The first vertebrates
And the, quite suddenly,400 to 500 million years ago, in the waters of the Ordovician Era, the first real vertebrated made their appearance. They were the ostracoderms (shell-skin), still very smell, a few inches long, but armored with the heavy, bony structure coviering their heads and the front part of their bodies that gave them their name.
The mystery is how this armor-plating came to be There is always some logic,some good reason behind the structures and organization of anatomy.One hypothesis says that their shield-like covering was to protect them against the high concentration of salt in the sea (the earliest ostracoderms did,in fact,inhabit the seas) or against a precipitation of calcium salts deriving from their excessive concentration in the seas of the period. But other animals living in the seas at the same time may well have been the real reason ; these were the arthropods (with limbs), including the Pterygotus, the old ancestors of lobsters and shrimps,gigantic at the time, about six feet long. These hunter fish, with their strong nippers and claws, were probably very bitter enemies of the first, defenseless vertebrates to whom robust armor-plating could offer protection.It may well have been the fight for survival agalist those first gigantic arthropods that caused the ostracoderms to form their bony shield.