[Ebook] Encyclopedia of Aquarium & Pond Fish – Introduction to FISHKEEPING – Evolution and classification
Fish are the oldest of all vertebrates (animals with backbones), with an ancestry dating back more than 500 million years. However, the earliest fish to appear in the world’s oceans were very different from those seen today, since they had no jaws or scales. They lacked specialized fins, so they relied solely on tail movements to propel them through the water. Internally, their spine was made of cartilage, rather than bone.