So as I was browsing recent articles and papers, I came across Adrien Tejedor’s systematic review of the microchiropteran natalid bats (Natalidae, Chiroptera), which is part of his PhD dissertation. These include a small number of extant and one extinct species of bat, and are primarily known for males bearing a large organ on the dorsum of their skulls (the “natalid organ”). But natalid bats also exhibit a feature of some other microbats, that of vertebral fusion, although they take this to the extreme and can exhibit almost complete thoracic and lumbar fusion into a series of barely mobile columns. With fusion of some posterior lumbars to the sacrum, and some posterior cervicals to the “notarial” fusion, this can result in what is essentially a single spinal rod, slightly or extremely arched. Tejedor characterizes this as a “bell-like,” as in one natalid, Chilonatalus, the ribs are also fused to one another, to the sternum, and create a solid structure.
It is easy to see where some comparisons to bird exist, in the formation of a rib apparatus around the thoracic cavity would require a unique method of inspiration/expiration to permit breathing, while creating a rigid structure upon which wing-related muscles can act from. This should exist in better fliers requiring more resistance to distortion caused by wing-flapping.
The ribcage and spine of other natalids is no less interesting, including the presence of distally expanded ribs that abut one another along nearly their entire length, sternae that are broad and can produce a deep keel on their cranial and posterior ends, or on each isolated xiphisternal element.
Now I was aware that some bats exhibited vertebral fusion cranial to the sacrum, but this was often just a series of thoracics to form a notarium, and it occurs in some fossil and extant bats, such as Moropidae; I was previously unaware that the fusion was this extensive in any species, or that it accompanied such extremely avian-like development of the spine and sternum. I have reproduced above Plate 18 from Tejedor (2011) to show this extreme development, because it is so fascinating.
Tejedor, A. 2011. Systematic of funnel-eared bats (Chiroptera: Natalidae). Bulletin of the American Museum of Natural History 353:1-140.