Chondrichthyes are further divided into two subclasses: Holocephali and Elasmobrachii.
The subclass Holocephali includes fishes known as chimaeras. They are characterized by the fusion of the upper jaw to the cranium (the upper part of the skull that encloses the brain), one pair of external gill openings, and no scales.
The subclass Elasmobranchii includes sharks and batoids. Elasmobranchs are characterized by cylindrical or flattened bodies, five to seven pairs of gill slits, an upper jaw not fused to the cranium, and placoid scales.
Of the rays, skates, and sawfishes, rays were the first to develop, beginning
in the Late Jurassic Period, some 150 million years ago.
Selachians are characterized by a fusiform body and 5 to 7 pairs of lateral gill slits.
The earliest evidence of the ancestors of modern sharks are isolated spines, teeth, and scales that appeared 350 to 400 million years ago in the Devonian Period, known as the "Age of Fishes".
Cladoselache, an ancient shark with fossils dating back 400 million years, was found near Cleveland, Ohio. This shark featured many characteristics of modern shark species such as a torpedo-shaped body and multiple rows of gill-slits. It did, however, lack scales over most of its body.
The teeth from enormous fossil jaws of Carcharodon megalodon tell us
that this extinct shark lived 10 to 70 million years ago.
Stethacanthus, a shark dating back 345 million to 280 million years ago, had a first dorsal fin shaped like an anvil. The first dorsal fin and the top of its head were covered with denticle scales. It is speculated that Stethacanthus used these features possibly as a threat display or to aid in mating.
Xenacanthus was an ancient shark that dominated freshwater rivers and lakes. It had an eel-like body, V-shaped teeth, and one long, thick spine that grew from the back of the skull. Presumably, this spine was a defense against other predators.
Carcharodon megalodon was a top predator in warm oceans some 10 to 70 million years ago. At first this shark was thought to be up to 36.5 m (120 ft.) long, but most scientists now believe the Carcharodon megalodon was no longer than 12.4 m (40 ft.).