Tutorial 21: how to measure the length of a centrum
March 11, 2013
For a paper that I and Matt are preparing, we needed to measure the centrum length of a bunch of turkey cervicals. That turns out to be harder than you’d think, because of the curious negative curvature of the articular surfaces.
Above is a C7 from a turkey: anterior view on the left; dorsal, left lateral and ventral views in the middle row; and posterior on the right. As you can see from the anterior, dorsal and ventral views, the anterior articular surface is convex dorsoventrally but concave transversely; and as you can see from the lateral view, the posterior face is concave dorsoventrally and convex transversely.
This means you can’t just put calipers around the vertebra. If you approach the vertebra from the top or bottom, then the upper or lower lip of the posterior articular surface will protrude past the centre of the saddle, and give you too long a length. If you approach from the side, the same will happen with the left and right lips of the anterior articular surface.
What are we trying to measure anyway?
But this raises the question of what it is we’re trying to measure. I said “we needed to measure the centrum length of a bunch of turkey cervicals”, but what exactly is centrum length? Why shouldn’t the upper and lower lips of the posterior articular surface count towards it?
What does centrum length mean?
The problem doesn’t only arise with bird cervicals. The same issue arises in measuring more sensible and elegant vertebrae, such as our old friend HMN SII:C8, or MB.R.2181:C8 as we must now learn to call it.
Although the back of the vertebra is nice and simple here — it’s obvious what line we’re measuring to at the back — we have three choices of where the “front” of the vertebra is, and a case can be made for any of them as being “the length of the vertebra”.
The longest measurement (here marked “T” for “total length”) goes to the front of the prezygapophyseal rami. The next one (“C” for “centrum length”) goes to the anteriormost point of the condyle. The distinction is important: as noted recently, the longest vertebra in the world belongs to Sauroposeidon if we use total length, but to Supersaurus if we use centrum length.
But in life, most of the condyle would be buried in the cotyle of the preceding vertebra. So should it count towards the length of the vertebra? If you consider a string of articulated vertebrae, the buried condyles don’t contribute to the overall length of the neck. So Matt and I call the length from the posterior margin of the condyle to the posterior margin of the cotyle the functional length (marked “F” above), which I believe is a new term.
Another way to think of the functional length is the distance from a given point on a vertebra (in this case the posterior margin of the cotyle) to the same point on the adjacent vertebra:
For our current project, Matt and I are interested in how the lengths of individual vertebrae contribute to total neck length, so for our purposes, functional length is definitely what we want.
By the way, Janensch is the only author I know of to have even recognised the importance of functional length. The measurement tables on pages 39 and 44 have columns for “Gesamtlänge des Wirbels ab Vorderende per Präzygapophyse”, “Gesamtlänge der Wirbel-Körpers in 1/2 Höhe” and “Länge der Wirbel-körpers ohne Condylus in 1/2 Höhe” — that is, “Total length of the vertebra from the anterior end of the prezygapophysis”, “Total length of the centrum measured at mid-height” and “Length of the centrum minus condyle at mid-height”. This is typical of his careful and methodical approach. Kudos!
Hey! I thought this was about turkeys
And so it is. Here is the functional length measurement for a turkey cervical:
It’s the shortest anteroposterior distance between the two articular surfaces.
Measuring functional length
Matt and I chatted about this at some length, and I am ashamed to say that we thought through all sorts of complicated solution involving subtracting measurements from known scaffold length and suchlike.
It took us a stupidly long to to arrive at the very obvious solution, which is just to modify the calipers to have a “tooth” that can protrude into the concavity of the anterior articulation between its left and right lips. Easily done with a flat-ended screw and a blob of wood glue:
With the measurements of all the vertebrae in my series, I can now fairly confidently expect that the sum of the individual lengths will come out at about the length of the complete neck.
You know, unless intervertebral cartilage turns out to be important or something.
- Janensch, Werner. 1950. Die Wirbelsaule von Brachiosaurus brancai. Palaeontographica (Suppl. 7) 3:27-93.
1. Matt and I are so used to opisthocoelous sauropod presacrals that when we’re talking about vertebrae — any vertebrae — we tend to say “condyle” and “cotyle” for the anterior and posterior articular surfaces, no matter what their morphology. When talking about crocodile cervicals or titanosaur caudals, we’re even likely to say ridiculous things like “the condyle is concave and the cotyle is convex”. Nonsense, of course: condyle means “A rounded prominence at the end of a bone, most often for articulation with another bone.” What we should say is “the condyle is at the back and the cotyle is in front”.