Sharks are elasmobranch fish with a cartilaginous skeleton, five to seven gill slits on the sides of the head, and unfused pectoral fins. Modern sharks belong to the clade Selachimorpha (or Selachii), which is the rays’ sister group. However, outside of the Selachimorpha, the word “shark” has been applied to extinct members of the Elasmobranchii subclass, such as Cladoselache and Xenacanthus, as well as other Chondrichthyes, such as the holocephalid eugenedontidans.

The earliest known sharks, according to this wider definition, stretch back more than 420 million years. Acanthodians are commonly referred to as “spiny sharks,” despite the fact that they are a paraphyletic collection that leads to cartilaginous fish in general. Sharks have evolved into approximately 500 different species since then.

They range in size from the tiny dwarf lanternshark (Etmopterus perryi), a deep water species of just 17 centimetres (6.7 in) in length, to the world’s biggest fish, the whale shark (Rhincodon typus), which reaches a length of around 12 metres (40 ft). Sharks may be found in all oceans and at depths of up to 2,000 meters (6,600 ft).

Although there are a few known exceptions, such as the bull shark and the river shark, which may be found in both seawater and freshwater, they typically do not dwell in freshwater. Sharks have a layer of dermal denticles on their skin that protects them from parasites and improves their fluid dynamics. They have many sets of teeth that may be replaced.

Apex predators—organisms at the top of their undersea food chain—include the tiger shark, blue shark, great white shark, mako shark, thresher shark, and hammerhead shark. Human activities are putting several shark populations in jeopardy. Shark populations have declined by 71% since 1970, owing primarily to overfishing.

Sharks were first discovered during the Ordovician period, 450–420 million years ago, when terrestrial vertebrates and a diverse range of plants inhabited the continents. Only scales have been found from the early sharks, and not all paleontologists believe that they are from real sharks, with some thinking they are from thelodont agnathans. The earliest commonly recognized shark scales are from the Silurian epoch, around 420 million years ago. The original sharks did not resemble contemporary sharks in appearance.

The cladodont, a kind of thin tooth with three tines resembling a trident, appears to be the most prevalent shark tooth at this period. The majority of today’s sharks date from about 100 million years ago. The majority of fossils are teeth, which are typically found in huge quantities. There have been partial skeletons and even full petrified remains unearthed.

Sharks are thought to produce tens of thousands of teeth over their lifespan, which explains the abundance of fossils. The teeth are made of apatite, a kind of calcium phosphate that is easily fossilized. When a shark dies, its skeleton disintegrates, dispersing the apatite prisms. Rapid burial in bottom sediments is required for preservation.

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