Everything you didn’t think to ask about the pterosaur sternum (and were afraid to ask)

Pterosaurs flew! No big shock there, but obviously flight places major constraints and selective pressures on the skeleton and we see that with the incredibly conservative nature of the pterosaur skeleton as a whole. So one would think that the associate flight apparatus in particular would be especially conservative and say more constrained than the feet or the neck, but it turns out an absolutely critical part of pterosaur anatomy is both basically all but unstudied and wildly variable, yes, it’s the sternum.

To try and correct that, I’ve just published a huge paper cataloguing and describing basically every sternum for every pterosaur out there. I’ve deliberately not covered every known one for a couple of very well-represented taxa like Rhamphorhynchus (where there’s a dozen or so known) but every taxon with a sternum (more than 60 it turns out!), however incomplete, is included and there are technical drawings of all of the well preserved ones. In this regard I need to give a massive, massive, massive shout-out to Skye McDavid who did all the technical illustrations for this paper and is a major reason why it looks so nice and I think helps communicate the anatomy of these bones. See her work and commission her to draw for you here.  Also a quick thank you to Rene and Bruce Lauer of the Lauer Foundation for providing access to, and photos of, a couple of really useful specimens that filled in a gap for me.

There’s been only a handful of descriptions of pterosaur sterna ever described properly. Hunting though the literature I repeatedly came across one line notes about it, even when one was well-preserved and featured in a photograph and only a couple of papers have looked at them in detail (and then not said much to be honest). Phylogenetic analyses of pterosaurs regularly included no sternum traits or only one or two, less than many simple traits like the unguals or pteroid. This is not a well-studied piece of the skeleton, despite it anchoring all the major flight muscles of (checks notes) a clade of flying animals! And ones that also were quadrupedal, so the sternum (and how it fits to the coracoids) and the associated musculature is also critical for terrestrial locomotion as well. This is the sort of thing that pterosaur works should probably not be overlooking!

What astounded me though, as hinted above, is just how incredibly variable they are between and *within* species. For an animal normally so limited in variation this is a key feature which is tremendously varied in overall shape and appearance and with loads of different details in the size, shape, arrangement and thickness of all kinds of bits to it that will affect where and how the coracoids fit, the muscles attach and the shape of the chest as a whole.

However, a major part of this seems to come down to the fact that the sternum is generally really poorly ossified and in fact I suggest it is often primarily cartilaginous in most animals (certainly juveniles) and only becomes bone, and thin bone at that, in near adult animals. That would explain a lot of the variation seen and the often complete absence of the sternum as a whole (or at least the sternal plate) in even some extremely well-preserved pterosaurs that aren’t missing any other features at all. That answers some questions (why the variation) but opens up others. Given how well ossified the flight apparatus is for even embryonic pterosaurs, how the hell have they ended up with a sternal plate of cartilage even in near mature large animals? The forces for flight muscles should be massive and the sort of thing to trigger early ossification not leave it till the last minute. And why is it so varied even in the adults where it’s well-preserved, id there a lot more going on in their muscles and so flight and ability on the ground that we have overlooked? And can we get some useful information out of this on their ecology and evolution, despite the poor preservation? These are questions I’ve left unanswered, but I am looking into them and I’d encourage others to do so as well.

I did, briefly, look at the ontogeny of the sternum and based on a nice (and so far not properly described) sternum seen under UV light it looks like the development is quite close to that hypothesised by Rupert Wild back in the 1970s based on a young Eudimoprhodon specimen. This would nicely align pterosaurs with other derived archosaurs and fits the general idea that they are indeed close to the Dinosauromorpha, but again there much more to do here.

The paper clocks in at 20 000 words and 21 figures (two thirds of which are multi-panel figures) so the MS is already very long and complex and I simply didn’t have the space or energy to get into phylogeny, origins, musculature, mechanics or pterosaur evolution in general even if I’d wanted to. Pointing out some very leading issues and hopefully priming things for future research and discussion is the best I could do after the mammoth description section but I would like to think it leaves the pterosaur sternum in a much better place than we found it and ready to spark renewed interest and research into this critical feature.

This is, to be sure, a pretty niche paper since the discussion in that context is a bit flimsy and I don’t think anyone is going to sit and read through all the descriptions for fun. But any new sternum coming up or any phylogeny or look at flight can now I think use this as a very comprehensive starting point to check what information is out there. Such ‘basic’ papers of anatomical description and illustration are so important (I use Wellnhofer’s 1970s classics and Bennett’s Pteranodon monograph almost every time I write a pterosaur paper) and so I hope this paper will add something useful in that regard. For now though, I’m mostly glad it’s off of my ‘to do’ list.

The paper is fully open access and available online here: Hone, D.W.E. 2023. The anatomy and diversity of the pterosaur sternum. Palaeontologica Electronica, 26.1.A12.