 20th fossil he had found that day, so he didn't think much of it. As he kept digging, he realized it was a massive dinosaur femur that stretched over six feet long.
Lacovara still wasn't all that thrilled — isolated bones are found all the time — until more and more pieces started popping up. By the end of that first day, he had added a tibia, a fibula and a half dozen tail vertebrae to his collection.
“At that point, I'm pretty excited,” he said. “But I had no idea that we were going to walk away with 130 bones.”
The huge creature also had a smaller companion, which Lacovara and his colleagues also dug up. Both got caught in quicksand, which is how their bones became so well-preserved.
“This is clearly a spectacular find, and I know what it takes to collect these things,” said Chiappe. “It's grueling work.”
The skeleton of Dreadnoughtus schrani is remarkably complete for a dinosaur of this size — over 70 percent of the bones were dug up, excluding the head. Typically these long-necked, long-tailed dinosaurs (called sauropods) have tiny skulls, so those bones tend not to survive. For instance, Dreadnoughtus's was probably about the size of a horse's head.
Also, such massive giants tend to leave only fragments of their skeleton behind, making their weight hard to pin down. In general, the smallest circumference of the humerus and femur tend to correlate well with an animal's weight, so paleontologists use this method for estimation.
So there may be more massive dinosaurs out there than Dreadnoughtus, but their masses can't be accurately calculated without those particular bones. The previous contender for biggest land animal with a mass that could be determined by bone circumference was another Argentine titanosaur called Elaltitan lilloi first described in 2012. It was estimated at 47 tons.
To look for hints of age, Lacovara and his colleagues looked for any traces of bone growth. As the animal gets older, parts of the skeleton will fuse together, and its bone growing cells will morph from fluffy to flat.
“With Dreadnoughtus, there's no indication that there was any cessation or slowing of growth [in the bones],” he said. “When it died at 65 tons, it was growing fast, which is kind of scary.”
Titanosaurs, a subgroup of sauropods that includes Dreadnoughtus, are thought to grow rapidly and reach full adult size in only 20 or 30 years. But surprisingly, their eggs are not overly large. The largest dinosaur eggs found have been about the size of a soccer ball.
“The hatchling from an egg the size of a football is not very big,” said Chiappe. “But in two decades, the hatchling would grow to become an enormous animal the size of two, three school buses put together.”
Chiappe, who has worked in Patagonia for a number of years and is originally from Argentina, discovered a large sauropod hatchery in 1997 that served as evidence that these dinosaurs laid eggs and huddled together in large nesting colonies.
“I would imagine [Dreadnoughtus] would have had a very similar nesting behavior: the congregation of hundreds or thousands of 60-ton females gathering together to nest in a lost valley somewhere,” he said.
The researchers have a whopping 10 Dreadnoughtus papers in the pipeline, four of which are already written, that take the analysis a step further. One of the upcoming studies looks at the dinosaur's locomotion through robotic and computer simulations, using 3D scans of the bones that are publicly available for free download along with the current study.
“Kids are going to be able to download the Dreadnoughtus bones and play with them,” said Lacovara.
Due to its monstrous size, Chiappe expects Dreadnoughtus to join the ranks of popular sauropod favorites such as Diplodocus and Brachiosaurus on the road to “Jurassic Park”-like stardom.
“This is the kind of creature that will soon make it into Hollywood,” he said. “You can guarantee that the next documentary on dinosaurs will feature this new creature.”
Kim is a freelance science journalist currently based in Philadelphia.