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

Haplocanthus sacrum; Homo sacrum

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

The sacral and the profane

November 10, 2007

I see now that Mike has beaten me to the punch in providing your at-least-weekly dose of sauroponderous vertebrawesome. And a nice job it is. Still, I feel funny about you not getting a new picture (ahem), so I’m posting my late entry anyway. For some reason, despite–or perhaps because of–my ardent devotion to cervicals, I have taken it on myself to push the anatomical boundaries of SV-POW! again.

amnh-diplodocus-500.jpg

Here’s a partial pelvis of Diplodocus in the American Museum of Natural History, in left lateral view. The big, vertically-oriented slab is the ilium. The semicircular concavity on the bottom of the ilium is the top half of the acetabulum, or hip socket. The bottom half of the acetabulum was formed by the other two pelvic bones, the pubis, which pointed down and forward, and the ischium, which pointed down and back. Behind the ilium you can see five sacral vertebrae, and the sacral ribs that connect the ilium to the vertebral column. The neural spines of the middle three vertebrae are fused together. The first and fifth have free neural spines, but all five are fused together at the centra.

The number of sacral vertebrae increased several times in the evolution of sauropodomorphs (sauropods and “prosauropods”). Almost all “prosauropods” have three sacrals, basal sauropods have four, most neosauropods have five, most titanosaurs and some elderly individuals of Camarasaurus have six, and Neuquensaurus is reported to have seven. It is a little weird that sacral count increases so regularly and straightforwardly up the tree. The sacrum is the only bony connection between limbs and the vertebral column (recall that the shoulder girdle “floats” in a sling of muscles and ligaments), so it’s pretty important mechanically. You might expect that bigger sauropods would have more sacral vertebrae and smaller sauropods would have fewer, but that’s not the case. Monsters like Apatosaurus and Brachiosaurus only have five, and little ole titanosaurs like Malawisaurus and Saltasaurus have six (“little” in this case means up to the size of an elephant–big for an animal, kinda pathetic for a sauropod). Now, granted, there were much bigger titanosaurs, like Argentinosaurus, Puertasaurus, and Paralititan, but the sixth sacral shows up early in titanosaurian history, in the little guys. So there does not seem to be much of a connection between sacral count and body size. Which raises the question: what was driving the increase?