Beware over-interpreting photos of fossils
January 2, 2017
Welcome to 2017! Let’s start the year with a cautionary tale. I’ll leap straight to the moral, then give an example: it’s very easy to reach the wrong conclusion about fossils from photos. That’s because no single photo can give an accurate impression of distortion. For that, you need at least a much bigger selection of photos; or better still, a 3d model; or of course best of all, the fossil itself.
Here’s the motivating example:
A correspondent — I will not divulge his or her name unless the person in question chooses to reveal it — had looked over the slides for our 2016 SVPCA talk on new Barosaurus specimens, which claims that Jensen’s Dry Mesa “Supersaurus” cervical BYU 9024 actually belongs to Barosaurus.
Matt and I felt, based largely on the degree of neural spine bifurcation, that the BYU vertebra compares most similarly to C9 of the AMNH specimen — the middle one in the top row of the composite illustration above. But my correspondent put together the composite, and wrote [lightly edited for clarity]:
I’ve already compared BYU 9024 with the AMNH cervicals, I attach a photo, because for me it is also very similar to C14: the centrum is much more similar to C14 than C9, I think. What do you think about this?
Like I said: you always need to be careful about interpreting any one view of a fossil. In this case, BYU 9024 is misleading in lateral view because the CPOLs are folded upwards and inwards, and the ventrolateral flanges are (to a lesser extent) folded downwards and inwards — making the posterior part of the centrum look much taller (and rather narrower) than it really is.
This is hard to see in photos, because the fossil is so smashed up and the matrix is so visually similar to the bone, but take a look at the posterior view (with anterior to the right of the photo):
Here are the key parts, annotated, as best I can make out. (And bear in mind that even I am not sure, after having spent a whole day with the fossil, and with literally hundreds of photos to consult.)
As you can see, the centrum accounts for only a small proportion of the apparent height of the posterior end of the vertebra — and even that is probably exaggerated, as the eccentricity of the condyle indicates that crushing has increased its height at the expense of its width.
Put it all together, and Jensen’s much-derided sculpture of what the vertebra should have looked like is actually pretty good: