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).

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.