The development of a muscular pumping pharynx and strengthening of gills resulted in increased efficiency in retrieving food and oxygen. The number of gill pouches and gill arches had surplus to requirements, thus, the spare gill arches evolved into jaws. As a result of the constant pumping of the muscular pharynx for drawing water in, pressure is applied to the anterior gill arches, resulting in them forming hinges to counter the pressure they experience. With increased efficiency in movement, respiration and the organism as a whole, an increased energy requirement follows. This means that an increase in food intake is necessary for optimal continuous growth of the organism. All these were driving factors to the evolution of jaws from gill arches.
The 1st anterior gill arch, the premandibular arch, moves forward and up to form the roof of the mouth. This provides protection of the developing brain from the possible dangers of the new food types being consumed, such as the sharp edges of calcareous shells.
The 2nd gill arch, the mandibular arch, moves forward and forms at least the lower jaw. This supports the mouth and allows for larger prey item to be consumed.
The 3rd gill arch, the hyomandibular arch, moves forward and behind the jaw, fusing to the skull to prevent the jaw from moving backwards into the organism itself when catching or consuming prey.
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