Monday, April 11, 2011

Functional Morphology of Anurognathid Pterosaurs

I recently gave a talk with my preliminary results regarding anurognathid biomechanics at the GSA Northeastern Division Conference. There's nothing particularly shocking in it, but I have decided to post some of the highlights from my abstract and presentation here since this information is now technically "public". Obviously I am sitting on more data and results than appears here, which will be in a forthcoming manuscript.

On to the frog-mouths...

Anurognathid fossils include several exceptionally well-preserved specimens, some of which include extensive soft tissue preservation. This exceptional amount of morphological information makes anurognathids prime candidates for functional biomechanical analysis. Furthermore, anurognathids displayed a suite of unusual characteristics that make them of particular interest for functional study. These traits included extensive pycnofiber coverings, fringed wing margins, shortened distal wings, shortened faces, and enlarged orbits. Prior authors have suggested that anurognathids were adapted to catching small insects on the wing. My quantitative analysis that supports this general behavioral inference, and provides details regarding probable anurognathid locomotion.

First off, bone strength analysis in Anurognathus ammoni reveals that each proximal wing was capable of supporting nearly 22 body weights of force. The wing spar of A. ammoni was substantially stronger in bending than that of an average bird of the same size, and the calculated relative bone strength from Anurognathus ammoni overlaps significantly with that of living birds that capture prey on the wing (p>0.92) but differs significantly from all other avian morphogroups (p<0.04).

This might might seem like an obvious result, given all of the traits of the anurognathid skeleton already associated with insect capture, but it is important to remember that "insect capture" is an incredibly wide spectrum of feeding ecologies. There are, after all, quite a number of insects out there (as in, more than any other animal group) and so there are a diverse array of insect predators, as well. Only a subset of insectivorous vertebrates capture prey with a rapid pursuit on the wing - many bats, for example, are gleaners that pull insects from substrates. Some insectivorous bats and birds hawk insects only over short distances, or feed mostly on slow-flying prey. That Anurognathus ammoni seems mechanically similar to animals like fast-flying bats, kestrels, and swallows may not be all that shocking, but it's still useful information.

Anurognathid launch appears to have been particularly rapid and steep (more on this another time), and once airborne, anurognathid pterosaurs could likely generate high lift coefficients. Leading edge structure reconstructed from the soft tissue of Jeholopterus suggests that anurognathids were capable of generating a leading edge vortex (LEV) as observed in some living bats and birds (particularly swifts and flycatchers). I cannot calculate exactly how strong the LEV was - there simply is not enough detail in the soft tissue to tell - but in living vertebrate flyers a sustained LEV can pump up the lift generation by about 40%.

Analysis of flapping efficiency suggests that the expansion of the proximal wing, coupled with reduction of the distal wing elements, would have increased flapping power at the cost of slightly increased drag. The proportions of the wing and details of the shoulder may be indicative of the ability to hover for brief intervals (again, I shall be cruel and make everyone wait for details on this one). Overall, these results are consistent with reconstructions of anurognathids as highly maneuverable flyers, preferentially foraging on small aerial prey, likely at high speeds and accelerations. Conclusions regarding the effects of the extensive insulation on boundary layer control and such are pending analysis.

1 comment:

  1. Given the peculiar jaw anatomy, rather than general head-shape in anurognathids, the biomechanics of anurognathid feeding would be of partiuclar interest as it differs so much from other pterosaurs and, in fact, any other archosaur (including birds). Bennett's reconstruction of the skull as asystem of struts and braces with a large series of bent bars around the jaw margin and one solid brace in the middle (braincase) is also vastly unique, and I'd love to see your work on it (when it comes out, of course).