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.


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?

4 Responses to “The sacral and the profane”

  1. Allen Hazen Says:

    Matt, do you know if that sacrum is on display, or hidden away in the research collection? There is an isolated sauropod sacrum displayed at the AMNH in a case (i.i.r.c.) in the corridor between two of the big vert. paleo. halls, but my visual memory isn’t good enough to tell whether it is this one or not.

  2. Matt Wedel Says:

    Hi Allen,

    Funny you should mention that sacrum! I almost used that one instead of this one, but I didn’t have a good enough picture that (a) showed the entire sacrum and (b) didn’t have any annoying reflections from the glass case. I do have a photo of the sign that goes with that sacrum, and it reads:

    “This was the first dinosaur fossil excavated by the American Museum of Natural History. It is part of a fragmentary skeleton of a giant plant-eating dinosaur that includes many other bones; the skull, however, was never found.

    “AMNH 223, collected by H.F. Osborn and B. Brown, 1897, Como Bluff, Wyoming.”

    You may have noticed that the sacrum pictured above has the number AMNH 3532 painted on it, but the specimen tag says AMNH 7532. I had bigger fish to fry when I was at the AMNH, so the answer to that mystery will have wait for another day.

  3. brightmoon Says:

    ooooh how did i miss this site …cool, ill learn more tetrapod anatomy

  4. […] a matter of fact, we’ve seen this sacrum before, too, in a photo from Matt’s much earlier AMNH visit. But only from a left […]

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