Agnatha is a paraphyletic infraphylum of non-gnathostome vertebrates, or jawless fish, in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct. Among recent animals, cyclostomes are sister to all vertebrates with jaws, known as gnathostomes.
Gnathostomata are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. Most gnathostomes have retained ancestral traits like true teeth, a stomach, and paired appendages. Other traits are elastin, a horizontal semicircular canal of the inner ear, myelin sheaths of neurons, and an adaptive immune system which has discrete lymphoid organs, and uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.
A craniate is a member of the Craniata, a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia, and the much more numerous Gnathostomata. Formerly distinct from vertebrates by excluding hagfish, molecular and anatomical research in the 21st century has led to the reinclusion of hagfish as vertebrates, making living craniates synonymous with living vertebrates.
Cephalaspidomorphs are a group of jawless fishes named for Cephalaspis of the osteostracans. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys, because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. They are the closest relatives of jawed fishes, who may have emerged from within them; if this is true, they would survive if the jawed fish are included.
Markuelia is a genus of fossil worm-like bilaterian animals allied to Ecdysozoa and known from strata of Lower Cambrian to Lower Ordovician age containing five species.
Placoderms are vertebrate animals of the class Placodermi, an extinct group of prehistoric fish known from Paleozoic fossils during the Silurian and the Devonian periods. While their endoskeletons are mainly cartilaginous, their head and thorax were covered by articulated armoured plates, and the rest of the body was scaled or naked depending on the species.
Anaspida is an extinct group of jawless fish that existed from the early Silurian period to the late Devonian period. They were classically regarded as the ancestors of lampreys, but it is denied in recent phylogenetic analysis, although some analysis show these group would be at least related. Anaspids were small marine fish that lacked a heavy bony shield and paired fins, but were distinctively hypocercal.
The class Osteostraci is an extinct taxon of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian.
Galeaspida is an extinct taxon of jawless marine and freshwater fish. The name is derived from galea, the Latin word for helmet, and refers to their massive bone shield on the head. Galeaspida lived in shallow, fresh water and marine environments during the Silurian and Devonian times in what is now Southern China, Tibet and Vietnam. Superficially, their morphology appears more similar to that of Heterostraci than Osteostraci, there being currently no evidence that the galeaspids had paired fins. A galeaspid Tujiaaspis vividus from the Silurian period of China was described in 2022 as having a precursor condition to the form of paired fins seen in Osteostraci and gnathostomes. Earlier than this, Galeaspida were already in fact regarded as being more closely related to Osteostraci, based on the closer similarity of the morphology of the braincase.
Doryaspis is an extinct genus of primitive jawless fish that lived in the Devonian period. Fossils have been discovered in Spitsbergen.
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.
Westergaardodina is a species-rich genus of spine, U or W-shaped paraconodont known from Middle Cambrian to Lower Ordovician strata.
Romundina is a small, heavily armored extinct genus of acanthothoracid placoderms which lived in shallow marine environments in the early Devonian (Lochkovian). The name Romundina honors Canadian geologist and paleontologist Dr. Rómundur (Raymond) Thorsteinsson of Calgary, Alberta, Canada. Romundina are believed to have lived on Earth between 400 and 419 million years ago. The closest known relative to Romundina is the acanthothoracid Radotina. The type and only described species is R. stellina.
Thyestiida is an order of bony-armored jawless fish in the extinct vertebrate class Osteostraci.
Shimenolepis granifera is an extinct yunnanolepid placoderm from the Xiaoxi Formation, Li County, Hunan, China. Its age is discussed, while originally considered as late Llandovery, it is later considered to belong to the Ludlow Epoch instead. It was the first described Silurian placoderm, and was the earliest known placoderm until Xiushanosteus was described, known from distinctively ordered plates.
Eugaleaspidiformes is an extinct order of jawless marine and freshwater fish, which lived in East Asia from the Telychian to the Lower Devonian period. The order was first named by Lui in 1965.
Yongdongaspis, colloquially known as the Binhai Yongdong, is an extinct genus of galeaspid vertebrates within the monotypic family Yongdongaspididae. The type species is Y. littoralis and it was found in the Silurian-aged Huixingshao Formation of China. This makes it the oldest known fish fossil discovered in Chongqing to date.
Compagopiscis is an extinct genus of placoderm known from the Gogo Formation. It lived in the Upper Devonian of Western Australia. The genus is monotypic, with its only species being Compagopiscis croucheri.
Foxaspis is a genus of duyunolepidid galeaspid from the early Devonian Xiaoshan Formation in Guangxi, Southern China. The type and only species is F. novemura, known from two well-preserved specimens.
