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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.
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.
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.
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.
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Jamoytius kerwoodi is an extinct species of primitive, eel-like jawless fish known from the Patrick Burn Formation in Scotland, dating to the Llandovery epoch of the Early Silurian period.
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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.
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.
Furcacaudidae, the "'fork-tailed' agnathans," is an extinct family of thelodontid agnathans from the Lochkovian stage of the Early Devonian epoch and Wenlockian epoch of the Silurian, known from fossils found in Northern Canada. It is the type family of the order Furcacaudiformes, and itself currently includes 6 known species. It was officially described in 1998 by Mark V. H. Wilson and Michael W. Caldwell.
Archipelepidiformes is an order of extinct jawless fishes in the class Thelodonti.
Cowielepis is an extinct genus of jawless fish in the class Anaspida. It is from the Cowie Harbour fish bed of Stonehaven, Scotland, which age is considered as the Silurian, or Early Devonian (Lochkovian).
Cornovichthys is an extinct genus of jawless fish in the super class Anaspidomorphi. It is known from the Middle Devonian of Scotland.
Tiiu Märss is an Estonian geologist and palaeoichthyologist.
Panderodus Is an extinct genus of jawless fish belonging to the order Conodonta. This genus had a long temporal range, surviving from the middle Ordovician to late Devonian. In 2021, extremely rare body fossils of Panderodus from the Waukesha Biota were described, and it revealed that Panderodus had a more thick body compared to the more slender bodies of more advanced conodonts. It also revealed that this conodont was a macrophagous predator, meaning it went after large prey.
This list of fossil fish research presented in 2022 is a list of new fossil taxa of jawless vertebrates, placoderms, cartilaginous fishes, bony fishes, and other fishes that were described during the year, as well as other significant discoveries and events related to paleoichthyology that occurred in 2022.
Achanarella is a genus of extinct jawless fish from the Achanarras Quarry in Scotland, dating to the Middle Devonian. The only known species in the genus is Achanarella trewini.
References
- Super-Class Agnatha. T. Jeffery Parker and William A. Haswell, Textbook of Zoology Vertebrates pp 164–202
- ↑ Woodward AS (1900). "On a new Ostracoderm (Euphanerops longaevus) from the Upper Devonian of Scaumenac Bay, Province of Quebec, Canada". Annals and Magazine of Natural History. Series 7. 5 (29): 416–419. doi:10.1080/00222930008678308.
- ↑ van der Brugghen, Gambit (2015-04-29). "Ciderius cooperi gen. nov., sp. nov., the earliest known euphaneropid from the Lower Silurian of Scotland". Netherlands Journal of Geosciences - Geologie en Mijnbouw. 94 (3): 279–288. doi: 10.1017/njg.2015.18 . ISSN 0016-7746.
- ↑ Sansom RS, Gabbott SE, Purnell MA (June 2013). "Unusual anal fin in a Devonian jawless vertebrate reveals complex origins of paired appendages". Biology Letters. 9 (3): 20130002. doi:10.1098/rsbl.2013.0002. PMC 3645028 . PMID 23576777.
