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The other group, the hybodonts, appeared about 320 million years ago and lived mostly in the oceans, but also in freshwater.

The results of a 2014 study of the gill structure of an unusually well-preserved 325 million year old fossil suggested that sharks are not "living fossils", but rather have evolved more extensively than previously thought over the hundreds of millions of years they have been around.

However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus, as well as other Chondrichthyes such as the holocephalid eugenedontidans.

Under this broader definition, the earliest known sharks date back to more than 420 million years ago.

These sharks attained gigantic proportions and include the extinct megatoothed shark, C. This giant shark reached a total length (TL) of more than 16 metres (52 ft).

Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes.

The oldest white shark teeth date from 60 to 66 million years ago, around the time of the extinction of the dinosaurs.

In early white shark evolution there are at least two lineages: one lineage is of white sharks with coarsely serrated teeth and it probably gave rise to the modern great white shark, and another lineage is of white sharks with finely serrated teeth. megalodon is also primarily known from its fossil teeth and vertebrae.

card shark, loan shark, etc.), which was later applied to the fish due to its predatory behaviour.

Estimates suggest that sharks grow tens of thousands of teeth over a lifetime, which explains the abundant fossils.

The teeth consist of easily fossilized calcium phosphate, an apatite.

At this time the most common shark tooth is the cladodont, a style of thin tooth with three tines like a trident, apparently to help catch fish.

The majority of modern sharks can be traced back to around 100 million years ago. Partial skeletons and even complete fossilized remains have been discovered.

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