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 C. megalodon. Many whale bones have been found with clear signs of large bite marks (deep gashes) made by teeth that match those of C. megalodon, and various excavations have revealed C. megalodon teeth lying close to the chewed remains of whales, and sometimes in direct association with them. Fossil evidence of interactions between C. 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 during its time of existence. However, C. megalodon, being at the top of the food chain, likely had a profound impact on the structuring of marine communities. Fossil evidence indicates a correlation between the emergence of C. megalodon and extensive diversification of cetaceans around the world. Juvenile C. megalodon preferred regions where small cetaceans were abundant, and adult C. megalodon preferred regions where large cetaceans were abundant. Such preferences may have developed shortly after they appeared in the Oligocene. In addition, C. megalodon was contemporaneous with macro-predatory odontocetes (particularly raptorial sperm whales and squalodontids), which were also likely among the apex predators of that time, and provided competition. In response to competition from giant macro-predatory sharks, macro-predatory odontocetes may have evolved some defensive adaptations; some species became pack predators, and some species 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, C. 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. C. megalodon likely also had a tendency for cannibalism.
Sharks often employ complex hunting strategies to engage large prey animals. Some paleontologists suggest that the hunting strategies of the great white shark may offer clues as to how C. megalodon might have hunted its unusually large prey (i.e., whales). However, fossil evidence suggests that C. megalodon employed more effective hunting strategies against large prey than those of the great white shark.
Paleontologists have conducted a survey of fossils to determine attacking patterns of C. megalodon on prey. 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 the attacking behavior of C. megalodon. The predator primarily focused its attack on the tough bony portions (i.e. shoulders, flippers, rib cage, and upper spine) of the prey, which great white sharks generally avoid. Dr. Bretton Kent elaborated that C. megalodon attempted to crush the bones and damage delicate organs (i.e. heart, and lungs) harbored within the rib cage of the prey. Such an attack would have immobilized the prey, which would have died quickly due to injuries to these vital organs. These findings also clarify why the ancient shark needed more robust dentition than that of great white sharks. Furthermore, the 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 and more advanced cetaceans appeared. C. 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 bite marks that were caused by attacks of C. megalodon. This paleontological evidence suggests that C. megalodon would attempt to immobilize a large whale by ripping apart or biting off its propulsive structures before killing and feeding on it.
Fossil evidence suggests that the preferred nursery sites of C. megalodon were warm water coastal environments, where potential threats were minor and food sources plentiful. Nursery sites have been identified in the Gatun Formation of Panama, the Calvert Formation of Maryland, 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 C. megalodon teeth indicate that C. megalodon pups were around 2 to 4 metres (7 to 13 ft) in total length (TL) at birth. Their dietary preferences display an ontogenetic shift. Young C. megalodon commonly preyed on fish, giant sea turtles, dugongs, and small cetaceans; mature C. megalodon moved to off-shore cetacean high-use areas and consumed large cetaceans.
However, there is an exceptional case which suggests that juvenile C. megalodon may occasionally have attacked much larger balaenopterid whales. Three tooth marks apparently from a 4-7 m 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 C. megalodon.