—the lognkosaurs—became exceptionally huge, tripling their mass and reaching the 38-to-60-ton bracket. This was the group that gave rise to Argentinosaurus and Patagotitan.

It’s not clear why these animals became so big, although flowering plants became very diverse at roughly the same time, and the climate became warmer. Brassey also wants to know how these animals became so huge. “Did they modify the shape of their skeleton, or the way in which they grew?” she wonders. “Did their metabolism change? Or did they alter their behaviour? Or all of the above?”

Also: How big could they possibly get? “Maybe someone can find a bigger one,” says Carballido. “But I feel like maybe this is the limit.”