Right, lets crack straight on with dealing with the details of this lovely little thing. First off, if you have read this post on pterosaur ontogeny, you should be able to recognise that while Bellubrunnus is small (in fact it’s tiny, with a wingspan under 30 cm and a skull just a fraction over 2 cm), it’s also very young. The head is proportionally huge, and the eyes (represented by the sclerotic rings) are massive too, lots of neurocentral sutures are open (as can be seen by displaced centra in the dorsal series) and the wrists, pelvis and scapulocoracoids are unfused, and even the skull is coming apart. Unusually for such a small pterosaur though, the tarsals are well ossified, though they are rather amorphous in shape, which is a classic feature of young pterosaurs.
This then is a very young animal. That does of course complicate issues a little as obviously some things change during growth and we don’t want to misdiagnose this by thinking it has some unique features which are in fact simply a result of its age. On the other hand though, following mostly from the work by Chris Bennett, we have a good idea of the ontogenetic changes undergone by Rhamphorhynchus as it grew so we do know what kinds of things change as they grow (and so can be avoided, or used carefully) and which don’t (and can be used pretty freely).
Lets start with the most obvious thing – Bellubrunnus is a rhamphorhynchoid pterosaur. It has a proportionally small head (compared to the size of the animal as a whole, and the body specifically), short neck, short wrist and pteroid, short metacarpal IV, long tail, long 5th toe and other characteristics. Moreover, it’s also a rhamphorhynchine – a derived member of this basal assemblage and close to things like Rhamphorhynchus and the recently described Qinlongopterus. Bellubrunnus exhibits a number of characters associated with this clade such as the shape of the deltopectoral crest and the proportional length of the wing finger.
Bellubrunnus is also really rather like Rhamphorhynchus which might be no surprise given the rocks it heralds from (though more on this later) – the area is kinda famous for producing specimens this genus in serious numbers. The two share a number of characters previously used to diagnose the latter alone such as edentulous jaw tips, a femur shorter than the humuers, and an H-shaped prepubis. However, the two also have some rather notable differences that clearly mark them as different taxa. Among other things, Bellubrunnus has only 22 teeth (or perhaps even fewer) compared to some 34 in Rhamphorhynchus, it has a rather different tail anatomy (more on this to follow as well) a different humeral shape, and several major proportions of the limbs are quite different.
The point about proportions is quite a significant one. Proportional ratios between various elements are common characters for pterosaur taxonomy and systematics, but unhelpfully, we also know that some of these change during ontogeny in at least some groups. So we do need to be careful and this is no exception. The best example here is the ratio of the humerus to the femur – in Rhamphorhynchus this seems to get larger as the animal gets bigger. Small (and so young) specimens tend to have a lower ratio and adults a high one. Bellubrunnus is among the smallest pterosaurs known and comparable in size to the smallest (and youngest) specimens of Rhamphorhynchus, but it’s ratio is the higher than any specimen of the other genus.
While I’ve not mentioned it here, in the paper we do compare this to other members of the rhamphorhynchine clade and provide similar numbers of difference in things like tooth count, anatomical features, skeletal proportions and the like. A number of these taxa are rather fragmentary and hard to say too much about, but do always differ from Bellubrunnus.
So in short while we can be sure this is a very young animal, we can also be confident with our assignment to the rhamphorhycnhoids and rhamphorhynchines, and while this would appear to be a close relative of Rhamphorhynchus, it’s also clearly quite different in a number of characters, marking it out as a new and distinct taxon. One last point needs to be raised here too as a launch-pad for the next post, Bellubrunnus is from Brunn, and Brunn, well it’s not actually part of the traditional Solnhofen that is home to Rhamphorhynchus. It’s older and the two were not contemporaneous.
Dave Hone's Archosaur Musings 
Dave, GREAT paper!
I agree with you in every detail about distinguishing this specimen from such a range of taxa, so laying out clearly these reasons will be great to help dissuade others from presuming it is merely a juvenile of subset A or B or whatever. As I read it, every question I had about reasoning ended up being covered in a further section, and I found myself firmly agreeing with the peculiarity of the specimen.
My only caveat is that, despite the distinctiveness of the material, I am extremely hesitant to support taxonomic nomenclature on the basis of a juvenile specimen, distinct or not. Pterosaurs may be a special case as, being highly altricial, may preserve derived adult features quite early in their ontogeny. Despite this, changes do occur (although I note how you hedged your bets in the diagnosis) and it is difficult to determine how these features match up eventually in an older animal. There aren’t, for example, any clear autapomorphies but merely a distinguishing apomorphic suite; such a complex of characters seems more in keeping with a super-specific taxon rather than distinguishing a new species, implying a general morphology.
Well I’m generally conservative with juvenile material, but we felt the completness of the material meant that we really could pull out a whole bunch of features compared to pretty much every other pterosaurs. This does share fully half of something like 20 autapomotphies that Bennett listed for Rhamphorhynchus which suggest the two are very close, yet the differences are major (many fewer teeth, different tail morphology). And then there’s the wingtips – we’re being very conservative in not counting them as an autapomophy, something one of the referees and a couple of colleagues suggested we should do. Obviously I would say this, but i’m really confident this is distinct.
Late addition:
“here aren’t, for example, any clear autapomorphies but merely a distinguishing apomorphic suite”
Not true Jamie, that’s not what it says. The definition in the paper is “Rhamphorhynchine pterosaur that differs from other rhamphorhynchines by the following diagnostic features: 22 or fewer teeth in total, no elongate zygopophyses or elongate chevrons in the tail, distal half of the humeral shaft straight, humerus significantly longer (1.4 times the length) of the femur, femoral head with no neck.”
That’s not a differential diagnosis of combination characters – if it was it waould say “following combination of characters”. These are all characters that differ from all other rhamphorhynchines. It’s a series of apomorpies, not a differential diagnosis and it’s a much more solid diagnosis than you are giving it credit for.
Dave, the little thing at the end of its tail (forgot what its called) is seen in the artwork, but it can’t be seen in the fossil. Considering how well preserved it is, are we sure it has that thing at the end?
It’s generally called a tail vane. And I think we can be pretty sure it had one – Rhamphorhynchus certainly did so that’s a good sign, and more importantly, every rhamphorhynchoid pterosaur (anuroganthids aside) we have with good soft tissue preservation has a tail vane. It’s in Sordes, Rhamphorhyncus and Pterorhynchus at least which are all i that general group of derived taxa so I’m confident it’d be there. For the record, Matt and I based the shape on young juvenile Rhamphorhynchs vanes.
Hi, this is very interesting also to a layman like me, theres just a small error in the last paragraph: it says three times RhamphorhyCNHus instead of Rhamphorhynchus.
best greetings