Scientists have discovered the fossilized remains of a new long-necked, long-tailed dinosaur that has taken the crown for largest terrestrial animal with a body mass that can be accurately determined.
Measurements of bones from its hind leg and foreleg revealed that the animal was 65 tons, and still growing when it died in the Patagonian hills of Argentina about 77 million years ago.
“To put this in perspective, an African elephant is about five tons, T. rex is eight tons, Diplodocus is 18 tons, and a Boeing 737 is around 50 tons,” said study author and paleontologist Kenneth Lacovara at Drexel University. “And then you have Dreadnoughtus at 65 tons.”
Dreadnoughtus, meaning “fears nothing,” is named after the impervious early 20th century battleships. Although it was a plant-eater, a healthy Dreadnoughtus likely had no real issues with predators due to its intimidating size and muscular, weaponized tail.
But its enormous bulk also had a downside. Based on the width and strength of its skeleton, toppling over would likely spell death for such a heavy animal.
“If you look at its really big ribs, there's no way they're going to withstand 65 tons of weight on top of them,” he said. “It would have been a catastrophic event in the life of a Dreadnoughtus if it fell over.”
However, it probably didn't do much walking around since its 37-foot-long neck could already provide access to a wide bounty of vegetation.
“How do you come up with a body size that is so enormous when you're a terrestrial animal?” said Luis Chiappe, director of the National History Museum of Los Angeles's Dinosaur Institute, who was not involved in the study. “You need to have a structural design that allows you to support a body like that, and you have to be potentially adapted to eat 24 hours a day, nonstop, with a minimal amount of sleep.”
The study was published online Thursday in Scientific Reports.
On the first day of the 2005 field season in southern Argentina, Lacovara spotted a little lump of bone sticking out of the ground. It was maybe the 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.
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