The shark is thought to have evolved around 400 million years ago and predates the dinosaur. Various species of shark have become extinct throughout history for different reasons, with the skeletal construction of the shark reducing the amount of available fossils.
The majority of ancient sharks, such as a primitive form of shark called the Cladodonts that is now extinct, are only identifiable by the teeth that are constructed from bone. The skeleton of a shark is made from cartilage instead of bone that allows the frame of the shark to be strong and flexible. When a shark dies, its skeleton tissue decomposes faster than the tissue can be replaced by minerals to form a fossil, meaning many ancient sharks have not left fossils apart from their teeth. Sharks have rows of teeth that are lost and regrown throughout their life. In the case of extinct sharks, often the only fossils found are lost teeth.
One of the best known extinct sharks is the Megalodon, which existed between 25 million and 1.6 million years ago. The size of the Megalodon species is disputed as being between 30 feet and 90 feet in length because the only fossils that have been discovered are teeth and parts of vertebrae. The distribution of Megalodon was global with fossils found in India, Europe, New Zealand, Australia, North America and South America in warm ocean waters. The extinction of Megalodon is closely associated with the last ice age to cover the Earth, with a lack of food and warm shallow coastal breeding grounds disappearing as global temperatures declined. A scarcity of food is also associated with the extinction of Megalodon, which was cannibalistic in nature and turned to eating its own species in times of food shortages.
The earliest complete shark fossil has been designated a Cladoselache, according to Enchanted Learning. The Cladoselache grew to between 1.5 and 6 feet in length, with two dorsal fins that are identified by large spines upon them. A defining feature of the shark was its distinctive three-pointed teeth, which had a large central point and two smaller points on either side. The Cladoselache had also evolved to have 5 gills on each side of its head.
There are other species of shark that have become extinct, such as Hybodus that became extinct around the same period as the mass dinosaur extinction of 65 million years ago. The shark species Orthacanthus was identified by a long spine growing from the back of its skull and a long fin stretching across the length of its back.