ostracoderm
noun
os·tra·co·derm ˈä-strə-kō-ˌdərm 
ä-ˈstra-kə-
: 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
Note: 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.
Word History
Etymology
from the base of New Latin Ostracodermi, subclass name, after earlier Ostracoderma, barnacle family, both formed from adaptations of Greek ostrakódermos "with a shell like a potsherd, hard-shelled," from óstrakon "earthen pot, potsherd, hard shell of a mollusk or tortoise" + -o- -o- + -dermos "having skin (of the kind specified)," adjective derivative of dermat-, dérma "skin" — more at ostracon, derm-
First Known Use
1890, in the meaning defined above