The Extinction of Megalodon
It is highly unlikely a single cause did in Megalodon. During its 14-million-year tenure, Megalodon was distributed nearly world-wide: its fossil teeth are known from Europe, Africa, North and South America, southern Asia, Indonesia, Australia, New Caledonia, and New Zealand. In general, widely distributed species are better able to persist through periods of environmental change than species with small or patchy distributions. Under adverse conditions, a few localized populations may dwindle and eventually disappear, but a cosmopolitan species like Megalodon is likely to endure. This mighty predator was therefore probably snuffed by an unfavorable combination of many different factors, at least some of which had far-reaching effects.
Major Environmental Changes
The world Megalodon inherited was very different from the one it left. Between Megalodon's appearance in the mid-Miocene (16 million years ago) and its extinction in the Plio-Pleistocene (1.6 million years ago), a barrage of large-scale changes occurred that affected the marine environment. Like shards of glass floating on molasses, the continents continued their languorous wanderings across the face of the planet, changing both land and sea as they went. Africa slammed into Europe during the Oligocene Epoch, reducing the interconnected Tethyan seaway to isolated bodies of water (today represented by the Mediterranean, Black, Caspian, and Aral seas). During the Miocene, the Indian subcontinent crashed into Asia, slowly crumpling the inland topography to help form the Himalayas. Similar tectonic altercations pushed up the Rockies and Andes mountains. These orogenic effects drastically altered global patterns of wind and rain as the continents jostled and shuffled into their present, familiar positions. The widespread cooling that had begun in the early Oligocene (about 35 million years ago) - combined with changes in wind and precipitation patterns - reduced the temperature at mid-latitudes by about 27 degrees Fahrenheit (15 degrees Celsius) and eventually resulted in a period of major glaciation at the poles by the late Miocene, about 10 million years ago. Between 7 and 3 million years ago, the Isthmus of Panama rose from the seafloor, joining North and South America and eliminating the last traces of the circum-tropical seaway that may have enabled Megalodon to disperse world-wide.
These terrestrial changes provoked ripple effects in the sea. Newly-formed mountains redistributed precipitation, creating rain shadows on their leeward sides. This redistribution of water caused many shallow inland seas to dry up, creating expansive salt flats that created insurmountable osmotic challenges for aquatic life. The Atlantic continued to spread on either side of the mid-oceanic rift - a gigantic tear in the seafloor into which magma from deep in the Earth is extruded, pushing apart the Old and New Worlds as inexorably as a couple in which the partners have stopped speaking to one another. Among other effects, this seafloor spreading caused the Gulf Stream to slow, thereby reducing upwelling of cold, nutrient-rich water near the southeastern parts of North America. Such upwellings were absolutely vital to supporting the rich web of marine life that ultimately provided a food source for Megalodon. It is probably no coincidence that the cetotheriids (early baleen whales) became extinct about the same time as Megalodon. Glaciation occupied much of the planetary water supply, locking it away in massive sheets of opalescent ice. Consequently, the sea level dropped some 650 feet (200 metres) and allowed many shallow coastal areas to dry up. Thus, through lowered temperature and sea level, Megalodon lost large areas of suitable habitat.
To a greater or lesser extent, all living lamnids - including the White Shark - have a modified circulatory system that enables them to retain metabolic heat and extend their range into chilly waters. With the exception of the Shortfin Mako (Isurus oxyrinchus), which makes a good living even in tropical waters, all extant lamnids are primarily cold-water animals. Although some lamnids - like the White Shark - occasionally visit warmer waters, very few actually live there. Like the primates slathered in coconut oil on tropical beaches, warm water lamnids are generally tourists. And, like their human counterparts, they eventually go home. In contrast, megalodon does not seem to have extended its range into cool temperate waters. Despite its enormous body mass, megalodon may not have shared the lamnids' ability to retain significant metabolic heat. This shortcoming may have effectively trapped Megalodon in discrete, ever-decreasing puddles of warm coastal waters. If, as Robert Purdy's paleoecological study suggests, Megalodon was limited to warm waters and relied on coastal areas as pupping grounds - no matter from whence it descended or what it looked like - it had a very sandtiger-like life history. And this may have led to Megalodon's ultimate undoing.
Tropical waters are relatively nutrient-poor compared with those of temperate zones. In the western North Atlantic, loss of upwelling by the end of the Pliocene (about 1.6 million years ago) may have left Megalodon trapped in waters unable to supply sufficient food to fuel its titanic mass. Due to an inability or unwillingness to expand its range to richer, colder waters, Megalodon may well have starved to death. In addition, lowered sea-level at about the same time may have left Megalodon without nursery areas relatively free from predators large and well-armed enough to snack on its pups. Because most coastal water available at the end of the Pliocene was near or over where the continental shelf drops off into oceanic depths, large pelagic predators such as Killer (Orcinus orca) and False Killer (Pseudorca crassidens) whales may have been able to snap up the 12-foot (4-metre) Megalodon pups. In modern sharks, access to predator-free nursery areas is known to be an important factor in determining survival of pups. Thus, due to environmental changes, Megalodon may have been 'hit' from both sides: the adults were too large to get enough to eat and the juveniles were too small not to be eaten by other predators.
Bad for Megalodon, Good for Us?
The rise of the Isthmus of Panama stimulated many ecological changes. In addition to sundering two oceans, this event linked the Americas, allowing animal migrations that forever changed New World ecology. Terrestrial species that had long been separated by impassable depths met, and in the face of new predators and diseases, many vanished. Paleontologist Steven Stanley has recently proposed that the rise of the Panamanian Isthmus may have also spurred the rise of human intelligence half the globe away. According to Stanley's intriguing but controversial theory, when Panama severed the link between the Atlantic and Pacific, the flow of deep ocean currents was changed dramatically which, in turn, had profound effects on Earth's climate. Warm currents were diverted away from the Arctic causing this region - and eventually the whole northern hemisphere - to chill. Between 3.2 and 2.5 million years ago, the Earth plunged into an ice age, triggering the oscillating cycle of glacial advances and retreats that continues to this day. This ice age brought cooling and drying, even to faraway Africa. Eastern African forests shrank and our early ancestors, the ape-like australopithicines, were forced to abandon lives spent mostly in trees. The few who survived evolved into Homo, man-like creatures that walked upright. With their hands essentially free for the first time in evolutionary history, these early humans could care for infants who were helpless and slow to mature. And only with that extra time during childhood development could humans begin evolving large, complex brains. Thus, what may have been an unlucky break for Megalodon, may have been a lucky one for us.
In the Shadow of Megalodon
We will probably never know for sure why Megalodon became extinct. Scattered distribution, insufficient food, or increased vulnerability of its pups may all have played a role. Or perhaps some other combination of factors conspired to snuff Megalodon. Yet the White Shark survives to this day. This species may have survived due to its ability to exploit the rich feeding available in cool temperate waters. Yet the White Shark is no more invincible than Megalodon proved to be. Like other sharks, the Great White matures late in life and has few pups per litter, severely limiting its ability to replace itself. This reproductive strategy has proven very sound in stable environments with few natural enemies. But evolution has simply not equipped the Great White to deal with the most complete and deadly predator the world has ever known. It may be the White Shark's ultimate misfortune to share a planet with us.