Temporal range: Silurian - Recent
|Atlantic sturgeon, Acipenser oxyrinchus|
|Nile bichir, Polypterus bichir|
Chondrostei are primarily cartilaginous fish showing some ossification. The 52 species are divided among two orders, the Acipenseriformes (sturgeons and paddlefishes), and the Polypteriformes (reedfishes and bichirs).
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.
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.
- "Chondrosteans: Sturgeon Relatives". paleos.com.