How to Calculate a Stegosaurus’s BMI

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​Among the dinosaurs were the largest animals ever to walk on land, so it's always been taken for granted that they were the heaviest too. That said, you can't just shove long extinct creatures onto a scale, so pinning down the exact weights of these giants has been a longstanding challenge for paleontologists.

But now, thanks to 3D scanning techniques, the era of ambiguous dinosaur weights may be coming to an end. In a study published today in Biology Letters, paleontologists narrowed down the expected body mass of a Stegosaurus specimen named Sophie after the daughter of a Natural History Museum (NHM) benefactor (though the gender of the dinosaur hasn't actually been determined).

According to the study's authors, Sophie clocked in at 1,560 kilograms, or 3,439 pounds, when "she" died some 150 million years ago.

That's actually on the small side for a Stegosaurus, because Sophie was a young adult. This Jurassic dweller measured a paltry 18.4 feet from head and tail, and stood nine-and-a-half feet tall. Had she grown to adulthood, she may have reached lengths of 30 feet, almost twice as large.

But what Sophie lacks in size, she makes up for in condition, and that was the key to unraveling her body mass. A full 85 percent of her original skeleton was recovered, making her the most complete Stegosaurus specimen in the world. This lucky case of pristine preservation and recovery gives paleontologists a lot more hard data to work with, and accordingly, much more accurate estimates.

The study's authors, led by NHM paleontologist Charlotte Brassey, also used an arsenal of cool technological tricks to further hone in on the dinosaur's specs. When the specimen arrived at the museum in 2013, a decade after it was first discovered, Brassey and her colleagues scanned it with both x-rays and lasers to produce highly resolved composite models of its frame in three dimensions.

"Because this incredible specimen is so complete, we have been able to create a 3D digital model of the whole fossil and each of its 360 bones, which we can research in excellent detail without using any of the original bones," Brassey said in an NHM statement.

The NHM team also exposed the inaccuracies inherent in traditional methods of estimating dinosaur mass by comparing the results of volumetric analysis and a technique known as linear bivariate mass estimation. The former involves isolating likely body volumes with computer modeling and comparisons to extant animal morphology. The latter is a common method in which the circumference of the limbs are used to calculate mass, and it turned out to be wrong by an enormous factor.

The problem was that like any budding adolescent, Sophie was somewhat gangly. Her legs were growing faster than the rest of her body, which skewed the linear bivariate results, which suggested she was almost two times heavier than she most likely was. The team warned that these kinds of missteps might be very common in paleontology.

"Calculating body mass in animals that have been dead for many millions of years is no easy task," said study co-author Susannah Maidment in a statement. "Our study is the first to attempt different methods on the same animal, and has highlighted how and why different body mass estimation methods come up with different results."

"The age of the animal when it died is very important," she added.

These precise estimates of body mass aren't just paleontological parlour tricks—they will lead to a much richer understanding of dinosaur behavior and biology. "If we want to estimate how fast an animal runs, you need body mass," Brassey told BBC News. "If you want to say something about their metabolism, you need to know their body mass."

"So, yes, we're really glad that we've been able to get hold of this very early on, and now what I'm looking to do is begin to strap muscles on to our computer models so that we can get her walking to say something about locomotion," she said.

As of December 2014, Sophie the Stegosaurus is now on display as the "star specimen" of NHM's Earth Hall. She may have died young, but this girl (or boy) definitely knows how to stage a comeback.