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| An azhdarchid pterosaur takes off in a festively-coloured woodland, because it's Christmas. Prints are available. |
The paper
My art accompanies the work of Nick Geist and his team on the respiratory mechanism of large pterodactyloid pterosaurs (Geist et al. 2014). Lung ventilation in pterosaurs is an interesting topic. The torso skeleton of many pterodactyloids is locked up pretty tightly thanks to their vertebrae fusing together, their scapulocoracoids being tightly braced between their sterna and backbones and a series of robust, mostly immobile ribs. How were their lungs or air sacs inflated within such a rigid skeleton? Because this configuration isn't a million miles from the torso skeletons of some birds, some authors (Claessens et al. 2009) have suggested that pterosaurs may have breathed in basically avian manner: muscles anchoring to small ribs set between the sternum and larger thoracic ribs move the sternum up and down, pumping air around the body in the process.This has been accepted fairly widely for the last five years, but now Geist et al. (2014) have presented an alternative argument. They suggest that pterosaur sternal ribs are ill suited for anchoring such muscles because they are very slender - we might even call them fragile - and often entirely cartilaginous, the latter observation borne out by their poor representation in fossil record. Indeed, large portions of the pterosaur chest seem cartilaginous and rarely preserved - the bony sterna of many species (Dorygnathus and Scaphognathus spring to mind) are tiny, and cannot possibly have supported the flight musculature indicated by their powerful shoulders and forelimbs. There must have been large cartilage extensions to these in life. Moreover, in many respects pterosaur torso construction resembles those of crocodilians more than birds, such as the manner with which the thoracic ribs articulate with the vertebrae and the essentially vertical orientation of the ribs themselves. This configuration does not permit the rib rotation required to move the sternum in respiration, and actually adds further rigidity to the anterior pterosaur torso. A bird-like respiratory mechanic may be unlikely for pterosaurs then.
So how were pterosaurs breathing, then? Perhaps the only part of their bodies which wasn't locked up tight and permitted the expansion and contraction required for breathing was their bellies. Behind the sternum sits a suite mobile bones: the belly ribs (gastralia) and the prepubes, a pair of paddle-shaped bones articulated with the pelvis, along with a few 'floating' sternal ribs. Perhaps, like crocodiles, but unlike birds, pterosaurs used this region of their body to control the pressure in their lungs. Crocodiles use contraction of their abdominal muscles to move a large, body-spanning liver forward to compress their lungs, while relaxation of their abdominal wall then allows the liver to retract and the lungs to expand, bringing in their next breath. It seems this action accounts for about 65% of air moved in and out of their lungs, with the rest coming from costal - rib - movements. Given that it seems only pterosaur bellies were flexible enough to inflict substantial changes on body volume, it is not inconceivable to think they used a similar 'belly-pump' (or extracostal pump) as their principle means of controlling air flow into their lungs.
What does this mean for pterosaur lung structure overall? It's well known that pterosaur skeletons and bodies were pneumatised to the same extent, if not more, than avian dinosaurs, prompting suggestions that pterosaurs also had solid avian-like lungs and similar unidirectional flow-through pulmonary mechanics (Claessens et al. 2009). Do the observations of Geist et al. (2014) refute this? Well, not really, but they don't support them, either. As Geist et al. point out: we really don't know anything concrete about pterosaur lung structure, and it's actually pretty hard to tell anything about them from bones alone. A bird-like lung may have been present in pterosaurs and would certainly be consistent with extensive skeletal pneumaticity. However, we need to be careful about exclusively linking extensive pneumaticity with bird-like respiratory organs: flying fish, which of course have no lungs at all, also have pneumatised skeletons thanks to outgrowths of their swim bladders (Geist et al. 2014). Moreover, our uncertainty is not helped by a general lack of knowledge about reptile lungs. This year has seen several revelations about the lungs of extant reptiles being more complex, and sometimes more avian-like, than previously thought. We might need a better handle on reptile lung diversity, and the phylogenetic distribution of different lung structures within Sauropsida, before we start making inferences about the lungs of long extinct reptile lineages. In sum, while the avian-like pterosaur lung remains a viable hypothesis, it's not the only option on the table. We might be able to gain insights into how the body cavity of pterosaurs was manipulated to move air in and out, but their precise lung anatomy remains largely mysterious (Geist et al. 2014). There's a lot more we could say about this, but you'll have to track down the full paper for further details.
The cover image
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| Festivodactylus in situ. |
This cover has been a long time coming, with Nick asking me for potential cover art for the paper at the end of last year. I duly obliged by lending the flying Anhanguera from my book. Nothing much happened while the paper was crunched through the publication mill, until in November the cover art arrived. Looked like I was due for the December issue, which, as noted above, Anatomical Record always jazzes up with festive colours - green, red and white. This involved tweaking the colours of my original art to meet these, as well as some stretching and cropping to fit the AR cover format. Without going into details, I wasn't really happy with the results. Uh oh. Pessimist I am, I foresaw the worst. I stress that these expectations weren't because of previous experience of working with Nick or AR, but my experiences with other clients and agencies. Protesting about art use normally leads to Bad Things: unhappily forced compromises, loss of commissions, or having to fix 'problems' without pay. When writing back to Nick and AR with my concerns, I pretty much expected the whole cover project to fall apart. I pitched, without optimism, the idea of doing another image, for a fee, to replace the modified one. Despite linking to the 'State of the Palaeoart' article I helped pen this year to substantiate my request for payment, I was expecting the same old response: lack of money, thanks but no-thanks.
To my complete surprise, Nick, his colleagues and AR were on board with everything. The 'palaeoart situation' was new to them all, but I - we, the palaeoart community - had their sympathy. Within a day, AR had been able to put things on hold for a week while I drafted a new image to their specifications and size, Nick and his team rapidly found a generous payment for the work at short notice, and we all ended up with a product we were happy with.
I mention all this for two reasons. Firstly, Nick, his team and AR deserve accolade for being so refreshingly cool and respectful of palaeoartistry. Secondly, independent palaeoartistry can seem a most hopeless industry at times: we get ripped off by everyone from toy companies and movie makers to museums and publishers; our marketplace is mainly structured around exploitation of individuals, and sympathy or assistance from those in the position to change this can be hard to find. But, as this case shows, it's not all hopeless. Increasing awareness of the issues facing palaeoartistry does help rectify them, change can happen, and we have more supporters than we know. I'm optimistic that eventually we'll all have more stories like this one than the negative situations currently reported so frequently.
Best to you all for the festive period, see you all in 2015!
References
- Claessens, L. P., O'Connor, P. M., & Unwin, D. M. (2009). Respiratory evolution facilitated the origin of pterosaur flight and aerial gigantism. PloS one, 4(2), e4497.
- Geist, N. R., Hillenius, W. J., Frey, E., Jones, T. D., & Elgin, R. A. (2013). Breathing in a box: Constraints on lung ventilation in giant pterosaurs. The Anatomical Record, 297, 2233-2253.