The ancestry of fish can be traced back over 500 million years to the Cambrian Period. Fish were the first animals to develop a vertebral column. Fish are fully aquatic and usually breathe through gills.
The most primitive fish belong to a group known as Agnatha. The Agnathans are commonly known as the "jawless fishes" and the only surviving members are lampreys and hagfish. The Agnathans never developed a jaw apparatus as seen in higher vertebrates.
"Jawed fishes," or Gnathostomes are divided into 4 classes. Members of the Class Placodermi (armored fishes such as Bothriolepis) and Acanthodii (spiny sharks) became extinct by the end of the Paleozoic Era. Members of Chondrichthyes (sharks, skates, and rays) and Osteichthyes (bony fish) still flourish today.