Sara gave an excellent talk in which she explained that there is a 400 million year fossil history of sharks. Thought to have arisen during the Devonian period there is some evidence that they could have evolved much earlier even into the Ordovician. There are about 1000 living species. She then went on to describe their morphology and how they differ from other fish. One major difference residing in the fact that they have a cartilaginous and not bony skeleton. This, of course leads to problems of fossilisation such that the main shark fossils are of teeth, dermal denticles,fin spines and calcified vertebrae. Trace fossils are rare but coprolites are frequent.
The most important genus during the Palaeozoic was that of Cladoselache, fossils of which can be found in the Cleveland Shales around Lake Erie in North America. By the start of the Carboniferous sharks had diversified and proliferated greatly and then remained relatively stable until the mass extinction at the end of the Permian. Two groups now emerged one moving into fresh water whilst the other remained marine. By the late Triassic the first modern sharks appear. These had a much more flexible jaw which increased their advantage when in competition for food.
By the Cenozoic Era all the modern sharks are present and also the biggest shark of all time Carcharodon Megalodon which is regarded as one of the largest and most powerful predators in vertebrate history.