Chondrostei

Chondrostei are primarily cartilaginous fish showing some degree of ossification. It is thought that the cartilaginous condition is derived, and that the ancestors of this group were bony fish with fully ossified skeletons. Members of this group share with the Elasmobranchii certain features such as the possession of spiracles, a heterocercal tail and the absence of scales. Nevertheless, the fossil record suggests they have more in common with the teleosts. The Chondrostei is probably a paraphyletic grouping; the fifty-two living species are divided among two orders, the Acipenseriformes (sturgeons and paddlefishes), and the Polypteriformes (reedfishes and bichirs).

Chondrostei
Temporal range: Silurian - Recent
Sturgeon2
Atlantic sturgeon, Acipenser oxyrinchus
Nile bichir
Nile bichir, Polypterus bichir
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
(unranked): Actinopteri
Subclass: Chondrostei
Orders

Acipenseriformes
Polypteriformes

Characteristics

The main distinguishing feature of this group is the cartilaginous nature of the skeleton, although some older fish show a degree of calcification. The ancestors of the chondrosteans are thought to be bony fish, but that this characteristic of an ossified skeleton was lost in later evolutionary development, resulting in a lightening of the frame. Elderly chondrostean individuals show beginnings of ossification of the skeleton, which suggests this process is delayed rather than wholly lost in these fishes.[1]

Taxonomy

This group has at times been classified with the sharks: this because the chondrosteans, like the latter, mostly lack bone, and their structure of the jaw is more akin to that of sharks than of other bony fish; further, both groups lack scales (excluding the Polypteriforms). Additional shared features include spiracles and, in sturgeons and paddlefishes, a heterocercal tail (the vertebrae extend into the larger lobe of the caudal fin). However the fossil record suggests these fish have more characters in common with the Teleostei than their external appearance might suggest.[1]

Chondrostei is paraphyletic, meaning this subclass does not contain all the descendants of their common ancestor; reclassification of the Chondrostei is therefore not out of the question.

The name comes from Greek chondros meaning cartilage and osteo meaning bone.

Classification

References

  1. ^ a b "Chondrosteans: Sturgeon Relatives". paleos.com. Archived from the original on 2010-12-25.
Acipenser

Acipenser is a genus of sturgeons. With 17 living species (3 others are only known from fossil remains), it is the largest genus in the order Acipenseriformes. They are native to Europe, Asia and North America, and most species are threatened.

Acipenseroidei

Acipenseroidei is a suborder of Acipenseriformes animals that contains:

Acipenseridae

Acipenserinae

Acipenserini

Scaphirhychini

Husinae

Huso

Polyodontidae

Polyodontinae

Polyodontini

PsephuriniThese animals are well known for the production of caviar.

Actinopterygii

Actinopterygii (), or the ray-finned fishes, constitute a class or subclass of the bony fishes.The ray-finned fishes are so called because their fins are webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii (lobe-finned fish). These actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton (e.g., pelvic and pectoral girdles).

Numerically, actinopterygians are the dominant class of vertebrates, comprising nearly 99% of the over 30,000 species of fish. They are ubiquitous throughout freshwater and marine environments from the deep sea to the highest mountain streams. Extant species can range in size from Paedocypris, at 8 mm (0.3 in), to the massive ocean sunfish, at 2,300 kg (5,070 lb), and the long-bodied oarfish, at 11 m (36 ft).

Amu Darya sturgeon

The Amu Darya sturgeon or false shovelnose sturgeon (Pseudoscaphirhynchus kaufmanni) is a species of fish in the Acipenseridae family. It is found in Tajikistan, Turkmenistan, and Uzbekistan.

Baikal sturgeon

The Baikal sturgeon, Acipenser baerii baicalensis, is a sturgeon indigenous to Lake Baikal in Siberia, Russia. It is a subspecies of the widely distributed Siberian sturgeon (Acipenser baerii). It resides primarily in the northern end of the lake, making considerable movements along the shore, and migrating up the Selenga River to spawn. Not long ago, sturgeons weighing 125 kilograms were not uncommon. The Baikal Sturgeon is now listed as endangered.

Bastard sturgeon

The bastard sturgeon, fringebarbel sturgeon, ship sturgeon, spiny sturgeon, or thorn sturgeon (Acipenser nudiventris) is a species of fish in the Acipenseridae family. Formerly abundant in the Black, Aral and Caspian seas, its range is now primarily limited to the Ural River (in Russia and Kazakhstan), with a possible relict populations in the Rioni River in Georgia and the Safid Rud in Iran. The healthiest population is one in Lake Balkhash in Kazakhstan, well outside its natural range, where they were introduced in the 1960s for commercial purposes. It has been reported that this fish has the highest relative fecundity for any sturgeon species.

Chondrosteidae

Chondrosteidae is a family of extinct actinopterygian fishes in the order Acipenseriformes.

Chondrosteus

Chondrosteus is a genus of extinct actinopterygian (ray-finned fish) belonging to the family Chondrosteidae.

Dwarf sturgeon

The dwarf sturgeon, little shovelnose sturgeon, or small Amu-dar shovelnose sturgeon (Pseudoscaphirhynchus hermanni) is a species of fish in the family Acipenseridae. It is found in Tajikistan, Turkmenistan, and Uzbekistan.

Huso

Huso is a genus of large sturgeons from Eurasia. It contains two species, which both are critically endangered:

Huso dauricus (Georgi, 1775) (kaluga)

Huso huso (Linnaeus, 1758) (beluga)Recent data indicate a polyphyletic origin of the genus Huso, and it is suggested that the two Huso species should be included in the genus Acipenser.

Japanese sturgeon

The Japanese sturgeon, or Amur sturgeon (Acipenser schrenckii) is a species of fish in the Acipenseridae family that can be found in the Amur River basin in China and Russia. Claims of its presence in the Sea of Japan need confirmation. The species has 11–16 dorsal, 34–47 lateral, and 7–16 ventral scutes. Their dorsal fins have 38–53 rays and 20–35 anal fin rays. They also have greyish-brown backs and pale ventral sides. The species could reach up to 3 m in length, and weight over 190 kg. The species is considered to be critically endangered.

Peipiaosteus

Peipiaosteus is an extinct genus of prehistoric ray-finned fish, closely related to living sturgeon and paddlefish. Its fossils are found in the Early Cretaceous Jiufotang Formation, Pani Lake, Liaoning Province, China.

Polypterus bichir

Polypterus bichir, the Nile bichir, is a fish which lives in the Nile and some of its tributaries in Africa. It is a dark grayish color on the top, with a dark vertical marking and bands on the flank. This marking is more prominent on juveniles, and fades as the fish grows.

Polypterus ornatipinnis

Polypterus ornatipinnis, the ornate bichir, is a bony fish found in Lake Tanganyika and the Congo River basin in Central and East Africa.

Pseudoscaphirhynchus

Pseudoscaphirhynchus is a genus of fishes in the sturgeon family (Acipenseridae). The three species, all of which are threatened, are restricted to Central Asia.

Scaphirhynchus

Scaphirhynchus is a genus of sturgeons native to the United States of America. All species in this genus are considered to be threatened. The pallid sturgeon is Endangered and the Alabama sturgeon is Critically Endangered.

Sterlet

The sterlet (Acipenser ruthenus) is a relatively small species of sturgeon from Eurasia native to large rivers that flow into the Black Sea, Azov Sea, and Caspian Sea, as well as rivers in Siberia as far east as Yenisei. Populations migrating between fresh and salt water (anadromous) have been extirpated.Due to overfishing (for its flesh, caviar, and isinglass), pollution, and dams, the sterlet has declined throughout its native range and is considered vulnerable by the IUCN. Restocking projects are ongoing, and it has been introduced to some regions outside its native range, but the latter have generally not become self-sustaining. Today, the majority of the international trade involves sterlets from aquaculture.

Syr Darya sturgeon

The Syr Darya sturgeon (Pseudoscaphirhynchus fedtschenkoi) is a species of fish in the family Acipenseridae. It is found in Kazakhstan, Tajikistan and Uzbekistan, where it is endemic to the Syr Darya River and, before its drainage, the Aral Sea. Due to the loss of its breeding site and damming projects over the length of the river, it is currently considered Critically Endangered and likely extinct, as no sightings have been reported since 1960. The sturgeon is among the 25 “most wanted lost” species that are the focus of Global Wildlife Conservation’s “Search for Lost Species” initiative.

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