C. megalodon

Appearance

Swiss naturalist Louis Agassiz gave the shark its initial scientific name, ''Carcharodon megalodon'', in 1835, in his research work ''Recherches sur les poissons fossiles'' , which he completed in 1843. ''C. megalodon'' teeth are morphologically similar to the teeth of the great white shark, and on the basis of this observation, Agassiz assigned ''C. megalodon'' to the genus ''Carcharodon''. While the scientific name is ''C. megalodon'', it is often informally dubbed the "megatooth shark", "giant white shark" or "monster shark".Among extant species, the great white shark is regarded as the best analogue to ''C. megalodon''. The lack of well-preserved fossil ''C. megalodon'' skeletons led scientists to rely on the great white shark as the basis of its reconstruction and size estimation.Gottfried and colleagues further estimated the schematics of ''C. megalodons 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 that of the great white. Its fins were proportional to its larger size. Scrutiny of the partially preserved vertebral ''C. 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 long and represents a young individual. The team stresses that relative and proportional changes in the skeletal features of ''C. megalodon'' are ontogenetic in nature in comparison to those of the great white, as they occur in great white sharks while growing. Fossil remains of ''C. megalodon'' confirm that it had a heavily calcified skeleton while alive.

Naming

Shark researcher David Ward elaborated on the evolution of ''Carcharocles'' by implying that this lineage, stretching from the Paleocene to the Pliocene, is of a single giant shark which gradually changed through time, suggesting a case of chronospecies. This assessment may be credible.

Habitat

Sharks, especially large species, are highly mobile and experience a complex life history amid wide distribution. Fossil records indicate that ''C. megalodon'' was cosmopolitan, and commonly occurred in subtropical to temperate latitudes. It has been found at latitudes up to 55° N; its inferred tolerated temperature range goes down to an annual mean of 12 °C . It arguably had the capacity to endure such low temperatures by virtue of mesothermy, the physiological capability of large sharks to conserve metabolic heat by maintaining a higher body temperature than the surrounding water.

''C. megalodon'' had enough adaptability to inhabit a wide range of marine environments , and exhibited a transient lifestyle. Adult ''C. 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.

Fossil remains show a trend for specimens to be larger in the southern hemisphere than in the northern and in the Pacific relative to the Atlantic . They do not, however, suggest any trend of changing body size with absolute latitude, or of change in size over time . The overall modal length has been estimated at 10.5 m, with the length distribution skewed towards larger individuals, suggesting an ecological or competitive advantage for larger body size.

Food

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 even more effective hunting strategies against large prey than the great white shark.

Paleontologists surveyed fossils to determine attacking patterns. One particular specimen – the remains of a 9 metres long prehistoric baleen whale  – provided the first opportunity to quantitatively analyze its attack behavior. The predator primarily focused on the tough bony portions 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 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 the great white shark. Furthermore, attack patterns could differ for prey of different sizes. Fossil remains of some small cetaceans suggest that they were rammed with great force from below before being killed and eaten.

During the Pliocene, larger and more advanced cetaceans appeared. ''C. megalodon'' apparently further refined its hunting strategies to cope with these large whales. Numerous fossilized flipper bones and caudal vertebrae of large whales from the Pliocene have been found with ''C. megalodon'' bite marks. This paleontological evidence suggests that ''C. megalodon'' would immobilize a large whale by ripping apart or biting off its locomotive structures before killing and feeding on it.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 of whales showed a trend towards polar regions. The cooling of the oceans during the Pliocene might have restricted the access of ''C. megalodon'' to polar regions, depriving it of its main food source of large whales. As a result of these developments, the food supply for ''C. 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. Paleontologist Albert Sanders suggests that ''C. megalodon'' was too large to sustain itself on the declining tropical food supply. The resulting shortage of food sources in the tropics during Plio-Pleistocene times may have fueled cannibalism by ''C. megalodon''. Juveniles were at increased risk from attacks by adults during times of starvation.

Evolution

''C. megalodon'' is represented in the fossil record primarily by teeth and vertebral centra. As with all sharks, ''C. megalodon's'' skeleton was formed of cartilage rather than bone; this means that most fossil specimens are poorly preserved. While the earliest ''C. megalodon'' remains were reported from late Oligocene strata, around 28 million years old, a more reliable date for the origin of the species is the early Miocene, about 23 million years ago. Although fossils are mostly absent in strata extending beyond the Tertiary boundary, they have been reported from subsequent Pleistocene strata. It is believed that ''C. megalodon'' became extinct around the end of the Pliocene, probably about 2.6 million years ago; reported post-Pliocene ''C. megalodon'' teeth are thought to be reworked fossils. ''C. megalodon'' had a cosmopolitan distribution; its fossils have been excavated from many parts of the world, including Europe, Africa and both North and South America, as well as Puerto Rico, Cuba, Jamaica, the Canary Islands, Australia, New Zealand, Japan, Malta, the Grenadines and India. ''C. megalodon'' teeth have been excavated from regions far away from continental lands, such as the Mariana Trench in the Pacific Ocean.


The most common fossils of ''C. megalodon'' are its teeth. Diagnostic characteristics include: triangular shape, robust structure, large size, fine serrations, and visible V-shaped neck. ''C. megalodon'' teeth can measure over 180 millimetres in slant height or diagonal length, and are the largest of any known shark species.

Some fossil vertebrae have been found. The most notable example is a partially preserved vertebral column of a single specimen, excavated in the Antwerp basin, Belgium by M. Leriche in 1926. It comprises 150 vertebral centra, with the centra ranging from 55 millimetres to 155 millimetres in diameter. However, scientists have claimed that considerably larger vertebral centra can be expected. A partially preserved vertebral column of another ''C. megalodon'' specimen was excavated from Gram clay in Denmark by Bendix-Almgeen in 1983. This specimen comprises 20 vertebral centra, with the centra ranging from 100 millimetres to 230 millimetres in diameter.Even after decades of research and scrutiny, controversy over ''C. megalodon'' phylogeny persists. Several shark researchers insist that ''C. megalodon'' is a close relative of the great white shark. However, others cite convergent evolution as the reason for the dental similarity. Such ''Carcharocles'' advocates have gained noticeable support. However, the original taxonomic assignment still has wide acceptance.

Uses

Expert consensus has pointed to factors such as a cooling trend in the oceans and a shortage of food sources during Plio-Pleistocene times having played a significant role in the demise of ''C. megalodon''.

However, a recent analysis of the distribution, abundance and climatic range of ''C. megalodon'' over geologic time suggests that biotic factors, i.e. dwindling numbers of prey species combined with competition from new macro-predators , were the primary drivers of its extinction. The distribution of ''C. megalodon'' during the Miocene and Pliocene did not correlate with warming and cooling trends; while the abundance and distribution of ''C. megalodon'' declined during the Pliocene, ''C. megalodon'' did show a capacity to inhabit anti-tropical latitudes. ''C. megalodon'' was found in locations with a mean temperature ranging from 12 to 27 °C , indicating that the global extent of suitable habitat for ''C. megalodon'' should not have been greatly affected by the temperature changes that occurred.

The extinction of ''C. megalodon'' set the stage for further changes in marine communities. Average body size of baleen whales increased significantly after its disappearance. Other apex predators gained from the loss of this formidable species, in some cases spreading to regions where ''C. megalodon'' became absent.