Ontogenetic basis of head posture in chiroptera

I evaluate chiropteran ontogeny and phylogeny from the point of view that soft tissues effect the position, size, shape, and orientation of each bony element in the head skeleton early in development. I use radiographic cephalometry to detect patterns of cranial growth in fetal bats that have been differentially stained for bone and cartilage. Based upon the cephalometric relationships among the brain, sensory capsules, and pharynx, embryos of taxa that echolocate nasally are easily distinguished from embryos of taxa that echolocate orally by the end of the first trimester of development. In nasal-emitting taxa, differential growth of the brain and the pharynx eventually distorts the skull by rotating the basicranium ventrally about the cervical axis, by depressing the rostrum below the basicranial axis, and by rotating the lateral semicircular canals to retain their horizontal orientation. Together, these actions align the nasal cavity and nasopharynx with the axis of the body in flight. In contrast, the skull of oral-emitting taxa is constructed around an axis aligned with the oral cavity. Together, fetal and adult material provide an example of the correlated progression of functionally connected features (Thompson, 1966). Structural changes in the pharynx have cascaded throughout the other functional spaces in the head (otic, optic, nasal, and oral) and have left the remainder of cranial development to accommodate these newly imposed spatial requirements. These patterns of skull growth are taxonomically distinct and form the basis for the present phyletic analysis of the order. I believe that the evolution of head posture in bats has been constrained by the demands of vocalization. Accordingly, the ontogeny of the chiropteran skull has been canalized along two distinct developmental paths--oral-emitting and nasal-emitting bauplane. Organic material is not putty and natural selection is not omnipotent Stephen J. Goul