As mentioned some time ago in the ‘what is an archosaur anyway’ post one of the defining characteristics is the mandibular fenestra, basically a hole in the back of the lower jaw. However, this feature is absent in pterosaurs (which are certainly archosaurs) which begs the obvious question or where did it go?
I should first mention that although the mandibular fenestra is a defining character of archosaurs, its disappearance does not make pterosaurs not archosaurs. Of course evolution keeps going and thus ultimately all things are transient as new features arise and old ones change or are lost. Birds are dinosaurs, but have lost their teeth and tails, yet the basic definition of a dinosaur would probably include both of those features. That is because with these definitions we are dealing primarily with the point of origin – what did the original archosaur look like, and thus what features did it and all of its descendents inherit from it, even if those were subsequently changed or lost.
Right, so back to the fenestra itself, what’s going on? Given that the evolution of pterosaurs is essentially one of ever lighter constructions and better flight capabilities, one would expect a hole to be kept as long as a possible. Getting rid of any excess bone, no matter how small will make a bit of a difference to the mass, and lowering it is always good so one would expect the fenestra not just to be retained in pterosaurs, but in fact to get bigger, not disappear.
The obvious answer to this is that pterosaurs actually reduced their jaws as a whole making the bones relatively low and thin. Sticking a hole in a very thin set of bony plates might make them incredibly weak, and so if you close up that fenestra you can reduce the weight of the jaw overall and keep the jaw relatively strong, than keeping a normal jaw and putting a hole in it. So it makes sense for pterosaurs to close it up, but in that case how? There is absolutely no evidence of it anywhere.
Except there just might be. In 2003 pterosaur supremo Peter Wellnhofer described an isolated jaw of the basal (ish) pterosaur Eudimorphodon from the Late Triassic of Austria that had, yes, wait for…wait a bit longer…a mandibular fenestra. Probably. The specimen is conveniently in the BSPG in Munich so obviously I’ve taken a look at it and provide a nice photo here with an arrow to show you the bit we are talking about. The fenestra is pretty obvious, and it’s clearly not the result of a break or damage, so why the problem?
Well first off it’s unusual because we actually have a quite a few specimens of Eudimorphodon including some wonderfully preserved skulls of at least two different species and several different ages and none of them have a mandibular fenestra. Judging from the size of this jaw, the animal was an adult or close to an adult, yet we don’t see it in other adults or juveniles, so that’s a bit odd. If it was a juvenile animal you might argue that it was something present in young animals that grew out with age and thus is rarely seen, but we can’t apply that here. Secondly, Eudimorphodon is not that basal as pterosaurs go there are several clades that evolved before the eudimorphodontids and none of them have a mandibular fenestra – it would be off for pterosaurs to get rid of it early in their origins and then rapidly reacquire it in only one species. Not impossible, but unlikely.
The third possibility is the most intriguing and perhaps the most frustrating, because right now we can’t really resolve the issue. The jaw was described as being in medial view, that is, we are looking at the side that would face into the mouth. This is important because we do not know what the state of the bone on the *outside* would be. The jaw is made of quite a few bones and towards the rear several of them overlap each other and obviously for the fenestra to be genuine it would have to go all the way through the jaw. But although we can clearly see that there is nothing broken on the medial surface, but that might not be true of the lateral surface. The sheet of bone that would make up the lateral face of the jaw might have come off, and thus the fenestra is an illusion and simply ‘half a hole’ rather than a full one.
This would make a lot more sense all round for a number of reasons. Evolutionarily it would mean that the fenestra did indeed close up early in pterosaur history (laterally) but perhaps was retained through to the eudimorphodontids at least medially. The fossil record of medial jaws in early pterosaurs is pretty much non-existent but this requires no reversals or complex changes to be true and it also demonstrates that pterosaurs do have a secondarily closed fenestra which is what you would expect of an archosaur. It also fits with what we know about other Eudimorphodon specimens since they are seen in lateral view and there is no sign of a fenestra. It is also quite plausible given that although this jaw is in apparently good condition, the rest of the specimen was disarticulated and broken or eroded in places, so a fragile sheet of bone in the jaw could easily be missing.
So how can we check this out to see if the lateral face is indeed broken and thus what happened to the pterosaur mandibular fenestra? All will be revealed tomorrow. Because I want to pad this out really. The more cunning / desperate among you will soon hunt the paper down.