Shark Attacks and Invades Mississippi River - World Documentaries

Sharks are a group of fish defined 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. Under this more comprehensive definition, the earliest known sharks date back to more than 420 million years ago.
Since then, sharks have actually diversified right into over 505 species. They range in dimension from the little dwarf lanternshark (Etmopterus perryi), a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the biggest fish on the planet, which reaches about 12 metres (39 ft) in length. Sharks are found in all seas and are common to depths of 2,000 metres (6,600 feet). They normally do not live in freshwater although there are a few known exceptions, such as the bull shark and the stream shark, which could be and endure found in both seawater and freshwater. They breathe through five to seven gill slits. Sharks have a covering of facial denticles that safeguards their skin from damage and bloodsuckers along with boosting their fluid mechanics. They have several sets of changeable teeth.
Famous species such as the great white shark, tiger shark, blue shark, mako shark, and the hammerhead shark are peak killers-- microorganisms at the top of their undersea meals chain. Many shark populations are intimidated by human tactics.
Proof for the existence of sharks days from the Ordovician period, 450-- 420 million years ago, before land animals alreadied existing and before numerous plants had colonized the continents. Just scales have been recovered from the very first sharks and not all paleontologists concur that these are from real sharks, assuming that these scales are actually those of thelodont agnathans. The oldest generally accepted shark scales are from regarding 420 million years ago, in the Silurian period. The initial sharks looked very different from modern sharks. Most of modern sharks could be traced back to around 100 million years ago. Most fossils are of teeth, often in numbers. Partial skeletons or even complete fossilized remains have been uncovered.