Sauropod Vertebra Picture of the Week

Or, how a single lateral fossa becomes two foramina: through a finely graded series of intermediate forms. Darwin would approve. The ‘oblique lamina’ that separates the paired lateral foramina in C6 starts is absent in C2, but C3 through C5 show how it grows outward from the median septum. How do I know it grows outward, instead of being left behind during the pneumatization of the more posterior cervicals? Because with very few exceptions, all neosauropod cervicals start out with a single lateral fossa on each side, as illustrated in this post. But many of them end up with two or more foramina. Diplodocus is a nice example of this (from Hatcher 1901: plate 3):

I should clarify that the vertebrae above show that character transformation in this individual, at this point in its ontogeny. The vertebrae of CM 555 are about two-thirds the size of those of CM 3018, the holotype of A. louisae. In CM 3018, even C4 and C5 have completely divided lateral fossae, corresponding to the condition in C6 of CM 555.

As Mike and I discussed in our 2013 neural spine bifurcation paper, isolated sauropod cervicals require cautious interpretation because the morphology of the vertebrae changes so much along the series. The simple morphology of anterior cervicals reflects both earlier ontogenetic stages and more primitive character states. As Mike says, in sauropod necks, serial position recapitulates both ontogeny and phylogeny. So if you have a complete series, you can do something pretty cool: see the intermediate stages by which simple structures become complex.

If you’re thinking this might have something to do with my impending SVPCA poster, you’re right. Here’s the abstract.

For more on serially increasing complexity in sauropodomorph cervicals, see this post.