New paper out today: Aureliano et al. (2022) on vertebral internal structure in the earliest saurischians

December 9, 2022

Micro-computed tomography of the vertebrae of the basalmost sauropodomorph Buriolestes (CAPPA/UFSM 0035). (A) silhouette shows the position of the axial elements. Artist: Felipe Elias. (B), three-dimensional reconstruction of the articulated cervical vertebral series and the correspondent high-contrast density slices in (D–I). Diagenetic processes partially compromised the internal structures in these cervicals. (C), 3D reconstruction of the articulated anterior dorsal vertebrae and the correspondent high-contrast density slices in (J–M). Small circumferential chambers occur both ventrally in the dorsal centrum (J) and laterally in the neural arch pedicles (D). All images indicate apneumatic chaotic trabeculae architecture. Some of the latter develop into larger chambers in the centrum (E,J,K). Nutritional foramina are broader at the bottom of the neural canal in the posterior cervicals (F,G). All slices were taken from the approximate midshaft. Anterior views in (D–H,J,K). Lateral view in (L). Ventral view in (H,I,M). Anterior/posterior orientation was defined based on the axial position, not the anatomical plane. cc circumferential chamber, ccv chamber in the centrum, ctr chaotic trabecula, d diapophysis, ltr layered trabeculae, nc neural canal, nf nutritional foramen, s neural spine. Scale bar in (A) = 500 mm; in (B–M) = 10 mm. Computed tomography data processed with 3D Slicer version 4.10. Figures were generated with Adobe Photoshop CC version 22.5.1 X64. (Aureliano et al. 2022: fig. 4)

Here’s a nice early holiday present for me: 51 weeks after our first paper together, I’m on another one with Tito Aureliano and colleagues:

Aureliano, T., Ghilardi, A.M., Müller, R.T., Kerber, L., Pretto, F.A., Fernandes, M.A.,Ricardi-Branco, F., and Wedel, M.J. 2022. The absence of an invasive air sac system in the earliest dinosaurs suggests multiple origins of vertebral pneumaticity. Scientific Reports 12:20844. https://doi.org/10.1038/s41598-022-25067-8

As before, I’m in the “just happy to be here” last author position, and quite happy to be so, too. I’ve had a productive couple of years, mostly because my colleagues keep inviting me to write a little bit, usually about pneumaticity, in exchange for a junior authorship, and that’s actually a perfect fit for my bandwidth right now. That dynamic has let me work on some cool and varied projects that have broadened my experience in satisfying ways. But enough navel-gazing!

Also as before, Tito made a really nice video that explains our findings from the paper and puts them in their broader scientific context:

For a long time now I’ve been interested in the origin of postcranial skeletal pneumaticity (PSP) in dinosaurs and pterosaurs (e.g., Wedel 2006, 2007, 2009, Yates et al. 2012, Wedel and Taylor 2013) — or is that origins, plural? Tito and crew decided to take a swing at the problem by CT scanning presacral vertebrae from the early sauropodomorphs Buriolestes and Pampadromaeus, and the herrerasaurid Gnathovorax. (Off-topic: Gnathovorax, “jaw inclined to devour”, is such a badass name that I adopted it for an ancient blue dragon in my D&D campaign.) All three taxa have shallow fossae on the lateral sides of at least some of their presacral centra, and some neural arch laminae, so they seemed like good candidates in which to hunt for internal pneumatization.

I’ll cut right to the chase: none of three have internal pneumatic chambers in their vertebrae, so if there were pneumatic diverticula present, they weren’t leaving diagnostic traces. That’s not surprising, but it’s nice to know rather than to wonder. The underlying system of respiratory air sacs could have been present in the ancestral ornithodiran, and I strongly suspect that was the case, but invasive vertebral pneumatization evolved independently in pterosaurs, sauropodomorphs, and theropods.

Detail of the vertebrae and foramina of the basalmost sauropodomorph Buriolestes (CAPPA/UFSM-0035). Cervical (A–C), anterior (D–F) and posterior (G–I) dorsal vertebrae in right lateral view. Note that nutritional foramina are present throughout the axial skeleton (dark arrows). Anterior/posterior orientation was defined based on the axial position, not the anatomical plane. Scale bar = 5 mm. Figures were generated with Adobe Photoshop CC version 22.5.1 X64. (Aureliano et al. 2022: fig. 4).

Just because we didn’t find pneumaticity, doesn’t mean we didn’t find cool stuff. Buriolestes, Pampadromaeus, and Gnathovorax all have neurovascular foramina — small holes that transmitted blood vessels and nerves — on the lateral and ventral aspects of the centra. That’s also expected, but again nice to see, especially since we think these blood vessels provided the template for invasive vertebral pneumatization in more derived taxa.

The findings I’m most excited about have to do with the internal structure of the vertebrae. Some of the vertebrae have what we’re calling a pseudo-polycamerate architecture. The polycamerate vertebrae of sauropods like Apatosaurus have large pneumatic chambers that branch into successively smaller ones. Similarly, some of the vertebrae in these Triassic saurischians have large marrow chambers that connect to smaller trabecular spaces — hence the term ‘pseudo-polycamerate’. This pseudo-polycamerate architecture is present in Pampadromaeus, but not in the slightly older Buriolestes, which has a more chaotic internal organization of trabecular spaces. So even in the apneumatic vertebrae of these early saurischians, there seems to have been an evolutionary trajectory toward more hierarchially-structured internal morphology.

Micro-computed tomography of the vertebrae of the herrerasaurid Gnathovorax (CAPPA/UFSM-0009). (A) silhouette shows the position of the axial elements. Artist: Felipe Elias. (B) 3D reconstruction of the anterior cervical vertebra and the correspondent high-contrast density slices in (D-I). Diagenetic artifacts greatly compromised the internal structures. (C) 3D reconstruction of the articulated posterior cervical vertebrae and the correspondent high-contrast density slices in (J–O). Minerals infilled between trabecular vacancies generate reddish anomalies. All images indicate irregular, chaotic, apneumatic architecture. Note the apneumatic large chambers in the centrum (ccv) and the smaller circumferential chambers at the bottom (cc). All slices were taken from the approximate midshaft. Anterior views in (D,H,I). Right lateral view in (E,L,M). Ventral view in (F,G,J,K). cc circumferential chambers, ccv chamber in the centrum, ce centrum, ctr chaotic trabeculae, d diapophysis, dia diagenetic artifact, nc neural canal, nf nutritional foramen, poz postzygapophysis, prz prezygapophysis. Scale bar in (A) = 1000 mm; in (B–O) = 10 mm. Computed tomography data processed with 3D Slicer version 4.10. Figures were generated with Adobe Photoshop CC version 22.5.1 X64.

But wait, there’s more! We also found small circumferential chambers around the margins of the centra, and what we’re calling ‘layered trabeculae’ inside the articular ends of the centra. These apneumatic trabecular structures look a heck of a lot like the circumferential pneumatic chambers and radial camellae that we described last year in a dorsal vertebra of what would later be named Ibirania (Navarro et al. 2022), and which other authors had previously described in other titanosaurs (Woodward and Lehman 2009, Bandeira et al. 2013) — see this post.

So to quickly recap, in these Triassic saurischians we find external neurovascular foramina from the nerves and vessels that probably “piloted” the pneumatic diverticula (in Mike’s lovely phrasing from Taylor and Wedel 2021) to the vertebrae in more derived taxa, and internal structures that are resemble the arrangement of pneumatic camerae and camellae in later sauropods and theropods. We already suspected that pneumatic diverticula were following blood vessels to reach the vertebrae and produce external pneumatic features like fossae and foramina (see Taylor and Wedel 2021 for a much fuller development of this idea). The results from our scans of these Triassic taxa suggests the tantalizing possibility that pneumatic diverticula in later taxa were following the vascular networks inside the vertebrae as well. 

A morphological spectrum of vertebral structure, as I thought of it 15 years ago. The Triassic saurischians described in the new paper by Aureliano et al. 2022 would sit between Arizonasaurus and Barapasaurus. (Wedel 2007: text-fig. 8)

“Hold up”, I can hear you thinking. “You can’t just draw a straight line between the internal structure of the vertebrae in Pampadromaeus, on one hand, and Apatosaurus, or a friggin’ saltasaurine, on the other. They’re at the opposite ends of the sauropodomorph radiation, separated by a vast and stormy ocean of intermediate taxa with procamerate, camerate, and semicamellate vertebrae, things like Barapasaurus, Haplocanthosaurus, Camarasaurus, and Giraffatitan.” That’s true, and the vertebral internal structure in, say, Camarasaurus doesn’t look much like either Pampadromaeus or Ibirania — at least, in an adult Camarasaurus. What about a hatchling, which hasn’t had time to pneumatize yet? Heck, what about a baby Ibirania or Rapetosaurus or Alamosaurus? Nobody knows because nobody’s done that work. There aren’t a ton of pre-pneumatization baby neosauropod verts out there, but there are some. There’s an as-yet-unwritten dissertation, or three, to be written about the vascular internal structure of the vertebrae in baby neosauropods prior to pneumatization, and in adult vertebrae that don’t get pneumatized. If caudal 20 is the last pneumatic vertebra, what does the vascular internal structure look like in caudal 21?

Cervical vertebrae of Austroposeidon show multiple internal plates of bone separated by sheets of camellae. Bandeira et al. (2016) referred to those as ‘camellate rings’, Aureliano et al. (2021) called them ‘internal plates’, and in the new paper (Aureliano et al. 2022) we call similar structures in apneumatic vertebrae ‘layered trabeculae’. (Bandeira et al. 2016: fig. 12)

To me the key questions here are, first, why does the pneumatic internal structure of the vertebrae of titanosaurs like Ibirania — or Austroposeidon, shown just above in a figure from Bandeira et al. (2016) — look like the vascular internal structure of the vertebrae of basal sauropodomorphs like Pampadromaeus? Is that (1) a kind of parallelism or convergence; (2) a deep developmental program that builds vertebrae with sheets of bone separated by circumferential and radial spaces, whether those spaces are filled with marrow or air; (3) a fairly direct ‘recycling’ of those highly structured marrow spaces into pneumatic spaces during pneumatization; or (4) some other damn thing entirely? And second, why is the vertebral internal structure of intermediate critters like Haplocanthosaurus and Camarasaurus so different from that of both Ibirania and Pampadromaeus— do the pneumatic internal structures of those taxa reflect the pre-existing vascular pattern, or are they doing something completely different? That latter question in particular is unanswerable until we know what the apneumatic internal structure is like in Haplocanthosaurus and Camarasaurus, either pre-pneumatization (ontogenetically), or beyond pneumatization (serially), or ideally both. 

A Camarasaurus caudal with major blood vessels mapped on, based on common patterns in extant tetrapods. A list of the places where blood vessels enter the bone is also a list of places where sauropod vertebrae can possibly be pneumatized. We don’t think that’s a coincidence. From Mike’s and my presentation last December at the 3rd Palaeo Virtual Congress, and this post. (Wedel and Taylor 2021)

I was on the cusp of writing that the future of pneumaticity is vascular. That’s true, but incomplete. A big part of figuring out why pneumatic structures have certain morphologies is going to be tracing their development, not just the early ontogenetic stages of pneumatization, but the apneumatic morphologies that existed prior to pneumatization. BUT we’re also nowhere near done just doing the alpha-level descriptive work of documenting what pneumaticity looks like in most sauropods. I’ll have more to say about that in an upcoming post. But the upshot is that now we’re fighting a war on two fronts — we still need to do a ton of basic descriptive work on pneumaticity in most taxa, and also need to do a ton of basic descriptive work on vertebral vascularization, and maybe a third ton on the ontogenetic development of pneumaticity, which is likely the missing link between those first two tons.

I’m proud of the new paper, not least because it raises many, many more questions than it answers. So if you’re interested in working on pneumaticity, good, because there’s a mountain of work to be done. Come join us!

References

Posted by Matt Wedel
Filed in cross sections, CT, pneumaticity, prosauropod
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Apatosaurus cervical ribs and the tyranny of 2D images

December 8, 2022

Some quick backstory: lots of sauropods have long, overlapping cervical ribs, like the ones shown here in Sauroposeidon (diagram from this old post):

These long cervical ribs are ossified tendons of ventral neck muscles, presumably longus colli ventralis. We know they’re ossified tendons because of their bone histology (Klein et al. 2012), and we suspect that they’re longus colli ventralis because those tendons look the same in birds, just less ossified, as in this rhea (same specimens as these even older posts: 1, 2):

Diplodocoids have apomorphically short cervical ribs, which never extend very far past the end of their respective centra and sometimes don’t overlap at all. Still, we assume the long ventral neck muscles were there, just without long ossified tendons. Which brings me to Apatosaurus, which has cervical ribs that are anteroposteriorly short but famously massive, extending very below and/or to the sides of the cervical centra — for a truly breathtaking example see this post. Here are C3 through C7 in CM 3018, the holotype of Apatosaurus lousiae (Gilmore 1936: plate 24):

At least for me, it’s hard to resist the temptation to mentally scoot those vertebrae together into articulation, and imagine that the very swoopy-looking and maybe even down-turned cervical ribs allowed the ventral tendon bundles to wrap around the bottom of each cervical rib protuberance, something like this:

But it’s just not so, because like all 2D images, Gilmore’s plate distorts 3D reality. If you get to see the mounted skeleton in person, it’s clear that the cervical ribs are all more or less in line, and none of them are pointed at the big protuberances, which stick way out ventrolaterally.

Here I’ve drawn in the likely trajectories of the longus colli ventralis tendons. My little red pathways don’t precisely match the cervical ribs as mounted, but there’s a lot of distortion and restoration going on. For example, comparing with Gilmore’s plate we can see that the cervical ribs of C5, which point downward compared to all the others, only do that because someone forced them to — the whole anterior portion of the rib, where the shaft would actually join to the capitulum and tuberculum, is reconstructed. Even if I’m a little off, it’s clear that the cervical ribs shafts point backward, they’re all more or less in two parallel lines, and none of them point down and out toward the ventrolateral processes. The photo contains a mountain of useful morphological information that you’d never get from the lateral views.

My takeaways from all this:

  1. If a person has only seen 2D images of a specimen, and especially if those 2D images have only been orthogonal views with no obliques, their little island of knowledge is surrounded by at least a sizeable lake of ignorance, if not a small ocean.
  2. That doesn’t mean that seeing specimens in person is the only antidote — 3D models and 3D prints are extremely useful, and for specimens that are difficult to manipulate because of their size or fragility, they may be more useful than seeing or handling the specimen, at least for some questions.
  3. For Apatosaurus specifically, those ventrolateral processes cry out for explanation. They’re fairly solid knobs of bone that stick way out past the ossified tendons of the ventral-most neck muscles. That’s a super-weird — and super-expensive — place to invest a bunch of bone if you’re not using it for something fairly important, especially in a lineage that had just spent the last 80-100 million years making their necks as light as possible.
  4. Pursuant to that last point, we’re now in — ugh-ouch-shame — our 8th year of BrontoSMASH!!, with still just the one conference presentation to show for it (Taylor et al. 2015). Prolly time we got moving on that again.

References

Posted by Matt Wedel
Filed in #Brontosmash, Apatosaurus, Carnegie Museum, cervical, cervical ribs, diplodocids, mounts, museums, necks, rhea, stinkin' theropods
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The cylindrical zygapophyses of Trionyx spinifera

December 7, 2022

I was googling around some photos, confirming to myself that turtles don’t have cervical ribs, when I stumbled across this monstrosity (and when I use that word I mean it as a compliment):

Softshell turtle Trionyx spinifera, cervicodorsal transition in ventral view, anterior to right. Copyright © Mike Dodd, used by kind permission. Original at https://www.amanita-photolibrary.co.uk/animals/trionyx_spinifera_1496.htm

The specimen is from the collection amassed by Caroline Ponds, formerly a reader in Zoology at Oxford, who picked up most of her specimens as roadkill in Milton Keynes. She has donated this collection to WildCRU (Wildlife Conservation Research Unit) just outside Oxford, just 90 minutes away from me.

The hot news here is of course the zygapophseal articulation between what I am interpreting as the last cervical and the first dorsal. Let’s take a closer look:

As you can see the prezygapophyses of the first dorsal are cylindrical, wrapping smoothly around from a fairly traditional anterodorsal-facing aspect through anterior, anteroventral, ventral, and even posteroventrally-facing. There is no hint of inclination towards the midline as in sane prezygapophyses.

And, providing perfect mates to those prezygs, the postzygapophyses of the last cervical wrap around producing a negative cylinder that encloses the positive one.

This leaves me with questions. Lots of them. For example:

  1. Did I even identify the vertebrae right, or is that “first dorsal” really the last cervical, based on its not carrying a rib? It looks like it’s trying to bear a rib, but not quite carrying it off. (For now I will assume my identification is correct.)
  2. What is the centrum articulation like here? Sadly, it’s obscured in the photo. My guess would be positive cylinder on the front of the dorsal, and a small contact point on the back of the cervical — but it really is just a guess.
  3. Is this unique to Trionyx spinifera, or do all cryptodiran turtles do this to some extent?
  4. If this condition is common among cryptodires, are there  species that take it to an even greater extreme?
  5. What do pleurodire turtles do here?
  6. Why haven’t I spent more of my like looking at the cervicodorsal transitions of turtles?
Posted by Mike Taylor
Filed in 100% totally real, cervical, dorsal, help SV-POW!
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Here’s that ventral-view apatosaur cervical anaglyph you ordered

December 4, 2022

Just to wash our mouths out after all the theropod-related unpleasantness yesterday:

What we’re seeing here, in glorious 3D, is the 7th cervical vertebrae of BYU 1252-18531. This is an apatosaurine at the Brigham Young University Museum of Paleontology which the museum has catalogued as “Apatosaurus excelsus” (i.e. Brontosaurus excelsus), and which Tschopp et al. (2015) tentatively referred to Brontosaurus parvus, but which I suspect is most likely good old Apatosaurus louisae.

It’s in the rarely seen ventral view, which really emphasizes the ludicrously over-engineered cervical ribs. Get your 3D glasses on and marvel at how they come lunging out of the screen at you, like giant insects in a 1950s B-movie.

So beautiful.

Posted by Mike Taylor
Filed in Apatosaurus, BYU Museum of Paleontology, cervical, collections, ventral views
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How did baby Utahraptor caudals articulate? The answer will sicken and disgust you

December 3, 2022

Over on Mastodon (sign up, it’s great!), Jim Kirkland posted a baby Utahraptor caudal vertebrae for #FossilFriday. Here it is:

And after a bit of virtual prep work:

My first reaction was just “That’s pretty!“. My second, which I admit should have been my first, was “Wait a sec — how the heck do those things articulate?”

The issue is that both the prezygs and the postzygs overhang the centrum by so much. If we imagine three of these babies consecutively, there are basically two options.

First, the centra articulate closely, with what we might feel intuitively is a reasonable cartilage gap; and the zygs cross over:

Does something like this ever happen? Not in sauropods, for sure, but it could be correct — if the zyg facets are some way short of the tips of their processes, so that the most distal parts of each process are pre-epipophyses and epipophyses rather than prezygs and postzygs per se.

The other interpretation is this, with the zygs overlapping near the end as in sensible dinosaurs, and much more spaced out centra:

If this is right, then (in this respect) baby Utahraptor tails resembled camel necks in having big intervertebral spaces, which in life were filled with big cartilage plugs.

 

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SPOILER SPACE

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Have a think about this before reading on.

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SPOILER SPACE

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OK, here is the horrible truth.

Dromaeosaur tails do overlap their zyg processes as in the first mock-up above: but they do much, much worse than this!

Here is the truly perverted figure 37 of Ostrom’s classic 1969 monograph on Deinonychus — the publication that catalysed the whole Dinosaur Renaissance:

As you can see, the zygapophyseal processes are grotesquely elongated, and overlap in long stiffening bundles with those of successive vertebrae (part C of the figure). The actual zyg facets are small, and close to the origins of these processes (see parts A and B of the figure). And the chevrons are also hideously protracted beyond their natural length to form stiffening bundles beneath the tail that complement those above the tail.

To add insult to injury, the chevrons even face in the wrong darned direction, extending anteriorly along the tail rather than posteriorly as in all decent animals. Yes: in Ostrom’s illustrations, we’re seeing the vertebrae in right lateral view, i.e. anterior is to the right.

All of this confirms that I was so, so right two decades ago to focus so completely on proper dinosaurs instead of these nasty mutant ones. Ugh.

Posted by Mike Taylor
Filed in cartilage, caudal, stinkin' theropods
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