ostracoderm

:  any of various primitive, extinct jawless fish (class Agnatha) of the Lower Paleozoic with a bony covering of plates or scales serving as a protective armor and which on the broad head usually form a bony shield of fused plates ◆Distant relatives of the hagfish and lampreys, ostracoderms were typically less than 12 inches (30.5 centimeters) long. Unlike earlier vertebrates using the gills for both feeding and breathing, ostracoderms were the first known forms to use the gills exclusively for respiration.