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Megalodon successor– Shark the ancient sea monster

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    What do we know about sharks?
In movies, documentaries or different video materials all over internet the shark is described like a terrible man-eater. Maybe it is true, maybe not but most of the time is that the sharks attack humans because we are in their habit and according to a huge number of documented incidents most probably they confuse humans floating especially above the water with their favorite meal – seals, penguins, etc.

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     Are sharks prehistoric animals? The answer is: Yes, the sharks are prehistoric animals. The first shark known well by paleontologists were Megalodon, Pliosaurus and Mosasaurus which were lived in Ordovician period becoming “apex predators”; it means about 450 million years ago. It isn’t many fossils like confirmation for their existence because the cartilages don’t resist such as bones do it. So, many sharks’ fossils are their fossilized teeth. At begin, prehistoric sharks was more reptile and less fish, an enormous creatures. After Mesozoic and after dinosaurs, aquatic dinosaurs’ cousins were prepared for a slow evolution and the result is here: like their grandfathers, the nowadays sharks are killing machines.
    Sharks represent a class of fishes defined by a cartilaginous skeletal structure, around 5-7 grill clefts located on the head’s each side and pectoral fins that aren’t consolidated to it.
Sharks began to swim throughout the oceans approximately since 450 million years ago. Their longevity isn’t the only great part of the story, their evolution is too. During these past million years, they managed to develop several impressive abilities when it comes to being successful predators.

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    Although most of people have the misconception that dinosaurs dominated the prehistoric age, this isn’t true at all. They came along approximately 230 million years ago, and mammals occurred around this period as well. The hominid, which is the first animal with human appearance, appeared about 4.5 million years ago, while modern humans have shown up probably 60,000 years ago. Moreover, the civilization is found to be about 5,000 years old. Therefore, sharks have lived 100 times longer in comparison to hominids and about 3 times longer when it comes to dinosaurs, or 2 times longer if we count birds also.
The oldest fossil of prehistoric sharks turns out to be from some sort of shark scales that appeared about 455 million years ago in Colorado, on the Ordovician Period. Some scientists specialized in fossils don’t think that these particular scales belong to a shark in order to confirm it. However, there isn’t a single doubt that the 420 million years old scales coming from the Silurian Period appertain to sharks. These shark scales found during this period were discovered in Mongolia and Siberia. The oldest teeth of prehistoric sharks are recovered from the Devonian Period and are almost 400 million years old. They were discovered in Europe. These scales and teeth represent the only known evidence related to the early sharks.
Many other shark fossils are about 380 million years old, considering also a fossil of a shark cranium, most likely a xenacanth, discovered in Australia. Other fossils of this species or similar ones have been discovered throughout Saudi Arabia and Antarctica.




Description
Teeth
    Shark teeth aren’t directly attached to the jaw but are implanted in the gums, and during sharks lifespan they are replaced in a constant manner. The replacing teeth appear in numerous rows in a channel on the jaw interior and they slowly move ahead as a conveyor belt. There are sharks that lose more than 30,000 teeth throughout their life. The replacement teeth grow between 8-10 days and sometimes this process lasts even for several months. For the majority of species, teeth are replaced one by one unlike the replacement of a full row simultaneously, which can be noticed happening in the cookiecutter shark.
The shape of tooth relies a lot on the shark’s diet. The sharks that eat mollusks and crustaceans bear thick and flattened teeth utilized for crushing, while those that eat fish possess teeth similar to needles used for gripping, and those that eat larger victims like mammals bear pointed lower teeth used for gripping and trilateral upper teeth with notched edges for slashing. The teeth of sharks that feed on planktons, for instance like basking shark, are small and malfunctioning.

Skeleton
    Skeletons of shark differ a lot from those belonging to bony fish and terrestrial vertebrates. Sharks and cartilaginous fish such as skates and rays inherit skeletal structures consisting of cartilages and connective tissues. Even though the cartilage has half of the bone’s normal density, it is flexible and resistant. This saves up energy by reducing the weight of the skeleton. Due to the fact that sharks don’t bear rib cages, they can be easily squeezed under their own body weight on shore.

Jaws
    Like the jaws of rays and skates, those of sharks aren’t affixed to the braincase. Unlike shark’s vertebrae and gill bars, the surface of the jaw necessitates more aid because of its grand exposure to stressful physical activities and its demand for strength. The jaw’s surface has a layer containing small plates in shape of a hexagon known by the name of “tesserae”, which represent crystallized blocks consisting of calcium salts ordered like a mosaic. This makes the areas to have almost the same strength that is found in the ossified structure of other animals.

 

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    In general, sharks possess just a single layer of tesserae, but big specimens like the bull shark, tiger shark and the great white shark, bear jaws that got more than 3 layers as it depends on the body dimension. Immense white sharks may possess jaws with about 5 layers. The cartilage inside of the rostrum, as in snout, can be defined by sponginess and flexibleness in order to take in the energy of impacts.

Fins
    Fin skeletal structures are lengthened and held by rays that are soft and united known as ceratotrichia, fibers made of elastic protein looking like the horny keratin found in hair and feathers. The majority of sharks possess 8 fins. Sharks have the ability to float away from things straight in front of them due to their fins that don’t permit them to propel first in the direction of the tail.

Dermal denticles
    Sharks possess a complex dermal corset compose of collagenous filaments which are flexible and ordered like a coiling network that surrounds their body. This has the role of acting like an external skeleton, giving adherence for their muscles used for swimming and therefore sparing some energy. Sharks dermal teeth offer them hydrodynamic benefits because they minimize turbulence in water whenever swimming.

Tails
    Tails offer thrust, causing speed and acceleration to depend on the shape of tail. The shapes of caudal fin differ substantially among shark species because of their growth in different environments. Sharks have a heterocercal caudal fin where the dorsal part is normally perceptibly bigger than the ventral one. This is due to the vertebral column of the shark because it stretches inside the dorsal part, offering a larger surface area for muscle affixation. This grants more effective movement throughout these light cartilaginous fish. In opposition, nearly all bony fish have a homocercal caudal fin. Tiger sharks possess a big upper lobe that permits slow cruising and also abrupt speed bursts. When hunting various foods, the tiger shark must have the ability to twist and turn underwater in an easy manner. On the other hand, the porbeagle shark, which is feeding on schooling fish like mackerel or herring, possesses a big lower lobe in order to maintain pace with its prey that swims fast. Other tail features aid sharks to grab prey in a more direct manner, for instance like thresher shark uses his powerful, lengthened upper lobe for stunning fishes and squids.




Species
    There are over 440 shark species spread throughout 8 orders that are identified according to their evolution since they first appeared until now.
Hexanchiformes – this group consists in cow sharks and frilled sharks, which look like marine snakes

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Squaliformes – this group contains bramble sharks, dogfish roughsharks and prickly sharks
Pristiophoriformes – these represent the sawsharks, which have lengthened and toothed snout for cutting their prey
Squatiniformes – called also angel sharks, they represent sharks that are flattened and resemble stingrays and skates
Heterodontiformes – these are better known as bullhead or horn sharks
Orectolobiformes – they are normally know as carpet sharks, adding also zebra sharks, wobbegongs, nurse sharks and the whale shark.

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Carcharhiniformes – in popular terms, they are referred to as groundsharks, species that covers blue, tiger, bull, grey reef, withetip reef, blacktip reef, blacktail reef, Caribbean reef, oceanic whitetip sharks (when joint known as requiem sharks), catsharks, houndsharks and hammerhead sharks. These species are recognized by their prolonged snout and third eyelid which defends the eye when an attack is happening.

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Lamniformes – these ones are referred to as mackerel sharks and they consists in goblin shark, megamouth shark, basking shark, shortfin, the thresher sharks, longfin marko sharks and the famous white shark. They are defined by their big jaws and reproduction that is ovoviviparous. This group also contains the extinct megalodon called Carcharodon megalodon.



Habitat
    Sharks live throughout all seas. Usually, they aren’t found living in freshwater, besides some exceptions like the bull shark and the river shark, which have the ability to swim in seawater and freshwater. Normally, sharks live in depths up to 2,000 meters, as in 7,000 ft, and some of them are found even deeper than this. However, sharks are nearly lacking below 3,000 meters, as in 10,000 ft. The shark that swum the deepest is a Portuguese dogfish, which was found at 3,700 meters, as in 12,100 ft.



Diet
    Mostly, all sharks are carnivorous. Some sharks like the basking sharks, megamouth sharks and the whale sharks possess independently developed strategies that are distinct in order to filter feasting on plankton. For example, basking sharks apply ram feeding, while whale sharks practice suction for taking in plankton and little fishes, and megamouth sharks use suction feeding more effectively because of their luminescent membrane in their mouths, which is to attract victims more deep in the ocean. This kind of eating demands gill rakers, which are some long, slender fibers in shape of an effective sieve, exactly like the great whale’s baleen plates. These filaments are used for trapping inside of them the plankton and sometimes the shark swallows it in huge bites. These species have teeth relatively small due to the fact that they aren’t used for eating.
Other sharks that are greatly specialized in feeding are the cookie sharks, which eat flesh chopped out of bigger fish and marine mammals. Teeth of the cookiecutter are huge in comparison to the animal’s dimension. This shark’s lower teeth are peculiarly sharp. Even though, they haven’t been watched while feeding, they presumably latch on their victims and utilize their thick lips for making a seal, turning their bodies around to tear off flesh.
    There are a few seabed-dwelling species that are greatly efficient ambush predators. Angel sharks and wobbengongs mainly practice camouflage to lurk and suck their prey inside of their mouths. A lot of benthic sharks are feasting only on crustaceans which they squeeze with their flattened molariform teeth.
    Some other species of sharks eat squid or fish by entirely swallowing them. The viper dogfish possesses teeth which can be pointed outwards for striking and capturing the prey that afterwards is swallowed intact. The great white and other similar big predators usually swallow tiny prey entirely or get huge mouthfuls out of large victims. Thresher sharks apply their long tails for stunning shoaling fishes; meanwhile sawsharks are either stirring prey from the bottom of the sea or are slashing swimming prey with the usage of their rostra studded teeth.
Furthermore, a lot of sharks, along with the whitetip reef shark, are feeders that cooperate and hunt in large numbers in order to crowd and catch elusive victims. These social types of sharks are frequently migrant, cruising long distances inside of ocean basins from big schools. These migrations are partially needed for finding fresh sources of food.



Reproduction
    There exist 3 different methods in which each of the shark species reproduce. Although mating of sharks hasn’t been observed that much, it is known that most sharks reproduce when they are seen swimming parallel to each other and the male introduces a clasper inside the oviduct of the female.
Oviparity – it’s when sharks are laying eggs underwater
Viviparity – similar to mammals, sharks possess placental connections with growing pups. They feed and hold them until pups are born completely functional.
Ovoviviparity – this is the most applied way of reproduction between sharks. Eggs evolve inside of the mother’s oviduct and afterwards they hatch in the oviduct in which they complete their full development by utilizing the leftover egg content for pups to be born totally functional.



Extinction
    Sharks are threatened because of the result of human actions and activities, which include shark finning and capturing inside of fishing gear. These species represent apex predators, meaning that they are at the top of food cycle, and they have an important and big role in preserving the health of oceans. Without sharks, the whole food chain can be altered, impacting in a negative way the whole ecosystem. Shark exist for a long time, grow late and they give birth to a few younger ones, which are particularly vulnerable when it comes to exploitation.

The main threats are the following:
-    Commercial Fishing – this is the greatest threat to all sharks, rays, and skates because of the overfishing and over-consumption of their flesh, fins, and cartilage. Shark finning manages to kill at least 100 million sharks throughout the world each and every year.
-    Bycatch – this represents also a major menace to sharks. It’s the unplanned capture of species that weren’t actually targeted. Commercial fisheries, which target tuna and billfish, particularly have a huge impact on sharks.
-    Habitat Degradation – all sharks are depending a lot on healthy ecosystem for surviving purposes and finding food sources. Habitat degradation occurs due to climate change, pollution, and destroyed areas such as mangroves and reefs.



Interesting facts
•    In order to breath and not die, mostly all sharks have to keep propelling forward. They can only breathe through their opened mouths by swimming straight ahead. This allows water to run over their gills.
•    Sharks can float because of the large quantity of oil based in their liver.
•    Tiger sharks, great white sharks and bull sharks are the predators which attack humans the most.
•    The frilled shark is known by the name of Loch Ness monster due to its circular mouth that bears over 300 spiny teeth.
•    Sharks can be very picky when eating. There are moments in which they’ll either have a small bite out of their prey or just insert their teeth in to taste it before they begin eating for real. In case they dislike the taste, they spit the flesh out and carry on with their activities.
•    When baby sharks are born, they already have sharp teeth and can hunt right from that moment.
•    Sharks have the ability to see behind them nearly as well as they can see in front.
•    The sharks had named the “sea dogs” until the 16th century.

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Sources
http://www.sharksavers.org/en/education/biology/450-million-years-of-sharks1/
http://en.wikipedia.org/wiki/Shark
http://www.sharks-world.com/how_do_sharks_reproduce/
http://www.seethewild.org/15/shark-threats.html
http://sharkfacts.org/fun-shark-facts/
http://sharkfacts.org/more-facts-on-sharks/
http://www.sharks-world.com/how_do_sharks_reproduce/
http://www.sharkguardian.org/shark-facts-top-100-shark-guardian/
https://static-secure.guim.co.uk/sys-images/Guardian/Pix/pictures/2014/3/11/1394546448054/A-great-white-shark---loo-010.jpg
http://scienceblogs.com/lifelines/files/2014/01/13243547332073393725diving-with-great-white-sharks.jpg
http://www.photosof.org/view/big_shark_with_open_mouth_jump_from_the_water-wide.html
http://cdn.theanimals.pics/pictures/www.diverwire.com/wp-content/uploads/2011/12/Whale-Shark-Above.jpg
http://blog.conservation.org/wp-content/uploads/2012/02/hammerhead-shark.jpg
http://img1.wikia.nocookie.net/__cb20140122034737/rwbyfanon/images/4/4c/H3WWrHm.jpg
http://fishaowiki.com/wp-content/uploads/2014/03/frilled_shark.jpg
http://upload.wikimedia.org/wikipedia/commons/f/f1/Whale_shark_Georgia_aquarium.jpg

Published by Claudia Barbu

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