It's not known whether archaic sharks posessed the bouyant livers, but cartilaginous skeletons and large hydrodynamic fins are found early in the fossil record. Unlike most fishes, cartilaginous fishes reproduce by internal fertilization. The males have paired intromittent organs called claspers attatched to their pelvic fins. Clasper-like structures have also been discovered in one group of placoderms, the ptyctdontids. This apparent homology has been cited by some as evidence that cartilaginous fishes and placoderms are related.
Cartilaginous fishes have rows of teeth that grow throughout life.
When the teeth in the front row are damaged or lost, they're replaced by teeth from the back. This constant shedding of teeth has contributed to the relative frequency of shark teeth in the fossil record. In fact, most fossil sharks are known only from their teeth, since the cartilaginous skeletons are rarely preserved. Fossil Record of the Chondrichthys. Vertebrate Paleontology and Evolution.