"Chondrichthyes" means "cartilaginous fishes." Biologists used to assume that bone was not evolved until the Osteichthyes, or bony fishes

Question

What clues do we have that shark ancestors, at some level, had bone? How can we infer this from (a) evidence from the fossil record and (b) evidence from comparative anatomy?

Answer 1

a. The fossil jawless fishes (ostracoderms) are more closely related to gnathostomes than are the living jawless fishes (lampreys and hagfishes). While the living jawless fishes lack any form of hard tissue, the ostracoderms had a head shield of dermal bone and bony scales over their body. The same condition is seen in primitive bony fishes (plus some other fossil fishes, such as placoderms and acanthodians), which implies that bone is an ancestral character for vertebrates more derived than lampreys. Thus the absence of this type of bone must represent secondary loss in chondrichthyans.
b. Careful examination of the tiny scales on the skin of sharks (placoid scales, or dermal denticles) shows that they are formed by mineralized tissue, with an outer layer of enameloid and an inner layer of dentine, similar to the tissues seen overlying the bone in the dermal plates in ostracoderms and primitive bony fishes. This observation suggests that these scales are the remnants of a once more extensive dermal skeleton, probably containing bone as well as other hard tissues.