Not to be confused with the Osteichthyes ( `` bony fish '' ) , who were ancestors of the bony fish , and hence of tetrapods ( including human beings ) . By the upper Silurian the Agnathans had reached the high point of their evolution . The first armored agnathansâ '' the Ostracoderms , precursors to the bony fish and hence to the tetrapods ( including humans ) â '' are known from the middle Ordovician , and by the Late Silurian the agnathans had reached the high point of their evolution .