A comparison of large sauropods: Amphicoelias (red), Argentinosaurus (purple), Mamenchisaurus (green), Sauroposeidon (blue), Supersaurus (orange).
Read Caption
Art by Matt Martyniuk (CC BY-SA 3.0).
A comparison of large sauropods: Amphicoelias (red), Argentinosaurus (purple), Mamenchisaurus (green), Sauroposeidon (blue), Supersaurus (orange).

Biggest Dinosaur Ever? Maybe. Maybe Not.

Paleontologists working in Argentina have uncovered the bones of what may be the largest dinosaur ever. I want to stress the uncertainty in that opening sentence. Despite various news outlets already calling the contest, we don’t yet know which titanic dinosaur wins the superlative of “biggest creature ever to walk the Earth.”

Don’t misunderstand me – the new find is certainly worth getting excited about. Found by a farm worker in the vicinity of La Flecha, Argentina, and excavated by a crew from the Museo Paleontológico Egidio Feruglio, the 95 million year old site contains over 150 bones belonging to seven individuals of  the same long-necked, heavy-bodied species. Even better, the dinosaur might be new to science, and the presence of so many specimens in one spot could yield detailed insights into the growth, ecology, and behavior of the dinosaur.

But the dinosaur’s massive size is what has catapulted it into the headlines this weekend. Press photos show paleontologists posing for scale against a femur bigger than they are – a required photo-op for any researcher who works with such enormous bones – and an initial estimate from one such bone suggests that the new dinosaur was 130 feet long and weighed 77 tons. If accurate, this would make the titanosaur the top contender to the biggest dinosaur yet known. The trouble is that it’s too early to tell whether or not the estimates are on the mark.

View Images
A reconstruction of Supersaurus – in tan, against the wall – looms over the Jurassic Hall of the Museum of Ancient Life. Snapshot by Brian Switek.

The tragedy of the biggest sauropods is that they’re so scrappy. Argentinosaurus – often cited as being about 100 feet long and in the range of 80 tons – is only known from a relatively paltry collection of vertebrae, ribs, and an incomplete femur. Bruhathkayosaurus, a dinosaur that may have been as big or even bigger than Argentinosaurus, was only known from limb, hip, and tail elements, and those fossils disappeared (much like the near-mythical dinosaur giant Amphicoelias, estimated to be 190 feet long from a long-lost piece of vertebra).

Even sauropods known from a decent amount of material are still too incompletely known to get a direct idea of how long and heavy they were. In order to estimate the size of dinosaurs such as Futalognkosaurus, Supersaurus, and “Seismosaurus“, paleontologists have to turn to more-complete skeletons of smaller, closely-related dinosaurs and scale up. This scrappy record is important to keep in mind because, among other parts, tails make a big difference in this ongoing “my dinosaur is bigger than yours” contest.

A great deal of a sauropod’s length was in the tail, and how long that tail really was hinges upon how many vertebrae were in that part of the spine. The trouble is that complete dinosaur tails are very rare in the fossil record, and some of those precious fossils even suggest that the number of vertebrae in a dinosaur species’ tail could slightly vary from one individual to the next. When you’re dealing with dinosaurs that had vertebrae measured in feet, not inches, the number of tail vertebrae paleontologists reconstruct can make a big difference for a size estimate. And given that the new bonebed contains only 150 bones between seven individuals, the length of the sauropod’s tail – and other parts – is going to have to rely on what we know about other species.

Weight is another matter altogether. Determining a dinosaur’s body mass not only relies on filling in missing bones based upon close relatives, but also a particular researcher’s perception of whether the dinosaur in question had a beefy build or was leaner. That’s why paleontologists are familiar with shrinking sauropods.

View Images
Paleontologist Jim Jensen with the reconstructed foot of the now-discarded “Ultrasauros.” Photo by Doctorjrj.

When I was a kid, the 90-foot-long, the 180 ton “Ultrasaurus” was supposed to be the biggest dinosaur ever. Then it turned out that this dinosaur was a composite of multiple individuals found in the same quarry, with some bones belonging to the substantial Supersaurus and others to a big, but not record-breaking, Brachiosaurus.

Supersaurus itself has fared a bit better, now estimated at 110 feet and about 45 tons, but the same isn’t true for another childhood favorite of mine. In news pieces and books, “Seismosaurus” was said to be 120-170 feet long and weigh over 100 tons. Today, the dinosaur has been recognized as a big species of DiplodocusD. hallorum – that was closer to 108 feet long and significantly less hefty than earlier estimates. And don’t even get me started on the titanosaur that was heralded in headlines as maybe being the largest of all time despite being known from a single tooth.

Dinosaurs aren’t the only animals to get downsized. The specimen of the giant marine reptile Pliosaurus nicknamed “Predator X” was initially said to be 50 feet long in documentaries and news items, supposedly the largest pliosaur of all time, but was later downsized to 33-42 feet long. Prehistoric creatures ballyhooed as “the biggest ever” upon discovery have a tendency to shrink by time of publication.

None of this is to say that the newly-discovered sauropod in Argentina is out of the running for the title of the largest dinosaur yet found. The sauropod could very well earn that distinction, or at least put in a good show against the other contenders both named and as-yet-unnamed. But there’s no way to say for sure just yet.

Size extrapolations from a single femur are not the final word, especially when there are pieces of comparably-sized dinosaurs out there. The new dinosaur will undoubtedly have lots of missing pieces that will have to be filled in and then used as a basis to estimate body mass, and I honestly wouldn’t be surprised if this turned out to be another roughly 110-foot-long sauropod with a weight about 50 tons. The fact that the best-known giant sauropods cluster in this range might hint that getting much bigger than that was biologically difficult – perhaps a marker of an upper limit for how big dinosaurs could be.

Then again, this new dinosaur could blow past that barrier and raise new questions about how these animals lived so large. We’ll have to wait for the published details and the ensuing scientific discussion. Ultimately, though, the best part of the discovery is that paleontologists have turned up many bones from multiple individuals, offering paleontologists a wealth of material to investigate how these ancient animals lived. Size isn’t everything.