Chondrostei are primarily cartiliginous fish showing some ossification. There're 52 species divided among two orders, the Acipenseriformes (sturgeons and paddlefishes) and the Polypteriformes (reedfishes and bichirs). It's thought that the ancestors of the chondrosteans were bony fish but this characteristic was lost in later evolutionary development, resulting in a lightening of the frame. Elderly chondrosteans show beginnings of ossification of the skeleton which suggests that this process's… (More on Chondrostei)