IN THE storage halls of the Smithsonian鈥檚 National Museum of Natural History
in Washington DC, among endless racks and shelves filled with specimens, lies an
ancient whale with quite a story to tell. Admittedly, the creature doesn鈥檛 look
too impressive these days. Its fragile skull is encased in a protective plaster
jacket and the various bony fragments are numbered and filed away in drawers.
But this baleen whale, which lived between 12 million and 15 million years ago,
had an encounter with one of the most ferocious predators of all time, and the
tale of its violent death is inscribed on its bones.
From dozens of marks on its skeleton it is clear that the cetacean lost its
life in the grip of a massive predatory shark. The culprit, Carcharodon
megalodon, may have grown to 17 metres or more and would have thought
nothing of challenging a 9-metre whale. Megalodon has been seen as a scaled-up
version of the great white shark of Jaws fame. Imagine a beast up to
three times the size of the movie star, snacking on holiday makers off the coast
of North Carolina. Now think again. Because new findings both from the bones of
the Smithsonian鈥檚 whale and from fossilised shark teeth suggest that megalodon
was far more terrifying than that.
Megalodon was the largest carnivorous fish that ever lived on Earth. Weighing
as much as 65 tonnes and sporting teeth that could exceed 15 centimetres from
tip to root base, the shark was a superpredator. For almost 20 million years it
dominated most of the world鈥檚 subtropical and temperate oceans. Then, somewhere
between 3 million and 1 million years ago it became extinct. Perhaps the
competition from early killer whales was too much. Maybe it fell victim to
global cooling, or shifts in whale populations to colder latitudes where the
shark could not follow. It is hard to tell, for the prehistoric giant left very
few clues to how it lived or died.
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Like all sharks and their relatives, the skates and rays, megalodon鈥檚
skeleton was made of cartilage, not bone, and cartilage rarely survives in the
fossil record. Nearly all of what we now know about megalodon comes from
studying its fossilised teeth. Unlike the teeth of mammals, sharks鈥� teeth are
not fixed in their jaws. Instead, resilient cartilage fibres anchor them to the
jaw surface. As the teeth wear out, they are shed and replaced鈥�
conveyor-belt fashion鈥攚ith new ones that form in rows inside the animals鈥�
mouths. Some sharks produce and discard tens of thousands of teeth in a
lifetime. As a result, shark teeth are among the most common vertebrate
fossils.
When palaeontologists come across a tooth from an extinct shark, they try to
understand the ancient fish by comparing it with the teeth of modern sharks.
Megalodon teeth are triangular with serrated edges. 鈥淭he great white shark is
the closest living analogue we have to megalodon,鈥� says Michael Gottfried, a
palaeontologist from the University of Michigan. He has used the similarities
between the teeth of the two fish to recreate an entire megalodon from a
smallish jaw on display at the Smithsonian鈥檚 National Museum of Natural History.
鈥淲e assumed megalodon had similar proportions to great whites,鈥� says Gottfried.
Using a formula relating tooth size to body length in great whites, his team
determined that their model should be about 11.5 metres long.
If megalodon looked like its smaller modern counterpart, did it also behave
like a great white? Megalodon teeth are often found in fossil beds rich in whale
bones, suggesting that the ancient sharks preyed on large marine mammals, just
as adult great whites frequently do. Palaeontologists assumed that both adopted
similar hunting tactics. But Bretton Kent from the University of Maryland is
challenging this assumption.
Kent describes himself as a functional morphologist, a biologist armed with
the equations of an engineer. He studies animal teeth through a simple set of
parameters that quantitatively relate tooth form to function, whether the
subjects are sharks, dogs or sabre-toothed tigers. 鈥淏asically what you do,鈥� says
Kent, 鈥渋s model the tooth as if it鈥檚 a beam attached at one end鈥攖o the
jaw鈥攁nd apply standard engineering equations to it to describe its
behaviour under different types of stress.鈥� For Kent, the key to megalodon鈥檚
behaviour lies in the subtle differences between its teeth and those of great
whites, rather than the more obvious similarities.
For sharks and all toothed creatures, teeth represent an investment of
energy, material and time. When human engineers build a machine, they typically
over-design mechanical components, explains Kent. That is, they calculate the
minimum necessary characteristics of struts, bolts or beams, and then include
safety factors to ensure that the various components can withstand three or more
times the maximum expected stresses.
In nature, on the other hand, over-design is a luxury few animals can
afford. A creature with overdesigned features, whether they are teeth or
feathers or muscles, will be at a disadvantage when competing with animals whose
features are just adequate for their needs, with the remaining resources devoted
to other vital activities, such as breeding, for example. Kent acknowledges that
teeth are a tiny fraction of a shark鈥檚 total bulk, and it might seem that slight
variations in tooth structure would have an equally slight impact on a shark鈥檚
viability. But evolutionary success hinges on such modest advantages.
If you look at tooth design from an engineering point of view, it can lead to
powerful insights into the lifestyles of ancient animals. Kent combines tooth
dimensions into three primary parameters: bending strength ratio, slenderness
ratio and mechanical advantage. The bending strength ratio reflects a tooth鈥檚
thinness, front to back. This is high in teeth adapted for cutting, which tend
to be broad but thin. They are more likely to break when stressed across their
natural cutting direction than through it鈥攋ust as a plastic knife will
more readily snap if pressure is applied to the side rather than to the cutting
edge.
The slenderness ratio is a measure of length to breadth. Needle-like teeth
with high slenderness ratios are good for grasping small prey, like mackerel or
flounder, but buckle under loads applied along their length, as would happen if
an animal munched on the bone of a large sea mammal. Mechanical advantage
relates the length of a tooth to the length of its root. Long teeth with short
roots have low mechanical advantage and are more easily pulled from the jaw than
teeth with comparatively larger roots.
The teeth of great whites have the high bending strength ratios of cutting
teeth, high slenderness ratios, implying they are not designed for crushing, and
roots that are short when compared with the overall tooth height. This means a
great white鈥檚 teeth have a low mechanical advantage and will likely be ripped
out by struggling prey. Such a combination of parameters, says Kent, 鈥渋s ideal
for a shark that hunts with a slash-and-release type of attack that concentrates
on the softer portions of the prey鈥�. Surveys of the corpses of marine mammals
killed by great whites confirm that, when feeding on sea lions, small whales and
porpoises, the sharks most frequently attack the delicate abdomens or rear
flippers of their victims. Though they often tear off chunks of flesh during an
attack, the sharks rarely grapple with large prey, waiting instead for the
animal to die of injuries, or carrying animals in their jaws until they bleed to
death, before feeding at their leisure.
Megalodon teeth, though superficially similar to those of great whites, have
an entirely different set of parameters. Though they share the serrated cutting
edge, megalodon teeth are comparatively thicker for their size with much lower
slenderness and bending strength ratios. They also have roots that are
substantially larger relative to total tooth heights, and so have a greater
mechanical advantage.
Kent explains that teeth with these traits not only make good cutting tools,
but are also well suited to grasping powerful prey and unlikely to crack when
they bite down on bone. The differences between the teeth of great white and
megalodon suggest fundamental differences in their diet, attack behaviour, or
indeed both. But Kent reasons that as large mammals formed the menu of both
sharks, the difference in their teeth must be related to variations in their
attack behaviour.
Finding a living eyewitness to a megalodon attack is, obviously, out of the
question. This is where the Smithsonian鈥檚 baleen whale comes in. The specimen,
which was recovered from the shores of Chesapeake Bay in southern Maryland,
comes from a now extinct species of baleen whale, and was both witness and
victim of a megalodon attack. As it happens, megalodon tooth marks on whale
bones are well documented, but no other near-complete whale skeletons bearing
the shark鈥檚 dental imprint have been identified. This rare specimen provided the
first opportunity to quantitatively analyse megalodon feeding behaviour.
鈥淢egalodon was the only shark large enough to attack a whale of this size,鈥�
says Kent鈥檚 student Monica Newell as she sorts through drawers dedicated to the
bits that make up the whale鈥檚 skeleton. 鈥淗ere鈥檚 one on a metacarpal-4, a right
flipper bone.鈥� The mark Newell points out is little more than a scratch, but the
gouge has a V-shaped cross section that neatly matches the profile at the tip of
a megalodon tooth.
In a survey lasting two years, Newell identified more than 70 megalodon bite
marks on the whale鈥檚 skeleton. Nearly two-thirds of those are on the whale鈥檚
shoulders, front flippers and upper spine. Newell also looked at whale fossils
in the Smithsonian collection that don鈥檛 come from complete skeletons鈥攎any
of which show signs of megalodon attacks. Here she also found that most
megalodon tooth marks appeared on whale bones from the shoulder and chest
regions. The shark apparently focused its attack on the bony portions great
whites generally avoid.
Kent believes that Newell鈥檚 surveys may explain why the ancient shark needed
teeth that were more robust than the great white鈥檚. While great whites usually
attack with lunging, slash-and-release bites, megalodon seems to have
latched onto its victims like a terrier on a rat. Kent suspects that the
prehistoric shark attacked the bony shoulder and rib cage of its prey, trying to
crush the bones and damage the heart, lungs or other delicate organs. In
contrast to a slower death from the jaws of a great white, the victim of a
megalodon would have died suddenly from massive internal injuries.
鈥淚 think [Kent鈥檚] kind of analysis is very good,鈥� says Thomas Frazzetta of
the University of Illinois at Champagne-Urbana. 鈥淭here are amazingly few
analytic works on how shark teeth work.鈥� Frazzetta, who studies the teeth of
living sharks, has written theoretical papers on the efficiency of various types
of cutting teeth, including the serrated teeth of great whites. 鈥淥f course, one
often might find that teeth are overdesigned for how they鈥檙e used on a daily
basis,鈥� he cautions. He points out that teeth may also be required for some rare
tasks which are crucial for survival. Nevertheless, Frazzetta applauds Kent鈥檚
engineering and forensic approach to palaeontology.
Others who study the behaviour and biology of great white sharks are less
enthusiastic about Kent鈥檚 findings. Shark expert A. Peter Klimley of the
University of California鈥檚 marine laboratory in Bodega Bay, is wary of drawing
conclusions based on such meagre evidence. 鈥淭o know whether or not megalodon was
an ambusher you need to know its pigmentation,鈥� says Klimley. He does accept
that the Smithsonian whale had bite marks in places a great white would not
normally bite its prey. 鈥淏ut it could be that outlier, that really unusual
animal you鈥檙e looking at,鈥� he says.
In contrast to Klimley鈥檚 conservatism, Mark Teaford of Johns Hopkins
University in Baltimore is almost cavalier when discussing Kent鈥檚 work. 鈥淧eople
oftentimes say, `This fossil isn鈥檛 like anything I can find today.鈥� Well, those
are the interesting ones,鈥� says Teaford, who applies tooth-shape analyses to
primate teeth. He believes that the minimal data on megalodons should not
discourage scientists from speculating about their subjects. 鈥淚n this sort of
case, where you鈥檙e dealing with a rather unique creature, then sure, you make do
with what you鈥檝e got,鈥� he says. 鈥淪ome people would probably be more cautious,
and hem and haw about it a bit . . . but by the same token, part of the business
we鈥檙e in is throwing ideas out there.鈥�
We can only imagine the antics of this ancient superpredator. Humans were
just beginning to conquer the land when megalodon鈥檚 reign of terror was coming
to an end. In the past century or so, occasional sightings have been recorded of
sharks claimed to be living examples of the prehistoric beast. None has been
substantiated, and most are attributed to misidentification of the
filter-feeding whale shark, which can grow to megalodon size or larger, but is a
threat only to plankton and small fish. So with nothing more ferocious than the
odd great white to contend with, perhaps it is safe to go back into the water
after all.
