The Astonishing Case of HMN SII:D8, part 1: Spinoparapophyseal Laminae!
May 26, 2008
Of all the sauropod vertebrae in the world, perhaps the single most intriguing is a dorsal vertebra of the Brachiosaurus brancai type specimen HMN SII. It was designated the 20th presacral (i.e. 7th dorsal) by Janensch (1950), but that was on the assumption that the dorsal column consisted of 11 vertebrae. Since Janensch never had any actual reason for assuming 11 dorsals, and Migeod’s (1931) Tendaguru brachiosaurid had 12 dorsals, it seems best to assume for now that brachiosaurids in general had 12 dorsals, so that the vertebra Janensch thought was D7 (i.e. the fifth-last dorsal) was more likely D8. Anyway, that’s what I’ll be calling it.
Here is that vertebra in right anterodorsolateral view — that is, from above, in front, and to the right.
It’s a bit of a close-up, so you can hardly see the centrum down there, and only the bottom half of the neural spine is included. On the positive side, you get a good view of those honking great prezygapophyses and the nice, big gap between them. You also see the neat, parallel spinoprezygapophyseal laminae (SPRLs) running up the front of the neural spine.
But I want to draw your attention to that lamina coming down towards us from the side of the SPRL. What is it? It can’t be a nice, well-behaved spinodiapophyseal lamina (SPDL), because we already have one of those over on the left-hand side, coming down from the side of the neural spine and flaring out laterally towards the diapophysis. No indeed: the lower end of the lamina I’m interested in here is actually running towards the parapophysis. OK, it’s broken off, but you can see where it would be if it was there. So this thing is a spinoparapophyseal lamina, or SPPL for short.
Well, this is weird. Wilson (1999) listed, illustrated and discussed nineteen different vertebral laminae, but there is no hint of an SPPL anywhere in that paper. So far as I’m aware, this feature is unique to this one vertebra. So is it just a one-off freak? I don’t think so, because on the other side of the neural spine, you see its mate, the left SPPL, heading off towards its parapophysis. But then why don’t we see this on any of the other dorsals of the same specimen? Well, it might have been there in life. The truth is that the B. brancai dorsals are rather more smushed up than you’d gather from the rather lovely drawings in Janensch (1950) — remind me some time to show you my photos of the co-ossified D11/D12 pair — and the relevant portion is only really preserved in D8. It’s possible that it was SPPLs all along the dorsal sequence.
Stay tuned for more hot news about how weird HMN SII:D8 is!
- Janensch, Werner. 1950. Die Wirbelsaule von Brachiosaurus brancai. Palaeontographica (Supplement 7) 3: 27-93.
- Migeod, Frederick. W. H. 1931. British Museum East Africa Expedition: Account of the work done in 1930. Natural History Magazine 3 (19): 87-103.
- Wilson, Jeffrey A. 1999. A nomenclature for vertebral laminae in sauropods and other saurischian dinosaurs. Journal of Vertebrate Paleontology 19 (4): 639-653.
To help you get your bearings, here is a photo that I should have included in the original post: the same vertebra from nearly the same angle, but taken from further out so that you can see the whole thing. The big photo was taken from a little higher up, looking slightly down on the vert.