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.