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Anurognathids and their snare-drum palates

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This post was inspired by a recent PBS special on hummingbirds.

When hummingbirds eat insects, which they do for the protein, they approach their prey with their narrow jaws wide open (perhaps the only time you’ll ever see them that way). Only when the insect passes out of their line of vision and touches their beak do the jaws snap shut (see the video here).

By contrast, anurognathid pterosaurs, like Batrachognathus (Fig. 1), had wide, frog-like jaws, ideal for capturing aerial insects in the manner of modern apodids (swifts), as we examined earlier. Today we’ll take a look at a mechanism that may have helped make anurognathid palates more sensitive to the presence of even tiny insects within their gaping jaws.

The palate of anurognathid pterosaurs is oddly and wonderfully constructed.
In place of a broad maxillary palate, like other pterosaurs had, anurognathids had broad ectopalatines (ectopterygoids fused to palatines) and broad vomers. Below that architecture, they had wire-like medial maxillary elements (orange in figure 1). These wire-like bones are typically identified as palatines by other pterosaur workers who have not had the experience of reconstructing several anurognathid palates as shown here.

Figure 1. Batrachognathus, an anurognathid pterosaur demonstrating its snare drum palate in which the maxillary palatal struts acted like vibratory snares subsurface to a taut palatal skin. Perhaps this increased sensitivity to insect contact on the palate.  Top view (above) two lateral views (left and below) and two anterior views of the mouth, in normal open position and forced to 180 degrees, unnaturally open. Only anurognathids had such wire-like maxilla palatal processes. Large jaw closing muscles filled the palatal spaces lateral to the pterygoids (posterior palatal elements).

Whenever the anurognathid mouth is opened wide, visual contact with the prey is gone. The tongue, covering the lower half of the open jaws was obviously sensitive to touch. The upper half, consisting of the palate, might have had increased sensitivity with the addition of those wire-like maxillary bones acting like snares(as in snare drums) subsurface to taut palatal skin. Whenever a drumstick hits a snare drum skin, the snares vibrate. Perhaps the wire-like morphology of the maxillary processes acted in a similar fashion to alert the brain to the presence of even tiny insects hitting the palate skin.

Just a odd hypothesis to explain an odd morphology.
What else could it be? Send along any other ideas.

Having such fragile bones in the palate would have restricted prey to small insects that did not put up so much of a struggle (nothing that could break those fragile splints. Other pterosaurs (from the long-jawed Dorygnathus clade) developed a more robust palate based on medial maxillary shelf/plates that permitted them to handle larger more wiggly prey, like fish.

Secondary palate shifts respiration posteriorly
In most derived pterosaurs the internal nares were shifted posteriorly due to the presence of a complete secondary palate created by medially developing plates arising from the maxilla. Other reptiles, including synapsids, have developed a secondary palate. That’s the roof of your mouth your tongue is touching now. Such a shift redirects respired air to the back of the throat, permitting such a pterosaur to continue breathing while carrying prey in the front of its jaws.

Anurognathids probably did not have such a secondary palate.
The internal choanae, through which respired air traveled, remained just behind the premaxillary teeth, on both sides of the wide vomers. Unfortunately we have no evidence whether or not the soft tissue that would have covered the wire-like maxillary processes of anurognathids also continued forward to cover the internal nares. The design of the Batrachognathus palate (Fig. 1), which appears to avoid the choanae, suggests the original internal nares remained unobstructed. Other anurognathids, like the flathead pterosaur, appear to follow this pattern.

However…
Others, like Jeholopterus, traverse the choanae with those wire-like maxillary processes. Perhaps this shift benefitted Jeholopterus, the vampire pterosaur, during feeding by allowing it to draw in dinosaur blood while continuing to breathe.


Source: http://pterosaurheresies.wordpress.com/2013/06/21/anurognathids-and-their-snare-drum-palates/


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