Sharks

Sharks are a group of fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the rays. 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.^[1] Acanthodians are often referred to as "spiny sharks"; though they are not part of Chondrichthyes proper, they are a paraphyletic assemblage leading to cartilaginous fish as a whole.

Since then, sharks have diversified into over 500 species. They range in size from the small dwarf lanternshark (Etmopterus perryi), a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (40 ft) in length. Sharks are found in all seas and are common to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can survive and be found in both seawater and freshwater.^[2] Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They have numerous sets of replaceable teeth.^[3]

Well-known species such as the great white shark, tiger shark, blue shark, mako shark, and the hammerhead shark are apex predators—organisms at the top of their underwater food chain. Many shark populations are threatened by human activities.

Taxonomy:

Kingdom: Animalia

Phylum: Chordata

Class: Chondrichthyes

Subclass: Elasmobranchii

Superorder: Selachimorpha

Some shark fact:

#1 Sharks have been on our oceans for over 400 million years. Some of the earliest sharks were discovered dating back to the Devonian age.

#2 Not only did sharks first appear 400 million yeas ago, they are still here today. They have survived 5 massive planet extinction events. These extinction events killed most life on earth. The last one around 65 million yeas ago killed the dinosaurs. But not sharks. They were here to stay.

#3 While many of us have learned to fear sharks, they’re the ones who should fear us. People are sharks’ most deadliest predator. In fact humans kill more than 100 million sharks each year.

#4 Most sharks species will drown if they stop moving. Great white, mako and salmon sharks don’t have the muscles they need to pump water through their mouth and over their gills. As long as they keep swimming, water keeps moving over their gills, keeping them alive.

#5 Sharks’ livers contain lots of oil. This makes the liver a relatively buoyant organ, which helps sharks keep their balance in the water.

Well, that's all from me. I actually feel bad because all I did was just copy-pasting, heheheh.

For more shark fact: http://www.sharkguardian.org/shark-facts-top-100-shark-guardian/

(source: Wikipedia, sharkguardian.org)