Agnatha (/ˈæɡnəθə, æɡˈneɪθə/;[3] from Ancient Greek ἀ- (a-) 'without' and γνάθος (gnáthos) 'jaws') is a paraphyletic infraphylum[4] of non-gnathostome vertebrates, or jawless fish, in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct (conodonts, anaspids, and ostracoderms, among others).

In direct contrast, many extinct agnathans sported extensive exoskeletons composed of either massive, heavy dermal armour or small mineralized scales.

Some fossil agnathans, such as osteostracans and pituriaspids, did have paired fins, a trait inherited in their jawed descendants.

Lampreys reproduce in freshwater riverbeds, working in pairs to build a nest and burying their eggs about an inch beneath the sediment.

A possible agnathan that has not been formally described was reported by Simonetti from the Middle Cambrian Burgess Shale of British Columbia.

The first armored agnathans—the ostracoderms, precursors to the bony fish and hence to the tetrapods (including humans)—are known from the middle Ordovician, and by the Late Silurian the agnathans had reached the high point of their evolution.

Most of the ostracoderms, such as thelodonts, osteostracans, and galeaspids, were more closely related to the gnathostomes than to the surviving agnathans, known as cyclostomes.

Approximately 500 million years ago, two types of recombinatorial adaptive immune systems (AISs) arose in vertebrates.

Instead, the AIS of jawless vertebrates is based on variable lymphocyte receptors (VLRs) that are generated through recombinatorial usage of a large panel of highly diverse leucine-rich-repeat (LRR) sequences.

[19] Three VLR genes (VLRA, VLRB, and VLRC) have been identified in lampreys and hagfish, and are expressed on three distinct lymphocytes lineages.

Phylogeny based on the work of Mikko Haaramo and Delsuc et al.[41][42] Hyperotreti/Myxini (hagfishes) Petromyzontomorpha (lampreys) ?†Conodonta †Pteraspidomorpha ?†Jamoytiiformes ?†Euphanerida †Anaspida †Thelodonti †Galeaspida †Pituriaspida †Osteostraci Gnathostomata (vertebrates with jaws) While the "Agnatha" Conodonta was indeed jawless, if it would have continued to live, its descendants would still be closer related to e.g. humans than to lampreys, and also contemporary it was closer related to the ancestor of humans.