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20.01.2016 00:55 - Largest prehistoric animals Vol.1 Vertebrates part11 Sharks ch.4 Megalodon - continuation
Автор: valentint Категория: Забавление   
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Последна промяна: 05.12.2016 19:41


Largest known specimens

Gordon Hubbell from Gainesville, Florida possesses an upper anterior megalodon tooth whose maximum height is 184.1 millimetres (7.25 in).In addition, a megalodon jaw reconstruction contains a tooth whose maximum height is reportedly 193.67 millimetres (7.625 in).This jaw reconstruction was developed by fossil hunter Vito Bertucci who was known as "Megalodon Man".

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Gottfried and colleagues introduced a method to determine the mass of the great white after studying the length–mass relationship data of 175 specimens at various growth stages and extrapolated it to estimate C. megalodon's mass. According to their model, a 15.9 metres (52 ft) long megalodon would have a mass of about 48
metric tons (53 short tons) a 17 metres (56 ft) long megalodon would have a mass of about 59 metric tons (65 short tons) and a 20.3 metres (67 ft) long megalodon would have a mass of 103 metric tons (114 short tons).

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A team of Japanese scientists, T. Uyeno, O. Sakamoto, and H. Sekine, discovered and excavated partial remains of a megalodon, with a nearly complete associated set of its teeth, from
Saitama, Japan in 1989.Another nearly complete associated megalodon dentition was excavated from Yorktown Formations of Lee Creek, North Carolina in the United States and served as the basis of a jaw reconstruction of C. megalodon at the American Museum of Natural History in New York City.These associated tooth sets solved the mystery of how many teeth would be in the jaws of the megalodon in each row. As a result, highly accurate jaw reconstructions became possible. More associated megalodon dentitions were found in later years. Based on these discoveries, scientists S. Applegate and L. Espinosa published an artificial dental formula (representation of dentition of an animal with respect to types of teeth and their arrangement within the animal's jaw) for megalodon in 1996.Most accurate modern C. megalodon jaw reconstructions are based on this dental formula.

The dental formula of C. megalodon is: 2.1.7.43.0.8.4.

As evident from the formula, C. megalodon had four kinds of teeth in its jaws.

Anterior - A

Intermediate - I (megalodon's tooth technically appears to be an upper anterior and is termed as "A3" because it is fairly symmetrical and does not point mesially (side of the tooth toward the midline of the jaws where the left and right jaws meet), but this tooth is still designated as an intermediate tooth.However, the great white shark's intermediate tooth does point mesially. This point was raised in the Carcharodon vs. Carcharocles debate regarding the megalodon and favors the case of Carcharocles proponents.)

Lateral - L

Posterior - P

Megalodon had a very robust dentition and had a total of about 276 teeth in its jaws, spanning 5 rows. Paleontologists suggest that a very large megalodon had jaws over 2 metres (7 ft) across.

In 2008, a team of scientists led by S. Wroe conducted an experiment to determine the bite force of the great white shark, using a 2.4 metres (8 ft) long specimen, and then isometrically scaling the results for its maximum confirmed size and the conservative minimum and maximum body mass of C. megalodon, placing the bite force of the latter between 108,514 N (24,400 lbf) and 182,201 N (41,000 lbf) in a posterior bite. Compared to 18,216 N (4,100 lbf) for the largest confirmed great white shark and 5,300 N (1,200 lbf) for the placoderm fish Dunkleosteus.

In addition, Wroe and colleagues pointed out that sharks shake sideways while feeding, amplifying the post-cranial generated forces. Therefore the total force experienced by prey is likely higher than the estimate.The extraordinary bite forces in C. megalodon must be considered in the context of its great size and of paleontological evidence suggesting that C. megalodon was an active predator of large whales.

Megalodon's exceptionally robust teeth are serrated which would have improved efficiency in slicing its prey's flesh. Paleontologist B. K. Kent suggested that these teeth are comparatively thicker for their size with much lower slenderness and bending strength ratios. Their roots are substantially larger relative to total tooth heights, and so have a greater mechanical advantage. Teeth with these traits are good cutting tools and are well suited for grasping powerful prey and would seldom crack even when slicing through bones.



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Gottfried and colleagues further estimated the schematics of megalodon's entire skeleton.To support the beast's dentition, its jaws would have been massive, stouter, and more strongly developed than those of the great white, which possesses a comparatively gracile dentition. The jaws would have given it a "pig-eyed" profile.Its
chondrocranium would have had a blockier and more robust appearance than the great white.Its fins were proportional to its larger size.Scrutiny of the partially preserved vertebral megalodon specimen from Belgium revealed that C. megalodon had a higher vertebral count than specimens of any known shark. Only the great white approached it.

Using the above characteristics, Gottfried and colleagues reconstructed the entire skeleton of C. megalodon, which was later put on display at the Calvert Marine Museum at Solomon's Island, Maryland in the United States.This reconstruction is 11.5 metres (38 ft) long and represents a young individual. The team stresses that relative and proportional changes in megalodon skeletal features are ontogenetic in nature in comparison to that of great white, as they occur in great whites while growing. Fossil remains of C. megalodon confirm that it had a heavily calcified skeleton while alive.

Sharks, especially large species, are highly mobile and experience a complex life history amid wide distribution.Fossil records indicate that it was cosmopolitan and commonly occurred in subtropical to temperate latitudes.Prior to the formation of the Isthmus of Panama, the seas were relatively warmer. This would have made it possible for the species to live in all oceans.

Megalodon had enough adaptability to inhabit a wide range of marine environments (i.e. coastal shallow waters, coastal upwelling, swampy coastal lagoons, sandy littorals and offshore deep water environments), and exhibited a transient lifestyle.Adult megalodon were not abundant in shallow water environments and mostly lurked offshore. C. megalodon may have moved between coastal and oceanic waters, particularly in different stages of its life cycle.

 

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Sharks are generally opportunistic predators. However, scientists propose that C. megalodon was "arguably the most formidable carnivore ever to have existed."Its great size, high-speed swimming capability and powerful jaws coupled with formidable killing apparatus made it a super-predator with the capability to consume a broad spectrum of fauna.

Fossil evidence indicates that C. megalodon preyed upon cetaceans (i.e., dolphins), small whales (including cetotherrids,squalodontids and Odobenocetops), and large whales (including sperm whales, bowhead whales, and rorquals), pinnipeds, porpoises sirenians and giant sea turtles.Marine mammals were regular prey targets for megalodon. Many whale bones have been found with clear signs of large bite marks (deep gashes) made by teeth that match megalodon's.Various excavations have revealed megalodon teeth lying close to the chewed remains of whales and sometimes in direct association with them.Fossil evidence of interactions between megalodon and pinnipeds also exist In one interesting observation, a 127 millimetres (5.0 in) C. megalodon tooth was found lying very close to a bitten earbone of a sea lion.

C. megalodon faced a highly competitive environment.However, its position at the top of the food chain,likely had a profound impact on the structuring of marine communities.Fossil evidence indicates a correlation between C. megalodon emergence and extensive diversification of cetaceans.Juvenile megalodon preferred habitats where small cetaceans were abundant, and adult megalodon preferred habitats where large cetaceans were abundant. Such preferences may have developed shortly after they appeared in the Oligocene.In addition, C. megalodon were contemporaneous with macro-predatory odontocetes (particularly raptorial sperm whales and squalodontids), which were also likely among the era's apex predators and provided competition.In response to competition from giant macro-predatory sharks, macro-predatory odontocetes may have evolved defensive adaptations; some species became pack predators and some attained gigantic sizes, such as Livyatan melvillei.By the end of the Miocene, raptorial sperm whales vanished from the fossil record and left an ecological void.

Like other sharks, megalodon also would have been piscivorous. Fossil evidence indicates that other notable species of macro-predatory sharks (e.g. great white sharks) responded to competitive pressure from C. megalodon by avoiding regions it inhabited.Megalodon likely also had a tendency for cannibalism.

Sharks often employ complex hunting strategies to engage large prey animals. Some paleontologists suggest that great white shark hunting strategies may offer clues as to how C. megalodon hunted its unusually large prey.However, fossil evidence suggests that C. megalodon employed more effective hunting strategies against large prey than those of the great white shark.

Paleontologists surveyed fossils to determine attacking patterns.One particular specimen — the remains of a 9 metres (30 ft) long prehistoric baleen whale (of an unknown Miocene taxon) — provided the first opportunity to quantitatively analyze its attack behavior. The predator primarily focused on the tough bony portions (i.e. shoulders, flippers, rib cage, and upper spine) of the prey, which great white sharks generally avoid. Dr. B. Kent elaborated that C. megalodon attempted to crush the bones and damage delicate organs (i.e. heart, and lungs) harbored within the rib cage. Such an attack would have immobilized the prey, which would have died quickly from injuries to these vital organs. These findings also clarify why the ancient shark needed more robust dentition than that of great whites.Furthermore, attack patterns could differ for prey of different sizes. Fossil remains of some small cetaceans (e.g. cetotheriids) suggest that they were rammed with great force from below before being killed and eaten.

During the Pliocene, larger and more advanced cetaceans appeared.Megalodon apparently further refined its hunting strategies to cope with these large whales. Numerous fossilized flipper bones (i.e., segments of the pectoral fins), and caudal vertebrae of large whales from the Pliocene have been found with megalodon bite marks. This paleontological evidence suggests that megalodon would immobilize a large whale by ripping apart or biting off its locomotive structures before killing and feeding on it.

Fossil evidence suggests that the preferred nursery sites of C. megalodon were warm water coastal environments, where threats were minor and food plentiful.Nursery sites were identified in the Gatun Formation of Panama, the Calvert Formation of Maryland, Banco de Concepción in Canary Islands and the Bone Valley Formation of Florida. As is the case with most sharks, C. megalodon also gave birth to live young. The size of neonate megalodon teeth indicate that megalodon pups were around 2 to 4 metres (7 to 13 ft) in total length at birth.Their dietary preferences display an ontogenetic shift.Young megalodon commonly preyed on fish, giant sea turtles dugongs and small cetaceans; mature megalodon moved to off-shore cetacean high-use areas and consumed large cetaceans.

However, an exceptional case in the fossil record suggests that juvenile megalodon may occasionally have attacked much larger balaenopterid whales. Three tooth marks apparently from a 4–7-metre (13–23 ft) long Pliocene macro-predatory shark were found on a rib from an ancestral great blue or humpback whale that showed evidence of subsequent healing. Scientists suspect that this shark was a juvenile megalodon.

The subject of C. megalodon extinction remains under investigation.Several possible causes for its decline and eventual disappearance have been proposed.

The Earth has been in a long term cooling trend since the Miocene Climactic Optimum, 15–17 Ma ago.This trend may have been accelerated by changes in global ocean circulation caused by the closure of the Central American Seaway and/or other factors (see Pliocene climate), setting the stage for glaciation in the northern hemisphere. Consequently, during the late Pliocene and Pleistocene, there were ice ages which cooled the oceans significantly.Expansion of glaciation during the Pliocene tied up huge volumes of water in continental ice sheets, resulting in significant sea level drops.The major reason cited is the decline in ocean temperatures at global scale during the Pliocene.This cooling trend adversely impacted C. megalodon, as it preferred warmer waters and as a result it may have declined in abundance until its ultimate extinction during the Pleistocene.Fossil evidence confirms the absence of C. megalodon in regions around the world where water temperatures had significantly declined during the Pliocene.Furthermore, these oceanographic changes may have restricted many of the suitable warm water nursery sites for megalodon, hindering reproduction.Nursery areas are pivotal for the survival of many shark species, in part because they protect juveniles from predation.

Baleen whales attained their greatest diversity during the Miocene with over 20 recognized genera in comparison to only six extant genera.Such diversity presented an ideal setting to support a gigantic macropredator such as C. megalodon.However, by the end of the Miocene many species of mysticetes had gone extinct surviving species may have been faster swimmers and thus more elusive prey.Furthermore, after the closure of the Central American Seaway, additional extinctions occurred in the marine environment and faunal redistribution took place; tropical great whales decreased in diversity and abundance.Whale migratory patterns during the Pliocene have been reconstructed from the fossil record, suggesting that most surviving species showed a trend towards polar regions.The cooling of the oceans during the Pliocene restricted the access of C. megalodon to polar regions, depriving it of its main food source, the great whales.As a result of these developments, the food supply for megalodon in regions it inhabited during the Pliocene, primarily in low-to-mid latitudes, was no longer sufficient to sustain it worldwide.C. megalodon was adapted to a specialized lifestyle, and this lifestyle was disturbed by these developments.The resulting shortage of food sources in the tropics during Plio-Pleistocene times may have fueled cannibalism by megalodon. Juveniles were at increased risk from attacks by adults during times of starvation.

Large raptorial delphinids (members of genus Orcinus) evolved during the Pliocene and likely filled the ecological void left by the disappearance of raptorial sperm whales at the end of the Miocene.A minority view is that competition from ancestral killer whales may have contributed to the shark's decline (another source suggests more generally that "competition with large odontocetes" may have been a factor). Fossil records indicate that these delphinids commonly occurred at high latitudes during the Pliocene, indicating that they could cope with the increasingly prevalent cold water temperatures. They also occurred in the tropics (e.g., Orcinus sp. in South Africa).

However, expert consensus suggests that factors such as a cooling trend in the oceans and a shortage of food sources during Plio-Pleistocene times played a significant role in megalodon's demise.Paleontologist Albert Sanders suggests that C. megalodon was too large to sustain itself on the declining tropical food supply. Other apex predators seem to have gained from the extinction of this formidable species.

















Тагове:   сова,   prehistoric animals,   giant animals,   sharks,


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