Monthly Archives: March 2023

New paper out today: Aureliano et al. (2023) on pneumaticity in the early dinosaur Macrocollum

Figure 1. Skeletal reconstruction of the unaysaurid sauropodomorph Macrocollum (CAPPA/UFSM 0001b) showing vertebral elements along the spine and putative reconstruction of the air sac systems involved. (a) Pneumatic posterior cervical vertebra and a cross-section CT slice in b. (c) a pneumatized anterior dorsal vertebra with cross-section CT slice in d, and detail of the pneumatic foramen in e. (f) Detail of the pneumatic foramen in a reconstructed 3D model of the element. (g) Anterior cervical element (apneumatic). (h) Posterior dorsal vertebra shows no traces of PSP. The sacral series (i), as well as the anterior (k) and mid-caudal (j) series are apneumatic. a, g, h, j, and k are in left lateral view. c, e and f are in right lateral view. i is in dorsal view. ABD, abdominal diverticula; CER, cervical diverticula; LUN, lung; pf, pneumatic foramen. The reconstruction was made by Rodrigo T. Müller. Scale bar of the skeletal reconstruction = 500 mm; a–j = 20 mm. (Aureliano et al. 2023)

New paper out today:

Tito Aureliano, Aline M. Ghilardi, Rodrigo T. Müller, Leonardo Kerber, Marcelo A. Fernandes, Fresia Ricardi-Branco, Mathew J. Wedel. 2023. The origin of an invasive air sac system in sauropodomorph dinosaurs. The Anatomical Record https://doi.org/10.1002/ar.25209

This paper is basically the second part of a one-two punch with our paper on vertebral internal structure in early saurischians from last December (Aureliano et al. 2022). In that paper we found no evidence of invasive pneumaticity in the basal sauropodomorphs Buriolestes and Pampadromaeus, nor in the herrerasaurid Gnathovorax, although we did find some pretty interesting non-pneumatic anatomy inside the vertebrae. In this study we did find invasive pneumaticity in the basal sauropodomorph Macrocollum — but not in the way that I expected.

I’ve been noodling around about the origins of pneumaticity in saurischian dinosaurs for a while now. Early on, I expected that the origin of pneumaticity would be found in the lateral fossae in the centra of presacral vertebrae. I even drew a figure illustrating that hypothesis in my 2007 prosauropod pneumaticity paper:*

TEXT-FIG. 8. Diagram showing the evolution of fossae and pneumatic chambers in sauropodomorphs and their outgroups. Vertebrae are shown in left lateral view with lines marking the position of the cross-sections, and are not to scale. The omission of ‘prosauropods’ from the figure is deliberate; they have no relevant apomorphic characters and their vertebrae tend to resemble those of many non-dinosaurian archosaurs. Cross-sections are based on first-hand observation (Giraffa and Arizonasaurus), published sections (Barapasaurus, Camarasaurus and Saltasaurus) or CT scans (Apatosaurus and Haplocanthosaurus). Giraffa based on FMNH 34426. Arizonasaurus based on MSM 4590 and Nesbitt (2005, fig. 17). Barapasaurus based on Jain et al. (1979, pls 101–102). Apatosaurus based on CM 11339. Haplocanthosaurus based on CM 572. Camarasaurus based on Ostrom and McIntosh (1966, pl. 24). Saltasaurus modified from Powell (1992, fig. 16). (Wedel 2007)

*When I announced the publication of that paper to friends and colleagues, I quipped, “Were prosauropods pneumatic? The fossils don’t say. Somehow I stretched that out to 16 pages.” Mike later told me that because of that self-deprecating description, he’d never been able to take that paper very seriously.

Yates et al. (2012) blew up that clean hypothetical sequence. The best available evidence at the time showed that pneumaticity was actually pretty widespread in basal sauropodomorphs, but the most diagnostic pneumatic features were not on the centrum. Rather, they were the laminae and subdivided fossae just ventral to the diapophyses. 

Fig. 9. Middle posterior dorsal vertebra of Antetonitrus ingenipes (BP/1/4952); A, right lateral; B, posterior views; C, left posterior infradiapophyseal fossa; D, right posterior infradiapophyseal fossa in oblique posterolateral and slightly ventral views; E, Close up of invasive left posterior infradiapophyseal subfossa. Abbreviations: cpol, centropostzygapophyseal lamina; dp, diapophysis; hs, hyposphene; il, internal lamina; midf, middle infradiapophyseal fossa; nc, neural canal; ncas, neurocentral articuloar surface; ns, neural spine; pcdl, posterior centrodiapophyseal lamina; pidf, posterior infradiapophyseal fossa; podl, postzygadiapophyseal lamina; poz, postzygopophysis; pp, parapophysis; prz, prezygopophysis; sf, subfossa. Scale for A, B, C and D, 100 mm; for C, 20 mm. (Yates et al. 2012)

That finding would dovetail with my work with Jessie Atterholt on paramedullary diverticula in birds and other dinosaurs (finally published last year but gestating much longer; Atterholt and Wedel 2022) and with my work with Mike on the developmental sequence of spinal cord -> spinal arteries -> pneumatic diverticula (Taylor and Wedel 2021), culminating in this figure:

Figure 4. Fossae and foramina adjacent to the neural canal in ornithodiran archosaurs. Fossae are shown in dark grey, foramina in black. Neural canals are labelled “nc”. A: Pterosauria, represented by cervical vertebra 9 of Pteranodon sp. YPM 2767 in anterior view (traced from Bennett 2001: figure 42). B: Theropoda, represented by dorsal vertebra 14 of Allosaurus fragilis UUVP 6000 in anterior view (traced from Madsen 1976: plate 23). C: Basal Sauropodomorpha, represented by a posterior dorsal vertebrae of Aardonyx celestae BP/1/6566 in posterior view (traced from Yates et al. 2012: figure 7). D: Neosauropoda, represented by cervical vertebra 5 of Diplodocus carnegii CM 84 in posterior view (traced from Hatcher 1901: plate 6). (Taylor and Wedel 2021)

…and this passage (Taylor and Wedel 2021: p. 8):

It is also notable that paired pneumatic fossae or foramina occur lateral or dorsolateral to the neural canal in every archosaurian clade with postcranial pneumaticity (Figure 4). These fossae and foramina occur in taxa with and without lateral cavities in the centra, and with and without laminated neural arches, so they are probably the most consistent osteological correlates of pneumaticity across non-avian ornithodirans. The consistent appearance of vertebral pneumaticity in areas adjacent to the neural canal corroborates the hypothesis that segmental spinal arteries were crucial in “piloting” pneumatic diverticula as they developed.

But I never looped that back to prosauropods. For a long stretch — 10 years — I wasn’t working on prosauropods or the origin of pneumaticity, in part that was because I was working on other things, but more importantly, because I had no new data on prosauropods. Then Tito Aureliano invited me to collaborate, and here we are. 

What’s surprising to me about the pneumaticity in Macrocollum is that although some of the vertebrae have pneumatic fossae in their centra, the most consistent and most invasive pneumaticity is in the neural arches. Arguably I should have seen that coming, especially after the bit I just quoted about how pervasive is pneumaticity adjacent to the neural canal. But even after that, I thought of neural arch pneumaticity as a sort of sideshow or opening act, just warming things up before the real pneumatization took off in the centrum.

Figure 3. Micro-CT scan of the anterior (second) dorsal vertebra of the unaysaurid sauropodomorph Macrocollum (CAPPA/UFSM 0001b). (a) and (b) show cross-sections of the entire vertebra in anterior view at the approximate midpoint. (e) and (f) show midshaft slices in lateral view. (f) shows three fossae in the neural arch (cprf, cdf and cpof). c, centrum; cdf, centrodiapophyseal fossa; cdl, centrodiapophyseal lamina; ctr, chaotic trabeculae; cpof, centropostzygapophyseal fossa; cpol, centropostzygapophyseal lamina; cprf, centroprezygapophyseal fossa; d, diapophysis; dia, diagenetic artifact; nc, neural canal; ncf, neural canal foramen; pf, pneumatic foramen; po, postzygapophysis; pocdf, postzygapophysealcentrodiapophyseal fossa; pr, prezygapophysis; prcdf, prezygapophysealcentrodiapophyseal fossa; ptc, protocamera; s, neural spine. Scale bar = 10 mm.

Not so, says Macrocollum. Some of the centra have deeply incised lateral fossae, which can be strikingly asymmetrical, but lots of the vertebrae have foramina up under the diapophyses that communicate with pneumatic chambers inside the neural arch. Chambers, plural, in a complex arrangement. That’s a pretty amazing thing to find in such an early sauropodomorph.  And it’s especially exciting to me because it means that possibly I’ve been conceiving of the evolution of vertebral pneumaticity precisely backwards, for decades. I’d much rather be wrong in an interesting way than right in a boring way — especially if I get to be an author on the paper that surprises me.

Here’s my takeaway thought: loads of prosauropods and early theropods have fossae up under the diapophyses. Heck, externally, that’s about all you can see in Macrocollum. And as Yates et al. (2012) pointed out, those fossae are not often prepared completely. But CT reveals that in Macrocollum, those fossae house foramina that communicate with internal chambers. Maybe that form of pneumaticity is actually widespread, and we (= humans) don’t know because we haven’t scanned very many things yet. The horizon is open, and the story can only get richer and stranger from here. What a delightful thing to realize after doing this for 25 years.

References

Would we know about flow-through lungs if birds were extinct?

Last night a thought occurred to me, and I wrote to Matt:

If birds had gone extinct 66 Mya along with all the other dinosaurs, would it ever have occurred to us that they had flow-through lungs? Is there — can there be, outside of amazing soft-tissue preservation — any way for bone fossils to tell us about this?

(Yes, we have evidence for air-sacs in the pneumatization of vertebrae and other bones, but I doubt that would have led us to the idea of the flow-through lung. I’m not even convinced it would have led us to the idea of air-sacs, if we didn’t have extant birds as a model.)

Matt wrote back and gave me permission to write up his reply into an SV-POW! post, which you are now, obviously, reading. Here’s what he said.

No, we’d have no idea about the flow-through lungs from fossils.

In fact, it’s particularly bad for birds. Big saurischian dinosaurs had lots of postcranial skeletal pneumaticity (PSP), and some extant birds have a lot of PSP, but most Mesozoic birds have limited to zero diagnostic PSP. A few have some external foramina on the vertebrae that might be pneumatic, but might just be lateral foramina for the equatorial arteries. It doesn’t help that most Mesozoic birds are smashed flat and often have other elements overlapping the vertebrae — most often the proximal portions of their own ribs.

So ironically, even if we somehow came up with the stacked notions that (1) PSP implied air sacs, and (2) air sacs implied flow-through lungs, we’d be much more likely to infer flow-through lungs in Diplodocus and Tyrannosaurus than in Archaeopteryx or most other Mesozoic birds.

But wait, it gets worse! The work by Colleen Farmer, Emma Schachner, and colleagues that demonstrated unidirectional flow in the lungs of crocs, monitor lizards, and iguanas would presumably still get done, but those animals have flow-through lungs without PSP and without particularly elevated metabolisms (although monitors are trying hard). Without the example of birds showing us how that primitive flow-through system can be further refined and supercharged to power tachymetabolism, we’d still learn of flow-through lungs, but we’d have no reason to connect them to PSP or any particular metabolic strategy.

I’ve probably mentioned this before, but it really irks me that we assume that birds are the pinnacle of lung evolution. Why? Birds survived the K/Pg extinction because they were small and could hide and eat seeds and grubs for a while, not because they had better lungs than everything else (otherwise mammals, lizards, etc. would have done even worse). To me it would be a heck of a coincidence if the one group of ornithodirans that survived — for reasons unrelated to lung function — just happened to have the most efficient lungs. It’s always been tantalizing to me that extant birds start out with 12 embryonic air sacs, which through development usually merge into the usual 9 (unpaired clavicular, and paired cervical, anterior thoracic, posterior thoracic, and abdominal sacs). This seems like an embryonic footprint of a greater diversity — and possibly even a greater complexity — of respiratory anatomy in the ancestral ornithodiran, saurischian, or theropod (or all of the above).

We were probably wrong about caudal pneumaticity in Ca13 of the Brontosaurus excelsus holotype YPM 1980

This is one of those posts where the title pretty much says it all, but here’s the detailed version.

Recap: the 2013 paper

In Matt’s and my 2013 paper Caudal pneumaticity and pneumatic hiatuses in the sauropod dinosaurs Giraffatitan and Apatosaurus (Wedel and Taylor 2013b), we wrote about the Brontosaurus excelsus holotype 1980:

Much more convincing, however, are two isolated lateral fossae: one on the left side of caudal 9, the other on the right side of caudal 13 (Figure 10). Both of these are much larger than the aforementioned foramina – about 6 cm across – and have distinct lips. There is absolutely no trace of similar fossae in any of the other caudals, so these fossae represent a bilateral pneumatic hiatus of at least seven vertebrae

And we illustrated the right side of Ca13 in our figure 10:

Wedel and Taylor (2013:figure 10). An isolated pneumatic fossa is present on the right side of caudal vertebra 13 in Apatosaurus excelsus holotype YPM 1980. The front of the vertebra and the fossa are reconstructed, but enough of the original fossil is visible to show that the feature is genuine.

Fast forward to 2023

The Yale Brontosaurus has been dismounted and sent to RCI in Canada for some long overdue TLC. It’s being re-prepared, and Brian Curtice has seen the material close up. The news from Brian is not good: I quote some of his emails. First, on 26 January:

The 1980 caudal 13 it isn’t pneumatic. That whole hole is plaster. The 2 verts in front of it have similar damage but on the opposite side. It looks like they were damaged during preservation, excavation, or preparation.

Then on 27 January:

Quick caudal pneumatic update: other than the fact 1980 has a large number of what I dub nutrient foramina there isn’t any shiny surfaces, no odd sculpting, fluting, etc. the bone is exquisite in these areas but will soon be painted black.

Later that day:

It was also exceptionally difficult to sometimes tell what was actual bone. Barbour [1890 — ed.] is spot on at what Marsh had done. The preparators sometimes couldn’t be sure without acetone and an air scribe… I did the best I could but my goodness it was tough and may have errors. Thus I stayed towards what I was positive on.

On 3 February, I wrote back to Brian asking:

My question about the “pneumatic fossa” in caudal 13 is: why did they sculpt it like that? It would have been the simplest thing in the world to give it a simple flat lateral aspect, like the other caudals, so what made them put the fossa in? One possible answer is that that’s what the bone was actually like, but smashed up, and they “repaired” it. I guess we are unlikely ever to know.

He replied the same day:

There are 3 caudals (11-13, pics attached) with similarly damaged bone, punky and smashed and “beat up”, with 11 and 12 having the damage on the left and ventral and 13 on the right. I suspect they were lying close to one another. I couldn’t tell if it was trampling, but it didn’t seem like it was from being hacked from the ground.
[…]
As to why they did it? I suspect because 13’s damage wasn’t as jagged, they could plaster over it easier? We’ll never know for sure.

Brian sent a photo of the re-prepared caudal 13, showing … well, see for yourself:

Truthfully, I don’t find this especially compelling. But that’s about the inadequacy of photos for this kind of work. My inclination is to trust Brian’s interpretation, while wondering how Matt and I were both fooled back in June 2012 when we visited YPM together and spent significant time gazing at this caudal.

So what now?

The good news for us is that this doesn’t really change any of our arguments or conclusion in the 2013 paper. We said that there is previously undocumented evidence of caudal pneumaticity in apatosaurines[1] — and there still is, in the other specimen we figured, FMNH P25112, in our figure 9. And the significant conclusion of the papers was the intermittent and unpredictable pneumatization along the tails of sauropods is compelling evidence for extensive “cryptic pneumaticity” — that is, for soft-tissue pneumatization alongside vertebrae that did not penetrate the bone. That conclusion is still good.

But still: one of the data-points we relied on in making that argument no longer looks solid, and it feels like the honest thing is to document that. It probably doesn’t warrant a follow-up paper or even an erratum. But it does warrant a blog-post, and this is it.

Thanks to Brian for bringing it to our attention!

Notes

[1]. In the paper we said “in Apatosaurus“, not “in apatosaurines”. But that was back when Apatosaurus was the only recognized apatosaurine, so it amounted t0 the same thing. If we were writing it in the post-Tschopp-et-al. world of today, we’d say “in apatosaurines”.

References

 

The equine interspinal ligaments of Ray Wilhite

Our old friend Ray Wilhite sent us this glorious photo of a horse neck that he dissected recently, with permission to post here:

The big yellow sheet at the top is the nuchal ligament, which in many mammals provides axial tension for the cervical vertebrae, and which has been hypothesized (e.g. by Alexander 1985:13) to have existed and provided similar support in at least some sauropods.

But what caught Ray’s eye was the smaller interspinal ligaments running horizontally between the neural spines of the consecutive vertebrae. The literature doesn’t talk about these much because the irresistible glamour of the nuchal ligament grabs everyone’s attention, but they’re there in pretty much everything, being primitive for tetrapods.

Here they are again in absolutely glorious detail. (Seriously, click through for the full-sized version. You can all but make out individual cells.)

Many thanks to Ray for sharing these photos with us!

An arresting image of an apatosaur vertebra

Here’s a cool photo of an apatosaur cervical in anterior view. This is from R. McNeill Alexander’s wonderful book Bones: The Unity of Form and Function, which was published in 1994. The whole book is packed with gorgeous full-color photos like this, and you can still get new copies for cover price (f’rinstance).

I remember stumbling across this image not long after I started working on sauropod vertebrae back in the late 90s, and being completely taken aback by the size of the cervical ribs. Up to that point I’d mostly been grokking the long, graceful cervicals of brachiosaurs, and the ridiculously overbuilt apatosaurine cervical morphology was a real kick in the brainpan. That’s well-trod ground here at SV-POW!, but this is still a beautiful photo. I suspect that the vertebra has been at least somewhat restored — some of the texturing on the condyle and under the diapophyses looks suspiciously like it was applied with tools or maybe just human fingers — but in general this is a pretty faithful representation of what an apatosaur cervical looks like from the front.

One thing that always strikes me about views like this is that you could take the centrum of this vertebra, strip off the neural arch and all the apophyses, and stick it through either one of the cervical ribs loops without scraping the sides. If life, the cervical rib loops held the (comparatively small) vertebral arteries and the (comparatively gigantic) intertransverse diverticula. We know this because that’s how birds are built, and because different apatosaurine specimens show pneumatic traces almost all the way around the inside of the cervical rib loop. The same is true in theropods like Majungasaurus, as Pat O’Connor showed in a lovely figure in his 2006 paper (O’Connor 2006: fig. 16). The volume of air in each of the paired cervical rib loops would have simply dwarfed the volume of air inside or even alongside the centrum. I wanted to visualize that better so I took my trusty old CT cross-section of OMNH 1094 and pasted it on top of this vert, stretching it a bit in GIMP to improve the fit:

Another thing that this photo shows nicely are the pneumatic fossae on the anterior surfaces of the cervical ribs. I’ve seen those features on loads of apatosaur cervical ribs, but I’ve never seen them discussed anywhere. I have thoughts on why those fossae are there, but that story will have to keep for another time.

References