Bat elbow styliforms?
Picture of a Lasiurus bat. Note rod-like element on the brachiopatagium, stretching from the elbow.
In my discussion on flying volaticotheres, I’ve pointed out that a possible way to infer volant capacities in mammals with patagia was the presence of styliform elements in the brachiopatagium.
Styliforms are cartilaginous/osseous rod-like elements developed in the wing membranes of nearly all gliding and flying amniotes, brought into attention thanks to the discovery of the notorious gliding theropod Yi qi. These structures presumably evolved to strengthen wing membranes and prevent fluttering, as well as extend the patagium in at least flying squirrels and anomalures.They are generally speaking original structures, independently acquired in the various clades.
They are present in a variety of arrangements and positions: in the wrist (flying squirrels and Yi qi), in the propatagium (pterosaurs; their iconic pteroid bone is a particularly specialized styliform element), in the uropatagium (the chiropteran calcar; the pterosaurian fifth toe is styliform-like in appearance, though not an original structure) and, most notably, in the middle of the brachiopatagium, projecting from the wrist (colugos, anomalures, marsupial gliders, eomyids, gliding glirids).
This latter arrangement in particular was discussed in the above post. I argued that it’s presence (or absence) could be a factor in determining on whereas an animal was a glider or flyer. It seemed intuitive to assume that such an awkwardly placed structure would be detrimental to wing strokes; this reasoning has been applied as a counter-argument for Yi qi being a powered flyer.
As it turns out, the one group of unambiguous flying mammals, bats, do seem to have elbow styliforms:
These structures are clearly solid rod-like styliform elements, similar to those observed in gliders:
Colugo, anomalure and Eomys respectively.
Yet, these structures on bat wings are not discussed on the Yi qi paper, and don’t appear to really discussed or even named anywhere!
Further adding to the drama is the fact that at least some bats appear to lack these structures all together!
This is quite fascinating and unexpected, and I feel is worth further discussion.
Jackson, Stephen Matthew and Schouten, Peter. Gliding Mammals of the World, Csiro Publishing, 2012
Xu, X.; Zheng, X.; Sullivan, C.; Wang, X.; Xing, L.; Wang, Y.; Zhang, X.; o’Connor, J. K.; Zhang, F.; Pan, Y. (2015). “A bizarre Jurassic maniraptoran theropod with preserved evidence of membranous wings”. Nature. 521: 70–3. doi:10.1038/nature14423. PMID 25924069.