vertebrates. Just like the molluscs and arthropods, the early fish had a hard outer covering. This armor plating around their front ends was made of bone. Perhaps it was a defense against sea scorpions. They first appeared around 500 million years ago, during the Ordovician period as small fish-like animals called ostracoderms.
They had probably ate by sucking in mud through their mouths. They filtered out particles of food as the water left through their gills. Jaws appeared later, and so did paired fins which were used for swimming. Some had two fins, some four or more. Many of them had flattened bodies, and were probably not very agile.
Early fish contained a swim bladder -- a bag of air to help them float. The strong bone casing around the head allowed the brain, eyes and other senses to develop.
Some jawless fish still survive today. They are the lampreys and hagfish.