Sharks are eight orders of cartilaginous fishes in the class Elasmobranchii. Sharks first appear in the fossil record of about 430 million years, during the Silurian Period. The Silurian is sometimes known as "Age of Fishes," because this is when the first kinds of fish-like animals appeared and then rapidly radiated into a great diversity of forms. Today, there are about 400 living species of sharks, divided into eight orders and 30 families. New species continue to be discovered as marine biologists begin to explore the relatively unknown abyssal waters of the oceans.

The living orders of sharks are:

• Squatiniformes, including 13 species of angelsharks, with a flat body, extremely wide and elongate pectoral fins, two dorsal fins, no anal fin, and a body length up to 8 ft (2.4 m); an example is the Pacific angelshark (Squatina californica).
• Pristiophoriformes, including five species of sawsharks, with a narrow cylindical body, two dorsal fins, no...