Evolutionary Origins: 

Agnatha are thought to be the earliest class or vertebrates,  scientists have discovered fossils of Agnathans from the Cambrian period which occurred over 500 million years ago. They are of great evolutionary interest since they are extremely distinguishable from other craniates because of what they lack, such as jaws, lateral fins supported by fin rays, vertebrae, a horizontal semi-circular canal in the ear and genital ducts.