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.
Gnathostomata are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living gnathostomes have true teeth, paired appendages, the elastomeric protein of elastin, and a horizontal semicircular canal of the inner ear, along with physiological and cellular anatomical characters such as the myelin sheaths of neurons, and an adaptive immune system that has the discrete lymphoid organs of spleen and thymus, and uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.
Acanthodii or acanthodians is an extinct class of gnathostomes. They are currently considered to represent a paraphyletic grade of various fish lineages basal to extant Chondrichthyes, which includes living sharks, rays, and chimaeras. Acanthodians possess a mosaic of features shared with both osteichthyans and chondrichthyans. In general body shape, they were similar to modern sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteians.
Thelodonti is a class of extinct Palaeozoic jawless fishes with distinctive scales instead of large plates of armor.
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.
Phlebolepis is an extinct thelodont agnathan genus belonging to the family Phlebolepididae. Whole fossils are found in Late Silurian aged strata from Saaremaa, Estonia. Phlebolepis elegans was average-sized for a thelodont, 7 cm long.
Furcacauda is a genus of thelodontid agnathan from the Lower Devonian of Canada, and is the type genus of the order Furcacaudiformes. Furcacaudiform thelodontids were deep water jawless vertebrates with symmetrical fork and lobed-finned tails and scales smaller than typical loganellid and nikoliviid thelodonti scales. Furcacaudiform thelodonts are noted as having a laterally compressed body, large anterior eyes, slightly posterior, lateral, and vertical to a small mouth, and a condensed curved row of branchial openings (gills) directly posterior to the eyes. Many but not all had laterally paired fins. Wilson and Caldwell also note the presence of a caudal peduncle and a long caudal fin made of two large lobes, one dorsal and one ventral separated by 8 to 14 smaller intermediate lobes, giving the appearance of a striated half-moon shaped tail resembling the tail of a heterostracan. A large square cavity within the gut connecting a small intestine to an anal opening lead many to believe that it is this genus that exhibits the first vertebrate stomach. According to Wilson and Caldwell their discovery, based on sediment infillings of fossils of the Furcacauda heintze, gives credence to the evolutionary development of stomach before jaws.
Drepanolepis is an extinct genus of thelodont which lived in Canada during the Early Devonian period.
A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as well as possible hydrodynamic advantages. The term scale derives from the Old French escale, meaning a shell pod or husk.
Longodus is an extinct genus of thelodont, placed in its own family – Longodidae – which existed in what is now Estonia during the Ludlow epoch of the upper Silurian period. The type and only species is Longodus acicularis. They are most noted for their long, needle-like scales, which run vertically along their trunk.
Thelodontiformes is an extinct order of jawless fish of the Silurian.
Furcacaudidae is a family of thelodontid agnathan from the Lower Devonian. It is the type family of the order Furcacaudiformes.
Archipelepidiformes is an order of extinct jawless fishes in the class Thelodonti.
Archipelepis is a genus of extinct thelodont agnathans, and are the most primitive recognized thelodonts of which whole body fossils are known. Fossils of bodies and scales are currently known from Late Telychian to Wenlock-aged marine strata of northern Canada.
Phlebolepididae is an extinct thelodont agnathan family in the order Phlebolepidiformes.
Erepsilepis is an extinct thelodont agnathan genus in the family Phlebolepididae.
Birkeniiformes is an extinct order of jawless fish belonging to the class Anaspida.
Anaspidomorphi (anaspidomorphs) is an extinct superclass of jawless fish.
Loganelliidae is an extinct family of thelodonts in the order Thelodontiformes. They are distinguished by the star or cross shaped ridges located on their head scales.
Oeselia mosaica is a fossil jawless fish and is the only species in its genus and in the family Oeseliidae. It is a member of an extinct family of thelodonts or jawless fishes. The family and genus were established along with the description of the species by Tiuu Märss in 2005.
