Evolution and Ancestors
Sharks are thought to be the decedents of Placoderms, one of their oldest known ancestors is the Cladoselache shark, this shark lived 400 million years ago and had distinct differences from the sharks that are in Earth today. This shark was small with a full skeleton had no claspers and no placoid scales.
After the Cladoselache shark came the Stethacanthus shark, this shark lived from 345 million years ago to 280 million years ago. This shark was the first known shark to have claspers, they also had a rather unusual dorsal fin, it is in the same place as the dorsal fins today's sharks have but it has an unusual shape. Some say that it was used to resemble a bigger animal to scare off predators since the shark was only three feet long, other say it was used as a sign of dominance when mating, but its true purpose is still unknown.
After this shark came the Xeancanthus shark this shark lived approximately 450 million years ago. This shark had "v" shaped teeth, its body resembled an eel, and it had a long spine for protection. These sharks have several similarities to the sharks of today.
All sharks overtime including today's sharks have gills, teeth, and cartilage.
This is a branching tree of the history of sharks.