How big was Alamosaurus?
September 2, 2009
Here’s a skeletal reconstruction of Alamosaurus modified from Lehman and Coulson (2002:fig. 11). I cloned the neck and rotated it a few degrees to see where it would put the head.
The skeleton in the figure is scaled to the size of the individuals in the Smithsonian and at UT Austin. The scale bar is 1 meter, which by my calculations gives that individual the following dimensions:
- Total length: 15.8 meters (52 feet)
- Neck length: 5.2 meters (17 feet)
- Shoulder height: 4 meters (13 feet)
- Head height (with neck raised): 8.4 meters (27.5 feet)
Here are a couple of articles on a giant sauropod found in Big Bend in 1999. This critter is generally assumed to be Alamosaurus but it could be something new (I have no evidence either way); the material is currently under study at the Dallas Museum of Nature and Science.
http://www.nps.gov/bibe/naturescience/alamosaurus.htm
http://www.geocities.com/stegob/texasdino.html
According to the articles, 10 cervical vertebrae were found in a string 23 feet long. From the pictures, those ten vertebrae look like the ten largest, which should account for almost all of the neck except for the first few cervicals behind the head. Let’s assume that this big individual therefore had a neck just a little longer than 23 feet, and we find that it is almost exactly 1.5 times bigger than the one listed above. If its proportions follow those of the Lehman and Coulson recon, its measurements would be:
- Total length: 24 meters (79 feet)
- Neck length: 7.8 meters (25.5 feet)
- Shoulder height: 6 meters (19.5 feet)
- Head height: 12.6 meters (41 feet)
In the second article Homer Montgomery speculates that the complete neck would have been more than 30 feet long. That’s certainly not impossible, since 30-foot-plus necks are known for the largest individuals in several clades (e.g., Mamenchisaurus, Supersaurus, Sauroposeidon, probably Puertasaurus, possibly Futalognkosaurus, but probably not Aegyptosaurus) If so, then you could just about double all of the proportions from the first individual described above, which would give a truly prodigious animal. The 52-foot animal probably had a mass around 15 tons, so the 79-footer would have been about 50 tons (1.5^3 = 3.375), and the hypothetical 100-footer would have been 120 tons, which is up in Amphicoelias/Bruhathkayosaurus territory. For what it’s worth, I think the numbers for the 79-foot animal are more plausible, but who knows. Anytime you’ve got a partial neck that is longer than the complete neck of Diplodocus, you’re dealing with a wacky big animal.
Reference
Lehman, T.M. & Coulson, A.B. 2002. A juvenile specimen of the sauropod Alamosaurus sanjuanensis from the Upper Cretaceous of Big Bend National Park, Texas. Journal of Paleontology 76(1): 156-172.
September 3, 2009 at 12:21 am
Yay, titanosaurs! :)
Since you are talking about sauropod size, Matt, what do you think of the recent paper on allometric equations for predicting body mass of dinosaurs (I’m inclined to think they are, for the most part, hogwash)? Mike told me he thinks their estimate is unreasonable for brachiosaurs, but what about titanosaurs? And does this mean that Amphicoelias instead of weighing 120 tons would *only* weigh 60 tons?
Also, in the skeletal for Alamosaurus it appears to have 11 or 12 cervicals, yet Rapetosaurus appears to have 15 or 16 cervicals. Did the number of titanosaur cervical vertebrae really differ by that much, or do we not really know how many cervical vertebrae Alamosaurus had (yet)?
Great post, BTW. (And it would be really nice if you could answer all these questions :) )
September 3, 2009 at 4:19 am
Briefly: I don’t have much faith in any mass estimation method, even the ones I use! But allometric equations seem to be particularly unreliable. This isn’t just me whinging–Hurlburt found the same thing, in what is AFAIK still the only study that compares the performance of multiple mass estimation methods on individual extant critters of known mass (he also found that the most reliable methods are often off by 20% at least).
I feel pretty confident that the HM SII individual of Brachiosaurus massed between 20 and 40 tons in life, and I strongly suspect that the actual number was in the bottom quarter of that range, but there are so many unknowns that narrowing it down any further or with any more certainty strikes me as incautious at best. Now, if I ran the numbers and got, say, 25.5 tons, I’d report that to however many decimal points I could justify, but I try to keep a firm mental distinction between “This is the mathematical output of this function given the input measurements and assumptions” and “This is actually what the animal weighed in life”. In other words, I make use of the output of mass estimation methods–and phylogenetic reconstruction methods, and posture determination methods (how else can I do science?)–but I try not to mistake them for the truth (how else can I do honest science?). IMHO, people who accept the results of these things uncritically are just as deluded as people who reject them uncritically. If you want to make progress, you’ve got to do the best you can without getting fanatical in either direction. That said, if there’s an allometric equation that predicts only 60 tons for Amphicoelias, then I will probably pay it even less attention than normal, asymptotically approaching zero.
The primitive number of cervicals in titanosaurs is not known for sure, but Gomani made a pretty good case for 13 based on Malawisaurus. Cervical counts are pretty plastic in sauropods in general and in Somphospondyli in particular, so who knows. More cervicals would change the estimated neck length for Alamosaurus but would have negligible impact on the mass (like maybe 1-2% max).
September 3, 2009 at 6:28 am
Interesting, I had no idea Alamosaurus might have gotten so big. “Even” the 79-foot/50 tons is bigger than I had thought…