The Astonishing Case of HMN SII:D8, part 2: The Missing Infradiapophyseal Lamina!
May 29, 2008
Just when you thought it was safe to go back into the Humboldt bone-room … hot on the heels of Part 1: Spinoparapophyseal Laminae!, comes another dose of terror thanks to everyone’s favourite mid-to-posterior brachiosaurid dorsal vertebra, HMN SII:D8. First, here is a pretty picture of the whole vertebra in right lateral view:
And here is the same bone in the same view, as figured by Janensch (1950:fig. 56):
Before I start laying the laminae on you, I should say what an awesome moment it was when I first came face to face with this vertebra, having been so intimately familiar with Janensch’s figures. It was weird — like meeting Cameron Diaz or something. Seeing D4 was even stranger, because it’s so much bigger than you think from the figure — but that’s another story for another day. I promised you laminae, or, rather, the absence of one particular lamina, and that’s what you’re going to get.
Look at the neural spine. You’ll see a prominent lamina running down the side facing you, sticking out of the page. That of course is the spinodiapophyseal lamina, and it comes down and joins the horizontal lamina complex(*) at the diapophysis. But now look below the horizontal laminae, and you will see that there is nothing supporting the diapopophysis from below. Doesn’t that make you feel sort of ill? Yes, there is a sort of vertical lamina below the horizontal lamina complex, but before it reaches the diapophysis, it forks into anterior and posterior branches, which (A) reach the horizontal laminae ahead of and behind the diapophysis, and (B) extend so tiny a distance out from the vertebra that their mechanical supporting function must have been negligible.
Now what the heck is that all about?
(*) The “horizontal lamina complex” is the set of, uh, horizontal laminae that connect the prezygapopophysis, via the parapophysis and diapophysis, to the postzygapophysis. In pre-Wilson (1999) days, these three laminae were sometimes referred to individually as the horizontal laminae, in a semi-formal way, but this term has been quite rightly supplanted by Wilson’s much more explicit PRPL, PPDL and PODL. Still, the term Horizontal Lamina Complex is still useful for referring to them as a unit, in those dorsals that are sufficiently posterior for the four landmarks to be roughly in a horizontal line.
References
Same as last time.
Sauropod Vertebra Picture of the Week 
June 22, 2008 at 10:58 pm
I had an uncle with a horizontal lamina complex.
He never got over it.
January 18, 2009 at 6:04 am
A lot of this bone looks like it was made of plaster. Look at Janensch’s drawing, it looks like some of the “zones” outlines match up with the cracked, “real bone”-looking areas on the vert. Is this thing all bone, or like 85% plaster?
January 18, 2009 at 9:16 am
Nima,
Yes, there is plenty of plaster on this bone, and it does make things trickier to interpret. But the features that I described in this mini-series are all well supported by good, solid bone.
January 19, 2009 at 6:19 am
In that case it looks like the the spinodiapophyseal lamina could use some plaster too. It looks severely chipped and cracked compared to the sketch by Janensch (no doubt the result of the years and humidity).
How did you get this thing off the mount? Was it during the remounting process or did they replace it with a replica?
January 19, 2009 at 9:26 am
The spinodiapophyseal laminae are in pretty good nick — do you mean the infraparapophyseal lamina (the one running near-vertically down the right-hand side of the photo)? Yes, it’s chipped, and could probably use a bit of work, but it’s great that they haven’t plastered over it: nothing is more frustrating than not knowing which part of a specimen is real and which is not; or, even when the plaster is easy to spot, not knowing how much real bone is underneath it.
As to getting it off the mount — that would have been double-plus impossible. Happily, Janensch’s original mount used rather poor plaster models of the presacral vertebrae, as the originals are too heavy, fragile and important to mount; happier still, the recent remount took the opportunity to install much, much better models of the presacrals, which I will hopefully feature in a forthcoming post. Anyway, the presacrals are available for study (except for a few that seem to have been lost).
June 4, 2009 at 9:15 am
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June 13, 2009 at 8:38 pm
[…] I have a paper on Brachiosaurus in press that mentions in passing the spinoparapophyseal laminae, absence of an infradiapophyseal laminae and perforate anterior centroparapophyseal laminae of the 8th dorsal vertebra of the Brachiosaurus […]
January 16, 2013 at 10:15 am
[…] discussed in detail here on account of its unique spinoparapophyseal laminae, its unexpectedly missing infradiapophyseal lamina and its bizarre perforate anterior centroparapophyseal […]