What Was the Biggest Dinosaur? Fragmentary Fossils Make It Hard to Tell

Nagatitan was immense. The recently announced titanosaur, the largest dinosaur yet uncovered in Thailand, likely stretched more than 88 feet in length and weighed almost 30 tons. The operative word is “likely,” though, because as big as the dinosaur was, only a smattering of its bones made it into the fossil record. Without a complete skeleton, it’s difficult to tell what the creature’s exact measurements were.

The newly named Nagatitan is far from the only large dinosaur whose actual size seems a little hazy. In a broad sense, the biggest dinosaurs were sauropods—the long-necked and long-tailed titans that walked on four legs and ate plants. But pinning down exactly which one was the most gargantuan is not an easy task. From the time the Jurassic Brachiosaurus was uncovered in 1900 and said to be biggest of all, the title holder of this sauropod superlative has continued to shift. In the late 1900s, scientists gave fossils names like “Ultrasaurus,” “Seismosaurus” and Supersaurus—the first two of which are no longer used—and touted them as being far larger than anything found before. But as paleontologists refined their calculation techniques, the estimates for these dinosaurs were downsized. Even more recently, long-necked herbivores such as Argentinosaurus, Futalognkosaurus and Patagotitan have become the top contenders, but all are known only from incomplete skeletons.

Missing bones can be a big problem for estimating sauropod size. Consider Patagotitan, the current favorite for the largest known dinosaur, for which a single neck vertebra could be nearly four feet long. The number of vertebrae in dinosaurs’ necks, backs and tails varied among species, meaning that if Patagotitan had even one more or one fewer vertebra than paleontologists think it did, that could significantly alter scientists’ projection of how big the dinosaur really was.

Without a complete skeleton, every assessment of size becomes an estimate. Most modern estimates are based on a close relationship between limb bone dimensions and body size that holds true even among modern animals. In short, the heavier an animal is, the thicker their limb bones must have been to handle the weight. Some large dinosaurs were similar in length, but each had different body proportions that could be tons different from one species to the next. And to fill in the blanks of incomplete remains, they can look to the skeletons of the animal’s close relatives. That raises a question: Why don’t we have better skeletons of the biggest dinosaurs?

A gargantuan, 100-foot-long dinosaur would have been a very sturdy reptile. But when such giants passed away, leaving 40 tons or more of flesh and bone out on the landscape, an even grander amount of sand or silt was required to bury and preserve them before they decomposed. Local flooding, perhaps caused by heavy wet-season rains, was required to move enough sediment to cover the bodies.

“It takes a lot of debris to bury a boulder,” says Museums of Western Colorado paleontologist Julia McHugh. “Now imagine a 65-foot-long sauropod like Apatosaurus.”

McHugh says her own experience of a 500-year flood along the Iowa River in 2008 underscored just how exceptional the circumstances would have needed to be to bury the largest dinosaurs in sediment. “This flood was massive and lasted for weeks,” McHugh says, recalling students on the University of Iowa campus forming human chains to move books to the top floor of the library. Still, the floodwaters left sediment only about six inches deep near the river channel. “How much larger or how many more successive major floods were needed to bury a sauropod dinosaur?” McHugh wonders.

When she began working with the Jurassic Mygatt-Moore Quarry, which lies outside Fruita, Colorado, and is rife with Apatosaurus remains, the questions about burial came back into focus. In a 2020 study, McHugh and her colleagues investigated how long the bones preserved in the quarry had been exposed to the elements before they became covered by sediment. If scientists can work out that timeline, it might explain the conditions needed for sauropods to be preserved and how long it took for those conditions to arrive.

“We were shocked by the results,” McHugh says. Many of the Mygatt-Moore bones showed insect damage that would have taken five to six months to form. Some of the bones were so worn that they were probably exposed for up to six years before finally being covered up, the team concluded. And during that window of decay, a great deal of a sauropod’s body can disappear.

“When a large animal dies, it becomes an instant source of opportunity for its entire ecosystem,” McHugh says. Not only were flies and beetles attracted to sauropod carcasses, but snails, mammals and other dinosaurs came to scavenge, too. Many fossil bones bear scrapes and bite marks from feeding Allosaurus and other large carnivores that likely relied on the deaths of multi-ton sauropods to fill their bellies, in addition to catching smaller prey. The death of a big sauropod dinosaur was a major event for these scavengers, which often stripped carcasses down to the bone. “We have bite marks on ribs, limbs and even pelvic bones,” McHugh says. And, if fossil carnivore feces filled with skeletal shards are any indication, they often ate the bones, too.

Even when enough sediment covered a dinosaur, the weight of the debris could be too much. “If a flood or landslide moves enough sediment to cover a sauropod, it is likely to crush or winnow away smaller, more delicate bones like the skulls and end of the tail, so the very situations that preserve large animals also bias the record of them,” says Adelphi University paleontologist Michael D’Emic.

Given the low odds of preservation, it’s astounding that experts have found as many sauropod bones as they have. But the spotty fossil record can sway their view of the past. Paleontologists used to think that sauropod dinosaurs had disappeared from North America around 100 million years ago, only to return millions of years later, but research has suggested this “sauropod hiatus” is better attributed to fewer environments that could have preserved sauropod fossils. Many factors, from how dinosaurs were preserved to where an expert decides to look for fossils, can also influence where the biggest specimens turn up.

Perhaps more complete titans are out there. For now, however, the top contenders for the largest dinosaur of all are known from partial remains. The fossils of some, such as Bruhathkayosaurus, crumbled away before they were even studied and cannot be accurately compared to those of other dinosaurs. The largest known dinosaurs, it seems, exceeded 100 feet in length and weighed more than 40 tons, but more than one species has been estimated to have attained such a stature. In terms of body length, declaring a champion could come down to a matter of inches.

The heaviest dinosaur, however, may be easier to identify. “Body mass is easier to compare,” D’Emic says, because “thicker bones support more weight than thinner bones.” When paleontologists find the remains of a sauropod’s legs, the thickness of those bones offers a way to better estimate the mass of the animal, even when the rest of the skeleton is missing. On that score, the largest Argentinosaurus specimens are likely the heaviest dinosaurs known. The bones imply that the dinosaur weighed more than 80 tons. The distinction doesn’t cement Argentinosaurus as the definitive largest dinosaur, but it does provide some important numbers for the extreme end of sauropod size.

Focusing on crowning a single dinosaur as the largest may be a misguided venture to begin with. Even within a dinosaur species, individual animals might have been vastly different sizes. “Paleontologists don’t currently have a good handle on the distribution of body sizes in populations of most species,” D’Emic says, so they lack a good idea of what was an average or exceptional size for most sauropods. Even if enough Argentinosaurus fossils had been found to reveal the average size for an adult, for example, there could have been rare individuals that were significantly bigger and have gone undiscovered. As is the case with variations in living animals, the very largest would have been uncommon, which makes them even harder to find.

And yet, such giants sometimes turn up. In 2014, McHugh and her team uncovered a 6-foot-7-inch-long Apatosaurus femur, the largest thigh bone of its species ever found. It’s something of a wonder the bone made it into the fossil record. “Limbs store a lot of meat, and even though the bones are strong, the meat is in high demand,” McHugh says. The large femur does bear bite marks, but it somehow was buried before being completely consumed. If the animal had the same proportions of other Apatosaurus, a dinosaur for which a complete skeleton has been found, it was likely 15 feet longer than the average.

It’s a sign that some of the more informative bones are out there, and D’Emic notes that new sauropod fossils are being announced at a faster rate than ever before. Even though the fossilization process was unkind to the biggest dinosaurs, destroying many of the bones we’d hope to find, glimmerings of the largest animals to ever walk the Earth are still turning up. And, given that sauropods larger than any living or extinct terrestrial mammal have evolved more than 30 times, there are undoubtedly many giants yet to find.