Protofish and internal bony skeletons
An important development in one group of protofish was the development of bony rods stiffening the pillars of flesh between the gill slits. The first pair of which hinged forward and were supported with muscle tissue, and produced the first jaws. The evolution of jaws permitted fish and their descendants to utilize larger and harder food, and thus enabled them to become adapted to many new and diversified ways of living. This advance was of sufficient importance so that fish and tetrapods (four-legged animals) are together called gnathostomes (= jaw + mouth). Some of the bony scales in the skin which covered these animals enlarged and became the first teeth. Lateral flaps of skin evolved into the first true fins and their swimming skills improved. These animals were called Placoderms and may have pioneered the gas bladder for vertical movement in water and eventually evolved into lungs. The most impressive of the placoderms was the Arthrodira which reached 9 m and possessed large jaws equipped with serrated teeth.
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Placodermi Conservation status: Fossil | ||||||||
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The Placodermi are armoured prehistoric fishes known from fossils dating to the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked. Placoderms were the first of the jawedfish, their jaws likely evolving from the first of their gill arches.
