The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya), to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by a few million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.
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.
Sarcopterygii — sometimes considered synonymous with Crossopterygii — is a clade including both a group of bony fish commonly referred to as lobe-finned fish, and tetrapods. They are characterised by prominent muscular limb buds (lobes) within their fins, which are supported by articulated appendicular skeletons. This is in contrast to the other clade of bony fish, the Actinopterygii, which have only skin-covered bony spines supporting the fins.
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 emerged from within them and they would survive if the jawed fish are included.
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.
Heterostraci is an extinct subclass of pteraspidomorph, Ostracoderm, jawless vertebrate that lived primarily in marine and estuary environments. Heterostraci existed from the mid-Ordovician to the conclusion of the Devonian.
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.
Minicrania is an extinct genus of tiny antiarch fish, with armor averaging up to about 2 centimetres (0.79 in) long, which lived during the Lochkovian epoch in Early Devonian Yunnan Province, China and northern Vietnam.
The Xitun Formation is a palaeontological formation which is named after Xitun village in Qujing, a location in South China. This formation includes many remains of fossilized fish and plants of the Early Devonian period. It was originally referred to as the Xitun Member of the Cuifengshan Formation.
Shuyu is an extinct genus of early jawless vertebrate from the Early or Middle Silurian period. It is the basalmost known eugaleaspidiform galeaspid and it lived in what is now northwestern Zhejiang Province, Southeast China. It is known from more than 20 headshields and at least 20 of them include three-dimensionally preserved neurocrania. The specimens of Shuyu were collected from the lower part of Maoshan Formation, located in Changxing District. Shuyu zhejianensis was first assigned by Pan, 1986 to a species of Sinogaleaspis. The genus was first named by Zhikun Gai, Philip C. J. Donoghue, Min Zhu, Philippe Janvier and Marco Stampanoni in 2011 and the type species is Shuyu zhejianensis.
The Xikeng Formation is located in Xiushui County, Jiangxi Province, and contains alternating beds of purplish red, grayish green and yellow green sandy and muddy rocks. The Xikeng Formation is dated to the Late Silurian - Early Devonian period.
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.
Xiangshuiosteus wui is an extinct monospecific genus of brachythoracid arthrodire placoderm from the Late Emsian stage of the Early Devonian epoch, discovered in Wuding County of Yunnan province, China. It has recently been reassessed as a dunkleosteid.
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.
This list of fossil fish research presented in 2022 is a list of new taxa of jawless vertebrates, placoderms, acanthodians, fossil 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.
