Can we distinguish taphonomic distortion and (paleo)pathology from normal biological variation?

February 12, 2021

Taylor 2015: Figure 8. Cervical vertebrae 4 (left) and 6 (right) of Giraffatitan brancai lectotype MB.R.2180 (previously HMN SI), in posterior view. Note the dramatically different aspect ratios of their cotyles, indicating that extensive and unpredictable crushing has taken place. Photographs by author.

Here are cervicals 4 and 8 from MB.R.2180, the big mounted Giraffatitan in Berlin. Even though this is one of the better sauropod necks in the world, the vertebrae have enough taphonomic distortion that trying to determine what neutral, uncrushed shape they started from is not easy.

Wedel and Taylor 2013b: Figure 3. The caudal vertebrae of ostriches are highly pneumatic. This mid-caudal vertebra of an ostrich (Struthio camelus), LACM Bj342, is shown in dorsal view (top), anterior, left lateral, and posterior views (middle, left to right), and ventral view (bottom). The vertebra is approximately 5cm wide across the transverse processes. Note the pneumatic foramina on the dorsal, ventral, and lateral sides of the vertebra.

Here’s one of the free caudal vertebrae of an ostrich, Struthio camelus, LACM Ornithology Bj342. It’s a bit asymmetric–the two halves of the neural spine are aimed in slightly different directions, and one transverse process is angled just slightly differently than the other–but the asymmetry is pretty subtle and the rest of the vertebral column looks normal, so I don’t think this rises to the level of pathology. It looks like the kind of minor variation that is present in all kinds of animals, especially large-bodied ones.

This is a dorsal vertebra of a rhea, Rhea americana, LACM Ornithology 97479, in posteroventral view. Ink pen for scale. I took this photo to document the pneumatic foramina and related bone remodeling on the dorsal roof of the neural canal, but I’m showing it here because in technical terms this vert is horked. It’s not subtly asymmetric, it’s grossly so, with virtually every feature–the postzygapophyses, diapophyses, parapophyses, and even the posterior articular surface of the centrum–showing fairly pronounced differences from left to right.

That rhea dorsal looks pretty bad for dry bone from a recently-dead extant animal, but if it was from the Morrison Formation it would be phenomenal. If I found a sauropod vertebra that looked that good, I’d think, “Hey, this thing’s in pretty good shape! Only a little distorted.” The roughed-up surface of the right transverse process might give me pause, and I’d want to take a close look at those postzygs, but most of this asymmetry is consistent with what I’d expect from taphonomic distortion.

Which brings me to my titular question, which I am asking out of genuine ignorance and not in a rhetorical or leading way: can we tell these things apart? And if so, with what degree of confidence? I know there has been a lot of work on 3D retrodeformation over the past decade and a half at least, but I don’t know whether this specific question has been addressed.

Corollary question: up above I wrote, “It looks like the kind of minor variation that is present in all kinds of animals, especially large-bodied ones”. My anecdotal experience is that the vertebrae of large extant animals tend to be more asymmetric than those of small extant animals, but I don’t know if that’s a real biological phenomenon–bone is bone but big animals have larger forces working on their skeletons, and they typically live longer, giving the skeleton more time to respond to those forces–OR if the asymmetry is the same in large and small animals and it’s just easier to see in the big ones.

If either of those questions has been addressed, I’d be grateful for pointers in the comments, and thanks in advance. If one or both have not been addressed, I think they’re interesting but Mike and I have plenty of other things to be getting on with and we’re not planning to work on either one, hence the “Hey, you! Want a project?” tag.


6 Responses to “Can we distinguish taphonomic distortion and (paleo)pathology from normal biological variation?”

  1. This doesn’t really answer your questions specifically but if you know reasonably for sure that some aspect of your bone should be circular, then that structure can work as a strain ellipse and give you an idea of how much deformation has occurred. I looked at orbits in a bunch of extant animals and they are generally roughly circular, and thus were a good candidate for understanding deformation in ankylosaur skulls. Doesn’t totally answer questions about asymmetry and pathology but can be a starting point! Paper is here:

  2. Matt Wedel Says:

    Interesting! Thanks, Victoria.

    It would be nice to be able to assume that sauropod centra were circular, but we enough good specimens that aren’t to know that’s a bust. In Opisthocoelicaudia the dorsal centra are gently bilobed, almost an incipient step toward a saddle-shaped articulation like that of birds. And sauropod neural canals are very often much taller than wide, so that won’t work either.

    I wonder if you could use paired elements like zygs. Assume that the two zygs had articular surfaces with roughly equal outlines and roughly equal orientations, at roughly the same distance from the centrum. But how “roughly” is good enough, and would that method work at all? I think you’d need to know how much zygs can vary left-to-right in a single vert in healthy, non-pathological individuals. And ideally across a lot of species.

    Humans probably have the most documentation, but the least useful data, since we’ve only recently evolved bipedality and we’re still not very good at it. So I’m guessing that we have more spinal asymmetry and pathology than a lot of other mammals that have been doing their thing for more than just a few million years.

  3. Eric Says:

    In theory distortion after the vertebrae is buried in sediment/rock will come from very regular forces. For instance, while a vertebrae in an animal can have different forces on the right and left the vertebrae in sediment be exposed to the same crushing force everywhere. Assuming a sufficiently good computer model of crushing forces you could tell the computer to assume that the vertebrae is bilaterally symmetrical and see whether any particular force vector would explain all or most of the asymmetry seen in the fossil. This would be a brute-force approach, but in theory you could end up being able to say, “This vertebrae was probably squished in this direction and originally looked like this, the remaining asymmetry is not explainable by post-sedimentation pressure.”

  4. Matt Wedel Says:

    I think that makes sense. Almost like a PCA–maybe the major asymmetry is a shearing from anterior, dorsal, right to posterior, ventral left, and we attribute that to plastic deformation of the bone in the surrounding sediment, but once that’s removed there is still a more subtle asymmetry in which all the structures on the right side are 2% larger than the ones on the left, and we chalk that up to biological variation, and possibly handed-ness. Although I like your more circumspect formulation of “the remaining asymmetry is not explainable by post-sedimentation pressure”.

  5. Drew Moore Says:

    Is that Rhea vert from a captive or a wild specimen? I’ve looked at a lot of bird skeletons, and while I can’t say I’ve tried to keep my eye out for relative amounts asymmetry, I’ve noticed that captive skeletons are often tragically janky.

  6. Matt Wedel Says:

    Hi Drew, I don’t know if that one was captive or wild, but I could find out the next time I’m over there. I have noticed the same thing, about captive animals of all kinds.

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