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

Monday, March 27, 2023 23:43

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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.

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