The really isn't a good answer for this. New species are constantly being discovered that may or may not be the so-called "ancestor fish". Since I started college three years ago I have heard of seven different species now that are considered to be our earliest chordate ancestors, the most common one referenced though is the Pikaia. Agnatha is a group of vertebrates known as lampreys which are still alive today, though organisms such as Pikaia were very similar to the modern lampreys.