Ostracoderms are the armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic and thus does not correspond to one evolutionary lineage. However, the term is still used as an informal way of loosely grouping together the armored jawless fishes.
Cardipeltis bryanti, a lower Devonian ostracoderm from the Bighorn Mountains of Wyoming. Ventral (underside) exposed.
Image: Galeaspida 1
Image: Pituriaspida
Image: Larnovaspis stensioei
Agnatha is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct species. Among recent animals, cyclostomes are sister to all vertebrates with jaws, known as gnathostomes.
Agnatha
Image: Eptatretus hexatrema 15727711
Image: Pacific lamprey facing right
Image: Thyestes verrucosus 22DB