How to weigh dinosaurs with lasers

By Ed Yong | June 5, 2012 7:00 pm

The camera adds 10 tonnes…

The largest mounted dinosaur skeleton in the world towers over visitors in the central hall of Berlin’s Museum of Natural History. It belongs to Giraffatitan, an animal formerly known as Brachiosaurus (the big one from the opening act of Jurassic Park). From the bones, we can tell how long and tall Giraffatitan was, but how much did it weigh?

The dinosaur’s flesh has long decayed, but Bill Sellers from the University of Manchester has developed a new way of reconstructing its physique and estimating its weight. By laser-scanning the skeleton, and wrapping skin around its virtual bones, he calculated that this particular Giraffatitan weighs in at a hefty 23,200 kilograms, or 23.2 tonnes. And no matter what the university’s press office would like you to believe (more on this later), that’s virtually identical to the best current estimates.

There are two typical approaches for estimating the weight of fossil animals. You can compare the lengths of certain bones with those of known animals, assume that its mass scales accordingly. This is the predictive regression approach, and it can be unreliable. Skeletal features can vary greatly and they may not relate to weight in the same way between different animal groups. The alternative is the volumetric approach: you draw an outline of its body, estimate how much volume it took up, and multiply that with its predicted density. It’s better, but drawing the outline is both subjective and laborious.

Sellers has devised a third option. He scans an entire skeleton and his software automatically stretches a virtual skin over the outline as tightly as possible. This estimates the volume of the animal, albeit of an emaciated unrealistic individual.

When Sellers tested this technique with 14 mammal skeletons, from a wild boar to an African elephant, he found that it underestimated the weight of all the species. You’d expect that – after all, the virtual reconstruction doesn’t include any muscles or organs. The point is that the technique consistently underestimated the animals’ weight by around 21 per cent. You can just take the former, multiply it by 1.21 and get the latter.

The relationship between the predicted and actual weight is remarkable in its reliability. Sellers thinks that this is because most of the missing volume in the virtual models is from the limbs muscles, which make up a fairly fixed proportions of a mammal’s mass, regardless of its size.

Sellers then applied his method to Berlin’s famous Giraffatitan and got a value of 23.2 tonnes. Obviously, it’s not clear if a technique that was calibrated against large mammals would apply to dinosaurs, or other groups like reptiles or birds. Sellers acknowledges this, and plans to test his technique in a wider range of animal groups.

There are other potential issues. Mike Taylor from the University of Bristol and SV-POW has estimated the weight of Giraffatitan before, and thinks that around 70 per cent of its volume comes from the torso. And reconstructing the torso is very difficult for large dinosaurs, because the ribs are often poorly preserved or distorted. Taylor also says that using a single density value isn’t that appropriate for brachiosaurs. “The very long neck likely had a density no more than half that of the legs,” he says.

For the moment, it’s encouraging that the new estimate is very close to previous ones. You might not get that from the press release (and probably most of the resulting coverage). It leads with “Dinosaurs lighter than previously thought”, and follows with “Previous estimates of this Brachiosaur’s [sic] weight have varied, with estimates as high as 80 tonnes, but the Manchester team’s calculations – published in the journal Biology Letters – reduced that figure to just 23 tonnes.”

While it is true that the weight of Giraffatitan and Brachiosaurus have varied wildly over the years, the most recent estimates have been nowhere near the cherry-picked 80-tonne figure. Indeed, in 2009, Taylor concluded that Giraffatitan weighed 23,377 kilograms, or 23.3 tonnes. Sellers’ new estimate shaves off a mere 177 kg from that figure – around 2 humans from a dinosaur that weighed as much as 300.

Taylor used the volumetric method to get his result. If his result was exactly the same as the new figure, one might question whether Sellers’ method adds anything new. It does, however, have several benefits. “It requires no irreproducible judgements on the part of the person using it, and it’s ground-truthed on solid data from extant animals,” says Taylor. It’s also automated. If it truly works for dinosaurs, we can weigh these extinct beasts as quickly as the laser-scanner can be wheeled around a museum. Even with the caveats, Taylor says “It’s an important new method which I expect to see widely adopted.”

Reference: Sellers, Hepworth-Bell, Falkingham, Bates, Brassey, Egerton & Manning. 2012. Minimum convex hull mass estimations of complete mounted skeletons. Biology Letters http://dx.doi.org/10.1098/rsbl.2012.0263

Image from Berlin Natural History Museum postcard

HT Matthew Cobb for the story tip

More on sauropods

CATEGORIZED UNDER: Dinosaurs, Palaeontology

Comments (15)

  1. “It requires irreproducible judgements…” This is a good thing?

  2. “It requires irreproducible judgements…” This is a good thing?

    An unfortunate typo in the article, which I hope Ed will fix. The quote I gave him was “a useful new technique that … requires no irreproducible judgements”.

  3. I would be very interested in seeing how their methodology differs (if at all) from last year’s study on Tyrannosaurus, where they typically found values much higher than traditional estimates. If it is a different version of the laser scan technique, it would be cool to see them do estimates for the same specimens of Tyrannosaurus as the Hutchinson et al. paper and compare the results.

  4. Sorry Mike, I’ve fixed it now. Very sloppy. My apologies.

  5. pionere

    Just out of curiosity, wouldn’t it be possible to measure the bone density differences at the top and the bottom part of the animal, and guess the weight based on this information and the age of the animal?

  6. Be fair now! Saying that new research has doubled the age of the Universe or quartered the weight estimate of one of the world’s biggest (and most famous) dinosaurs IS good PR for the supporting institution! It certainly got people’s attention. Whereas saying that scientists used a whole new and rather novel technique to shave 1% off the previous weight estimate would be interpreted by the average reader as “scientists waste time and money playing with new toys to confirm what they already knew”. Sadly, the gender-neutral hominid on the Clapham Omnibus doesn’t want to read stories about science making slow, steady, methodical progress when the same story can be spun as a radical overturning of everything we thought we knew.

  7. pionere, the problem isn’t that bone density varies in different regions of the animal (though that’s true), it’s that the entire animal’s density changes. The neck of Giraffatitan was probably 50% air — some of it inside the vertebrae, some in soft-tissue diverticula. But that “50%” is guesswork, and no-one has even attempted a rigorous estimation yet.

    BTW., for anyone who wants to see my comments in full, they are now up at Sauropod Vertebra Picture of the Week.

  8. Georg

    The big problem
    in density of animals is lung (contains a lot of air) versus
    the rest. The success of Sellers method shows that lung volume
    for mammals has a certain, fixed relation to body volume.
    (What about birds?)
    If Sellers relation holds for dinosaurs, this is another hint
    to the dinosaurs being warm blood animals, because that
    influences the oxygen demand (thus lung volume) per
    body volume very much.
    Georg

  9. pionere

    Mike, thanks for your answer, but I did not mean it that way. My suggestion was to compare the tear/wear of the bones at different places of the animal. Since the bottom has to carry the whole body while the top not, this comparison could lead to an estimation of the weight of the animal.

  10. I’ve never heard that technique suggested before. It’s a neat idea, but I have to say I am skeptical that it would be possible to extract useful information from it. The only difference in the mass a limb bone has to carry at the top and bottom is that of the limb segment itself, which is a very small proportion. Then the sizes and shapes of the top and bottom of the bone are different. And in life they would have been constantly repaired as micro-damage accumulated. And they would have been covered by extensive cartilage caps that had very different shapes from what we can see of the actual bone.

  11. John

    There’s an error in the math: “The point is that the technique consistently underestimated the animals’ weight by around 21 per cent. You can just take the former, multiply it by 1.21 and get the latter.” If you underestimate by 21 per cent, you have 79 per cent left. If you multiply 79 per cent by 1.21, you get roughly 95.6 per cent, not 100. You would in fact have to multiply by 1.266, roughly, to obtain the correct figure.

  12. Jerrold Alpern

    Is it possible to apply these techniques to get a new approximation of the weight of the AMNH Apatosaurus? Visitors ask about this all the time. At the moment, as an Education Volunteer in the Saurischian Hall, I tell them that estimates have ranged between 18 and 36 tons. Is a more exact answer now available?

  13. If I understand the article correctly, then it is saying that there are three ways for determining the mass of a dinosaur. Yet I am not sure from the description if it is listing the method that I would think would be the most reliable. The one listed method says to start by drawing an outline of the body – that is an area – then estimate the volume. Sorry but that does not make sense.

    Are not the authenticated plastic models of dinosaurs the result of paleontologists extensively debating the muscle attachment, that make up the shape of the dinosaurs? Since this is the core of their expertise I would think that the authenticated plastic models put forth by paleontologists should be fairly accurate in regards to shape and scaling ratio of the actual dinosaurs. With an accurate model a given dinosaur it is simple straight forward process for determining the mass of an actual dinosaur.

    Archimedes Principle tells that we can obtain the mass of the displaced water by submerging our dinosaur model in water while using a string to hang it from a scale. Knowing the mass of the displaced water we can determine the volume of the dinosaur model, or better yet go directly to determining the mass of the actual dinosaur. This is possible because the density of the bones, muscle mass, and whatever else of dinosaur can not be much different from the bones, muscle mass, and whatever else of reptiles and mammals of today. In all cases the overall density is very close to the density of water. If one wants to argue that dinosaurs had bird like lungs then a dinosaur’s density is reduced to be only about 0.7 or 0.8 g / cm3. After processing our data and using a density range between 0.7 to 1.0 we obtain a fairly tight range for the mass of each species of dinosaur. In addition, it is easy to use an accurate model of an elephant, or any large animal, to show that this method works like a charm in producing accurate mass estimates.

    Archimedes method is much easier, cheaper and even much more precise than this elaborate laser / software for determining the mass of a dinosaur. So I have to ask what are these paleontologists’ motives for using this complicated and yet clearly inferior laser / software method. Bottom line is that I have absolutely no confidence in these paleontologists’ latest guess as to mass of the dinosaurs.

  14. Jerrold Alpern

    What are these “authenticated plastic models of dinosaurs” ? Who designed them, who manufactured them, who “authenticated” them and where can they be found?

  15. From Wikipedia: The Carnegie Collection is a series of authentic replicas based on dinosaurs and other extinct prehistoric creatures, using fossils featured at the Carnegie Museum of Natural History as references. In addition to the Carnegie Collection there are the German museum’s Schleich dinosaurs and perhaps one or two other lines that are sold at the gift shops of dinosaur museums. The success of these products is primary determined by the general consensus of paleontologists regarding how well they replicate the actual dinosaurs.

    Regardless of which one of these lines of dinosaur models is used the results giving the dinosaurs’ mass comes out about the same, no more than about a 20% variation. Yet the dinosaur masses promoted by this article are three to even four times smaller than the experimental results produced by using dinosaur models to determine the volume, and in turn the mass, of the dinosaurs. Three or four times smaller is much more than just a little bit of fudging.

    What bothers me is that the science community is so overflowing with sheep. People are so willing to be followers that never ever question authorities. If people do not what to think for themselves then they should join the military where they just follow orders, or join the masses of mindless zombies that show up to church each Sunday so they can listen to their preacher as he thumps on the bible. But if someone wants to add something constructive to a scientific discussion then they must be willing to do further research or possibly conducting their own experiment in a process of questioning the statements made by authorities.

    In this case, just about anybody willing to put forth a little effort and capable of thinking for themselves can obtain the simple equipment needed for accurately determining the mass of various dinosaurs.

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