The battle for the title of world's largest dinosaur is complicated.
Here's why: Paleontologists rarely discover an entire skeleton. They're more likely to uncover bone fragments and then try to estimate a profile of height and weight. Moreover, there are three categories for largest dinosaur on record: the weightiest, longest and tallest.
Starting with the weightiest, the gold-medal winner is likely Argentinosaurus. This supermassive titanosaur (a titanosaur is a giant sauropod, a long-necked and long-tailed herbivorous dinosaur) that lived about 100 million to 93 million years ago, during the Cretaceous period, in what is now (you guessed it) Argentina. [What Really Killed the Dinosaurs?]
But estimates of Argentinosaurus' weight vary widely; the beast weighed 77 tons (70 metric tons), according to London's Natural History Museum; up to 90 tons (82 metric tons), according to New York City's American Museum of Natural History; and 110 tons (100 metric tons), according to BBC Earth.
It's no wonder these calculations are all over the place. Argentinosaurus is known from just 13 bones: six midback vertebrae, five fragmentary hip vertebrae, one tibia (a shinbone) and one rib fragment. "There's a femur that you'll see with it [in some sketches], but that femur was found 15 kilometers [9 miles] away. So, who knows who that belongs to?" said Kenneth Lacovara, a professor of paleontology and geology and the dean of the School of Earth & Environment at Rowan University in Glassboro, New Jersey.
Another contender is Patagotitan, a titanosaur that weighed a whopping 69 tons (62 metric tons) when it lived about 100 million years ago in what is now Argentina. However, this weight was calculated based on a composite of individuals (there were six found in all), rather than just one dinosaur, Lacovara noted.
Which raises the question: How do scientists calculate the weight of an extinct animal? According to Lacovara, there are three ways.
Minimum shaft circumference method: Scientists measure the minimum circumference of the humerus (the upper arm bone) and femur (the thigh bone) from the same individual. Then, they plug these numbers in to a formula. The result is highly correlative with the animal's mass. "It makes sense," Lacovara said, "since all quadrupeds have to put all of the weight of the body on just those four bones. [So], the structural properties of those four bones are going to correlate closely with the mass."
There are caveats, however. If the humerus and femur bone are from different individuals, as they were with Patagotitan, "the result is an estimate of a composite individual that never actually existed," Lacovara said. Moreover, if only a single bone (a humerus or a femur) is used, the proportions of the missing bone are a guess. "Obviously, this introduces even more uncertainty," he said. "Examples of this are Notocolossus and Paralititan."
The largest known dinosaur that has a humerus and femur bone from the same individual is the 77-million-year-old Dreadnoughtus, a 65-ton (59 metric tons) titanosaur that Lacovara and his team excavated in Argentina.
Volumetric method: In this approach, researchers determine the body volume of the dinosaur and use that number to calculate the animal's weight. This is challenging, because most titanosaur skeletons are incomplete. (Dreadnoughtus is the most complete, at 70 percent. Argentinosaurus is just 3.5 percent complete.) In addition, researchers have to guess how much space the lungs and other air-filled structures took up. Experts also have to speculate how "blubbery or shrink-wrapped" the skin on these dinosaurs was.
"In my view, this method is unworkable and lacks replicability, which is one of the hallmarks of science," Lacovara said.
Wild guesses: This is how scientists estimate the weight of dinosaurs that don't have any preserved humerus or femur bones. "Argentinosaurus, Futalognkosaurus and Puertasaurus are examples of this," Lacovara said. "They are clearly huge, but there is no systematic, replicable way to estimate their mass."
Moving on, what's the longest dinosaur? That honor likely goes to Diplodocus or Mamenchisaurus, which can be described as slender and elongated sauropod dinosaurs, Lacovara said. "Both are known from reasonably complete skeletons, and both would be about 115 feet [35 m] long." [How Did Dinosaurs Grow So Huge?]
In contrast, the titanosaurs were shorter. For example, Dreadnoughtus was "only" about 85 feet (26 m) long.
But this category is still rife with uncertainty. "Some dinosaurs claimed to be the longest are extremely fragmentary," Lacovara said. "For example, Sauroposeidon is known from just four neck vertebrae. So, really, who knows?" Meanwhile, Amphicoelias, a sauropod known from only a sketch of a single vertebra in a notebook from the 19th century paleontologist Edward Cope, is sometimes cited as the longest, tallest and heaviest dinosaur.
"The vertebra was apparently lost or destroyed in transport — or maybe never existed," Lacovara said. "You can't have a dinosaur represented by nothing, so as far as I'm concerned, Amphicoelias is not a thing."
As for that dinosaur's actual height, the devil is in the details.
"This, of course, depends on whether these animals could lift their necks up to maximum height," Lacovara said. "Their forelimb and shoulder structure looks like they were angling their necks upward, but we may never know the degree to which they could do this."
Editor's Note: This article was originally published on Oct. 10, 2012, and was updated on Jan. 27, 2019. Additional reporting by Katharine Gammon.
Originally published on Live Science.