Anyone who has been keeping up with pterosaur research in the last few years will have spotted the various exchanges going on over the pteroid bone. I am not sure I ma giving anything away or bringing something private out into the open air when I ‘reveal’ that the pterosaur community is petty small and there are some fairly big disagreements that have been going on for what seems, to a relative newcomer, like decades and this is simply the latest one. However it is one of great interest as this relatively innocuous bone is actually very important in pterosaur research.
“But what is a pteroid?” I (possibly) hear three or even four of you (possibly) ask. Come this way and all will be (possibly) revealed (possibly)….
The pteroid is a small bone that articulates with the pterosaurian wrist and helps control the propatagium (the ‘front wing’ that sits in the crux of the arm). There are quite a few different types of pteroid ranging from a simple spike, to a long rod, and even a few with a 90 degree twist in the base like a bendy drinks straw. Thanks to a lack of direct pterosaurian ancestors in the fossil record we are not sure where the pteroid comes from, it’s either a highly modified wrist bone, or a ‘neomorph’ (that is a new bone that has evolved exclusively in pterosaurs) but it’s actual identity is rather irrelevant to the key question. Here is a nice picture of a pteroid in a specimen of Pterodactylus (black arrow) with the propatagium (red arrow) lying between the wrist (white arrow, where the pteroid articulates) and the upper arm (on the right).
And that question is “In what direction did the pteroid point? To the front or to the middle?” (or more technically, anteriorly or medially). Though you might want to add to that “And who cares? Is it really important?”Well a lot of us (OK out of pterosaur researchers) care and in a few ways it is important, but mostly it is an issue because the direction of the pteroid profoundly affects the way in which pterosaurs could have flown, with the two different positions offering a very different flight profile. A forward pointing pteroid (especially one that could be flipped to point downwards) has been suggested several times in the literature on the grounds that it would massively increase the size of the propatagium and enable the wing camber to be significantly increased which in turn would increase the lift available to the pterosaur, providing more control and great efficiency, something that would be especially advantageous to some of the 12 and 13m wingspan giants of the end Cretaceous.
However, despite the intuitive appeal there was no real evidence for this, and certainly no figures to back up the idea until some recent computer model based research suggested that a forward pointing pteroid could add up to 30% lift when compared to a medial pteroid / small propatagium. What’s more, it was also suggested just how the pteroid could articulate on the complex pterosaurian wrist to produce the necessary effects. Suddenly the idea was a lot more credible than before and actually had some data to back up the concept, but as ever this was not the whole story at all.
For a start, not everyone agreed that the proposed articulation was not even necessarily possible and in fact the pteroid could only have articulated at a point that would make it point medially. (I’ll not go into the details because it gets really, really messy, but essentially you have several of ‘dips’ in the various wrist bones where something like the pteroid could articulate, and both a pteroid and a small bone called a sesamoid that needed to fit – which one went where, and how?). Secondly, the proposed orientation would put huge stresses on the pteroid and in several species it is incredibly long and thin or even sinusoidal and there was no way it could have supported the forces suggested to be going through it (and it may have also torn the propatagium with a pointed tip). Thirdly at least some pteroids simply couldn’t orientate forwards (not least those with a bend in the middle) so at the very least if the forward interpretation is true for some we will still have to have medial ones for others. So how can we resolve this issue?
Well the obvious response is to check the fossil record (naturally) and of course to bear in mind that just because something is possible or apparently good does not mean that it happens (i.e. a forward pointing pteroid would massively increase lift, but that does not mean pterosaurs necessarily evolved it). Sadly as you might have guessed the fossil record is not as clear cut as one might expect on the issue – surely sooner or later you will just have a wrist preserved with the pteroid jammed in one socket and the sesamoid in another, problem solved. Well, no and when you think about it for a rather obvious reason. The pteroid is of course bound up in the propatagium (a muscular sheet of tissue) and furthermore no matter its orientation would have been held in place by a bunch of tendons and ligaments, and possibly various muscles as well. That means when the animal dies and muscles rot or contract and ligaments and tendons shrink as they dry the pteroid gets yanked out and moved. It is only ever found in what appears to be a natural articulation in the ‘roadkill’ 2D specimens from places like the Solnhofen of Germany such as the one pictured. Here it might well be articulated, but since it is flat you still can’t see exactly where it joins the wrist or how, and the animal is small too that does not help.
If you check various abstracts appearing in various palaeontological meetings over the last few years (SVP, SVPCA and EAVP) you will see a swathe of pteroid descriptions moving back and forth between several researchers in addition to some major papers either published or making their way through the review process. Currently the consensus seems to be swinging back in favour of a medially facing pteroid and at the least this must have been true for some animals (those with exceptionally thin pteroids, or crooked ones). The real question now is exactly how and where that articulation occurred on the wrist, but this one is set to run for quite a while yet. It seems odd that so much attention and interest could be generated by such a small bone, especially one so often preserved so well. As ever, new finds are coming to the rescue with better preservation and new information available but it will still be some time before the dust settles on this one. I’ll keep you posted. In the meantime, you can try Chris Bennett’s pteroid challenge .