So onto the last major post about Bellubrunnus – the tail. The tails of rhamphorhychines are interesting as they have a somewhat unusual anatomy. Just like the dromaeosaurs (though obviously, convergently acquired) rhamphorhynchines have elongate zygopophyses and chevrons that overlap multiple vertebrae and bind the tail together so that it is a relatively stiffened structure. These extensions are basically rods of bone, and can long enough to overlap the four or five adjacent vertebrae to their origin.
However, Bellubrunnus appears to be unique among rhamphorhynchines in lacking these structures. It still has zygopophyses and chevrons (as can be just about seen in the figure) and compared to some pterosaurs at least these are long, but they are a fraction of the length of other closely related species. This begs the obvious question, of what happened – are these really reduced or is there some other factor at play? Well, there are a number of possible hypotheses to explain this and here I’ll go through them and why we have come to the conclusion that this is a genuine feature. Not all of the chevrons are there for sure, so some have been lost or remain cryptic for whatever reason, but several are quite well preserved and so the below discussions refer to the issue of the size and shape rather than presence / absence of chevrons.
First off, were these simply not properly ossified prior to preservation and were there, just as unpreserved cartilage? This seems really unlikely, even the smallest and youngest specimens of Rhamphorhycnhus (which includes specimens even smaller than Bellubrunnus) have fully ossified chevrons and zygopophyses. These seem to be present at a very early age in the pterosaurs that have them and so it would be odd if they had a different ossification pattern here. Indeed it would be doubly odd since if anything, Bellubrunnus has better ossification of its elements than is usual for small pterosaurs, with well-preserved and ossified tarsals and sternum. So it is far more likely that they are fully ossified, they are just much smaller.
Could these have been lost through decomposition or disassociation from the specimen, or may not have been preserved even if they were ossified? While a few pieces have begun to disarticulate and move (the gastralia and a few dorsal centra and the prepubes) nothing else on the specimen has really begun to move and indeed the caudal vertebrae as a whole is well articulated. It would be odd indeed if the only thing to have rotted or moved was the chevrons or parts of them, while the rest remained intact and complete. Similarly, with even tiny parts like the delicate palate and gastralia being preserved (and indeed at least a couple of chevrons) it also seems unlikely that these somehow resisted preservation when everything else is there. Certainly it’s possible, but even so, some are still there and the loss of many chevrons would not affect the shape of the ones that have survived and wouldn’t affect the zygopophyses.
Were these destroyed or modified through preparation? This is a tricky one to answer, but again, there’s no obvious reason to think so. The preparation job is superb (look at the skull!) and was done with great care and attention to detail. It’s always possible that thin and delicate structures were lost, but while the matrix has been all but scraped clean, at least some have survived (and again lots of other delicate things) and I find it hard to imagine they were modified in such a consistent manner.
Are these genuinely distinct then? That is the obvious conclusion given the problems with the other hypotheses. However, there is more than just negative arguments against these other ideas, but there are reasons to positively support the idea that these are genuinely short. Although greatly reduced in length, the anatomy of both the zygopophyses and chevrons is otherwise well consistent with the anatomy of these features in other rhamphorhychines. The zygopophyses are rather rod-like and then taper abruptly to a point, just as we see in others, only with a much shorter rod. And similarly the chevrons are long and thin and splint-like, terminating in a point at each end, just as we see in others, only again being rather shorter.
While as noted we are rather short of chevrons for whatever reason, those that remain and indeed the zygopophyses seem to have a genuinely distinct morphology to other rhamphorhynchines, including similarly small and young specimens of Rhamphorhycnhus. This then is a major anatomical difference that separates Bellubrunnus from its nearest relatives as well as providing a little more interest and intrigue in the origins of this structure since while it is present on all other rhamphorhycnhines, it’s also present in basal pterosaurs outside the group and so may well have had multiple origins and losses.