The first vertebrates were fishlike organisums that appeared over five-hundred-million years ago. They were Jawless.They were also cartilaginous and hardly preserved and had bony external armor that covered the head and most of "the trunk". They grew to ten inches long, They had thick flat bodies with only a set of side fins to help in manuvering. They probably swam poorly just above the sea-bed. The mouth helped gather oxegen and to retain bits of food. After true fish appeared about four-hundred-million years ago. At this time ostracoderms quickly became extinct.
Some ostracoderm plates are found in Devonian bone beds in Indiana. |
