I find Pterosaurs pretty fascinating.  One of the details of pterosaur anatomy i haven't been able to find much information on is their fuzz or "pycnofibres".  I read that it is not "fur" because pterosaurs aren't mammals, which is about all i have been able find out about it.

 Are there actually observed distinctions that differentiate it from mammal fur, and if so what are these?  Or do the fossils not provide that level of detail?  I have a hard time imagining something plausible that's relatively fur-like and that's different enough from fur to be distinguished in a fossil.

Thanks.

A good question and tricky to answer. In short, you are right it's pretty hard to tell pycnofibers from mammalian fur on a flat fossil slab, though the former tend to be rather thicker than the average hair. However, it's also pretty obvious that the two are not that same thing - there is obviously a huge evolutionary gap between the two that is filled with all kinds of lizards and reptiels that do not hav anyhting like this kind of covering. Fur and pycnofibers appeared independently and thus are worth having different names to distinguish them and avoid confusion.

There are a surprisingly high number of pterosaurs known that are preserved with this cover (aside from the best known Sordes and Jeholopterus) but most have only poor impressions and show little. What we really need to do is get soem samples of the better ones and get them under a scanning electorn microrscope or simialr to look at the microstructure but that has yet to happen. When it does we can probably spot more differences, but right now, there's not much to say.