Your sacrum is negligible

In the spirit of Your neck is pathetic and Your torso is also pretty lame, I note that your sacrum is negligible:

We have here the sacrum of the Haplocanthosaurus priscus holotype CM 572, in ventral view with the ilia still in place (so that the slightly hourglass-shaped dark regions you see on either side are the acetabular regions of the ilia, facing downwards). To the right is the sacrum of a good-sized adult male human such as my good self, in dorsal view.

The total length of the five fused sacral vertebrae of Haplo is 79.5 cm (Hatcher 1903:18), compared with about 11.5 cm for the human. Actually, Haplo is not a particularly big sauropod: the sacrum of Brachiosaurus altithorax holotype FMNH P25107, for example, comes in at 95 cm (Riggs 1904:236). But it’s big enough to make your sacrum hang its zygapophyses in shame. I picked Haplo just because Hatcher’s 1903 monograph on it is so beautifully illustrated, and the world is not as plentifully supplied with complete, uncrushed, well-preserved sauropod sacra as we might wish.

This post marks Haplo’s SV-POW! debut, but what kind of sauropod is it? As pointed out by Taylor and Naish (2005:4), various studies have reached different conclusions about this: Riggs (1904:229) classified it as a brachiosaurid; McIntosh (1990:347) as a cetiosaurid; the phylogenetic analysis of Wilson and Sereno (1998:54) recovered it as a non-camarasauromorph macronarian; Upchurch (1998:74) found it to be an “eosauropod” (i.e. a non-neosauropod sauropod); Wilson (2002:240) recovered it as a basal diplodocoid (outside Diplodocimorpha, the clade uniting Diplodocus with Rebbachisaurus); and Upchurch et al. (2004:297) found it in a derived position, as a camarasauromorph closer to Titanosauriformes than to Camarasaurus.

All this disagreement is not as bad as it seems, though. We can discount Riggs’s assignment to Brachiosauridae as this was offered at a time when few sauropods were known. All the other assignments, as disparate as they appear, place Haplo very close to the root of Neosauropoda, that is the divergence between the two great Neosauropod clades Diplodocoidea and Macronaria. Some have it at the base of one branch, some at the base of the another, some just outside; with the exception of Upchurch et al.’s (2004) placement as a macronarian more derived than Camarasaurus, these positions are all only one or two nodes apart on a consensus cladogram.

In fact, the most recent common ancestor of all neosauropods might have been something rather similar to Haplocanthosaurus. It can’t have been Haplo itself, as it came along fifteen million years too late to be the ancestor of early neosauropods such as Atlasaurus, but it’s possible that it’s little changed from that ancestor.

Bibliography

• Hatcher, J.B. 1903. Osteology of Haplocanthosaurus with description of a new species, and remarks on the probable habits of the Sauropoda and the age and origin of the Atlantosaurus beds. Memoirs of the Carnegie Museum, 2, 1­-72, pls 1­4.
• McIntosh, J.S. 1990. Sauropoda. Pp. 345­-401 in D. B. Weishampel, P. Dodson and H. Osmólska (eds.). The Dinosauria. University of California Press, Berkeley.
• Riggs, E.S. 1904. Structure and relationships of opisthocoelian dinosaurs. Part II, the Brachiosauridae. Field Columbian Museum, Geological Series 2, 6, 229­-247.
• Taylor, M.P., and Darren Naish. 2005. The Phylogenetic Taxonomy of Diplodocoidea (Dinosauria: Sauropoda). PaleoBios 25 (2): 1­-7.
• Upchurch, P. 1998. The phylogenetic relationships of sauropod dinosaurs. Zoological Journal of the Linnean Society 124:43­-103.
• Upchurch, P., P.M. Barrett, and P. Dodson. 2004. Sauropoda. Pp. 259-­322 in D.B. Weishampel, P. Dodson, and H. Osmólska (eds.). The Dinosauria, 2nd edition. University of California Press, Berkeley and Los Angeles.
• Wilson, J.A. 2002. Sauropod dinosaur phylogeny: critique and cladistic analysis. Zoological Journal of the Linnean Society 136: 217-­276.
• Wilson, J.A., and P.C. Sereno. 1998. Early evolution and higher-level phylogeny of sauropod dinosaurs. Society of Vertebrate Paleontology Memoir 5:1­-68.