Paleoecology of Megalodon

and the White Shark

Devourer of Giants
© Robert Nicholls,
"Devourer of Giants", by Robert Nicholls.  In this stunning painting, a herd of mastodons (Anancus) have been drowned in a flash flood and carried out to sea, where they are now attracting the Megalodons.  The painting depicts a huge female (left), a large male (right) and a juvenile (centre). Used with the gracious permission of the artist, who reserves all rights to this image.

Megalodon was to the White Shark what Godzilla is to a gecko, a much larger and more ominous variation on the same theme. Yet, for almost 10 million years, these two species were actually contemporaries. How could the smaller, less heavily-armed white shark survive in the shadow of the awesome megalodon? How did the white shark manage to persist after megalodon shuffled off this mortal coil? And how can we puzzle out what happened in the far-distant past?

Since history unfolds in patterns that reflect environmental conditions, it can be argued that nothing in the fossil record makes sense except in the light of ecology. Paleoecology is the science of reconstructing how prehistoric ecosystems functioned. Clues from the fossil record enable paleontologists to piece together a cohesive 'snap-shot' of which species composed a given biological community and what the local environment was like. For example, studying which species of plants or animals are usually found together allows paleontologists to reconstruct floral or faunal associations. Similarly, analyzing temperature-dependent ratios of certain isotopes (mass variants of an element) can reveal much about the local climate, such as temperature and humidity. As the shark fossil record consists almost entirely of teeth, understanding the paleoecology of these creatures is fraught with challenges. But there are a few fundamental truths about sharks that we can extend to their prehistoric brethren with reasonable confidence.

Sharks are generally opportunistic predators, taking a broad spectrum of prey and scavenging when they can get away with it. The shape and size of a shark's teeth are well-known to constrain its dietary options. (Otherwise, the Basking Shark would probably be a dire threat to people in or on the water!) The knife-like, serrated teeth of megalodon and the white shark are marvelously suited to scooping gobbets of flesh from food items too large to be swallowed whole. This dental equipment thus puts these sharks in a rather exclusive snack bracket. While sharks with more slender, smooth-edged teeth had to be content with bite-sized prey, megalodon and its saw-toothed kin could reduce even the largest animals to manageable pieces. The basic dental type of both megalodon and the white shark can be traced back - through a blur of similarly-toothed antecedents - to at least the early Eocene Epoch, about 50 million years ago.

During the Eocene, the world was uniformly warmer - even at relatively high latitudes - and many coastal areas that are now dry land were covered by warm, shallow seas. On land there appeared giant, flightless "terror birds" known as phorusrhacids. Resembling malevolent roadrunners, these carnivorous birds pursued galloping herds of Hyracotherium, which were primitive, dog-sized ancestors of the modern horse. Nimravids, generally smallish (bobcat to jaguar-sized) carnivores resembling saber-toothed cats, used their elongated canines to cut slabs of flesh from large, herbivorous prey such as ungulate ancestors of today's deer, pronghorn antelope, and horses. Early bats, such as Icaronycteris, also appeared at about this time. Rousing groggily from their daytime slumber, these mammals slapped the nightwind on diaphanous webbed fingers illuminated by an Eocene moon. And in the ancient seas appeared the earliest truly marine mammals. As unlikely as it seems, the sleek, streamlined whales evolved from a hyena-shaped ancestor (actually more closely related to the hippopotamus, complete with teensy hooves on the tips of its toes), and the football-shaped sirenians (manatees and dugongs) evolved from an elephant-like ancestor. Shark-toothed archeocetes (primitive whales) pursued quicksilver schools of fishes and jet-propelled armadas of squids while, closer inshore, sirenian 'sea cows' dim-wittedly grazed on expansive submerged meadows of sea grasses. It must have been an opportune time to be a large, saw-toothed shark. 

Carcharocles megalodon, reconstructed as a giant White Shark, in "hot pursuit" of Allodesmus, an elephant seal-like pinniped from the middle Miocene of California.

Evidence from the Fossil Record

In a 1996 paper, paleontologist Robert W. Purdy used the fossil record to explore the paleoecology of Megalodon, the White Shark, and related species in the western North Atlantic. Purdy divided these fossil sharks into two groups based on the form of their teeth: a giant-toothed line (characterized by relatively thick, heavily constructed teeth) and a small-toothed line (characterized by thinner, more lightly constructed teeth). Although other paleontologists differ on the ancestry of the individual species Purdy includes in each of these categories, they all agree that megalodon is the most recent representative of the giant-toothed group and the white shark is the most recent of the small-toothed. Purdy's study of these two groups of saw-toothed shark have allowed him to infer tantalizing details of their respective prey and feeding area preferences. And because the teeth of both the giant-toothed and small-toothed sharks change shape with growth, Purdy's study has granted us intriguing insights into where each of these prehistoric sharks grew up, what they fed on, and how their respective ranges changed over time.

Purdy's study suggests that, since their origin in the early Eocene, about 50 million years ago, marine mammals have been fed upon by at least some of the saw-toothed sharks. It seems that, almost as soon as they re-invaded the sea, mammals were fair game for these sharks. As a carnivorous rule-of-thumb, bigger teeth imply bigger food. In keeping with this pattern, the giant-toothed sharks apparently preferred larger prey than their smaller-toothed cousins. Fossils of giant-toothed antecedents of megalodon have been associated with archeocete remains in deposits dating back to the mid-Eocene (about 45 million years ago), suggesting that these sharks fed on ancient whales more-or-less from their earliest beginnings. In the late Oligocene Epoch, about 30 million years ago, the first baleen whales (suborder Mysticeti) appeared (although their filter plates are rarely - if ever - preserved, baleen whales are identifiable by the symmetrical, toothless skull). Among the earliest mysticetes was a group known as the cetotheriids, which were generally 10 to 30 feet (3 to 10 metres) long and resembled the modern Grey Whale (Eschrichtius robustus). About 15 million years later, large individuals of megalodon seem to have developed a particular penchant for dining on baleen whales. Mysticete flipper bones and tail vertebrae from Miocene-Pliocene deposits (about 5 million years old) in the continental US often bear deep gashes clearly made by hand-sized triangular, finely serrated teeth. These wounds suggest the killer immobilized its prey with gruesome efficiency by chomping off its propulsive structures. The triangular, finely serrated dental pattern strongly implicates adult megalodon as the most likely perpetrator.

In the late Oligocene, about the same time as the baleen whales originated, there evolved a new kind of marine mammal. About 26 million years ago, a bear-descended group known as the pinnipeds (represented today by the seals, sea lions, and walruses) took to the sea. Purdy presented no evidence that these early sea lion-like pinnipeds were fed upon by either giant-toothed or small-toothed sharks. However, shortly after the slow-swimming, rather sausage-like seals (family Phocidae) originated, about 15 million years ago, they began appearing on the menu of small-toothed antecedents of the White Shark. In Miocene-Pliocene deposits (about 5 million years old) of the Lee Creek site in North Carolina, Purdy has found two specimens of monk seal (subfamily Monachinae) with wounds inflicted by a small-toothed predecessor of the White Shark known as xiphodon (apparently an intermediate form between C. hastalis and C. escheri, although Purdy himself interprets the matter somewhat differently) - including a fossilized monk seal bone with the tip of a xiphodon tooth embedded in it. The remarkable Lee Creek site also provides the first fossil evidence of an area of "high use" by large marine vertebrates, including Megalodon and xiphodon.

As with any skill, predation has to be learned. Juvenile Megalodon had teeth with a narrower blade than the adults. Based on faunal associations in fossil-bearing strata, juvenile megalodon seem to have fed on small-to-medium sized toothed whales (suborder Odontoceti), such as pilot whales, dolphins and porpoises. In mid-Miocene deposits (about 14 million years old) of the Calvert formation, Bill Heim has found a cut-marked porpoise (family Phocenidae) vertebra with a 2.5-inch (6.4-centimetre) anterior Megalodon tooth touching it. The close association between the porpoise vertebrae and a juvenile-sized Megalodon tooth might indicate that it was embedded in the cetacean's body when it died (although scavenging is also a possibility). Young Megalodon may thus have learned their trade by practicing on smaller cetaceans, resembling career criminals who get their start roughing up schoolyard kids for lunch money before moving on to larger, more risky and lucrative game. Juvenile White Sharks also have narrower teeth than the adults, usually with a basal cusplet on either side of the main blade. Such teeth are well suited to grasping slippery-bodied fishes that can be swallowed whole. Although Purdy presented no direct evidence, it seems likely that - from the mid-Miocene onward - juvenile White Sharks fed largely on bony fishes and other sharks. Today, White Sharks are primarily fish-eaters but are able to begin taking marine mammals once they grow large enough (more than about 10 feet or 3 metres in length) to tackle such massive entrées. Thus, like Megalodon, the White Shark apprentices on smaller prey before graduating to larger game.

Different Habitats, Different Niches

Purdy's work has also revealed differences in the distribution and pupping grounds of giant-toothed and small-toothed sharks. The fossil record suggests that the giant-toothed form inhabited the waters surrounding Antarctica in the late Eocene (about 35 million years ago), when the ocean in general was warmer than it is today and these polar seas were temperate rather than freezing cold. But already a steady world-wide cooling trend had begun. Eventually, continental drift and other tectonic activities changed oceanic circulation patterns, resulting in a circum-Antarctic current, effectively condemning the continent to icy isolation. Paleontologist Albert Sanders has excavated about 100 fossil teeth of giant-toothed antecedents of Megalodon from late Oligocene deposits (about 27 million years old) in South Carolina. The seas covering South Carolina at that time were shallow and warm. Just offshore, cool, nutrient-rich water welled up from the depths, supporting vibrant communities of marine predators and their prey. Thus, since its appearance some 16 million years ago, Megalodon had been an inhabitant of relatively warm waters over continental shelves. In the western North Atlantic, fossilized teeth of juvenile Megalodon occur in warm-water coastal areas south of Cape Hateras, especially in regions with nearby upwelling, suggesting that mothers of this species used these areas as nurseries.

Fossil white shark teeth are abundant in late-Miocene and younger deposits (from about 10 million years ago onward) characterized by cool temperate water and are rare or absent in deposits of similar age characterized by warm water. Today, many of the smallest known white shark specimens (about 4 to 5 feet or 1.2 to 1.5 metres in length) have been taken off Long Island, at the northern end of the mid-Atlantic Bight. Based on this fact, Purdy suggests that, from the late-Miocene onward, the white shark used these relatively cold waters as nursery areas and seemed to have "avoided" warmer areas that were preferred by Megalodon. While there is no evidence that Megalodon actually displaced the white shark, Purdy's data make it clear that the white shark has been a primarily cool temperate water inhabitant since at least the early Pliocene Epoch (about 5 million years ago). Paleoecological studies thus indicate that the white shark did not live in Megalodon's shadow at all. These two heavyweight predators got along by feeding on different prey - megalodon on whales, the white shark on seals - which contributed to their living in different areas - Megalodon in warm water, the White Shark in cool.

Megalodon must have been one of the most awesome predators the ocean has ever produced. For 14 million years, megalodon was the uncontested ruler of the warm, shallow seas it inhabited. But no autocracy lasts forever. According to the fossil record, about 1.6 million years ago megalodon suddenly and mysteriously died out. But its smaller cousin, the White Shark, is still with us, haunting our dreams and inspiring our fear, admiration, and curiosity. The mystery of why Megalodon became extinct while the White Shark survives is one of paleontology's all-time great whodunnits.


ReefQuest Centre for Shark Research
Text and illustrations © R. Aidan Martin
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