Xenoposeidon week, day 4: the question everyone is asking … how big was it?
November 18, 2007
[Sorry about the late posting today: I had to leave the house at 7:15 to fly to Copenhagen for Christmas lunch — long story — and I am completing today’s post from my hotel room.]
There’s no getting away from it: everyone wants to know how big dinosaurs are. Xenoposeidon is based on a single partial vertebra, so there is no way to be at all sure about the size and shape of the whole animal; but we knew that everyone would want to know, so in the paper (Taylor and Naish 2007, natch) we made the best guesses we could. Details on how we did this follow below.
Then, a few months after the revised manuscript was accepted by Palaeontology, purely as a joke, I put together what I called “the first scientifically rigorous skeletal reconstruction of Xenoposeidon“. The joke is based on a very well-established style for skeletal reconstructions of complete or nearly complete skeletons, first popularised by Greg Paul, in which the animal is shown in left lateral view, with bones drawn in white and the soft tissue outlined as a black silhouette — as in these rather beautiful Morrison Formation diplodocids from Paul (1998: fig. 1b). F is Diplodocus carnegii CM 84 with skull CM 3542 scaled to fit, G is Barosaurus lentus AMNH 6341 (with a fair bit of guesswork to fill in the gaps) and E is Apatosaurus louisae CM 3018 with skull CM 11162 scaled to fit:
I thought it would be funny to do this for an animal known only from a single bone, showing the bone floating in the middle of a big black silhouette. Har har. (You may not find that funny. The key point here is that Matt and Darren both did, which is all I was aiming for. And it turns out that John Hutchinson agrees, which proves it.)
In fact the only things that are remotely scientific about this “reconstruction” are that the bone is in roughly the right place (i.e. a posterior dorsal) and the human is about the right size. The actual shape of the “reconstruction” is a total guess: given that Xenoposeidon could belong in any part of the huge clade Neosauropoda, the only reason I went with a brachiosaurid body-shape rather than, say, a diplodocid was that I like brachiosaurus more. They’re just cooler. (This is how science is done, kids! Don’t tell your parents!)
It wasn’t until much later, when the publicity was in full swing, that it occurred to me that if the media were going to use the “skeletal reconstruction” it would be better to give them one with all the bones, greyed out. I made this almost as soon as it had occurred to me, and made it available to newspapers and TV stations, but it was a bit too late in the day — hardly any of them used it. Here it is, anyway:
Now that all the bones are back in place, it’s easy to recognise this as a knock-off of Matt Wedel’s Brachiosaurus reconstruction, as discussed in a much-overlooked Prehistoric Times article and which has since cropped up in various places. If I’d had my wits about me, I’d have credited Matt for the original “reconstruction” that went out to newspapers, but since it was only done as a joke it didn’t occur to me. Sorry, Matt!
So how did we figure out the probable size of Xenoposeidon? Since we had only one vertebra to go on, and since even that was missing the neural spine and other processes, all we had to work with were the centrum measurements. We reconstructed the centrum to be 20 cm long, with a cotyle diameter of 16.5 cm (the average of 16 cm width and 17 cm height). Then we compared that with the dimensions of vertebrae from the same position in the dorsal column of other sauropods. For this, we chose Brachiosaurus brancai and Diplodocus carnegii, because they are both known from nearly complete skeletons, and pretty much bracket the range of neosauropod body shapes.
Guessing length is easy: we just assume that the total length of Xenoposeidon would be in the same proportion to the length of one of its posterior dorsals as in the comparison taxa. The Xenoposeidon dorsal is 60% as long as the 33 cm D7 (seventh dorsal) of the Brachiosaurus brancai type specimen HMN SII (Janensch 1950: 44). If Xeno were built like a brachiosaurid, then it would be 60 per cent as long as HMN SII, yielding a length of 15 m based on Paul’s (1988) estimate of 25 m for that specimen. Similarly, the Xenoposeidon dorsal is 74% as long as the 27 cm vertebrae in the Diplodocus carnegii type specimen CM 84 (the average of the lengths of D7 and D8 as stated in Hatcher 1901:38). Therefore, a Diplodocus-like Xenoposeidon would be about 20 m long, based on the widely quoted figure of 27 m for CM 84.
But these figures are subject to a fair degree of uncertainty, and shouldn’t be taken too seriously. Suppose for example that we compared instead with D9 of Diplodocus, which is 29 cm long: the corresponding length estimate for Xenoposeidon would have been reduced to 20/29 of 27 m, which is 18.5 m.
And now to the much more handwavy problem of estimating mass. We can roughly extrapolate the mass of an animal as being proportional to the centrum volume, which in turn is proportional to centrum length times the square of its average cotyle diameter, and this is what we did in the paper. For Brachiosaurus brancai, this means that we estimate Xenoposeidon mass as (20 x 16.5 x 16.5) / (33 x 27 x 27) = 22% the mass of Brachiosaurus brancai; or as (20 x 16.5 x 16.5) / (27 x 29.5 x 29.5) = 23% the mass of Diplodocus carnegii, depending on which it most resembled.
But weight just a moment! (Har har.) What actually is the mass of Brachiosaurus brancai? An astonishingly wide range of figures have been calculated, all based on the same specimen (HMN SII). In chronological order (and do let me know if I’ve missed any):
- 78 tonnes (Colbert 1962), based on the volume of sand displaced by a scaled plastic model
- 15 tonnes (Russell et al. 1980) based on the dimensions of limb-bones, plotted on a best straight line through limb-bone-thickness vs. mass for extant animals
- 47 tonnes (Alexander 1985), based on the volume of water displayed by a different model from the one that Colbert used
- 29 tonnes (Anderson et al. 1985), using a similar though slightly more rigorous method than that of Russell et al. (1980)
- 32 tonnes (Paul 1988), using, I think, graphic double integration or something similar (the paper is not very explicit)
- 74 tonnes (Gunga et al. 1995), based on a computer model built using data scanned with lasers — very high-tech!
- 37 tonnes (Christiansen 1997) by suspension of scale models
- 26 tonnes (Henderson 2004) by measurement of computer model built by hand
We ignored the Russell et al. (1980) and Anderson et al. (1985) estimates because they are not actual measurements of anything, and depend instead on additional assumptions about scaling. We ignored the Colbert (1962) estimate, too, as it was based on a grotesquely overweight model. Worse still is the Gunga et al. (1995) estimate, which starts with ultra-precise measurements of the skeleton and then throws it away by fleshing that skeleton out in a computer model composed entirely of circular conic sections. Needless to say this makes the neck, torso and tail all hugely too wide, and the resulting volume estimate is near worthless. When they redo this work using elliptical conic sections, I will be interested to see how much their result comes down by. That leaves the more reliable estimates of Alexander, Paul, Christiansen and Henderson, and the average of their estimates 35322 kg — which seems correct using the well-it’s-way-bigger-than-an-elephant-anyway method. Using that figure (as we did in the paper), we got a probable mass of 7600 kg for Xenoposeidon — and this is course is the source of the statement, much quoted in the media, that it was “about as big as an elephant”. A big elephant.
But remember how vague this is: the lightest of the published estimates of Brachiosaurus brancai mass is 42% of the value we used, and the heaviest is 2.2 times as great. If we’d used these values in extrapolating the mass of Xenoposeidon we could have arrived at 3200 kg or 16720 kg!
For Diplodocus carnegii, we just used the estimate of Wedel 2005, 12 tonnes, and got a mass of 2800 kg for Xenoposeidon. There is rather less divergence in the various published mass estimates for this animal; but whether that’s because people’s work on it is more consistent, or just because not so many people can be bothered estimating the mass of so relatively uncharismatic a sauropod, I wouldn’t like to say.
All this means that Xenoposeidon was small — for a sauropod. In our poxy extant ecosystems, elephants are considered tolerably large, but in the Mesozoic they would hardly have raised an eyebrow, and neither would Xenoposeidon. Why the small size? Is it a juvenile? Apparently not: in archosaurs (that is, crocodiles, birds and all descendents of their most recent common ancestor, including dinosaurs), the neural arches fuse to their centra only some way into growth (Brochu 1996), so a vertebra such as that of Xenoposeidon in which the arch is fully fused and the sutures completely obliterated indicates that the animals was mostly or fully grown. Just small. Like Paul Simon. That’s nothing to be ashamed of.
Coming up next on SV-POW!’s Xenoposeidon week: on day 5, Darren will talk about the joys and hazards of dealing with the media. Stay tuned. (And, folks, please do make the occasional comment. Just so we know someone’s listening.)
- Alexander, R. McNeill. 1985. Mechanics of posture and gait of some large dinosaurs: Zoological Journal of the Linnean Society 83: 1-25
- Anderson, J. F., A. Hall-Martin and Dale A. Russell. 1985. Long-bone circumference and weight in mammals, birds and dinosaurs. Journal of Zoology 207: 53-61
- Brochu, Christopher A. 1996. Closure of neurocentral sutures during crocodilian ontogeny: implications for maturity assessment in fossil archosaurs. Journal of Vertebrate Paleontology 16: 49-62.
- Christiansen, Per. 1997. Locomotion in sauropod dinosaurs. Gaia 14: 45-75
- Colbert, Edwin H. 1962. The weights of dinosaurs. American Museum Novitates, 2076: 1-16
- Gunga, Hans-Christian, K. A. Kirsch, F. Baartz, L. Röcker, Wolf-Dieter Heinrich, W. Lisowski, A. Wiedemann and J. Albertz. 1995. New Data on the Dimensions of Brachiosaurus brancai and Their Physiological Implications. Naturwissenschaften 82: 190-192
- Hatcher, Jonathan B. 1901. Diplodocus (Marsh): its osteology, taxonomy and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum, 1: 1-63 and plates I-XIII
- Henderson, Donald M. 2004. Tipsy punters: sauropod dinosaur pneumaticity, bouyancy and aquatic habits. Proceedings of the Royal Society of London, B (Supplement) 271: S180-S183. doi 10.10998/rsbl.2003.01.36
- Janensch, Werner. 1950. Die Wirbelsaule von Brachiosaurus brancai. Palaeontographica (Suppl. 7) 3: 27-93
- Paul, Gregory S. 1988. The brachiosaur giants of the Morrison and Tendaguru with a description of a new subgenus, Giraffatitan, and a comparison of the world’s largest dinosaurs. Hunteria 2 (3): 1-14
- Paul, Gregory S. 1998. Terramegathermy and Cope’s Rule in the land of titans. Modern Geology 23: 179-217
- Russell, Dale, Pierre Béland and John S. McIntosh. 1980. Paleoecology of the dinosaurs of Tendaguru (Tanzania). Memoires de la Societe Geologique de France 139: 169-175
- Taylor, Michael P., and Darren Naish. 2007. An unusual new neosauropod dinosaur from the Lower Cretaceous Hastings Beds Group of East Sussex, England. Palaeontology 50 (6): 1547-1564. doi: 10.1111/j.1475-4983.2007.00728.x
- Wedel, Mathew J. 2005. Postcranial skeletal pneumaticity in sauropods and its implications for mass estimates. pp. 201-228 in: Jeffrey A. Wilson and Kristina Curry-Rogers (eds.), The Sauropods: Evolution and Paleobiology. University of California Press, Berkeley