Nature’s CT machine

January 28, 2020

Because I’ve worked a lot on the anatomy and evolution of air-filled bones in sauropod dinosaurs, I’ve spent most of my career looking at images like this:

CT sections through a cervical vertebra of an apatosaurine (Apatosaurus or Brontosaurus), OMNH 1094. Wedel (2003b: fig. 6). Scale bar is 10cm.

…and thinking about images like this:

Physical sections through pneumatic vertebrae of Giraffatitan. Janensch (1950: figs. 71-73).

Turns out, that’s pretty good practice for fossil prospecting in the Salt Wash member of the Morrison Formation, where we frequently find things like this:

That’s a bit hard to read, so let’s pull it out from the background:

This is almost certainly a pneumatic vertebra of a sauropod, sectioned more-or-less randomly by the forces of erosion to expose a complicated honeycomb of internal struts and chambers. The chambers are full of sandstone now, but in life they were full of air. I say “almost certainly” because there is small chance that it could belong to a very large theropod, but it looks more sauropod-y to me (for reasons I may expand upon in the comments if anyone is curious).

I’m not 100% certain what section this is. At first I was tempted to read it as a transversely-sectioned dorsal, something like the Allosaurus dorsal shown in this post (link) but from a small, possibly juvenile sauropod. But the semi-radial, spoke-like arrangement of the internal struts going to the round section at the bottom looks very much like the inside of the condyle of a sauropod cervical or cervico-dorsal–compare to fig. 71 from Janensch (1950), shown above. And of course there is no reason to suspect that the plane of this cut is neatly in any of the cardinal anatomical directions. It is most likely an oblique cut that isn’t purely transverse or sagittal or anything else, but some combination of the above. It’s also not alone–there are bits and bobs of bone to the side and above in the same chunk of sandstone, which might be parts of this vertebra or of neighboring bones. Assuming it is a sauropod, my guess is Diplodocus or Brachiosaurus, because it looks even more complex than the sectioned cervicals and dorsals I’ve seen of Haplocanthosaurus, Camarasaurus, or the apatosaurines.

Sometimes we can do a little better. This is one of my favorite finds from the Salt Wash. This boulder, now in two parts, fell down out of a big overhanging sandstone cliff. When the boulder hit, it broke into two halves, and the downhill half rolled over 180 degrees, bringing both cut faces into view in this photo. And there in the boulder is what looks like two vertebrae, but is in fact the neatly separated halves of a single vertebra. I know I refer to erosion and breakage as “Nature’s CT machine”, but this time that’s really on the nose. Let’s take a closer look:

Here’s what I see:

It’s a proportionally long vertebra with a round ball at one end and a hemispherical socket at the other end: a cervical vertebra of a sauropod. Part of the cervical rib is preserved on the upper side, which I suspect is the left side. The parapophysis on the opposite side is angled a bit out of the rock, toward the camera. Parapophyses of sauropod cervicals tend to be angled downward, and if we’re looking at the bottom of this vertebra, then the rib on the upper side is the left. The right cervical rib was cut off when the boulder broke. All we have on this side are the wide parapophysis and the slender strut of the diapophysis aiming out of the rock toward the missing rib, which must still be embedded in the other half of the boulder–and in fact you can see a bit of it peeking out in the counterpart in the wide shot, above.

Can we get a taxonomic ID? I think so, based on the following clues:

  • The cervical ribs are set waaay out to either side of the centrum, by about one centrum diameter. Such wide-set cervical ribs occur in Camarasaurus and the apatosaurines, Apatosaurus and Brontosaurus, but not typically in Diplodocus, Brachiosaurus, or other Morrison sauropods.
  • The cervical rib we can see the most of is pretty slender, like those of Camarasaurus, in contrast to the massive, blocky cervical ribs of the apatosaurines (for example).
  • We can see at least bits of both the left and right cervical ribs in the two slabs–along with a section right through the centrum. So the cervical ribs were set wide from the centrum but not displaced deeply below it, as in Camarasaurus, and again in contrast to the apatosaurines, in which the cervical ribs are typically displaced far below (ventral to) the centrum (see this).
  • This one is a little more loosey-goosey, but the exposed internal structure looks “about right” for Camarasaurus. There is a mix of large and small chambers, but not many small ones, and nothing approaching the coarse, regular honeycomb we’d expect in Apatosaurus, Brontosaurus, or Diplodocus, let alone the fine irregular honeycomb we’d expect in Barosaurus or Brachiosaurus (although I will show you a vert like that in an upcoming post). On the other hand, the internal structure is too complex for Haplocanthosaurus (compare to the top image here).
  • As long as Camarasaurus is on the table, I’ll note that the overall proportions are good for a mid-cervical of Cam as well. That’s not worth much, since vertebral proportions vary along the column and almost every Morrison sauropod has cervicals with this general proportion somewhere in the neck, but it doesn’t hurt.

So the balance of the evidence points toward Camarasaurus. In one character or another, every other known Morrison sauropod is disqualified.

When it’s too dark to hunt for sauropods, you can look at other things.

Now, Camarasaurus is not only the most common sauropod in the Morrison, it’s also the most common dinosaur of any kind in the formation. So this isn’t a mind-blowing discovery. Still, it’s nice to be able to get down to a genus-level ID based on a single vertebra fortuitously sectioned by Mother Nature. In upcoming posts, I’ll show some of the more exciting critters that we’ve been able to ID out of the Salt Wash, ‘we’ here including Brian Engh, John Foster, ReBecca Hunt-Foster, Jessie Atterholt, and Thuat Tran. Brian will also be showing many of these same fossils in the next installment of Jurassic Reimagined. Catch Part 1 here (link), and stay tuned to Brian’s paleoart channel (here) for more in the very near future.

References

When I visited Dinosaur National Monument in October with Brian Engh and Yara Haridy, we spent a decent amount of time checking out DNM 28, a skull and associated bits of Camarasaurus. In particular, I got some shots of the axis (the second cervical vertebra behind the head), and it got me thinking about pneumaticity in this unusual element. Why I failed to get a full set of orthogonal shots is quite beyond my capacity, but we can roll with what I have. Before we go on, you might want to revisit Tutorial 36 to brush up on the general parts of the atlas-axis complex.


Here’s the axis in left lateral view (so, anterior to the left).

And a labeled version of the same. A few things to note:

  • One oddity of sauropod axes (and of axes of most critters) is that not only are the articular facets of the prezygapophyses not set forward of the neural arch, they’re set backward, well behind the forward point of the arch.
  • The dens epistrophei or odontoid process is sticking out immediately below the neural canal. This is the tongue of bone that articulated with the atlas (first cervical vertebra) in life.
  • Check out the prominent epipophysis above the postzygapophysis, which anchored the long dorsal neck muscles. (For more on epipophyses, see these posts, and especially this one.)
  • The diapophysis and parapophysis articulated with a cervical rib, which is not shown here. In fact, I don’t remember seeing it in the drawer that this vert came from. The atlantal and axial cervical ribs are small, apparently fused late in life if they fused at all, and are easily lost through taphonomic processes.
  • At least three pneumatic features are visible in this lateral view: the lateral fossa on the centrum, which is referred to as the “pleurocoel” in a lot of older literature; a ventral fossa that lies between the parapophysis and the midline ventral keel; and a fossa on the neural arch, behind the postzygodiapophyseal lamina. In the nomenclature of Wilson et al. (2011), this is the postzygocentrodiapophyseal fossa.

“Postzygocentrodiapophyseal fossa” is a mouthful, but I think it’s the only way to go. To be unambiguous, anatomical terminology needs to references specific landmarks, and the schemes proposed by Wilson (1999) for vertebral laminae and Wilson et al. (2011) for vertebral fossae are the bee’s knees in my book.

Nomenclatural issues aside, how do we know that these fossae were all pneumatic? Well, they’re invasive, there’s no other soft-tissue system that makes invasive fossae like that in archosaur vertebrae (although crocs sometimes have shallow fossae that are filled with cartilage or fat), and the same fossae sometimes have unambiguous pneumatic foramina in other vertebrae or in other sauropods.

Most of the features labeled above are also visible on the right side of the vertebra, although the ventral fossa is a little less well-defined in this view, and I can’t make out the prezyg facet. Admittedly, some of the uncertainty here is because of my dumb shadow falling across the vertebra. Specimen photography fail!

The paired ventral fossae are more prominent in this ventral view, on either side of the midline ventral keel (anterior is to the top).

And here’s a labeled version of the same ventral view.

Finally, here’s the posterior view. It’s apparent now that the neural spine is a proportionally huge slab of bone, like a broad, tilted shield between the postzygapophyses (which are also quite large for the size of this vertebra). The back side of the neural spine is deeply excavated by a complex fossa with several subfossae (kudos again to Jeff Wilson [1999] for that eminently useful term).

Here’s the same shot with some features of interest labeled. If I’ve read Wilson et al. (2011) correctly, the whole space on the back side of the neural spine and above the postzygs could be considered the spinopostzygapophyseal fossa, but here I’ve left the interspinous ligament scar (ILS) unshaded, on the expectation that the pneumatic diverticula that created that fossa were separated on the midline by the interspinous ligament. I might have drawn the ILS too conservatively, conceivably the whole space between the large deeply-shadowed subfossae was occupied by the interspinous ligament.

I’m particularly interested in those three paired subfossae, which for convenience I’m simply calling A, B, and C. Subfossa A may just be the leftover space between the spinopostzgyapophyseal laminae laterally and the interspinous ligament medially. I think subfossa B is invading the ramus of bone that goes to the epipophysis and postzygapophysis, but I didn’t think to check and see how far it goes (that might require CT anyway).

Subfossa C is the most intriguing. Together, those paired fossae form a couple of shallow pits, just on either side of the midline, and aimed straight forward. They can’t be centropostzygapophyseal fossae, which used to be called peduncular fossae, because they’re not in the peduncles on either side of the neural canal, they’re up above the lamina that connects the two postzygapophyses. Could they be ligament attachments? Maybe, but I’m skeptical for at least four reasons:

  1. Although interspinous ligament attachments often manifest as pits in the cervical vertebrae of birds, in sauropods they usually form rugosities or even spikes of bone that stick out, not inward. Furthermore, these pits are smooth, not rough like the interspinous ligament scars of birds.
  2. The interspinous ligament in tetrapods is typically a single, midline structure, and these pits are paired.
  3. Similar pits in front of the neural spine are present in some sauropod caudals, and they appear to be pneumatic (see Wedel 2009: p. 11 and figure 9).
  4. Pits at the base of the neural spine seem to be fairly uncommon in sauropod vertebrae. If they were attachment scars from the universally-present interspinous ligaments, we should expect them to be more prominent and more widespread.

But if these paired pits are not ligament scars, what are they? Why are they present, and why are they so distinct? Sometimes (often?) subfossae and accessory laminae look like the outcome of pneumatic diverticulum and bone reacting to each other (I almost wrote ‘playing together’), in what looks like a haphazard process of adaptation to local loading. But the symmetry of these pits argues against them being incidental or random. They don’t seem to be going anywhere, so maaaybe they are the first hoofbeats of the embossed laminae and “unfossae” that we see in the vertebrae of more derived sauropods (for which see this post), but again, their symmetry in size and placement isn’t really consistent with the “developmental program gone wild” appearance of “unfossae”. I really don’t know what to make of them, but if you have ideas, arguments, or observations to bring to bear, the comment field is open.

In summary, sauropod axes are more interesting than I thought, even in a derpasaurus like Cam. I’ll have to pay more attention to them going forward.

References

 

Unworn:

Worn:

Spent some time last week just admiring these things. They’re pretty cool.

EDIT: in answer to Mike’s question in the first comment below, here’s a photo of some more worn teeth, showing that the level of wear in the one shown above is not unusual. Also, all of these worn teeth still had full roots, with no sign of the root resorption that would have preceded shedding of the tooth, so they were evidently going to be used for a while yet, probably a few months at least — BUT see the very useful comment from Jens Kosch below on the likely rapidity of tooth replacement in Camarasaurus.

DINO collections - more worn Camarasaurus teeth

The stupidest head

August 21, 2019

Left: Homo sapiens, head, neck and upper trunk in right lateral view (unprepared specimen). Right: Camarasaurus sp., skull in left lateral view. Photograph at the Natural History Museum of Utah, Salt Lake City, Utah. 2016.

In a word, amazingly. After 6 days (counting public galleries last Sunday), 4300 photos, 55 videos, dozens of pages of notes, and hundreds of measurements, we’re tired, happy, and buzzing with new observations and ideas.

We caught up with some old friends. Here Mike is showing an entirely normal and healthy level of excitement about meeting CM 584, a specimen of Camarasaurus from Sheep Creek, Wyoming. You may recognize this view of these dorsals from Figure 9 in our 2013 PeerJ paper.

We spent an inordinate amount of time in the public galleries, checking out the mounted skeletons of Apatosaurus and Diplodocus (and Gilmore’s baby Cam, and the two tyrannosaurs, and, and…).

I had planned a trip to the Carnegie primarily to have another look at the Haplocanthosaurus holotypes, CM 572 and CM 879. I was also happy for the chance to photograph and measure these vertebrae, CM 36034, which I think have never been formally described or referred to Haplocanthosaurus. As far as I know, other than a brief mention in McIntosh (1981) they have not been published on at all. I’m planning on changing that in the near future, as part of the larger Haplocanthosaurus project that now bestrides my career like a colossus.

The real colossus of the trip was CM 555, which we’ve already blogged about a couple of times. Just laying out all of the vertebrae and logging serial changes was hugely useful.

Incidentally, in previous posts and some upcoming videos, we’ve referred to this specimen as Brontosaurus excelsus, because McIntosh (1981) said that it might belong to Apatosaurus excelsus. I was so busy measuring and photographing stuff that it wasn’t until Friday that I realized that McIntosh made that call because CM 555 is from the same locality as CM 563, now UWGM 15556, which was long thought to be Apatosaurus excelsus but which is now (i.e., Tschopp et al. 2015) referred to Brontosaurus parvus. So CM 555 is almost certainly B. parvus, not B. excelsus, and in comparing the specimen to Gilmore’s (1936) plates of CM 563, Mike and I thought they were a very good match.

Finding the tray of CM 555 cervical ribs was a huge moment. It added a ton of work to our to-do lists. First we had to match the ribs to their vertebrae. Most of them had field numbers, but some didn’t. Quite a few were broken and needed to be repaired – that’s what I’m doing in the above photo. Then they all had to be measured and photographed.

It’s amazing how useful it was to be able to reassociate the vertebrae with their ribs. We only did the full reassembly for c6, in part because it was the most complete and perfect of all of the vertebrae, and in part because we simply ran out of time. As Mike observed in his recent post, it was stunning how the apatosaurine identity of the specimen snapped into focus as soon as we could see a whole cervical vertebra put back together with all of its bits.

We also measured and photographed the limb bones, including the bite marks on the radius (above, in two pieces) and ulna (below, one piece). Those will of course go into the description.

And there WILL BE a description. We measured and photographed every element, shot video of many of them, and took pages and pages of notes. Describing even an incomplete sauropod skeleton is a big job, so don’t expect that paper this year, but it will be along in due course. CM 555 may not be the most complete Brontosaurus skeleton in the world, but our ambition is to make it the best-documented.

In the meantime, we hopefully left things better documented than they had been. All of the separate bits of the CM 555 vertebrae – the centra, arches, and cervicals ribs – now have the cervical numbers written on in archival ink (with permission from collections manager Amy Henrici, of course), so the next person to look at them can match them up with less faffing about.

We have people to thank. We had lunch almost every day at Sushi Fuku at 120 Oakland Avenue, just a couple of blocks down Forbes Avenue from the museum. We got to know the manager, Jeremy Gest, and his staff, who were unfailingly friendly and helpful, and who kept us running on top-notch food. So we kept going back. If you find yourself in Pittsburgh, check ’em out. Make time for a sandwich at Primanti Bros., too.

We owe a huge thanks to Calder Dudgeon, who took us up to the skylight catwalk to get the dorsal-view photos of the mounted skeletons (see this post), and especially to Dan Pickering, who moved pallets in collections using the forklift, and moved the lift around the mounted skeletons on Tuesday. Despite about a million ad hoc requests, he never lost patience with us, and in fact he found lots of little ways to help us get our observations and data faster and with less hassle.

Our biggest thanks go to collections manager Amy Henrici, who made the whole week just run smoothly for us. Whatever we needed, she’d find. If we needed something moved, or if we needed to get someplace, she’d figure out how to do it. She was always interested, always cheerful, always helpful. I usually can’t sustain that level of positivity for a whole day, much less a week. So thank you, Amy, sincerely. You have a world-class collection. We’re glad it’s in such good hands.

What’s next? We’ll be posting about stuff we saw and learned in the Carnegie Museum for a long time, probably. And we have manuscripts to get cranking on, some of which were already gestating and just needed the Carnegie visit to push to completion. As always, watch this space.

References

Coproliteposting time!

October 28, 2018

I wasted some time today making memes. I blame the Paleontology Coproliteposting group on Facebook.

Of course I started out by making fun of the most mockable sauropod. This one’s for you Cam-loving perverts out there. You know who you are.

This one was inspired by the thiccthyosaur meme, which irritatingly enough I cannot find right now. Oh no, wait, here it is.

I’m laughing through the tears.

For previous adventures in meme-ing, see this post.

Thanks to everyone who’s engaged with yesterday’s apparently trivial question: what does it mean for a vertebra to be “horizontal”? I know Matt has plenty of thoughts to share on this, but before he does I want to clear up a couple of things.

This is not about life posture

First, and I really should have led with this: the present question has nothing to do with life posture. For example, Anna Krahl wrote on Twitter:

I personally find it more comprehensible if the measurements relate to something like eg. the body posture. This is due to my momentary biomech./functional work, where bone orientation somet is difficult to define.

I’m sympathetic to that, but we really need to avoid conflating two quite different issues here.

Taylor, Wedel and Naish (2009), Figure 1. Cape hare Lepus capensis RAM R2 in right lateral view, illustrating maximally extended pose and ONP: skull, cervical vertebrae 1-7 and dorsal vertebrae 1-2. Note the very weak dorsal deflection of the base of the neck in ONP, contrasting with the much stronger deflection illustrated in a live rabbit by Vidal et al. (1986: fig. 4). Scalebar 5 cm.

If there’s one thing we’ve learned in the last couple of decades, it’s that life posture for extinct animals is controversial — and that goes double for sauropod necks. Heck, even the neck posture of extant animals is terribly easy to misunderstand. We really can’t go changing what we mean by “horizontal” for a vertebra based on the currently prevalent hypothesis of habitual posture.

Also, note that the neck posture on the left of the image above is close to (but actually less extreme than) the habitual posture of rabbits and hares: and we certainly wouldn’t want to illustrate vertebrae as “horizontal” when they’re oriented directly upwards, or even slightly backwards!

Instead, we need to imagine the animal’s skeleton laid out with the whole vertebral column in a straight line — sort of like Ryder’s 1877 Camarasaurus, but with the tail also elevated to the same straight line.

Ryder’s 1877 reconstruction of Camarasaurus, the first ever made of any sauropod, modified from Osborn & Mook (1921, plate LXXXII).

Of course, life posture is more important, and more interesting, question than that of what constitutes “horizontal” for an individual vertebra — but it’s not the one we’re discussing right now.

In method C, both instances are identically oriented

I’m not sure how obvious this was, but I didn’t state it explicitly. In definition C (“same points at same height in consecutive vertebrae”), I wrote:

We use two identical instances of the vertebrae, articulate them together as well as we can, then so orient them that the two vertebrae are level

What I didn’t say is that the two identical instances of the vertebrae have to be identically oriented. Here’s why this is important. Consider that giraffe C7 that we looked at last time, with its keystoned centrum. if you just “articulate them together as well as we can” without that restriction, you end up with something like this:

Which is clearly no good: there’s no way to orient that such that for any given point on one instance, the corresponding point on the other is level with it. What you need instead is something like this:

In this version, I’ve done the best job I can of articulating the two instances in the same attitude, and arranged them such that they are level with each other — so that the attitude shown here is “horizontal” in sense C.

As it happens, this is also just about horizontal in sense B — the floor of the neural canal is presumably at the same height as the top of the centrum as it meets the neural arch.

But “horizontal” in sense A (posterior articular surface vertical) fails horribly for this vertebra:

To me, this image alone is solid evidence that Method A is just not good enough. Whatever we mean by “horizontal”, it’s not what this image shows.

References