Fig. 6.

Schematic diagram depicting potential developmental mechanisms forming orofacial clefts in bats. Top row: Most bat species do not have an orofacial cleft in their skull. Palatal bones are composed of three sets of bones: the premaxilla (blue), the maxilla (pink), and the palatine (gray). Each is a paired structure originally, having left and right elements. In adults, these bones are fused to each other and form the palate. In bats, the sutures between the bones constructing their skull (including the palate region) usually become indistinct through complete fusion of the bones. Middle row: In bat species with the midline cleft, the growth of the premaxilla bones toward the midline is inhibited and this makes medially unfused premaxilla bones. Heterochrony in ossification of the premaxilla bone (shorter and/or delayed ossification of the bone compared to the ancestor) may result in such a small-scale morphological change in the tip of the face. Bottom row: In bat species with the bilateral cleft, the cleft is likely formed through three developmental steps: (1) the domain of the secondary palate expands antero-medially, possibly through changes in growth and patterning of the facial processes at the early stages of embryogenesis. This narrows the space for the primary palate (arrows in the left illustration). (2) The maxilla bones are elongated anteriorly (arrows in the central illustration) compared to in bats species without orofacial clefts as well as those with midline clefts, acquiring its anterior projection. Simultaneously, the position of the premaxilla bones is confined at the center of the tip of the face, due to reduction of the space for its lateral expansion. (3) The boundary between the (anterior) premaxilla and (posterior) maxilla is left as a joint connected through loose connective tissue (a white dashed line in the right illustration). The space between the (medial) premaxilla and the (lateral) maxilla bones is left as a cleft