Chondrostei Temporal range: | |
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Atlantic sturgeon, Acipenser oxyrinchus | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Actinopterygii |
Subclass: | Chondrostei Müller, 1844 |
Orders | |
See text |
Chondrostei is a subclass of non-neopterygian ray-finned fish. While the term originally referred to the paraphyletic grouping of all non-neopterygian ray-finned fish, it was redefined by Patterson in 1982 to be a clade comprising the Acipenseriformes (which includes sturgeon and paddlefish) and their extinct relatives. [1]
Taxa commonly suggested to represent relatives of the Acipenseriformes include the Triassic marine fish Birgeria and the Saurichthyiformes, but their relationship with the Acipenseriformes has been strongly challenged on cladistic grounds. [2] Coccolepididae, a group of small weakly ossified Jurassic and Cretaceous fish found in both marine and freshwater environments, have also been suggested to be close relatives of the Acipenseriformes. However, this has never been subject to cladistic analysis. [1] Near & Thacker (2024) also recovered the ptycholepiform Boreosomus as a stem-acipenseriform. [3]
The following taxa are known: [4]
Actinopterygii, members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of skin supported by radially extended thin bony spines called lepidotrichia, as opposed to the bulkier, fleshy lobed fins of the sister class Sarcopterygii. Resembling folding fans, the actinopterygian fins can easily change shape and wetted area, providing superior thrust-to-weight ratios per movement compared to sarcopterygian and chondrichthyian fins. The fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the articulation between these fins and the internal skeleton.
Acipenseriformes is an order of basal ray-finned fishes that includes living and fossil sturgeons and paddlefishes (Acipenseroidei), as well as the extinct families Chondrosteidae and Peipiaosteidae. They are the second earliest diverging group of living ray-finned fish after the bichirs. Despite being early diverging, they are highly derived, having only weakly ossified skeletons that are mostly made of cartilage, and in modern representatives highly modified skulls.
Neopterygii is a subclass of ray-finned fish (Actinopterygii). Neopterygii includes the Holostei and the Teleostei, of which the latter comprise the vast majority of extant fishes, and over half of all living vertebrate species. While living holosteans include only freshwater taxa, teleosts are diverse in both freshwater and marine environments. Many new species of teleosts are scientifically described each year.
Saurichthys is an extinct genus of predatory ray-finned fish from the Triassic Period. It is the type genus of the family Saurichthyidae, and the most speciose and longest lasting genus in the family. This family also includes the Permian Eosaurichthys (China) and the Jurassic Saurorhynchus from Europe and North America, though it may be more appropriate to treat these as subgenera of Saurichthys, due to the genus Saurichthys otherwise being paraphyletic.
Holostei is a group of ray-finned bony fish. It is divided into two major clades, the Halecomorphi, represented by the single living genus, Amia with two species, the bowfins, as well as the Ginglymodi, the sole living representatives being the gars (Lepisosteidae), represented by seven living species in two genera. The earliest members of the clade, which are putative "semionotiforms" such as Acentrophorus and Archaeolepidotus, are known from the Middle to Late Permian and are among the earliest known neopterygians.
The Palaeonisciformes, commonly known as "palaeoniscoids" are an extinct grouping of primitive ray-finned fish (Actinopterygii), spanning from the Silurian/Devonian to the Cretaceous. They are generally considered paraphyletic, but their exact relationships to living ray-finned fish are uncertain. While some and perhaps most palaeoniscoids likely belong to the stem-group of Actinopteryii, it has been suggested that some may belong to the crown group, with some of these possibly related to Cladistia and/or Chondrostei. Many palaeoniscoids share a conservative body shape and a similar arrangement of skull bones, though paleoniscoids as a whole exhibit considerable diversity in body shape.
Birgeria is a genus of carnivorous marine ray-finned fish from the Triassic period. Birgeria had a global distribution, with fossil known from Madagascar, Spitsbergen, Germany, Switzerland, Italy, Slovenia, China, Russia, Canada and Nevada, United States. The oldest fossils are from Griesbachian aged beds of the Wordie Creek Formation of East Greenland. Birgeria existed throughout the entire Triassic period, from the very beginning just after the Permian-Triassic mass extinction, up to the very end with its extinction during the Triassic-Jurassic mass extinction.
Stichopterus is an extinct genus of chondrostean ray-finned fish that lived during the Early Cretaceous epoch in Asia. It has been found in Russia and Mongolia.
Cyranorhis is an extinct genus of marine ray-finned fish that lived during the Serpukhovian age of the Carboniferous period. One species is known, C. bergeraci in the Bear Gulch Limestone what is now Montana, United States. It is named after French novelist Cyrano de Bergerac.
Cladistia is a subclass of bony fishes whose only living members are the bichirs of tropical Africa. Their major synapomorphies are a heterocercal tail in which the dorsal fin has independent rays, and a posteriorly elongated parasphenoid.
Saurichthyiformes is an extinct order of ray-finned fish which existed in Asia, Africa, Australia, Europe and North America, during the late Permian to early Middle Jurassic. Saurichthyiiformes comprise two families, Saurichthyidae and Yelangichthyidae. Yelangichthyidae is monotypic, containing only the genus Yelangichthys. The gar or needlefish-like Saurichthyidae is primarily known from the genus Saurichthys. Additionally, the subgenera SaurorhynchusCostasaurichthys, Eosaurichthys, Lepidosaurichthys, and Sinosaurichthys are frequently used to group species, and are sometimes considered separate genera. Species are known from both marine end freshwater deposits. They had their highest diversity during the Early and Middle Triassic. Their phylogenetic position is uncertain, while they have often been considered members of Chondrostei, and thus related to living sturgeons and paddlefish, phylogenetic analysis of well-preserved remains has considered this relationship equivocal. They may actually belong to the stem-group of Actinopterygii, and thus not closely related to any living group of ray-finned fish.
Ptycholepiformes are an extinct order of prehistoric ray-finned fish that existed during the Triassic period and the Early Jurassic epoch. The order includes the genera Acrorhabdus, Ardoreosomus, Boreosomus, Chungkingichthys, Ptycholepis, and Yuchoulepis. Although several families have been proposed, some studies place all these genera in the same family, Ptycholepididae.
Scheenstia is an extinct genus of neopterygian ray-finned fish from the Late Jurassic–Early Cretaceous of Europe. Fossils have been found in both marine and freshwater environments.
Ginglymodi is a clade of ray-finned fish containing modern-day gars (Lepisosteidae) and their extinct relatives in the order Lepisosteiformes, the extinct orders Semionotiformes and Kyphosichthyiformes, and various other extinct taxa. Ginglymodi is one of the two major subgroups of the infraclass Holostei, the other one being Halecomorphi, which contains the bowfin and eyespot bowfin and their fossil relatives.
Parasemionotiformes is an extinct order of neopterygian ray-finned fish that existed globally during the Triassic period. It comprises the families Parasemionotidae and Promecosominidae. Many of the included genera are monotypic and most species lived during the Early Triassic epoch.
Redfieldiiformes is an extinct order of ray-finned fish (actinopterygians) which lived from the Early Triassic to Early Jurassic. Redfieldiiforms were fairly typical Triassic fish in overall anatomy. They had a fusiform body shape with thick, ganoine-covered scales. The dorsal and anal fins were large, positioned opposite from each other, and shifted back, close to the tail. The caudal fin was hemiheterocercal, with the vertebral column and body scales extending into an upper lobe equal in size and shape to the lower lobe. They also had several characteristic skeletal traits, such as a hatchet-shaped preopercle, a series of fulcra fringing the fins, a reduced number of branchiostegal rays, and a snout ornamented with tubercles.
Coccolepididae is an extinct family of ray-finned fish, known from the Early Jurassic to Early Cretaceous, most of which were originally referred to the type genus Coccolepis. They had a widespread distribution, being found in North and South America, Australia, Asia and Europe. They are mostly known from freshwater environments, though several species have been found in marine environments. They are morphologically conservative, and have poorly ossified endo and exoskeletons, which usually results in poor preservation. This makes it difficult to distinguish species. They are generally small fish, with the largest known specimens reaching a length of 210 mm. Historically, they have been classified as members of “Palaeonisciformes”, a paraphyletic grouping of non-neopterygian fish, due to their plesiomorphic conservative morphology closely resembling those of many other groups of primitive fish. Some recent authors have suggested that they may belong to the order Chondrostei as relatives of the Acipenseriformes.
Lepidotidae is an extinct family of fish, known from the Jurassic and Cretaceous periods. Most species were originally assigned to the genus Lepidotes which was long considered a wastebasket taxon. Cladistic analysis has indicated that they are close relatives of gars, with both being members of the order Lepisosteiformes. Members of the family are known from both marine and freshwater environments. Their bulky body morphology and small median fins suggests that they were slow-moving fish that were capable of performing fine movements in order to grasp prey. Lepidotes sensu stricto had peg-like grasping marginal teeth and crushing palatal teeth, and is known to have consumed small crustaceans, while Scheenstia had low rounded crushing marginal teeth, indicating a durophagous diet.
Lamprimorpha is a superorder of marine ray-finned fishes, representing a basal group of the highly diverse clade Acanthomorpha. Represented today only by the order Lampriformes, recent studies have recovered other basal fossil species of the group dating as far back as the Cenomanian stage of the Late Cretaceous. Some of these fossil taxa, such as the paraphyletic genus Aipichthys, are among the oldest known fossil acanthomorphs, and overall they appear to have been a major component of the marine fish fauna at that time. Lamprimorpha is thought to be the sister group to the superorder Paracanthopterygii, which contains cod, dories, and trout-perches; however, Lamprimorpha may instead be sister to Acanthopterygii.
Fishes are a paraphyletic group and for this reason, the class Pisces seen in older reference works is no longer used in formal taxonomy. Traditional classification divides fish into three extant classes, and with extinct forms sometimes classified within those groups, sometimes as their own classes: