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Shark Evolution Through the Millennia

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Shark Evolution Through the Millennia

Shark evolution has been on the move for millions of years, with around 500 species now swimming in the oceans. Although sharks are under increasing pressure, mainly from human activities such as hunting, their history suggests that they can quickly adapt to environmental changes. This flexibility in shark evolution may prove beneficial amid climate change, which is already causing problems for many marine species that lack this adaptation.

Are sharks’ dinosaurs?
Considering how long they’ve been around some might wonder: are sharks’ dinosaurs?

According to Live Science, sharks hunted in the oceans for about 450 million years, while dinosaurs arrived only 240 million years ago. Also, most dinosaurs did not survive many of the extinction events that sharks did (although in smaller numbers). Despite their rudimentary and terrifying appearance (thanks Jazz!), they’re not even close relatives. A Harvard news article notes that chickens, ostriches, and crocodiles are still close relatives of Tyrannosaurus Rex.

The Natural History Museum in London describes how the “golden age” of sharks came about 359 million years ago, the age before the dinosaurs. At the time, sharks ruled the oceans after a mass extinction event wiped out many of their competitors. CNN points out that these animals were actually the first vertebrate predators on the planet, long before the activities of the dinosaurs.

Fossils reveal secrets of shark evolution
Following sharks through the fossil record is difficult. Unlike dinosaurs, sharks have a cartilaginous skeleton rather than one made of solid bone. Unfortunately, this structure does not fossilize well because it does not calcify. Finding evidence of shark species throughout their lives is therefore mostly limited to teeth and skin scales, which contain calcifying minerals such as calcium and bioapatite.

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In fact, examination of these denticle microfossils revealed evidence of an extinction event that wiped out nearly 90 percent of sharks 19 million years ago. According to a study published in science not only were shark populations extinct in the open ocean, but the species also became less diverse.

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Sometimes the remains of a complete skeleton are fossilized. Upon discovery, it could tell biologists a lot about ancient sharks. For example, a single fossilized vertebral column helped scientists determine the age and size at birth of the mighty Megalodon.

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Shark Evolution for Species Success
From the available fossil evidence, scientists have been able to piece together some of the stages of shark evolution and see how each stage helped sharks remain top predators.

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For example, being cartilaginous makes sharks relatively light, which helps them conserve energy while swimming long distances. Conical bodies and well-placed fins combined with dermal denticles, and larger tail fins make very efficient and powerful swimmers. This means that sharks can perform long migrations and swim at high speeds. As a result, they have few natural predators other than humans.

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They are also generalists, capable of occupying a wide variety of marine niches, and their teeth have evolved to cope with a very flexible diet. For example, fossil evidence shows that sharks developed protruding, flexible jaws early in the Jurassic period. This helped them tackle larger prey and eat larger animals.

Sharks are top predators with a lateral line sensory system that detects vibrations of prey in the water. They also have the ability to capture electromagnetic fields that help not only in the location of food but also in navigation.

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The sharks also survived the Paleocene-Eocene Thermal Maximum, which saw a dramatic increase in both global carbon dioxide and temperature around 56 million years ago. It is likely that their continued survival can be attributed to their ability to rapidly adapt to environmental change. For example, rising water temperatures caused sharks to rapidly decrease in body size over a relatively short evolutionary period of 7,000 years. Some shark species can also move between fresh and salt water with ease.

New shark discoveries, ancient and modern
New and new ones continue to be discovered

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