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Most scientists believe that sharks came into existence around 400 million years ago (200 million years before the dinosaurs) and descended from a small leaf shaped fish that had no eyes, fins or bones. Ancestral fish evolved and diverged into the two main groups seen today; Osteichthyes (bony fish) and Chondrichthyes (cartilaginous fish).
The fossil record documents over 3,000 types of shark and their relatives, however it is likely that many more existed but disappeared without trace. When a human or bony animal dies it leaves behind a skeleton, however a shark’s skeleton is made of cartilage rather than bone, which rots away when a shark dies. Usually, the only remains a shark leaves behind are its teeth, dermal denticles (placoid scales) and fin spines. However this is more than enough for scientists, as a single tooth can still determine which shark it belonged to and what that shark fed on.
The fossil records of sharks are abundant and diverse. Some fossil species that lived over 150 million years ago are identical to shark, skate and ray species that exist today. One of the earliest known species is the Cladoselache, which lived about 350 million years ago, like many other early sharks their mouth was located at the front of their snout. Ancestors the Megamouth Shark, Frilled Shark, Angelshark and devil rays still have this today, whereas most sharks have evolved a mouth under their snout.
One fossil tooth, belonging to the Megalodon Shark, was recorded at 17cm, which suggests an enormous predator as long as two buses. This species was a relative of the White Shark, however Whites are much smaller with teeth about 6cm long.
Sharks have either shared or solely owned the position at the top of the marine food chain for over 400 million years. The species that briefly joined them as apex predators have consisted of massive marine reptiles such as Mosasaurs and Plesiosaurs, but sharks are the only group that have survived to modern day. So what is it that makes sharks so special that they can survive global catastrophes when other top predators cannot? The answer lies in the fossil record and to fully understand these magnificent emperors of the ocean, we shall travel back in geological time, to a period called the Devonian.
There are four periods that make up the entire history of planet Earth. The first period, the Precambrian, began with the evolution of the Earth 4.6 billion years ago and is categorised by a complete lack of animal life. The second period, the Paleozoic, spans 545 to 250 million years ago and saw the evolution of life from single celled organisms to bony fish and sharks. The third period was the Mesozoic, famous for the reign of the dinosaurs, which lasted from 250 to 65 million years ago. The most recent period, the Cenozoic, spans from 65 million years ago to modern day.
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