Megalodon
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[Photo] Size comparison between a Megalodon and a human diver.

The megalodon, Carcharodon megalodon, (from ancient Greek μ??γα?? = "big" + 'οδο???? (genitive 'οδ??ντο??) = "tooth") was a giant prehistoric shark that probably lived between about 16 to 1.6 million years ago. It is the biggest known carnivorous fish to have ever lived.

Known physiology
The megalodon is known principally from fossil teeth and a few fossilized vertebral centra. As with all other sharks, the skeleton of megalodon was formed of cartilage and not bone, resulting in the poor skeletal fossil record. However, megalodon's large teeth have survived the ages. The teeth are in many ways similar to great white shark teeth but are much larger and can measure up to more than 17.78 cm (7 inch) long in slant. Here is an example of one such massive tooth:

Recent studies cited by Roesch (see external links below) suggest megalodon was a "close relative" of the great white shark. However, a growing number of researchers dispute this close great white shark???megalodon relationship, instead citing convergent evolution as the reason for the dental similarity. Nevertheless, it is extrapolations from the tooth size of megalodon to modern sharks that provide us with our conceptions about what this ancient superpredator was like.

Estimating the maximum size of a megalodon is a highly controversial subject. However, three shark experts, M. D. Gottfried, Leonard Compagno and S.C. Bowman, have tried to solve this issue by developing a special method for measuring the size of sharks (including megalodon) with much greater accuracy. By using this new method, the maximum size of this creature has been calculated to be at 15.9 m (52.1 feet) long and the body mass of about 48 tons. But this calculation is based on a (6.61 inch) long upper anterior megalodon tooth, which is not the biggest specimen found. So these bigger megalodon teeth indicate that this creature probably could grow more than 16 m (54 feet) long.

Still it is important to note that a very old jaw reconstruction (developed by Professor Bashford Dean in 1909) of this shark suggests a size of about 30 m (100 ft) but that jaw reconstruction is now considered to be inaccurate.

Assuming similar metabolic-weight ratios as the great white shark, it is estimated that a large megalodon would need to eat about one-fiftieth of its weight of food on average per day. From our knowledge of the food chain during megalodon's existence, it is generally believed that this shark's diet consisted mostly of whales, along with large fish and primitive pinnipeds and sirenians.

Taxonomic dispute
There is some disagreement as to how the megalodon should be classified in taxonomy.

The older view (more favored by marine biologists) is that the megalodon should be classified in the Carcharodon genus with the great white shark, though this has generated debate whether megalodon is a direct ancestor of the great white shark or whether the two species share a common ancestor.
Around 1923, the genus, Carcharocles, was proposed to classify the very similar shark Carcharodon auriculatus. Many paleontologists are now favouring Carcharocles for the Megalodon as well. Carcharocles proponents give megalodon's likely ancestor as Otodus obliquus from the Eocene epoch, and the ancestor of the great white shark not megalodon but Isurus hastalis, the "broad tooth mako".

Extinction hypotheses
One hypothesis is that the adult Carcharodon megalodon fed largely on whales and became extinct as the polar seas became too cold for sharks, allowing whales to swim out of the range of megalodon via migration. Other explanations are simpler, suggesting that any prolonged disturbance of the foodchain would wipe out a predator with such massive metabolic requirements. Some cryptozoologists suggest the shark might have died out more recently, or might even still be alive; see "Relict" below. The fossil teeth of the animal are often found in areas that had shallow seas, such as near Bakersfield in California. Megalodon was probably a specialist that fed mostly on baleen whales in shallow waters. A main prey item was Cetotherium. Since the time of its extinction there have been few such shallows supporting constant, large whale populations, and the loss of such habitats caused the animal to gradually become extinct, as the species could no longer find enough food to sustain itself. The process would have been gradual, leading to fewer megalodons, more genetic drift and isolated megalodon populations.

The shark also faced competition from the Killer Whale (or Orca) which evolved less than five million years ago. Populations of "transient" Killer Whales exploit sea mammals, and with pack behavior and high intelligence the Orca would have crowded the shark out of the same declining food source. Once that took place, the shark's huge adult size was of no advantage and in fact meant starvation. The large size of the shark meant it probably hunted alone. It is also likely that Killer Whales preyed on small juvenile megalodons, which shared their range and thus were potential food.

Relict
While most mainstream experts contend that available evidence suggests that the megalodon is extinct, the idea of a relict population seems to have seized the public imagination, but evidence supporting such ideas is generally seen as both scant and ambiguous.

Megalodon teeth have been discovered that some argue date as recently as 10,000 to 15,000 years ago. This claim is based on the discovery of two teeth by the HMS Challenger scientific expedition (these teeth were dated by estimating the amount of time it took for manganese to accumulate on them, although it is quite possible the teeth were fossilized before being encrusted).

Others have countered that these recent estimates for these teeth are inaccurate, and "claims of post-Pliocene C. megalodon ... are erroneous", being based on outdated testing and methodology. Research has suggested that megalodons were probably coastal sharks, and that deep-sea survival is extremely unlikely.

Some relatively recent reports of large shark-like creatures have been interpreted as surviving megalodons, but such reports are usually considered misidentification of basking sharks, whale sharks or Greenland sharks. One well-known example was reported by writer Zane Grey. It is possible, but unlikely, that some of these sightings might be due to abnormally large great white sharks.

http://en.wikipedia.org/wiki/Megalodon

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