Ribs

Length measurements of individual ribs were performed by using the point-function in Drishti (version 2.7), placing landmarks onto the outermost voxels of the bony elements and later connecting them to form a line. To compare rib measurements across selected ricefish species, thoracic ribs were numbered depending on their location relative to the base of the lateral process [23] of each specimen’s pelvic girdle. As we were focusing on potential differences of rib lengths in the region of the pelvic fins where the egg-bundle is situated during brooding, the pelvic girdle was chosen as a reference point from where the ribs were numbered. Ribs aligned with this position were defined as rib 0 (R0; colored orange in Fig. 5a). Ribs anterior to it were numbered incrementally with negative numbers (colored yellow in Fig. 5a), ribs posterior were labeled with positive numbers (colored blue in Fig. 5a). Potentially incomplete ribs (e.g. caused by fracture or ontogenic reasons) [23] attached to the anterior most vertebrae were not counted.

Rib length (Fig. 5b) was defined as the length of the costal arch between its articulation at its vertebra and the ventral tip of a rib. We measured rib length by orienting the specimen’s volume-render with its head pointing left and orthogonally to the viewing plane. First, landmarks were placed centrally onto the bones from a dorsal viewing angle on the rib portion close to its respective vertebra; then the render was rotated 90° along the longitudinal body axis to mark the remaining thoracic part of each rib. Rib gap (Fig. 5b) was defined as the distance between the left and the right rib of the same vertebra. This measure was taken by placing points onto the tip of each rib and measuring the distance as a straight line between them. Finally, rib height (Fig. 5b) was defined as the maximum extension of the rib from the vertebral column independent from its curvature. It was measured by calculating the direct distance between the ventral tip and the dorsal end of a rib in maximum intensity projections from a lateral perspective using the measurement-tool embedded in Fiji [75].