Chondrichthyes is a class of jawed fish that contains the cartilaginous fish or chondrichthyians, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or bony fish, which have skeletons primarily composed of bone tissue. Chondrichthyes are aquatic vertebrates with paired fins, paired nares, placoid scales, conus arteriosus in the heart, and a lack of opecula and swim bladders. Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.
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.
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.
Placodermi is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches.
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.
Lajia is a Bronze Age archaeological site in the upper reaches of the Yellow River, on the border between the Chinese provinces of Gansu and Qinghai. As at other sites of the Qijia culture, the people of Lajia had an agricultural economy based primarily on millet cultivation and sheep herding. They also kept pigs for use in ritual activities, including making oracle bones, and experimented with a high temperature-fired pottery described as proto-porcelain. The world's oldest known noodles were discovered at the site in 2005.
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.
Entelognathus primordialis is a early placoderm from the late Silurian of Qujing, Yunnan, 419 million years ago.
Qilinyu is a genus of early placoderm from the late Silurian of China. It contains a single species, Qilinyu rostrata, from the Xiaoxiang fauna of the Kuanti Formation. Along with its contemporary Entelognathus, Qilinyu is an unusual placoderm showing some traits more similar to bony fish, such as dermal jaw bones and lobe-like fins. It can be characterized by adaptations for a benthic lifestyle, with the mouth and nostrils on the underside of the head, similar to the unrelated antiarch placoderms. The shape of the skull has been described as "dolphin-like", with a domed cranium and a short projecting rostrum.
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.
The Intertemporal bone is a paired cranial bone present in certain sarcopterygians and extinct amphibian-grade tetrapods. It lies in the rear part of the skull, behind the eyes.
The Xiaowa Formation is a Carnian-age geological formation found in southern China. It is a sequence of limestone and marls from the Carnian stage of the Triassic. Its lower section was previously known as the Wayao Formation or Wayao Member of the Falang Formation. In 2002, the Wayao Member was renamed and raised to the Xiaowa Formation to prevent confusion with an Eocene unit of the same name. Crinoids and marine reptiles are abundant in the Xiaowa Formation, forming a lagerstätte known as the Guanling biota. Ammonoids and conodonts found in the formation constrain its age to the early Carnian. Reptiles of the Guanling biota include ichthyosaurs, thalattosaurs, placodonts, and Odontochelys. Sedimentary events within this formation have been tied to the Carnian Pluvial Event.
The Mongolepidida is an order of primitive sharks that lived during the Late Silurian and Early Devonian periods of Russia and Mongolia, with possible fragmentary remains dating back to the Katian epoch of the Late Ordovician. The three known genera are: Elegestolepis, Mongolepis and Polymerolepis. They are only known from fragmentary placoid scales, so their appearance is unknown. The oldest of these scales have been dated back to the Ludlow epoch, making the members of the Mongolepidida the oldest sharks known to date.
Terropterus is a genus of eurypterid, an extinct group of aquatic arthropods. The type and only species of Terropterus, T. xiushanensis, is known from deposits of Early Silurian age in China.
Bianchengichthys is a genus of maxillate placoderm fish from the late Silurian Period. Its fossils have been recovered from Yunnan Province, China, and it is represented by only one species: Bianchengichthys micros.
Qianodus is a jawed vertebrate genus that is based on disarticulated teeth from the lower Silurian of China. The type and only species of Qianodus, Q. duplicis, is known from compound dental elements called tooth whorls, each consisting of multiple tooth generations carried by a spiral-shaped base. The tooth whorls of Qianodus represent the oldest unequivocal remains of a toothed vertebrate, predating previously recorded occurrences by about 14 million years. The specimens attributed to the genus come from limestone conglomerate beds of the Rongxi Formation exposed near the village of Leijiatun, Guizhou Province, China. These horizons have been interpreted as tidal deposits1 that form part of the shallow marine sequences of the Rongxi Formation.
Fanjingshania is an extinct genus of acanthodian from the lower Silurian of China around 439 million years old, making it currently the oldest known acanthodian. It comprises a single species, Fanjingshania renovata which is known from over 1,000 isolated elements, including fin spines, branchiostegal plates, sclerotic plates, trunk scales, and tectal and postorbital tesserae. No teeth were ascribed to the taxon, but in the same horizon the tooth taxon Qianodus was found, and these taxa may be synonymous. In a phylogenetic analysis accompanying the description, it was placed in a clade with Climatius, Parexus, and Ptomacanthus.
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