Nature’s CT machine, part 2: an apatosaurine in the Salt Wash

January 28, 2020

In the last post, we looked at some sauropod vertebrae exposed in cross-section at our field sites in the Salt Wash member of the Morrison Formation. This time, we’re going to do it again! Let’s look at another of my faves from the field, with Thuat Tran’s hand for scale. And, er, a scale bar for scale:

And let’s pull the interesting bits out of the background:

Now, confession time. When I first saw this specimen, I interpeted it as-is, right-side up. The round thing in the middle with the honeycomb of internal spaces is obviously the condyle of a vertebra, and the bits sticking out above and below on the sides frame a cervical rib loop. I figured the rounded bit at the upper right was the ramus of bone heading for the prezyg, curved over as I’ve seen it in some taxa, including the YPM Barosaurus. And the two bits below the centrum would then be the cervical ribs. And with such big cervical rib loops and massive, low-hanging cervical ribs, it had to an apatosaurine, either Apatosaurus or Brontosaurus.

Then I got my own personal Cope-getting-Elasmosaurus-backwards moment, courtesy of my friend and fellow field adventurer, Brian Engh, who proposed this:

Gotta say, this makes a lot more sense. For one, the cervical ribs would be lateral to the prezygs, just as in, oh, pretty much all sauropods. And the oddly flat inward-tilted surfaces on what are now the more dorsal bones makes sense: they’re either prezyg facets, or the flat parts of the rami right behind the prezyg facets. The missing thing on what is now the right even makes sense: it’s the other cervical rib, still buried in a projecting bit of sandstone. That made no sense with the vert the other way ’round, because prezygs always stick out farther in front than do the cervical ribs. And we know that we’re looking at the vert from the front, otherwise the backwards-projecting cervical rib would be sticking through that lump of sandstone, coming out of the plane of the photo toward us.

Here’s what I now think is going on:

I’m still convinced that the bits of bone on what is now the left side of the image are framing a cervical rib loop. And as we discussed in the last post, the only Morrison sauropods with such widely-set cervical ribs are Camarasaurus and the apatosaurines. So what makes this an apatosaurine rather than a camarasaur? I find several persuasive clues:

  • If we have this thing the right way up, those prezygs are waaay up above the condyle, at a proportional distance I’ve only seen in diplodocids. See, for example, this famous cervical from CM 3018, the holotype of A. louisae (link).
  • The complexity of the pneumatic honeycombing inside the condyle is a much better fit for an apatosaurine than for Camarasaurus–I’ve never seen that level of complexity in a camarasaur vert.
  • The bump on what we’re now interpreting as the cervical rib looks suspiciously like one of the ventrolateral processes that Kent Sanders and I identified in apatosaurine cervicals back in our 2002 paper. I’ve never seen them, or seen them reported, in Camarasaurus–and I’ve been looking.
  • Crucially, the zygs are not set very far forward of the cervical ribs. By some rare chance, this is pretty darned close to a pure transverse cut, and the prezygs, condyle (at its posterior extent, anyway), and the one visible cervical rib are all in roughly the same plane. In Camarasaurus, the zygs strongly overhang the front end of the centrum in the cervicals (see this and this).

But wait–aren’t the cervical ribs awfully high for this to be an apatosaurine? We-ell, not necessarily. This isn’t a very big vert; max centrum width here is 175mm, only about a third the diameter of a mid-cervical from something like CM 3018. So possibly this is from the front of the neck, around the C3 or C4 position, where the cervical ribs are wide but not yet very deep. You can see something similar in this C2-C5 series on display at BYU:

Or, maybe it’s just one of the weird apatosaurine verts that has cervical rib loops that are wide, but not very deep. Check out this lumpen atrocity at Dinosaur Journey–and more importantly, the apatosaur cervical he’s freaking out over:

UPDATE just a few minutes later: Mike reminded me in the comments about the Tokyo apatosaurine, NSMT-PV 20375, which has wide-but-not-deep cervical ribs. In fact, C7 (the vertebra on the right in this figure) is a pretty good match for the Salt Wash specimen:

UpchurchEtAl2005-apatosaurus-plate2-C3-6-7

NSMT-PV 20375, cervical vertebrae 3, 6 and 7 in anterior and posterior views. Modified from Upchurch et al. (2005: plate 2).

UPDATE the 2nd: After looking at it for a few minutes, I decided that C7 of the Tokyo apatosaurine was such a good match for the Salt Wash specimen that I wanted to know what it would look like if it was similarly sectioned by erosion. In the Salt Wash specimen, the prezygs are sticking out a little farther than the condyle and cervical rib sections. The red line in this figure is my best attempt at mimicking that erosional surface on the Tokyo C7, and the black outlines on the right are my best guess as to what would be exposed by such a cut (or pair of cuts). I’ve never seen NSMT-PV 20375 in person, so this is just an estimate, but I don’t think it can be too inaccurate, and it is a pretty good match for the Salt Wash specimen.

Another way to put it: if this is an apatosaurine, everything fits. Even the wide-but-not-low-hanging cervical ribs are reasonable in light of some other apatosaurines. If we think this is Camarasaurus just because the cervical ribs aren’t low-hanging, then the pneumatic complexity, the height of the prezygs, and the ventrolateral process on the cervical rib are all anomalous. The balance of the evidence says that this is an apatosaurine, either a small, anterior vert from a big one, or possibly something farther back from a small one. And that’s pretty satisfying.

One more thing: can we take a moment to stand in awe of this freaking thumb-sized cobble that presumably got inside the vertebra through one its pneumatic foramina and rattled around until it got up inside the condyle? Where, I’ll note, the internal structure looks pretty intact despite being filled with just, like, gravel. As someone who spends an inordinate amount of time thinking about how pneumatic vertebrae get buried and fossilized, I am blown away by this. Gaze upon its majesty, people!

This is another “Road to Jurassic Reimagined, Part 2″ post. As before, Part 1 is here, Part 2 will be going up here in the near future. As always, stay tuned.

References

8 Responses to “Nature’s CT machine, part 2: an apatosaurine in the Salt Wash”

  1. Mike Taylor Says:

    If it makes you feel any better, I also fell for the upside down interpretation. But only for about eight seconds, when I had all the same thoughts that you outline above. I’m also sold on the apatosaurine identification. On the CRs being less ventrally positioned than we might expect: first, these things must be vulnerable to distortion; and second, it’s a good match for the Tokyo apatosaur described by Upchurch et al. (2005): see the cervicals reproduced at https://svpow.com/2012/04/05/neural-spine-bifurcation-in-sauropods-what-we-knew-a-month-ago/

  2. Matt Wedel Says:

    Oh, very good point on the Tokyo apatosaur! I had a feeling there was a documented specimen with not-so-low cervical ribs, but I couldn’t for the life of me remember which one.

    And it does make me feel better that you’re also happy with the ID. I’m trying to be as careful and objective as possible, but I’m alert to the possibility of anatomical pareidolia creeping in.

  3. Brad Lichtenstein Says:

    So I don’t spend much time thinking about fossils and their formation (maybe I’m not especially sad about that, only because I don’t know what I’m missing), so I can only hope one day to randomly contribute something worthwhile. Like a stopped watch being extremely precise for an extremely short instant every half day. So if fossilization generally requires an animal be buried fairly soon after death, then I’m generally assuming something died in or directly over a lot of mud – or was subject to a violent death by flood, of water, mud, or volcanic ejects – which at least raises the possibility of bones being snapped, as well as gravel being found inside. Or if it’s been eroding slowly, and was found in a normally-dry gulch, gravel gets washed along when not dry.

    (And depending on what way the strata face, it’s reasonable to assume a large dead creature might end up on its side or back.)

    But the other thought is about the “trace fossils” which I assume are only found on parts of an animal that were buried both intact, and soon after death. I’ve always wondered if they haven’t been found simply because few have carefully looked for them. Especially during the bone wars, I assume no one looked in their haste to uncover the bones. But even if you looked, any bits of skin for an exposure like this one, would look just like a thin line, if that. And given the pressures that squish the bones, where the heck would one even look for skin impressions?

    And I raise the second point because y’all are getting to me – I’m actually starting to get really annoyed by media depictions of dinosaurs, which I noticed go beyond shrink wrap, to “vacuum sealed”. I mean, find me any animal today whose non-muscled face is simply skin-on-bone.

    Now, I’m also irritated by more fleshy, or feathered, depictions – because THOSE are almost equally speculative. Point: y’all better FIND more skin! Yeah, the bones are what catches the eye….

  4. Mike Taylor Says:

    Oh, one other thing I forgot to mention. There is a small rounded projection of what looks like bone at about eleven o’clock in the original photo (and at five o’clock once it’s been rotated 180). I see you’ve not included it in your highlight of what part of the specimen is bone, so I may be misinterpreting. But if I’m right and it’s bone then I assume we’re seeing part of the left parapophysis.

    Brad, you’re right about the kinds of conditions where fossils form, but I doubt that the process of burial damages the bones much. Either there is a violent process that kills the animal, in which case the bones are safely encased in gloop and protected from trauma; or the animal has long died and decayed before the bones are buried, in which case a gradual deposition is suffient to cover them.

  5. Matt Wedel Says:

    That thing at 11 o’clock in the original photo is a splotch of packrat poop with gravel stuck in it. At one time there was probably a whole packrat midden here, and it fell off when erosion finally undercut it sufficiently.

    If you’re thinking that finding sauropod vertebrae beautifully sectioned but then vandalized by poop-smearing, hantavirus-carrying rodents is less than 100% pleasant, you are correct.


  6. […] most of the other dinosaur fossils we’ve found in the Salt Wash, including the camarasaur, apatosaur, and haplocanthosaur vertebrae I’ve shown in recent posts, the humerus and associated bones […]

  7. Vahe Demirjian Says:

    Given that NSMT-PV 20375 was referred to Apatosaurus ajax by Upchurch et al. (2005), and the you consider that specimen a good match for the Salt Wash apatosaurine, you ought to see if the Salt Wash apatosaurine and NSMT-PV 20375 could represent a distinct lineage of Apatosaurus, or the Salt Wash apatosaurine could be distinct from NSMT-PV 20375, because Brontosaurus (=Eobrontosaurus) yahnahpin is known from the Salt Wash Member of the Morrison Formation, found at stratigraphic levels much lower than other apatosaurines.

  8. Mike Taylor Says:

    Well, let’s hold our horses. I mentioned one feature of the Salt Wash apatosaurine that resembled the Tokyo apatosaurine — we are a long long way from referring them together. Meanwhile, I am not completely sold on the idea that the Toko specimen is A. ajax anyway — though I won’t say it any stronger than that, not having seen the material.

    But Eobrontosaurus is an interesting call. If I had more time spare (i.e. any at all) I would go and look up the description and see what it shows us about the cervical ribs.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: