Agnatha

Jawless fish Temporal range: ; Cambrian Stage 3–Present;
	A lamprey, a jawless fish belonging to Cyclostomi
	Life reconstruction of extinct agnathan genus, Pteraspis (Pteraspidomorphi)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Agnatha; Cope, 1889
Groups included
	†Anaspidomorphi; †Conodonta; Cyclostomi (hagfish and lampreys); †Galeaspida; †Myllokunmingiida?; †Metaspriggina?; †Nuucichthys?; †Osteostraci; †Palaeospondylus?; †Pituriaspida; †Pteraspidomorpha; †Thelodonti; †Yunnanozoonidae?; †Zhongxiniscus?;
Cladistically included but traditionally excluded taxa
	Gnathostomata (jawed vertebrates);

Agnatha (/ˈæɡnəθə, æɡˈneɪθə/; from Ancient Greek ἀ- (a-) 'without' and γνάθος (gnáthos) 'jaws') or jawless fish is a paraphyletic infraphylum of animals in the subphylum Vertebrata of the phylum Chordata, characterized by the lack of jaws. The group consists of both living (cyclostomes such as hagfishes and lampreys) and extinct clades (e.g. conodonts and cephalaspidomorphs, among others). They are sister to vertebrates with jaws known as gnathostomes, who evolved from jawless ancestors during the early Silurian by developing folding articulations in the first pairs of gill arches.

Molecular data, both from rRNA and from mtDNA as well as embryological data, strongly supports the hypothesis that both groups of living agnathans, hagfishes and lampreys, are more closely related to each other than to jawed fish, forming the superclass Cyclostomi.

The oldest fossil agnathans appeared in the Cambrian. Living jawless fish comprise about 120 species in total. Hagfish are considered members of the subphylum Vertebrata, because they secondarily lost vertebrae; before this event was inferred from molecular and developmental data, the Craniata hypothesis was accepted (and is still sometimes used as a strictly morphological descriptor) to reference hagfish plus vertebrates.