Chondrostei

Chondrostei Temporal range:
Atlantic sturgeon, Acipenser oxyrinchus
Nile bichir, Polypterus bichir
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Subclass:	Chondrostei
Orders
Acipenseriformes Polypteriformes

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).

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.

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, 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.

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

• Acipenseriformes
  • Acipenseridae — sturgeons
  • Polyodontidae — paddlefishes
  • Chondrosteidae(†)
  • Errolichthyidae(†)
• Cheirolepidiformes(†)
• Guildayichthyiformes(†)
• Luganoiiformes(†)
• Palaeonisciformes(†)
  • Acrolepidae(†)
  • Birgeriidae(†)
  • Palaeoniscidae(†)
• Perleidiformes(†)
• Phanerorhynchiformes(†)
• Pholidopleuriformes(†)
• Polypteriformes
  • Polypteridae - bichirs and reedfish
• Ptycholepiformes(†)
• Saurichthyiformes(†)
• Tarrasiiformes(†)