The ancestors of modern sharks appeared in the oceans some 350 million years ago. The Earth looked very different then than it is now: continents and oceans have not yet purchased our usual shapes. Ancient sharks swam where nowadays sow wheat and build cities. Therefore, fossils of sharks sometimes discover for tysychi kilometers from the sea. Paleontological scientists studying ancient forms – most often find shark teeth. Fossil shark teeth affect its size. Most krupnye- length of 15 cm. And a weight of 340 g – belonged to the monster in the mouth which could put a man gets up to his full height, and it reached 13 meters in length!
After hundreds of millions of years, descendants of the ancient sharks overcome thorny path of evolution continue to surf the seas and oceans. Sharks, of course, in many ways inferior to more progressive in structure bone fish. Sharks do not have kostey- their skeleton is composed entirely of cartilage (though their ancestors had the skeleton). The gill slits are not covered shark gill covers. And most importantly, sharks, like all cartilaginous fish, no swim bladder. In part, though, the swim bladder shark replace fat deposition in the liver. To stay in the water and sink to the bottom, the shark spends his life in constant motion. Sleep she also prihoditsya- not only coastal and reef sharks can afford to rest on the bottom underwater caves. Gills sharks are not so
soversheeny like a fish bone. and extracting only half of the water of dissolved oxygen (y kostnyh- 80%). The shark must continually move more and to its gills continuously washed with fresh water. Not by chance the shark in captivity become “sleepy” from lack of oxygen. Sharks caught in the network, often suffocate and die long before they pull out.
Shark is perfectly adapted to life in the ocean. The body of the shark is a perfect streamlined shape. Triangular teeth located in the 5- and 6 and 15 rows. The total number of teeth up to a few hundred, and kazhdyy- sharp razor. Rear front teeth replaced as wear and tear, as the cartridges in the revolver. For 10 years the shark can endure up to 24 thousand teeth. Unlike most dental they grow in the skin and do not grow into the bone. The strength of the compression jaws up to 18 tonnes! So even a medium-sized shark bites easily in half a dolphin or sea turtle. Rough scales sharks Therefore scientific called placoid – the same teeth, only smaller – like sandpaper. Sharks are very tenacious. There are cases when caught, gutted and thrown back into the sea shark again get hooked. The most amazing “achievement” akul- their reproduction. Some sharks lay “eggs” – eggs that is protected by a thick leathery shell, equipped with a “mustache” and allows you to attach to underwater vegetation. But most sharks do not trust the whims of fate themselves nurture their offspring and give birth to live young. Shark sees the world in black and white and fairly “Tuga’s ear.” But her sense of smell is very subtle and well developed lateral line, which gives her “sixth sense” that allows her to capture the tiniest fluctuations in water. Due to the lateral line organs shark can feel the movement of large fish at a distance of 300 meters. To top it all a shark on the head there is another so-called organ-cells Lorenzini by which she feels pressure changes, the electromagnetic permeability and temperature. Everyone knows akuly- predator. Shark attacks everything seems possible prey: fish, turtles, seabirds and marine mammals, their sobratev- sharks, and once attacked by a shark off the coast of Kenya who came into the sea elephant. In the stomach, caught a great white shark found the whole horse. In the stomach, the other sharks, tiger – head and the front part of the body of a crocodile, the sheep back foot, three gulls, two sealed kilogram cans of green peas and a tin of cigarettes.