At one time, Chondrichthyes were thought to have split off from other vertebrates before the evolution of bone. Now we have concluded that the Chondrichthyes split off after the evolution of bone had started. This change demonstrate

Chondrichthyes Class of vertebrate animals characterized by a cartilaginous endoskeleton, a skin covered by placoid scales, the structure of their fin rays, and the absence of a bony operculum, lungs, and swim bladder. It includes the subclasses Elasmobranchii (sharks, rays) and Holocephali (ghostfish).