Head Full of Air

Anurognathus ammoni cranial tissues

It’s that Anurognathus ammoni skull, again.

Pterosaur skulls are fairly light, comprised of bones filled with air passages; but less remarked is how, in most pterosaurs, most of the rest of that skull is essentially air, itself. In fact, the most dense part of the skull are the muscles themselves. But knowing how much of the head’s mass is muscle requires some idea of where the muscles go, so above I’ve done a quick, off the cuff assumption of muscle positions and size (“B” above), and it doesn’t look too favorably for these little guys when it comes to how much muscle motivates the jaw. Coincident with this is how far lateral the muscles are arranged: They don’t wrap around the top of the skull, but are set to the side, and thus are almost completely vertical — except for the palatal muscles, which are mostly horizontal and medial (see “C” above, which describes muscle actions; almost all adductors are vertically arranged, and thus suggests almost no retractive component). Almost everything else is taken up with ligaments, eyeballs, a few additional muscles, nerves, and the vascular complex feeding the face, integument, and nasopharyngeal canal, including the narial plexus. The rest is completely air: nasal passage, cranial air sac system (show above at “A” are only the main antorbital airsac and initial palatal sacs).

It seems, based on a cursory evaluation of other pterosaurs, especially some pterodactyloids, that anurognathids may have had a relatively DENSER head, perhaps a result of the giant eyeballs — assuming I’ve reconstructed them the right size. Adding fleshy integument to this, one might get a comparatively heavy head:

Jeholopterus ningchengensis, in three views.

Muscle reconstruction of Jeholopterus ningchengensis, in three views. A, dorsal; B, ventral; C, left lateral. Integument overlying muscle/ligaments hypothetical, position and extent of patagia as shown unlikely with current knowledge: cheiropatagium likely extended toward hindlimb, and probbaly attached at the fifth toe, not the uropatagium. Body is shown covered in filaments (pycnofibres).

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3 Responses to Head Full of Air

  1. The size of the scleral ring generally delineates the diameter of the iris, which is generally much smaller than the eyeball itself. Currently you have the iris at nearly the same diameter as the eyeball and you’ve cheated it down compared to Bennett’s reconstruction. You also cheated up the size of the antorbital fenestra. Increase the size of the eyeball to the correct diameter (try this in dorsal view so the two diameters touch medially and extend to the left and right surfaces and beyond, which is a huge distance in Bennett’s error-ridden reconstruction) and you’ll find that you have a head full of aqueous and vitreous humor and jaw muscles and/or brains much larger than in any other pterosaur and way too large for such small jaws (which you’ve cheated deeper) and altogether these were no doubt heavier than air, a problem in itself.

    On the other hand, the flathead reconstruction by yours truly does indeed provide a very large antorbital fenestra, small eyes and a head full of air.

    Your caption reads “Jeholornis,” which is not a pterosaur that I know of.

    And why attach the proximal wing membrane midway down the torso? Which specimen shows that morphology?

    And why are the pedal digit 5s tied into such a deep uropatagium? No specimen demonstrates this. I know accuracy is important to you. There is too much wild imagination here.

    To your credit you have the free fingers pointing palmar side ventral in flight.

    • Lars Schmitz (2009) goes into some depth to show that inner scleral diameter correlates very well with lens diameter, while the outer scleral diameter correlates well to an eyeball with maximum diameter ~29% wider. Lens diameter to inner ring diameter skews a bit wider, but predicts a lens diameter ~8% smaller than inner scleral ring diameter. The eyeball size, discussed more in the previous post, does talk about the eyeball size and Schmitz and Motani’s work in more detail, especially correlation of inner diameter to diel. The eyeball is shown with the lens largely filling the inner scleral ring, but this is artistic: I am not attempting to constrain something that isn’t a constant, and the art file has a small iris — I felt it would be too distracting — but also that the pupil diameter is, for the most part, based on the space within the scleral ring layer (drawn separately). Problematically, orbital diameter itself is NOT as good a predictor of eyeball size (as I noted in the previous post and in this one) but rather circularity of the orbit. In many birds, the anterior margin of the orbit is missing, as the lacrimal either doesn’t contact the jugal bar, the jugal bar is NOT THERE, or there is no other lacrimal ventral contact, such as to the maxilla (which is always there). The lack of clear determinates to orbit shape, that its size and shape is estimated in most studies, leaves me to doubt orbital diameter or shape studies, merely that eyeball fit to orbit curvature does seem to work out well (no data to present on this).

      As noted above, though — and to be shown better in a future post — the adductor chamber of the skull is fairly small in this reconstruction; it cannot get much larger, as Bennett’s second specimen has a fairly distinct and “obvious” supratemporal fenestra. The remaining mass of the skull, comprising ligaments connecting eye to skull, nerves, endocranial volume, and vascular tissues, additional smaller muscles without clear osteological correlates, the stapes, hyoid aparatus, additional muscles of the tongue and throat, integument and oral epithelium, and the narial plexus and the soft-tissue that supports their structure, and the possible supporting thin bones or cartilages that almost NEVER preserve despite being there — ’cause of how ridiculously thin they are — auditory canal and auricular structure, and, of course, the eye itself, should occupy a relatively greater volume of skull than, say, any other pterosaur out there. I can go into greater detail on this by showing relative orbital diameters, circularity, and fitting circles to other circles, but that’s down the road. Estimating volumes in flattened skulls is tricky business. Regardless, the specimen preserves a huge scleral ring in comparison to apparent length of the skull, a factor not to be dismissed by attempting to squeeze the scleral ring into a much, much smaller head (as your reconstruction does); indeed, in the specimen in question, the scleral ring appears to to be as long as just over 1/4th the length of the skull, and this is approximated in my reconstruction, though I’ve taken the liberty of “uncrushing” the specimen.

      As for the reconstruction, this has been around for some time (the issue with the caption occurred in its original presentation, was fixed then, but seems to have reappeared as it is part of the “native” caption). Not making excuses, but the structure of the integument and membrane of the wing especially are all hypothetical, as are the locations and sizes of muscles, tendons and such in the underlying structure. It was not a major concern at the time where, precisely, the membrane inserted on the body, whether it was the body wall, the the hip, the thigh, the knee, somewhere in between, or the ankle. At the time, I fancied the hip attachment of the cheiropatagium, and assumed the fifth toe connected to the uropatagium, though I know the latter is incorrect NOW — the former may not be as incorrect, but I am not so certain about clear indications for distal insertion, as there are unresolved issues in reconstruction of the soft-tissues close to the body wall (detail is sparse nearer the body wall). I have NOT gone around correcting this illustration because, as one might see, it is fairly complex. I’d do BETTER by removing the nonmuscle structure; I am leaving it this way, instead.

      Schmitz, L. 2009. Quantitative estimates of visual performance features in fossil birds. Journal of Morphology 270: 759-773. (available online)

  2. Pingback: Jaws, Jaws, and More Jaws! | The Bite Stuff

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