Ostracoderms - an extinct group with heavy armour
Within the agnathan group were other small fish-like animals called ostracoderms and possessed heavy armour-plating which may have originated from deposition of salts derived from their food. This marks the first presence of bone, the material that was destined to influence much of the evolution of the vertebrates. These early bony plates may have provided protection against the large (2 m) sea scorpions that co-existed at the same time. Heavy deposition of salts in the head region have permitted remarkable fossilisation of these animals where the structure of the brain and nerve and blood vessels can be identified. In addition a balancing mechanism composed of two arching tubes at right angles to the vertical plane has been recognized. The liquid within these tubes, moved over the sensitive inside surfaces enabled these animals to be aware of their posture in the water. These animals dominated the freshwater streams 500 million years ago and the largest representatives reached 600 mm in length. The single median fins down the midline of their back provided stability in locomotion, but only the group Cephalaspidomorpha had paired lateral appendages that may have had a similar function to the lateral fins of true fishes. All these animals had gills located in pouches
