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Sharks have been around for hundreds of millions of years, appearing in the fossil record before trees even existed. But what did they evolve from, are they 'living fossils', and how did they survive five mass extinctions?
Sharks belong to a group of creatures known as cartilaginous fishes, because most of their skeleton is made from cartilage rather than bone. The only part of their skeleton not made from this soft, flexible tissue is their teeth.
While often referred to as living fossils, sharks have evolved many different guises over the hundreds of millions of years that they have been swimming the oceans.
The earliest fossil evidence for sharks or their ancestors are a few scales dating to 450 million years ago, during the Late Ordovician Period.
Emma Bernard, a curator of fossil fish at the Museum, says, 'Shark-like scales from the Late Ordovician have been found, but no teeth. If these were from sharks it would suggest that the earliest forms could have been toothless. Scientists are still debating if these were true sharks or shark-like animals.'
Analysis of living sharks, rays and chimaeras suggests that by around 420 million years ago, the chimaeras had already split from the rest of the group. As there are no fossils of these animals from this period of time, this is based solely on the DNA and molecular evidence of modern sharks and chimaeras. It was also around this time that the first plants invaded the land.
The earliest shark-like teeth we have come from an Early Devonian (410-million-year-old) fossil belonging to an ancient fish called Doliodus problematicus. Described as the 'least shark-like shark', it is thought to have risen from within a group of fish known as acanthodians or spiny sharks.
'Acanthodians are not at all shark-like in shape, for example they have diamond-shaped scales and spines in front of all the fins,' says Emma. 'But they do have a cartilage skeleton, a shark-like skull and jaw, and at least some shark-like teeth, which were often fused together.'
By the middle of the Devonian (380 million years ago), the genus Antarctilamna had appeared, looking more like eels than sharks. It is about this time that Cladoselache also evolved. This is the first group that we would recognise as sharks today, but it may well have been part of the chimaera branch, and so technically not a shark. As active predators they had torpedo-shaped bodies, forked tails and dorsal fins.
The Carboniferous Period (which began 359 million years ago) is known as the 'golden age of sharks'. An extinction event at the end of the Devonian killed off at least 75% of all species on Earth, including many lineages of fish that once swam the oceans. This allowed sharks to dominate, giving rise to a whole variety of shapes and forms.
Some of the most bizarre prehistoric 'sharks' to appear during this time actually evolved out of the chimaera lineage. These include Stethacanthus, which had a truly peculiar anvil-shaped fin on its back, Helicoprion with a spiral buzz saw-like bottom jaw, and Falcatus, in which the males had a long spine jutting out of the back and over the top of the head.
Modern-day chimaeras are much less diverse and typically live in the deep ocean. Growing up to 1.5 metres long, they are not actually sharks. Their upper jaw is fused with the skull, and most chimaera also have venomous spines.
The end of the Permian Period (252 million years ago) saw yet another mass extinction event, wiping out around 96% of all marine life. But a handful of shark lineages persisted.
By the Early Jurassic Period (195 million years ago) the oldest-known group of modern sharks, the Hexanchiformes or sixgill sharks, had evolved. They were followed during the rest of the Jurassic by most modern shark groups.
It was at this point that they evolved flexible, protruding jaws, allowing the animals to eat prey bigger than themselves, while also evolving the ability to swim faster.
At the beginning Cretaceous of Period (145 million to 66 million years ago) sharks were once again widely common and varied in the ancient seas, before experiencing their fifth mass extinction event.
While much of life became extinct during the End-Cretaceous extinction event, including all non-avian dinosaurs, sharks once again persisted.
But they were still affected. Fossil teeth show that the asteroid strike at the end of the Cretaceous killed off many of the largest species of shark. Only the smallest and deep-water species that fed primarily on fish survived.
Sharks soon began to increase in size once again, and continued to evolve larger forms throughout the Palaeogene (66 to 23 million years ago). It was during this time that Otodus obliquus, the ancestor to megalodon (Otodus megalodon), appeared.
O. megalodon is one of the biggest sharks ever to have lived. At between 15 to 18 metres in length, is was around the same length as today's whale sharks. Scientists consider megalodon one of the most powerful predators to have evolved.
Despite what many might think, megalodon is not related to great white sharks. In fact it may have been in competition with the great white shark's ancestors, which evolved during the Middle Eocene (45 million years ago) from broad-toothed mako sharks.
The youngest living group of sharks are thought to be the distinctive hammerhead sharks.
There are at least eight different species of hammerhead shark, and while fossil teeth evidence suggests that their ancestors may have existed 45 million years ago, molecular data points to a much more recent appearance during the Neogene (which began 23 million years ago).
The strange shape of their head is thought to mainly help in electroreception (the detection of naturally occurring electric fields or currents) as they hunt for prey. It may also improve their vision, enhance their swimming and refine their ability to smell.
Since the End-Cretaceous mass extinction, sharks have come to dominate the oceans once again, returning to the role of apex predator along with large marine mammals.
The vast majority of shark fossils found are teeth. This is down to two main reasons.
Because most of the skeleton of sharks is made from soft cartilage, it takes special conditions for this to preserve. The teeth, however, are made from a much tougher material known as dentin, which is harder and denser even than bone. While this enables a powerful bite, it also increases the chance that the teeth will fossilise as they are less likely to decompose.
The other reason is simply numbers. Rather than having just a few sets of teeth that last all their life, sharks are continually producing new teeth. As an older one breaks or wears down, it simply falls out of the front of the mouth and onto the sea floor, as a new tooth takes its place.
Depending on species and diet, over its entire lifetime a shark can produce between 20,000 and 40,000 teeth.
This means that there is a much greater chance that a shark tooth will be preserved and turned into a fossil. Not only are the teeth the most common part of sharks to be found, they're one of the most common fossils of any organism.
There is no single reason sharks survived all five major extinction events - all had different causes and different groups of sharks pulled through each one.
One general theme, however, seems to be the survival of deep-water species and the dietary generalist. It is possible that shark diversity may also have played an important role.
Emma explains, 'I think it is safe to say that it is partly because sharks are able to exploit different parts of the water column - from deep, dark oceans to shallow seas, and even river systems. They eat a wide variety of food, such as plankton, fish, crabs, seals and whales. This diversity means that sharks as a group are more likely to survive if things in the oceans change.'
Rather than sharks simply being incredibly hardy, it is more likely that their amazing diversity is the key to their success. It's no wonder they've been dominating the ocean for hundreds of millions of years.
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