A very important group of agnathans are the ostracoderms which also appeared during the Ordovician. These took on a lot of forms. Heterostracans were another group of heavily armored agnathans. All of the ostracoderms and heterostracans are now extinct and the only two groups of agnathans which remain are the lampreys and the hagfish, both eel-shaped, mostly parasitic animals. The Acanthodians and Placoderms - Evolution of Jaws Jaws are thought to have evolved from gill arches and to have evolved only once in biological history.
Jawless fishes have bony gill supports between the gill slits.
It's thought that perhaps a forward gill support became attached to the skull in an agnathan fish, perhaps something like a conodont. Then this the upper part of the gill support became the upper jaw and the lower part of the gill support became the lower jaw. Jaws permitted a switch away from filter feeding to active predation upon smaller animals, including other fish. With the development of jaws, the pharynx no longer had a role as a strainer and part of the gut enlarged into a stomach to store and break down large pieces of food. The first known jawed fishes are the acanthodians .