Tetrapods originated in shallow waters in which limbs like those of the tetrapods were advantageous for locomotion and useful, along with a neck, for raising partly out of the water. The same strengthening is true for the ribs which they had. Their derived characters are four limbs with digits, a neck, a lack of gills (usually), and different developmental fates for the pharyngeal clefts.
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This information summarizes what is said on pp. 725-726.