The not missing mandibular fenestra of Eudimorphodon

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?

The mandibular fenestra in Eudimorphodon. From Nesbitt & Hone, 2010.

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.

20 Responses to “The not missing mandibular fenestra of Eudimorphodon”


  1. 1 Nick Gardner 15/12/2010 at 8:15 am

    Except you didn’t provide a link to the paper or a complete citation.😉

    Nesbitt, S. J., and D. W. E. Hone. 2010. An external mandibular fenestra and other archosauriform character states in basal pterosaurs. Palaeodiversity 3: 225–233.

  2. 5 Nick Gardner 15/12/2010 at 9:13 am

    Doesn’t help that the article WAS on Chinleana not that long ago.

    The same issue has an interesting (but suspect) paper by M. Maisch on Ichthyosaurs. http://www.palaeodiversity.org/pdf/03/Palaeodiversity_Bd3_Maisch.pdf

    • 6 jay 15/12/2010 at 2:48 pm

      No doubt you have you reasons, i’m unwilling to do any more than skim at this point through a 60-page monograph on animals outside my own interest – but i am nonetheless curious: aside from the millions of new taxa created, i need to know why this paper is suspect [without reading it!]

      • 7 jay 15/12/2010 at 2:50 pm

        Sorry i asked – the first sentence said it all:

        “The work presented her is a result – or rather an intermediate
        progress report – of ongoing studies, started
        in 1994, on the most successful and enigmatic group of
        Mesozoic secondary marine amniotes, the Ichthyosauria”

  3. 8 Nick Gardner 15/12/2010 at 9:15 am

    I found it on Google just by searching “Nesbitt Hone 2010 Eudimorphodon”, then I posted the first comment above.

    • 9 David Hone 15/12/2010 at 9:20 am

      It might be now, but not when I wrote this last night and checked myself.

      And I had no internet last week so didn’t see it on Chinleana. I’m not divine.

  4. 10 Mickey Mortimer 15/12/2010 at 9:33 am

    “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.”

    Err… only if it’s a Huenian definition including tons of plesiomorphies. Otherwise teeth and tails would be found in the “basic definitions” of something like Gnathostomata and Chordata respectively. Which birds still work well for, as members of those clades that lost those characters.

    • 11 David Hone 15/12/2010 at 9:39 am

      Well look, and I hope for the last time, this is supposed to provide a very general, non-technical introduction to these kinds of topics. I’m trying to give reasonable examples that people can latch onto, and very much not get into the depths of systematic taxonomy and character evolution. I wouldn’t write this in a paper any more than I would write about ‘plesiomorphic conditions’ here. what is appropriate and absolutely right for one audience is not the same as the other.

      • 12 Mickey Mortimer 15/12/2010 at 2:50 pm

        Hardly seems fair when Palmer’s post on pterosaur aerodynamics was as technical as anything I write in comments.

        Regardless of how big of words you want to use, the important thing is your analogy is flawed. Pterosaurs’ supposed lack of a mandibular fenestra seems important for excluding them from Archosauriformes specifically because a mandibular fenestra is an advanced character diagnostic for Archosauriformes. Tails and teeth are both primitive characters for dinosaurs, so their absence in an animal doesn’t immediately suggest anything about whether that animal is a dinosaur or not. You could have retained layman’s language AND accuracy by writing something like-

        “Ankylosaurids are dinosaurs, but have lost the hole in their hip socket, yet dinosaurs have always been diagnosed in part by the presence of that hole. That is because with diagnoses we are dealing with the point of origin – what did the original dinosaur look like, and thus what features did its descendents inherit from it, even if those were subsequently changed or lost.”

        Accuracy need not be lost when communicating with the public.

      • 13 David Hone 15/12/2010 at 4:06 pm

        Well I know what you mean about Colin’s post but then, i didn’t write that. I asked him to contribute and i wont’ censure him for that. My intent is always to make things simple and clear and provide examples or analogies that are easy for the non-expert to grasp. It wasn’t exactly criticism of your comment, but people do seem to expect this site and Pterosaur.net (and I can only assume others) to provide them with what they want how they want it and don’t necessarily consider that this may not be what the writer wants to say or get across.

        As to the specific point, I do know what you mean in terms of exact accuracy (tails are plesiomorphic for vertebrates as whole). But then I have found in the past that I have to explain that penguins are not mammals even though they can’t fly. In that context, even then explaining the difference between a plesiomorphic and derive character is more than I would normally do. So I don’t think my analogy was inappropriate or inaccurate, though I would say yours is better.

  5. 14 mattvr 15/12/2010 at 10:26 am

    I’d rather treat it like a surprise and wait for tomorrows post!

  6. 15 jay 15/12/2010 at 2:31 pm

    For me, the below-quoted passage is brilliant.

    In fact, i was just about to post that i thought..
    “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.”

    ..is an awesome passage, before noticing Mickey’s post, and almost didn’t bother posting anything. In this context it still is a good passage – it got me thinking about diagnoses in a way i wouldn’t have otherwise, and it got a message across that may have been missed had it been written in explicit technical lingo; as Dave said is designed for this medium.

    Kinda reminds me about some of the things that might be said following submission of a technical article – i.e., in media statements or required ‘plain-English summaries’, we’d state things that we wouldn’t in the corresponding peer-reviewed journal paper, because the aim is towards getting the message across in public outreach exercises as opposed to relying science info to peer-level scientists

  7. 16 Allen Hazen 15/12/2010 at 10:28 pm

    Ummm. Do lepidosaurs have a mandibular fenestra? There ARE people (D*v*d P*t*rs?) who think that Pterosaurs aren’t archosaurs at all, but derive from some basal diapsid, or perhaps even lepidosauromorphoform*. Would finding a “vestigial” (as in: partly closed, closed on the lateral surface) mandibular fenestra in pterosaurs be relevant evidence against their view?

    *Meaning: something that came just above the archo/lepido split in the tree, but below any of the interesting splits we think we know about on the lepido branch.

  8. 17 Nick Gardner 16/12/2010 at 8:40 am

    “Ummm. Do lepidosaurs have a mandibular fenestra? There ARE people (D*v*d P*t*rs?) who think that Pterosaurs aren’t archosaurs at all, but derive from some basal diapsid, or perhaps even lepidosauromorphoform*. ”

    Lepidosaurs (tuataras, lizards, and those crazy derived lizards known as snakes) don’t have a mandibular fenestra. You can look at the skulls of various living lepidosaurs here: http://digimorph.org/listbygroup.phtml?grp=Lizards,%20S&sort=SpeciesName

    Something closer to lepidosaurs and after the split between lepidosauromorphs and archosauromorphs would be a lepidosauromorph…

    As far as DP goes, gah, ignore his stuff. Too few characters, way too many spurious codings/interpretations of material.


  1. 1 In Pictures » Blog Archive » In Pictures | Dinosaur Junction’s New Online Children’s Game Trackback on 15/12/2010 at 4:20 pm
  2. 2 The pterosaur mandibular fenestra part 2 « Dave Hone's Archosaur Musings Trackback on 16/12/2010 at 8:09 am
  3. 3 Finding the fliers – pterosaur discoveries « Dave Hone's Archosaur Musings Trackback on 13/05/2011 at 7:55 am
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