How heavy was Giraffatitan brancai? I mean, really?
June 9, 2014
We’ve touched on this several times in various posts and comment threads, but it’s worth taking a moment to think in detail about the various published mass estimates for the single specimen MB.R.2181 (formerly known as HMN SII), the paralectotype of Giraffatitan brancai, which is the basis of the awesome mounted skeleton in Berlin.
Here is the table of published estimates from my 2010 sauropod-history paper, augmented with the two more recent estimates extrapolated from limb-bone measurements:
|Author and date||Method||Volume (l)||Density (kg/l)||Mass (kg)|
|Janensch (1938)||Not specified||—||—||`40 t’|
|Colbert (1962)||Displacement of sand||86,953||0.9||78,258|
|Russell et al. (1980)||Limb-bone allometry||—||—||13,618|
|Anderson et al. (1985)||Limb-bone allometry||—||—||29,000|
|Paul (1988)||Displacement of water||36,585||0.861||31,500|
|Alexander (1989)||Weighing in air and water||46,600||1.0||46,600|
|Gunga et al. (1995)||Computer model||74,420||1.0||74,420|
|Christiansen (1997)||Weighing in air and water||41,556||0.9||37,400|
|Henderson (2004)||Computer model||32,398||0.796||25,789|
|Henderson (2006)||Computer model||—||—||25,922|
|Gunga et al. (2008)||Computer model||47,600||0.8||38,000|
|Taylor (2009)||Graphic double integration||29,171||0.8||23,337|
|Campione and Evans (2012)||Limb-bone allometry||—||—||35,780|
|Benson et al. (2014)||Limb-bone allometry||—||—||34,000|
(The estimate of Russell et al. (1980) is sometimes reported as 14900 kg. However, they report their estimate only as “14.9 t”; and since they also cite “the generally accepted figure of 85 tons”, which can only be a reference to Colbert (1962)”, we must assume that Russell et al. were using US tons throughout.)
The first thing to notice is that there is no very clear trend through time, either upwards or downwards. Here’s a plot of mass (y-axis) against year of estimate (x-axis):
I’ve not even tried to put a regression line through this: the outliers are so extreme they’d render it pretty much useless.
In fact, the lowest and highest estimates differ by a factor of 5.75, which is plainly absurd.
But we can go some way to fixing this by discarding the outliers. We can dump Colbert (1962) and Alexander (1989) as they used overweight toys as their references. We more or less have to dump Russell et al. (1980) simply because it’s impossible to take seriously. (Yes, this is the argument from personal incredulity, and I don’t feel good about it; but as Pual (1988) put it, “so little flesh simply cannot be stretched over the animal’s great frame”.) And we can ignore Gunga et al. (1995) because it used circular conic sections — a bug fixed by Gunga et al. (2008) by using elliptical sections.
With these four unpalatable outliers discarded, our highest and lowest estimates are those of Gunga et al. (2008) at 38,000 kg and Taylor (2009)at 23,337. The former should be taken seriously as it was done using photogrammetrical measurements of the actual skeletal mount. And so should the latter because Hurlburt (1999) showed that GDI is generally the least inaccurate of our mass-estimation techniques. That still gives us a factor of 1.63. That’s the difference between a lightweight 66 kg man and and overweight 108 kg.
Here’s another way of thinking about that 1.63 factor. Assuming two people are the same height, one of them weighing 1.62 times as much as the other means he has to be 1.28 times as wide and deep as the first (1.28^2 = 1.63). Here is a man next to his 1.28-times-as-wide equivalent:
I would call that a very noticeable difference. You wouldn’t expect someone estimating the mass of one of these men to come up with that of the other.
So what’s going on here? I truly don’t know. We are, let’s not forget, dealing with a complete skeletal mount here, one of the very best sauropod specimens in the world, which has been extensively studied for a century. Yet even within the last six years, we’re getting masses that vary by as much as the two dudes above.