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?