How — and where — should we measure neural canal diameter?

Confession time: I have yet to find a satisfyingly regular and repeatable method for measuring neural canal diameters. A LOT of dinosaurian neural canals are not cylindrical but flare out on either end, like two trumpet bells set back-to-back. You can see a similar thing in this drawing of human vertebrae in mid-sagittal section from Gray’s Anatomy, which Mike and I repurposed as figure 9 in our 2022 vertebral orientation paper:

The most obvious solution would be to simply measure the minimum diameter of the neural canal at its midpoint, but that’s also often fraught with complications, because there may be troughs or fossae opening dorsally or ventrally, which can be so large and anteroposteriorly extensive that there is no point along the whole length of the neural canal at which it is cylindrical — or, if there is, the cylindrical stretch may not correspond to the maximum or minimum diameter, but be somewhere in between.

Here’s an example, MWC 9632, an incomplete proximal caudal vertebra of a diplodocid, shown here in posterior view.

The green line on the right-hand photo is my best approximation of the horizontal profile of the right side of the neural canal. It’s a smooth curve from the horizontally-narrowest spot (which is not the vertically narrowest spot — more on that in a sec), furthest in, to the lowermost extension of the centropostzygapophyseal lamina (CPOL), which continues downward as a faint ridge (black arrows). Where along that line is the “entrance” to the neural canal? I can’t think of a non-arbitrary way of deciding. What’s more, the vertebra is bilaterally asymmetric — the same line on the left side would not be a smooth curve, because the left CPOL has a medial bar that is lacking on the right (white arrows). I doubt that asymmetry is a product of taphonomic deformation — although the vertebra is missing a lot of side and top bits, what’s there is in pretty good shape. I see asymmetries like that and worse all the time in mastodon vertebrae that aren’t taphonomically deformed at all, so I think maybe this is just normal for sufficiently large animals.

But wait! It gets worse! Because the vertical profile of the neural canal is totally different, with a reasonably flat ceiling but a very deep trough in the floor of the canal. Note also that the neural arch is shorter than the centrum, so the ceiling is not as long as the floor. If we were forced to choose an anterior and posterior diameter for the canal, where would we measure them? Would they even be orthogonal to the long axis of the canal? (And, er, how do we decide on a single long axis for the canal?)

As Mike is fond of saying, I don’t know the answer, but I admire the problem. Unfortunately for us, a classifying species that likes things to fit into neat boxes, all the neural canal in big vertebrates has to do is house the spinal cord and other soft tissues (so many others: pneumatic diverticula, blood vessels, connective tissues, balance organs…), and there’s no requirement for the canal to be a neat cylinder, or be easy to measure, draw, photograph, or even think about.

Often when people are measuring neural canals, the anterior and posterior diameters (if those are even definable in a repeatable way) are less important than the minimum diameters or cross-sectional area, which constrain the size of the spinal cord and its associated meninges (see, e.g., Giffin 1990). It seems janky as heck, but I wonder if a cylindrically-rolled piece of paper might not be a plausible way to get the minimum height and width at least, regardless of the non-cylindricity (if that’s a word) of the neural canal at any one point. Mike and I have used that method for visualizing neural canal orientation, as you can see in Figure 11 from our vertebral orientation paper, but it seems like it might also work for barrel diameter.

In that 2022 paper, Mike wrote, “In a ‘trumpet shaped’ neural canal that is wider at one end than at the other, the paper uncurls further at the wider end, giving a visual indication of the variation in width.” That’s true, but if you forced the paper to be a cylinder (even if elliptical in cross-section), such that the walls were parallel, and you made sure the paper was actually touching the walls at the top, bottom, and sides, you could get minimum diameter measurements a lot faster and more easily than any other way I can think of. IF you had a good 3D model you could probably get a few tenths of a millimeter better accuracy, but I’ll bet the method would just be the digital version of the paper cylinder.

So, that’s as far as I’ve gotten. If you know of a better, more repeatable way of measuring neural canal diameter — that would actually work on MWC 9632, as a seemingly grueling but actually very representative test case — please sing out; I’d love to know.

References

• Giffin, Emily B. 1990. Gross spinal anatomy and limb use in living and fossil reptiles. Paleobiology 16(4):448-458.
• Taylor, Michael P., and Wedel, Mathew J. 2022. What do we mean by the directions “cranial” and “caudal” on a vertebra? Journal of Paleontological Techniques 25: 1-24.