How Much Did Dinosaurs Weigh?

How Scientists Estimate the Weight of Extinct Dinosaurs

Brontomerus, a plus-sized sauropod of the Mesozoic Era (Francisco Gasco).

Imagine that you're a paleontologist examining the fossilized remains of a new genus of dinosaur--a hadrosaur, say, or a gigantic sauropod. After you've figured out how the specimen's bones are put together, and what type of dinosaur you're dealing with, you eventually go on to estimate its weight. One good clue is how long the "type fossil" is, from the tip of its skull to the end of its tail; another is the published weight estimates for comparable types of dinosaurs.

If you've discovered a huge titanosaur from late Cretaceous South America, for example, you might venture a guess of 80 to 120 tons, the approximate weight range of South American behemoths like Argentinosaurus and Futalognkosaurus. (See a slideshow of The 20 Biggest Dinosaurs and Prehistoric Reptiles and an article discussing why dinosaurs were so big.)

Now imagine that you're trying to estimate the weight not of a dinosaur, but of an obese stranger at a cocktail party. Even though you've been around human beings all of your life, of all shapes and sizes, your guess is likely to be inaccurate: you might estimate 200 pounds when the person actually weighs 300 pounds, or vice-versa. (Of course, if you're a medical professional, your guess will be much closer to the mark, but still potentially off by 10 or 20 percent.) Extrapolate this example to the 100-ton titanosaur mentioned above, and you can be off by as many as 10 or 20 tons.

If guessing the weight of people is a challenge, how do you pull off this trick for a dinosaur that's been extinct for 100 million years?

How Much Did Dinosaurs Really Weigh?

As it turns out, recent research demonstrates that experts may have been drastically overestimating the weight of dinosaurs, for decades.

Since 1985, paleontologists have used an equation involving various parameters (the total length of the individual specimen, the length of certain bones, etc.) to estimate the weight of all kinds of extinct animals. This equation produces reasonable results for small mammals and reptiles, but veers sharply from reality when larger animals are involved. In 2009, a team of researchers applied the equation to still-extant mammals like elephants and hippopotamuses, and found that it vastly overestimated their weight.

So what does this mean for dinosaurs? At the scale of your typical sauropod, the difference is dramatic: whereas Apatosaurus (the dinosaur previously known as Brontosaurus) was once thought to weigh 40 or 50 tons, the corrected equation puts this plant-eater at a mere 15 to 25 tons (though, of course, it doesn't have any effect on its enormous length). Sauropods and titanosaurs, it seems, were much more slender than scientists have given them credit for, and the same probably applies to plus-sized duckbills like Shantungosaurus and horned, frilled dinosaurs like Triceratops.

Sometimes, though, weight estimates veer off the tracks in the other direction. Recently, paleontologists examining the growth history of Tyrannosaurus Rex, by examining various fossil specimens at various growth stages, concluded that this fierce predator grew much more quickly than was previously believed, putting on as many as two tons per year during its teenaged spurt.

Since we know that female tyrannosaurs were bigger than males, this means that a full-grown T. Rex female may have weighed as much as 10 tons, two or three tons heftier than previous estimates.

The More Dinosaurs Weigh, the Better

Of course, part of the reason researchers impute enormous weights to dinosaurs (though they may not admit to it) is that these estimates gives their findings more "heft" with the general public. When you're talking in terms of tons, rather than pounds, it's easy to get carried away and carelessly attribute a weight of 100 tons to a newly discovered titanosaur, since 100 is such a nice, round, newspaper-friendly number. Even if a paleontologist is careful to tone down his weight estimates, the press is likely to exaggerate them, touting a given sauropod as the "biggest ever" when in fact it wasn't even close.

People want their dinosaurs to be really, really big!

The fact is, there's still a lot we don't know about how much dinosaurs weighed. The answer depends not only on measures of bone growth, but on other still-unresolved questions, such as what type of metabolism a given dinosaur possessed (weight estimates can be very different for warm-blooded and cold-blooded animals), what kind of climate it lived in, and what it ate on a daily basis. The bottom line is, you should take the weight estimate of any dinosaur with a big grain of Jurassic salt--otherwise you'll be sorely disappointed when future research results in a slimmed-down Diplodocus.