Ontogenetic trends in order and timing of canal segment morphogenesis in SO and MD canals. Staging of the development of each canal segment is illustrated for the supraorbital (SO) canal in Tramitichromis (A), Aulonocara (B), and the mandibular (MD) canal in Tramitichromis (C) and Aulonocara (D) in larvae and juveniles (6 to 19.5 mm SL). Light to dark grays represent the progression among developmental stages for each canal segment (white = Stage I, darkest gray = Stage IV) on right and left sides, denoted by diagonal lines - see key. Note that in Tramitichromis canal segments become enclosed (Stage III) and ossified (Stage IV) at smaller fish sizes than in Aulonocara in both the SO and MD canals. In both species, the SO canal shows directionality in development (caudal to rostral) and the MD canal shows weak bidirectionality in Tramitichromis only, whereas no pattern was evident in Aulonocara.