Extinction of species-survival of the fittest: (Late Carboneferous Age-300 million years ago)

About 300 million years ago, towards the end of Carboniferous Period, the rapid diversification of sharks came to an end. The period of global catastrophe had begun. At the end of Permian period, about 245 million years ago, extensive volcanic eruptions, climatic changes and changes in sea level caused about 96% of the marine life to disappear- the largest mass destruction of all times. Many of these vanishing species were food for Sharks. Most other sharks became victims of their own specialization and joined the casualty list. However, a few fit ones survived and a second evolutionary jolt caused two main shark groups to fill the ecological niches.

One group included the Xenacanth sharks. They appear to have avoided extinction by moving into freshwater. One species, Xenacanthus, was a 75 cm (30 in) long, eel-like shark that resembled a modern conger eel. It had a dorsal fin that extended the length of its back and around the tail to join with an unusual double anal fin. Its most conspicuous feature was a single defensive spine growing backwards out of the top of its head.

The other survivors were in the sea. These were the hybodonts, unspecialized sharks that resembled modern sharks but were not their immediate ancestors. Their success demonstrated that the basic shark structure and way of life had evolved successfully tens of millions of years before modern sharks appeared.

By the start of the Jurassic period, 213 million years ago, the first 'modern' sharks were challenging the hybodonts. These newcomers had more flexible jaws that could be thrust forwards, giving them a distinct advantage in the competition for food. The hybodont lineage began to tail off, and sharks remarkably similar to modern species started to take over.