The pterosaur mandibular fenestra part 2

Yesterday I introduced the problem of the ephemeral mandibular fenestra of ptersaurs and wondered where it might have gone. Based on the paper out the other day that Sterling Nesbitt and I wrote (well, Sterling wrote, I helped out) we reveal what we think happened.

Firstly the Eudimorphodon specimen with the medial fenestra, is this real, and does it extend externally? The answer lies in several different areas. First off, the may not be Eudimorphodon. The specimen was described in the days before Raeticodactylus and other pterosaurs with Eudimorphodon-like teeth but that were not Eudimorphodon. Several things were tied to this taxon based on their teeth when hindsight suggests this was not a great idea. If we assume this is not Eudimorphodon (and there are reasons to think so) then the problem of fenestra-free Eudimorphodon specimens disappears.

Secondly, the jaw is not in medial, but lateral view. This is a genuine mandibular fenestra that fully penetrated the jaw. This came about thanks to Sterling’s superb eye for detail when he noticed that the sutures of the bone for the various jaw elements just didn’t match up with the medial pattern seen in other archosaurs, but was very similar to that seen on the lateral face.

Archosaur mandibular fenestra. From Nesbitt & Hone, 2010.

This is interesting news in itself, but still leaves us with the problem of a non-basal pterosaur with an apparently primitive jaw condition when none of the things that came before kept one. Not impossible, but certainly odd. Enter Dimorphodon. Way back when the great, and, let’s face it, a bit nuts, Richard Owen wrote a description of various Dimorphodon specimens he noted that one may have had a mandibular fenestra. However, noting himself that other pterosaurs seemed to lack one, he wrote this off as an internal one revealed by missing bones at the back of the jaw, just as Wellnhofer did over a century later. Again, revisiting this specimen and a third with new ideas means it looks very much like Dimorphodon had the same thing going on. Now we have two taxa with fenestrae, including in Dimorphodon, a very basal pterosaur.

This at least suggests that this really is a hang over from the ancestral archosaurian condition and that it was then lost several times in various pterosaur clades, though probably quite quickly given how rare it seems to be. This then is our, not actually all that helpful, but ultimately quite interesting, solution to the question of the problematic Swiss Eudimorphodon. Come in mandibular fenestrae, you time might just be up. (Possibly, and with the full knowledge that this really won’t go down well in at least a couple of pterosaur researcher camps).

There are some other bits in the paper on pterosaur origins and the distribution of various characters. In short you can get good mileage from difficult problems from new specimens and of course ideas can change as the available data increases and changes. To get more you should read the paper. Happily, it’s freely available, so off you trot to here to get your copy.

Nesbitt, S.J. & Hone, D.W.E. An external mandibular fenestra in pterosaurs supports placement within Archosauriformes. Palaeodiversity, 3: 225-233.