Opisthocoelicaudia is Just Plain Wrong
February 24, 2013
I was cruising the monographs the other night, looking for new ideas, when the humerus of Opisthocoelicaudia stopped me dead in my tracks. I think you’ll agree it is an arresting sight:
Opisthocoelicaudia right humerus in medial, anterior, lateral, and posterior views, from Borsuk-Bialynicka (1977: figure 7)
I’d seen it before, but somehow I had never grokked its grotesque fatness. I mean, damn, Opisthocoelicaudia, you really let yourself go. Especially compared to the slenderness and grace of this juvenile Alamosaurus humerus:
Alamosaurus left humerus in anterior and posterior views, from Lehman and Coulson (2002: figure 7).
Now, I realize that part of the slenderness of this Alamosaurus humerus might be because it’s a juvenile–other alamosaur humeri are a bit more robust–but it’s still a striking contrast. I couldn’t help but superimpose them, scaled to the same midshaft width:
I flipped the Alamosaurus humerus left-to-right to match that astonishing lump of Opisthocoelicaudia. The result reminds me of one of Abrell and Thompson’s Actual Facts:
If you put Woodrow Wilson inside William Howard Taft, he would have stuck out by a good 18 inches.
None of that probably signifies anything more than that I am easily amused. And also, Opisthocoelicaudia is Just Plain Wrong. You hear me, Opisthocoelicaudia? Don’t make me make you cry mayonnaise!
References
- Borsuk-Bialynicka, M. 1977. A new camarasaurid sauropod Opisthocoelicaudia skarzynskii, gen. n., sp. n. from the Upper Cretaceous of Mongolia. Palaeontologia Polonica 37: 5-64.
- Lehman, Thomas M. and Alan B. Coulson. 2002. A juvenile specimen of the sauropod dinosaur Alamosaurus sanjuanensis from the Upper Cretaceous of Big Bend National Park, Texas. Journal of Paleontology 76(1): 156-172.
Sauropod Vertebra Picture of the Week 
February 24, 2013 at 9:09 pm
“Now, I realize that part of the slenderness of this Alamosaurus humerus might be because it’s a juvenile.”
Unlikely. From Taylor (2009:796): “Discarding a single outlier, the ratio of proximodistal length to minimum transverse width (Gracility Index or GI) in humeri of B. brancai varies between 7.86 for the right humerus HMN F2 and 9.19 for the left humerus HMN J12, with the type specimen’s right humerus scoring 8.69, slightly more gracile than the middle of the range. (It is notable that the juvenile left humerus HMN XX19 has a GI of 8.63, and so is as gracile as the humeri of adult specimens, corroborating in B. brancai the findings of Carpenter and McIntosh (1994:277) for Apatosaurus, Ikejiri et al. (2005:176) for Camarasaurus, and Tidwell and Wilhite (2005) for Venenosaurus Tidwell, Carpenter and Meyer, 2001 that sauropod limb bones, unlike their vertebrae, scale isometrically during ontogeny.)”
February 24, 2013 at 9:14 pm
“Other alamosaur humeri are a bit more robust”–go read Lehman and Coulson. IIRC there are a few other cases now of juvenile sauropods having more slender limb bones than adults. I can’t remember the specifics, and I will post them if and when I do. Or maybe some helpful commenter will fill in. Anyway, I agree that isometric scaling of the limb bones seems to be the null for sauropods, but I also would not be surprised to find counter-examples, given that it is not the null for most other things.
February 25, 2013 at 6:55 pm
Could it be from an ankylosaur?
February 25, 2013 at 7:00 pm
‘Friad not, it’s part of a single articulated skeleton. See Borsuk-Bialynicka (1977)–it’s a free download, and it’s linked at the end of the post.
When I say it’s Just Plain Wrong, I’m not saying that Borsuk-Bialynicka screwed up the descriptive work. I’m saying that Opisthocoelicaudia is offensively lumpen. Unfortunately, we just have to accept it.
February 25, 2013 at 7:58 pm
Matt’s use of “just plain wrong” is an allusion to this old post.
February 26, 2013 at 10:24 pm
Ankylosaurs and stegosaurs had patently kludgy ‘lumpenhumeri’… and unusually muscular tails commonly provided with weapons of mush destruction.
Opisthocoelicaudia is reported to have posessed anomalously rugose and heavily muscled proximal caudals…. according to that nonauthoritative but very open-sourced gem, Wikipaedia…. since I don’t have Borsuk-Bialynicka ready to hand.
Look at the tail to understand the shoulders?? Control a heavy, strong posterior member with opposing side-thrusts at the other end??
February 26, 2013 at 10:31 pm
I don’t have Borsuk-Bialynicka ready to hand.
Easily fixed–it’s a free download from the link at the end of the post. :-)
Look at the tail to understand the shoulders?? Control a heavy, strong posterior member with opposing side-thrusts at the other end??
Certainly possible. However, I am reluctant to ascribe any major functional significance to the heavy humeri specifically, because the whole animal is built that way. A sauropodan hippopotamus, maybe?
February 26, 2013 at 11:06 pm
Nahh… hippos only use their tails the way trolls use their mice and keyboards.
I keep seeing the wonderful “Brachiosaur 30 ” rearing up… or similar for Bakker’s stegosaurs. Ta for the answer.
March 4, 2013 at 7:11 am
“hippos only use their tails the way trolls use their mice and keyboards”
Nicely graphic. :)
August 21, 2020 at 5:11 pm
[…] and on that basis Opisthocoelicaudia may have at least two feature supporting this interpretation: very robust limb bones and (if the interpretation of Borsuk-Bialynicka 1977: figure 5 is to be trusted) a transversely […]