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shark tooth
A shark tooth is one of the numerous teeth of a shark. A shark tooth contains resistant calcium phosphate materials.〔(【引用サイトリンク】title=Fossil folklore )〕 Sharks continually shed their teeth; some Carcharhiniformes shed approximately 35,000 teeth in a lifetime, as well as replace them by producing thousands of more.〔(【引用サイトリンク】title=Shark teeth )〕 There are four basic types of shark teeth: dense flattened, needle-like, pointed lower with triangular upper, and non-functional. The type of tooth that a shark has depends on its diet and feeding habits.
In some geological formations, shark's teeth are a common fossil. These fossils can be analyzed for information on shark evolution and biology; they are often the only part of the shark to be fossilized. Fossil teeth comprise much of the fossil record of the Elasmobranchii, extending back to hundreds of millions of years. Shark teeth are also useful in conducting research about the structure of teeth, shark migration patterns, and identifying shark species.
The most ancient types of sharks date back to 450 million years ago, during the Late Ordovician period, and are mostly known by their fossilised teeth. However, the most commonly found fossil shark teeth are from the Cenozoic era (the last 66 million years).
==Types and Functions==
There are four basic types of shark teeth that vary according to the diet of the shark; those species that have dense flattened teeth for crushing, those that have needle-like teeth for gripping, those that have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting, and those that have teeth that are tiny, greatly reduced, and non-functional.〔(【引用サイトリンク】title=Shark Savers :: Shark Teeth )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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