Chondrostei

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Chondrostei

Chondrostei are primarily cartiliginous

Cartilage

Cartilage is a type of dense connective tissue. It is composed of specialized cells called chondrocyte that produce a large amount of extracellular matrix composed of collagen fibers, abundant ground substance rich in proteoglycan, and elastin fibers....
fish showing some ossification

Ossification

Ossification is the process of bone formation, in which connective tissues, such as cartilage are turned to bone or bone-like tissue. The ossified tissue is invaginated with blood vessels....
. There are 52 species divided among two orders, the Acipenseriformes

Acipenseriformes

Acipenseriformes are an order of primitive Actinopterygii that includes the sturgeons and paddlefishes, as well as some extinct families.Notable characteristics of Acipenseriformes include:...
(sturgeon

Sturgeon

Sturgeon is the common name used for some 26 species of fish in the family Acipenseridae, including the genus Acipenser, Huso, Scaphirhynchus and Pseudoscaphirhynchus....
s and paddlefish

Paddlefish

Paddlefish are primitive Chondrostean ray-finned fishes. The paddlefish can be distinguished by its large mouth and its elongated snout called a rostrum ....
es) and the Polypteriformes (reedfish

Reedfish

The reedfish, Erpetoichthys calabaricus, ropefish , or snakefish is a species of freshwater fish in the bichir family and order ....
es and bichir

Bichir

The bichirs are a family, Polypteridae, of archaic-looking Actinopterygii, the sole family in the order Polypteriformes.All species occur in freshwater habitats in tropical Africa and the Nile River system, mainly swampy, shallow floodplains and estuary....
s).

It is thought that the ancestors of the chondrosteans were bony fish but this characteristic was lost in later evolution

Evolution

In biology, evolution is change in the heritability trait of a population of organisms from one generation to the next. These changes are caused by a combination of three main processes: variation, reproduction, and selection....
ary development, resulting in a lightening of the frame. Elderly chondrosteans show beginnings of ossification of the skeleton which suggests that this process is delayed rather than lost in these fish.

This group has at times been classified with the shark

Shark

Sharks are a type of fish with a full Cartilage skeleton and a highly Streamlines, streaklines and pathlinesd body. They respire with the use of five to seven gill slits....
s: the similarities are obvious, not only do the chondrosteans mostly lack bone, the structure of the jaw is more akin to that of sharks than other bony fish, and both lack scales

Scale (zoology)

In most biology nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration....
(excluding the sturgeon

Sturgeon

Sturgeon is the common name used for some 26 species of fish in the family Acipenseridae, including the genus Acipenser, Huso, Scaphirhynchus and Pseudoscaphirhynchus....
s).

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Encyclopedia

Chondrostei are primarily cartiliginous

Cartilage

Ossification

Acipenseriformes

Sturgeon

Paddlefish

Paddlefish are primitive Chondrostean ray-finned fishes. The paddlefish can be distinguished by its large mouth and its elongated snout called a rostrum ....
es) and the Polypteriformes (reedfish

Reedfish

The reedfish, Erpetoichthys calabaricus, ropefish , or snakefish is a species of freshwater fish in the bichir family and order ....
es and bichir

Bichir

Evolution

Shark

Scale (zoology)

Sturgeon

Teleostei

Teleostei is one of three infraclasses in class Actinopterygii, the ray-finned fishes. This diverse group, which arose in the Triassic period, includes 20,000 extant species in about 40 orders; most living fishes are members of this group....
than their external appearance might suggest. Additional shared features include spiracle

Spiracle

Spiracles are small openings on the surface of some animals that usually lead to respiratory systems.In elasmobranchs , a spiracle is found behind each eye, and is often used to pump water through the gills while the animal is at rest ....
s and (sturgeons) a heterocercal tail (the vertebra

Vertebra

A vertebra is an individual bone in the flexible column that defines vertebrate animals. The vertebral column encases and protects the spinal cord, which runs from the base of the cranium down the dorsal side of the animal until reaching the pelvis....
e extend into the larger lobe of the caudal fin).

Chondrostei is paraphyletic

Paraphyly

In phylogenetics, a group of organisms is said to be paraphyletic if the group contains its most recent common ancestor Common descent but does not contain all the descendants of that ancestor....
meaning that 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

Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
chondros meaning cartilage and osteo meaning bone.