Megalodon

[Life]

Carcharodon megalodon from the Upper Miocene of Denmark, with comments on elasmobranch tooth enameloid: coronoïn

ABSTRACT

C. megalodon, not previously known from deposits in Denmark, is recorded from a large, but imperfect tooth derived from the marine clay exposed at the type locality for the Upper Miocene Gram Formation. Detached, large vertebral centra from this locality, probably belonging to the same species, are considered as well as the nomenclature and phylogenetic relationships of the genus and species. A revised list of the Gram clay elasmobranch assemblage is included.

Elasmobranch tooth enameloid, now distinguished under the term coronoi'n and perfectly preserved in the C. megalodon tooth, is a mesodermally derived, hypermineralized hard substance. Its ultrastructural, histological and ontogenetic characteristics in elasmobranchs in general are touched upon and considered in comparison with similar features of other enameloids including acrodin of actinopterygians and varieties of pleromin occurring in dipnoan and chimaeroid dentitions. Coronoi'n and acrodin (both differing from pleromins in all essential features of ontogenetic formation and growth), are characteristic of elasmobranchs and actinopterygians, respectively, and these two groups separated phylogenetically far back in the Palaeozoic. It is not unexpected, therefore, that coronoi'n and acrodin are found to be structurally distinguishable from each other.

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Paleoworld: Prehistoric Sharks (1997)

Full length video:



Courtesy: Discovery Channel

Program link: www.youtube.com/watch?v=MwotvGwLq3A

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Prehistoric Sharks (1998)

Video gallery of prehistoric sharks

Courtesy: Discovery Channel

Program link: www.imdb.com/title/tt0326005/

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Hell's teeth

James Riordon

IN THE storage halls of the Smithsonian's 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't 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's 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's 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.

Like all sharks and their relatives, the skates and rays, megalodon's 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' 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— conveyor-belt fashion—with new ones that form in rows inside the animals' 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. "The great white shark is the closest living analogue we have to megalodon," 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's National Museum of Natural History. "We assumed megalodon had similar proportions to great whites," 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. "Basically what you do," says Kent, "is model the tooth as if it's a beam attached at one end—to the jaw—and apply standard engineering equations to it to describe its behaviour under different types of stress." For Kent, the key to megalodon's 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's total bulk, and it might seem that slight variations in tooth structure would have an equally slight impact on a shark's 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's 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—just 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's teeth have a low mechanical advantage and will likely be ripped out by struggling prey. Such a combination of parameters, says Kent, "is ideal for a shark that hunts with a slash-and-release type of attack that concentrates on the softer portions of the prey". 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's 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's dental imprint have been identified. This rare specimen provided the first opportunity to quantitatively analyse megalodon feeding behaviour.

"Megalodon was the only shark large enough to attack a whale of this size," says Kent's student Monica Newell as she sorts through drawers dedicated to the bits that make up the whale's skeleton. "Here's one on a metacarpal-4, a right flipper bone." 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's skeleton. Nearly two-thirds of those are on the whale's shoulders, front flippers and upper spine. Newell also looked at whale fossils in the Smithsonian collection that don't come from complete skeletons—many 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's surveys may explain why the ancient shark needed teeth that were more robust than the great white's. 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.

"I think [Kent's] kind of analysis is very good," says Thomas Frazzetta of the University of Illinois at Champagne-Urbana. "There are amazingly few analytic works on how shark teeth work." 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. "Of course, one often might find that teeth are overdesigned for how they're used on a daily basis," he cautions. He points out that teeth may also be required for some rare tasks which are crucial for survival. Nevertheless, Frazzetta applauds Kent's engineering and forensic approach to palaeontology.

Others who study the behaviour and biology of great white sharks are less enthusiastic about Kent's findings. Shark expert A. Peter Klimley of the University of California's marine laboratory in Bodega Bay, is wary of drawing conclusions based on such meagre evidence. "To know whether or not megalodon was an ambusher you need to know its pigmentation," says Klimley. He does accept that the Smithsonian whale had bite marks in places a great white would not normally bite its prey. "But it could be that outlier, that really unusual animal you're looking at," he says.

In contrast to Klimley's conservatism, Mark Teaford of Johns Hopkins University in Baltimore is almost cavalier when discussing Kent's work. "People oftentimes say, `This fossil isn't like anything I can find today.' Well, those are the interesting ones," 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. "In this sort of case, where you're dealing with a rather unique creature, then sure, you make do with what you've got," he says. "Some people would probably be more cautious, and hem and haw about it a bit . . . but by the same token, part of the business we're in is throwing ideas out there."

We can only imagine the antics of this ancient superpredator. Humans were just beginning to conquer the land when megalodon's 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.

Source: issue 2190 of New Scientist magazine, 12 June 1999, page 32

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Giant-toothed White Sharks and Wide-toothed Mako (Lamnidae) from the Venezuela Neogene: Their Role in the Caribbean, Shallow-water Fish Assemblage

ABSTRACT

The role of Neogene giant-toothed lamnid sharks in the Caribbean neritic fish assemblage is discussed based on predator-prey relationships in three different tropical paleoenvironments from the Lower Miocene to Lower Pliocene. New records of extinct giant-toothed white sharks Carcharodon megalodon (Agassiz), Carcharodon subarticulatus (Agassiz), and wide-toothed mako Isurus xiphodon (Agassiz) from coastal shallow waters, coastal upwelling, swampy, coastal lagoon, and sandy littorals of the Venezuela Neogene, suggest these species were large transient piscivores that may have had enough behavioral flexibility to occupy different environments. Giant tooth shark species, that probably fed mainly on fish, turtles, cetaceans and sirenids, did not exhibit large population sizes. The trophic role that large consumer sharks have carried out represents the higher trophic category, the type of selective pressure, and the control performed in the marine environment. The dynamics of these associations during Neogene is deeply different.

Source: www.springerlink.com/content/637327q841643568/

For full details: Click here. (PDF download warning)

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Jurassic Shark

Two million years ago, it wasn't safe to go in the water, thanks to a predator the size of a Greyhound bus. These days, you have to search the Peruvian desert to get face-to-face with the granddaddy of Jaws.

By Gail Harrington

I’d made a remarkable discovery in the Ocucaje Desert, 12 miles inland from Peru’s southern coast, but my guide was a quarter of a mile away, relieving himself in a gully. I jumped up and down, screamed, and did a few cartwheels, which made my head swirl, but still no Roberto. Here in one of the oldest and quietest places on Earth a wondrous desert of tiered escarpments, sedimentary rock, and level stretches between Precambrian volcanic mountains no one else would hear my squeals. Waiting for Roberto to reappear, I paced back and forth across a fossil-embedded steppe, a broken section of the ocean floor that was thrust 2,600 feet above sea level 12 million years ago. When he finally returned and saw my big surprise a three-inch-long fossilized tooth from a megalodon shark a beaming Roberto said, “Pachamama [“earth mother”] sent me to the water closet so you’d find this gift.”

Three weeks earlier, I’d read an online posting about Roberto Penny Cabrera, and called him to find out more about the ocean’s largest-ever predator. “Forget Jaws. The megalodon was more than twice as long as a great white shark, much larger than a Greyhound bus,” he told me. “With an enormous mouth 11 feet high and nine feet across, it could break a whale in two with one bite.” Roberto’s enthusiasm for “sharkies” was endearing. Later I learned he was actually saying “shark teeth” so fast that the words ran together, but by then I’d grown fond of the term and chose to hear it that way.

A self-taught geologist and finder of prehistoric shark teeth, Roberto is the eyes and ears of world-renowned paleontologists who share his obsession for the Carcharocles megalodon shark, which terrorized the sea between two million and 16 million years ago. Consumed with the geology of the Ocucaje Desert, this 50-year-old with an aristocratic Spanish heritage prefers a life of few possessions some desertworthy garb and gear, books, a chess set, and a signed declaration from Peru’s National Institute of Culture that names him an official protector of the Ocucaje. His home in Ica, 200 miles south of Lima, is decorated with satellite maps of the desert. Most of his megalodon collection is stored elsewhere perhaps someday he’ll open his own museum. Even so, Roberto considers himself a finder, not a collector.

I FIRST MEET ROBERTO a few miles from Ica at my hotel in Huacachina, a tiny palm-fringed lake surrounded by monstrous sand dunes. “So, how’d you hear about me?” he asks. “And why would a woman from New York City want to go 150 kilometers off-road to look for sharkies in one of the driest deserts in the world?” I blurt out: “When I was a little girl, my grandfather used to take me to a canyon in California, where we found fossils of fish and seashells. I was completely in awe of the fact that something millions of years old could still exist. My grandpa was the most fascinating man I’d ever met.” Upon hearing this story, Roberto melts. And suddenly I realize that my entrée to the depths of the Ocucaje hasn’t been a sure thing. “You’ve passed the test,” he announces. “I will pick you up tomorrow morning at eight.”

Only then does Roberto pull out a map and an album. “Gail, what you’re about to see will shock you. If I showed these photos to everyone in Huacachina, there’d be a line of people fighting to go with me.” I am mesmerized. We’re talking fossilized whale skeletons, giant shark teeth larger than a man’s hand, pre-Hispanic pottery partially exposed in the sand, and human bones and skulls exposed by grave robbers, or huaqueros, who plundered the 3,000-year-old tombs of the Paracas people.

Clad in hiking boots, a khaki shirt, an Australian oilskin hat, lightweight pants with zip-off legs, and with a 10-inch knife hanging from his belt, Roberto arrives the next morning driving the heavily modified 1981 Datsun truck he fondly calls Hermelinda. Provisioned with food and bottled water, camping gear, firewood, three spare tires, and a makeshift shower with a 27-gallon tank, we are good to go. We take off, first through the little town of Ocucaje, where locals we pass acknowledge Roberto’s desert foray with a salute. We won’t see another human being for two days.

Within an hour, we’re off-roading in a landscape formed over millions of years by colliding tectonic plates. Our first stop: a large fossilized whale skeleton that includes eye sockets, a skull, and a partial spine and vertebrae. So what happened to the rest? “Erosion wouldn’t destroy half a whale,” Roberto says. “No, the lower part had to be taken by something strong, and that’s the megalodon. See the sharp break in the spine? Sometimes you’ll even find teeth marks on the bones.” All around us are earthy-looking heaps, more fossilized skeletons, but Roberto is barely interested in the whales all he can think about are megalodons.

As Hermelinda bounces us deeper into the Ocucaje and temperatures soar to 96 degrees, caffeine-addicted Roberto guzzles quarts of warm Coca-Cola diesel for his body, he declares and I chug bottled water. When the heat gets to him, Roberto stops to take a shower fully clothed and climbs back into the truck, dripping wet. Every couple of hours, he checks in with one of two SOS contacts to make sure someone knows our whereabouts at all times.

Periodically, he makes a close-up inspection of an area before promising that shark teeth can be found there. What is he looking for? “First, you must find a spot with brown,” he says, fingering some powdery rust-colored soil, “organic remains, mostly plankton. And when you have plankton, you’ll find a chain of consequences of life fossils, shells, small pieces of bone, and also shark teeth. But you won’t see the teeth unless a strong wind hits the ground at just the right angle to bring them out into the open.” Taking them out of the desert is entirely legal, I learn, because within 10 to 12 weeks, wind and sun will break them into little splinters.

Roberto spots a dozen teeth before I find my first, an inch-long mako shark tooth. I quickly realize this sport can be addictive we don’t even stop for lunch. By late afternoon, I’ve found only a few small teeth, but Roberto reassures me. “Don’t worry, Gail. We’re just starting today. Tomorrow you’ll find the big one. Now it’s time for some dinner.”

We set up camp in a sheltered spot below a six-foot-high ledge of diatomite, a sedimentary rock that’s rich in ocean plankton remains. Within minutes, Roberto unloads lounge chairs, sleeping bags, a small table, firewood, and the food fresh-baked rolls as soft as cotton balls, plus cheese and assorted canned goods. Famished, I grab a can of frijoles, which Roberto opens with his knife, and I eat them straight from the can without even bothering to heat them up. As I wolf down my meal, I realize how extraordinary it is to be dining beneath a 23-million-year-old piece of the ocean floor, and I think that perhaps the remarkable surroundings help make the beans so sweet and tasty, despite their being cold. Later, Roberto builds a campfire and pours boxed red wine into our mugs he spikes his with Coca-Cola (more diesel) and then we toast: “Salud! Mañana, el megalodon.”
For a while, we stop talking about sharks, or anything else for that matter, indulging only in the delicious absence of sound. And when our campfire burns down to a few glowing embers, a moonless dark sky graces us with a larger-than-IMAX screening of a meteor shower.

Search and Rescue

“Wake up, Gail, you can’t miss the sunrise,” Roberto mumbles. I appreciate the 5:15 a.m. heads-up, but I’m not ready to emerge from my bag. Roberto goes back to sleep and starts snoring, but I am so wide awake that I give up. When the sun comes up, I wander far enough to feel entirely alone in the mysteriously beautiful desert that lacks living creatures, rainfall, or anything that has a scent. An hour later, Roberto tracks me down by following the aroma of my SPF 45 sunscreen, he claims to bring me a mug of campfire coffee.

“If you want a big find, Gail, get ready for a hard ride,” he warns, which doesn’t worry me until too late. As we bounce along in Hermelinda and up impossible inclines, I swear he’s this close to driving us off of a cliff. Then he spots something, a long shape that stands out in the layers of rocky sediment, and he screeches Hermelinda to a stop. “See the tooth sockets? That’s the jawbone of a dolphin, so we know that we’re on a broken piece of the ocean floor.” Roberto starts searching for other signs too patches of brown organic matter, fossils, small bone fragments, and the wind. It’s as if he can almost smell a big find, following these clues right to the exposed tip of a magnificent four-inch megalodon.

“Imagine how many sharks lived over millions of years. Then realize that each one had about 140 teeth that could be replaced hundreds of times,” Roberto says. “For every megalodon that lived, there could be thousands of teeth the only remnants, since sharks don’t have bones. Your chances of finding one are pretty good. So search this area thoroughly, and I’ll be back.”

About 50 feet from the place where Roberto made his find, I spot a pointed shape that stands out in the sediment. I am thrilled but cautious until I carefully brush off the powdery magnesium-rich sediment that had protected the tooth for millions of years, and I can fully see its size and serrated edge. At that moment, I know I’ve found a shiny, well-preserved megalodon. And where is my shark master in this moment of discovery? Indisposed.

When he finally returns, Roberto is even more excited than I am as he works cautiously with brush and tools to extricate the tooth, which comes out nearly intact. “Don’t worry about that small damage on the root. My restorer, Nestor, will make it perfect,” he promises. And he does. Back in Ica, I relinquish my 28 shark teeth to Nestor Diaz Cegarra, who cleans, polishes, and repairs them before I leave for Lima the next day.

The handover takes place at Ica’s best restaurant, El Otro Peñoncito, where Nestor spreads out the polished sharks’ teeth on the dark green tablecloth for my approval. Everyone is amazed diners, waiters, and sidewalk passersby who hear the fuss. Later, Roberto and Nestor see me off at the bus station and wave until I am out of sight. That’s when I unroll the cotton-wrapped teeth. Suddenly, I understand why Roberto calls himself a finder, not a collector. It’s not about hoarding and storing, it’s about rescuing a piece of the past … before it’s too late.

Original link: gailharrington.net/jurassicshark.aspx (not working)

Revisited link: www.besttravelwriting.com/btw-blog/great-stories/adventure%e2%80%94bronze-category-winnerjurassic-shark/

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Perfect shark (2006)

Video segment featuring Megalodon:



Courtesy: BBC

Program link: www.imdb.com/title/tt0814256/

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Megalodon growth sequence


Large image:



NOTE: Click on the image to enlarge it.

Courtesy: Florida Museum of Natural History

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Source: www.karencarr.com/tmpl1.php?CID=196

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Prehistoric Predators: Monster Shark (2008)

Full length video:



Courtesy: National Geographic

Main link: shop.nationalgeographic.com/ngs/product/dvds/animals-and-nature/animals-and-wildlife/prehistoric-predators%3A-monster-shark-dvd-exclusive

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Three-dimensional computer analysis of white shark jaw mechanics: how hard can a great white bite?

ABSTRACT

The notorious jaws of the white shark Carcharodon carcharias are widely feared, yet poorly understood. Neither its bite force, nor how such force might be delivered using relatively elastic cartilaginous jaws, have been quantified or described. We have digitally reconstructed the jaws of a white shark to estimate maximum bite force and examine relationships among their three-dimensional geometry, material properties and function. We predict that bite force in large white sharks may exceed c. 1.8 tonnes, the highest known for any living species, and suggest that forces may have been an order of magnitude greater still in the gigantic fossil species Carcharodon megalodon. However, jaw adductor-generated force in Carcharodon appears unremarkable when the predator's body mass is considered. Although the shark's cartilaginous jaws undergo considerably greater deformation than would jaws constructed of bone, effective bite force is not greatly diminished.

Source: www3.interscience.wiley.com/journal/121376881/abstract?CRETRY=1&SRETRY=0

For full details: Click here. (PDF download warning)

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Ancient Nursery Area for the Extinct Giant Shark Megalodon from the Miocene of Panama

Abstract

Background

As we know from modern species, nursery areas are essential shark habitats for vulnerable young. Nurseries are typically highly productive, shallow-water habitats that are characterized by the presence of juveniles and neonates. It has been suggested that in these areas, sharks can find ample food resources and protection from predators. Based on the fossil record, we know that the extinct Carcharocles megalodon was the biggest shark that ever lived. Previous proposed paleo-nursery areas for this species were based on the anecdotal presence of juvenile fossil teeth accompanied by fossil marine mammals. We now present the first definitive evidence of ancient nurseries for C. megalodon from the late Miocene of Panama, about 10 million years ago.

Methodology/Principal Findings

We collected and measured fossil shark teeth of C. megalodon, within the highly productive, shallow marine Gatun Formation from the Miocene of Panama. Surprisingly, and in contrast to other fossil accumulations, the majority of the teeth from Gatun are very small. Here we compare the tooth sizes from the Gatun with specimens from different, but analogous localities. In addition we calculate the total length of the individuals found in Gatun. These comparisons and estimates suggest that the small size of Gatun's C. megalodon is neither related to a small population of this species nor the tooth position within the jaw. Thus, the individuals from Gatun were mostly juveniles and neonates, with estimated body lengths between 2 and 10.5 meters.

Conclusions/Significance

We propose that the Miocene Gatun Formation represents the first documented paleo-nursery area for C. megalodon from the Neotropics, and one of the few recorded in the fossil record for an extinct selachian. We therefore show that sharks have used nursery areas at least for 10 millions of years as an adaptive strategy during their life histories.

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Introduction

Sharks, especially large species, are highly mobile organisms with a complex life history and wide distribution. During their lifetime they generally utilize three types of areas: adult feeding, reproduction and nurseries [1]. In modern species, nursery areas are historically defined by the presence of gravid females and free-swimming neonates. It is also an area that can be shared by several shark species, where young sharks spend their first weeks, months or years [2]. More recent studies have defined nursery areas as geographically discrete essential zones for shark survival [3] that provides them with two types of benefits: protection from predation (mainly larger sharks [2]) and abundant food resources. Productive, shallow-water ecosystems thus provide sharks significant protection from larger predators and/or abundant food resources, both of which are essential to survival [4].

The Gatun is a highly fossiliferous Neogene formation located in the Isthmus of Panama (Figure 1) with a diverse fauna of sharks [5]–[7]. It was located within a marine strait that connected the Pacific Ocean and the Caribbean Sea during the late Miocene (~10 Ma) [8]. Studies of different taxa, including the exceedingly diverse molluscan fauna, indicate that it was a shallow-water ecosystem (~25 m depth) with higher salinity, mean annual temperature variations, seasonality and productivity relative to modern systems in this region [7], [9]–[13]. Over the past 20 years, the Gatun Formation localities have been extensively used to extract sediment for construction. During the more recent years, these extraction activities have increased substantially. Based on our observations made during the two past years of fieldwork, we predict that these outcrops will soon likely be excavated completely. Therefore it is timely and urgent to study the fossils occurring in these outcrops before they are no longer available to science.

View Image (Figure 1)

Figure 1. Study area.

A. Location of Panama and the Gatun Formation. The shaded box represents the general study area in northern Panama. B. Expanded geological map (from “See Below” shaded box in Fig. A). This map shows the exposures of the Gatun Formation and surrounding rock units (modified from Coates et al., 1992). The two fossil localities collected from the Gatun Formation during this study include: (1) Las Lomas and (2) Isla Payardi.

Fossil sharks were first reported from Panama in 1862 [5]. In 1984, the first description of the elasmobranchs from the Gatun Formation was published [6]. More recently, in 2010 the biodiversity of the fossil sharks from the Gatun has been documented from large new collections, and comprise 16 recognizable taxa. This work also included paleoecological and paleodepth analyses that supported the interpretation of the paleoecology of the Gatun Formation as shallow-water habitat in a productive environment [7].

Although it is not very common, the extinct Carcharocles megalodon (Agassiz 1843) is one of the species that occurs in the Gatun Formation. The taxonomic assignment of this species has been debated for nearly a century, and there are three possible interpretations: (1) Some authors place C. megalodon and other megatoothed sharks with the extant white shark (Carcharodon carcharias) in the same genus (Carcharodon) and therefore the same family (Lamnidae) [14]–[16]. (2) Other authors place C. megalodon and megatoothed sharks in a different genus (Carcharocles) and family (Otodontidae) [17]–[23]. Although a minority point of view, some workers recognize (3) megatoothed sharks as a series of chronospecies of the genus Otodus, and place all megatoothed sharks except C. megalodon in this genus. Furthermore, C. megalodon is assigned to the genus Megaselachus, based on the loss of lateral cusplets [24]. We follow the second hypothesis; that Carcharocles megalodon and Carcharodon carcharias belong to separate genera in different families. However, both species belong to the order Lamniformes, and in the absence of living members of the Otodontidae, C. carcharias should be regarded as ecologically analogous species to C. megalodon. We base this analogy on the fact that both species share similar ecological niches with presumed similarities in body shape, feeding habits, and overall tooth and vertebral centrum morphology. Even though these species are not direct relatives, no other extant lamniform species share as many characteristics with C. megalodon as does C. carcharias.

C. megalodon is widely regarded as the largest shark to have ever lived. Based on tooth crown height (CH), this giant reached a total length (TL) of more than 16 m. One single tooth can exceed more than 168 mm of total height [14]. The diagnostic characters of C. megalodon teeth include: large size, triangular shape, fine serrations on the cutting edges, a convex lingual face, a slightly convex to flat labial face, and a large v-shaped neck [7]. Juvenile specimens of C. megalodon can have lateral cusplets [15], or not [22]. The size and shape of the teeth vary within the jaw: anterior teeth are large and symmetrical whereas the latero-posterior teeth are asymmetrical with slanted crowns. Moving antero-posteriorly through the jaw, there is a slight initial increase in size on either side of the mid-line, followed by a progressive decrease that continues to the last tooth, e.g. [25] (Figure S1). Fossil teeth of C. megalodon range in age from 17 to 2 Ma (middle Miocene to Pleistocene) and have a cosmopolitan distribution [7], [14], [16].

Of relevance of this study, two shark paleo-nursery areas have previously been proposed: the Paleocene Williamsburg Formation of South Carolina, based on the presence of juvenile teeth of four lamnoid taxa [26]; and the late Oligocene Chandler Bridge Formation of South Carolina, based on the abundance of juvenile Carcharocles teeth, accompanied by small odontocete and mysticete skulls, which are assumed to be their prey species [16]. However, neither of the collections from these two localities have been rigorously analyzed and thus the presence of paleo-nurseries remained anecdotal until the present report.

The presence of mammals as potential prey does not represent evidence of a shark nursery area. As known from modern studies of sharks, the main purpose of the nursery areas is not feeding [1]–[4]. Studies have shown that some shark species do not consume large quantities of food during their juvenile stages [27]–[28]. Even when high-productivity nursery areas provide ample food resources for juvenile sharks, some species select these habitats more for predator avoidance and not food consumption [3]–[4]. Furthermore, some shark species present an ontogenetic shift in feeding patterns [29]–[32]. For example, the lamnoid white shark (C. carcharias) feeds mostly on fishes (including other sharks) during their juvenile stage and on mammals during their adult stage [33]–[35]. Marine mammals are not commonly found in the Gatun Formation. On the other hand, bony fish otoliths [36] and other shark species [7] are abundant, representing a food source for the marine fauna that lived in this diverse environment.

In this study C. megalodon teeth were collected and measured from two localities within the Gatun Formation of Panama (Figure 1). Surprisingly, large teeth are uncommon with specimens recovered having CH ranging between 16 to 72 mm (Figure 2). The objective of this work is to determine if the late Miocene Gatun Formation was used as a nursery area by young C. megalodon. Accordingly, we compared the tooth sizes from the Gatun Formation with those found in older and younger formations to determine if the smaller size distribution observed is unique to the species during the late Miocene. In addition, we compared these sizes with tooth sets from individuals of different life stages to determine if the small size observed is related to age, or position within the jaw. Finally, we calculated the TL of all C. megalodon individuals from the Gatun Formation to estimate their life stage. The results obtained in this study from tooth measurement comparisons and individual total length estimates allowed us to determine the age class/size of individuals that inhabited the shallow-water habitats of the late Miocene Gatun Formation, ~10 million years ago.

View Image (Figure 2)

Figure 2. Carcharocles megalodon collection from the Gatun Formation.

Specimens and their respective collection numbers. One specimen (CTPA 6671) was not available to photograph.

Results and Discussion

Temporal comparisons of similar faunas

In many clades represented in the fossil record, animals oftentimes show a general tendency to become larger through time, i.e., also called “Cope's Rule” [37]–[38]. For example, within lamnoid sharks there is a chronoclinal trend towards increasing size of species within the genus Carcharocles from Carcharocles auriculatus to Carcharocles angustidens to its larger species, Carcharocles megalodon [16]. However, there is no evidence of such a microevolutionary trend within the single species C. megalodon through time, as we will show below.

In order to know if the small size observed in the fossil C. megalodon from the Gatun Formation is a special feature during the late Miocene in a potentially chronoclinally evolving species, we performed tooth size comparisons through time within other marine faunas that have sufficiently large numbers of specimens of C. megalodon. Given the fact that the C. megalodon from the Calvert Formation of Maryland are older (~14 Ma) and the C. megalodon from the Bone Valley Formation of Florida are younger (~5 Ma), comparing these populations with C. megalodon from the Gatun Formation can determine if there is a long-term, chronoclinal trend for size increase, or if C. megalodon from the Gatun Formation are unusually small. Figure 3 shows that both large and small tooth sizes are found in the faunas older and younger than the Gatun Formation, and thus there is no observed microevolutionary trend for increased size in C. megalodon over time. We therefore assert that the small size observed in the Gatun Formation is not related to microevolutionary shifts in body size. Consequently, we demonstrate stasis in body size within the species C. megalodon, which provides us important context in which to compare ancient populations from the localities described above.

View Image (Figure 3)

Figure 3. Temporal comparisons of similar faunas.

Comparisons of Carcharocles megalodon tooth measurements (CH: crown height, CW: crown width), in millimeters from the Gatun Formation (late Miocene), with isolated teeth from a younger (Bone Valley, early Pliocene) and an older formation (Calvert, middle Miocene), which represent three localities from which this species is relatively abundant.

Life stage comparisons

It is known that within an individual, C. megalodon teeth vary in size within the jaw, e.g. [15]–[16], [25] (Figure S1). It could therefore be argued that the small size observed in the Gatun Formation is related to tooth position, rather than juvenile life stage of the individuals. In order to test this, we compared tooth sizes of the Gatun Formation specimens with associated tooth sets from individuals of different life stages (juvenile and adult) from other localities. Our results indicate that most teeth from the Gatun Formation are close to the observed range of a juvenile dentition (Figure 4), regardless of tooth position within the jaw.

View Image (Figure 4)

Figure 4. Life stage comparisons.

Comparisons of Carcharocles megalodon tooth measurements (CH: crown height, CW: crown width) from the Gatun Formation with tooth sets of: a juvenile from the Bone Valley Formation and an adult from the Yorktown Formation. Note the size difference in relation with the tooth positions: larger teeth are the most anterior (e.g. A1, A2, L1, L2) whereas smaller teeth are the most lateral (e.g. L8, L9, l7, l8, l9). For more details on tooth positions, see figure S1.

Comparing the Gatun's isolated teeth with tooth sets of individuals from different life stages helps to determine if the tooth size observed is related with tooth position. Nevertheless, in order to determine the life stage of those animals were neonates, juveniles or adults; it is necessary to establish total length estimates as well, as presented below.

Life stage and total length estimates

The tooth size comparisons made in this research suggest that the small size of C. megalodon teeth from the Gatun Formation is not related to temporal differences within a chronoclinally evolving species or to the tooth position within the jaw (as described above); but rather they belong to juvenile sharks. When only the teeth of a shark species are preserved, life (ontogenetic) stages of individuals can be predicted in two different ways: (1) studying morphological features of the teeth during juvenile stages; and (2) extrapolating total length using the relationship between body size and tooth crown height.

(1) In C. megalodon, teeth of juveniles sometimes demonstrate lateral cusplets [15] or not [22]. For example, UF 237914 (a lateral tooth) exhibits lateral cusplets and is assumed to be from a juvenile. On the other hand, UF 237959 (a lower anterior tooth) and UF 237949 (an upper anterior) are both very small teeth that exhibit no lateral cusplets (Figure 2). The latter teeth are small thick, heart-shaped, and are considered to represent embryonic C. megalodon teeth (Hubbell teeth). These latter teeth retain the morphology of the species even at small sizes and do no demonstrate lateral cusplets [22].

(2) Gottfried et al. (1996) [14] made inferences about the skeletal anatomy of C. megalodon based on comparisons with ontogenetic trends in the white shark, Carcharodon carcharias. They deduced that a C. megalodon fetus could reach ~4 m, juveniles ~10.5 m, and adults more than 10.5 m (~17 m). Based on crown heights (CH) and following the work of Shimada (2003) [39], we estimate the total lengths (TL) of C. megalodon specimens from the Gatun Formation (Table 1). Based on Gottfried et al.'s inferences, the total length estimates made in this research suggest that the C. megalodon specimens from the Gatun Formation represent mostly juveniles (21 individuals), with total lengths less than 10.5 m, while a few specimens (7 individuals) are interpreted as adults, with an estimated total lengths beyond 10.5 m (Figure 5).

View Image (Figure 5)

Figure 5. Total length histogram.

Frequency of Carcharocles megalodon individuals at different life stages based on Gottfried et al. [14]. Neonates of C. megalodon reach until 4 m; juveniles until 10.5 m, and adults more than 10.5 m.

View Table 1

Table 1. Carcharocles megalodon isolated teeth measurements from the Gatun Formation, Panama.

There is some expectation to find adult individuals inside a paleo-nursery area, along with the juvenile sharks for two reasons: (1) sharks constantly produce and shed teeth [40], if gravid females lay their eggs or give birth in nursery areas, one would expect to find some larger teeth; and (2) while nursery areas do offer some protection from larger predators, they do not necessarily keep all large individuals out [3], in fact, nursery areas are productive environments where competition for food can be very high [4].

Concluding Remarks: Nursery area hypothesis

As described earlier, in addition to nurseries, extant sharks utilize adult feeding and reproduction areas [1]. Based on what is known about aggregations of the analogous species, the white shark (C. carcharias), we have also considered the hypothesis that the late Miocene Gatun Formation was used as an aggregation site by C. megalodon for feeding and/or reproduction rather than as a nursery area. Following their oceanic migrations, C. carcharias individuals aggregate in the eastern Pacific (also called SOFA or Shark CafÈ) [41]–[42]. They seasonally return to this pelagic area that is suggested to be used for feeding/foraging and mating [41]–[43]. Furthermore, C. carcharias also aggregates in various coastal “hot spots” where they feed around pinniped colonies [41]–[47]. Nevertheless, based on the presence of neonates of C. megalodon, the generally high proportion of juvenile individuals, the estimated shallow depth of the Gatun Formation and the scarcity of large mammals, we reject the hypothesis of the Gatun as a primarily area for reproduction or feeding.

In this study we show that the abundance of small tooth size observed in C. megalodon specimens from the Gatun Formation is not related to its temporal position within a chronoclinally evolving species or tooth position within the jaw. Thus, the C. megalodon from the Gatun Formation indicates the dominant juvenile life stage of individuals present from this fossil locality (with estimated body lengths between 2 and 10.5 meters). The C. megalodon and associated marine invertebrate and vertebrate faunas from the late Miocene Gatun Formation of Panama presents the typical characteristics of a shark nursery area, i.e., a shallow, productive environment that contains juveniles and neonates (the later indicating these individuals probably were born in the Gatun area). We therefore propose the Miocene Gatun Formation, as a nursery area that offered juvenile C. megalodon protection from larger predators and ample food resources (i.e. fishes).

Given that C. megalodon was the largest shark that has ever lived, it could be argued that this species may not have represented a potential prey for other sharks and therefore nursery areas would not be needed. In this study however, we report that neonate individuals of C. megalodon from the late Miocene Gatun Formation of Panama could be as small as 2 m long. Furthermore, many other shark species in the Gatun Formation apparently were sympatric with juvenile C. megalodon, including potential predators that can reach more than 6 m of total length (e.g. the great hammerhead shark (Sphyrna mokarran) and the extinct snaggletooth shark (Hemipristis serra)) [7]. Moreover, in spite of a juvenile dominance, adult individuals of C. megalodon (reaching until ~17 m of TL) are also found in the Gatun Formation, representing additional potential predators. With regard to modern species, large-bodied sharks such as the tiger shark (Galeocerdo cuvier) and the great hammerhead (S. mokarran) also use nursery areas [48]. Additionally, it has been demonstrated that the modern apex shark predator of the oceans (and the analogous species of C. megalodon in this study), the white shark, uses the Southern California Bight as a nursery ground [49].

In summary, this study represents the first definitive evidence of an ancient shark nursery area from the Neotropics. Sharks are a very successful group that has been common in our oceans for at least 400 million years [40]. This research presents evidence that sharks have used nursery areas since ancient times, i.e., for at least 10 million years, and therefore extends the record of this behavior and adaptive strategy based on fossil evidence.

Materials and Methods

Carcharocles megalodon teeth are relatively rare in the Gatun Formation. Of more than 400 teeth of fossil sharks collected from the Gatun Formation between 2007 and 2009 representing 16 taxa, a total of 28 specimens (Figure 2) of C. megalodon have been collected. Fossils do not provide a record of life as complete as when studying living organisms. For that reason and also because of the rarity of this species in the area of study, we consider our sample size adequate. In addition, it is urgent to study the fossils of a formation that will soon disappear due to the increasing excavations.

The two localities studied in the Neogene marine sediments of the Gatun Formation of Panama (Figure 1), crop out in a broad area in north-central Panama and have been proposed to be late Miocene, spanning from about 12 to 8.4 Ma [9]. The materials were collected mainly by surface prospecting by the Panama Canal Project Field Team of the Center of Tropical Paleobiology and Archaeology (CTPA) of the Smithsonian Tropical Research Institute (STRI), as well as the University of Florida (UF) scientists. Some of the specimens collected are deposited in the Florida Museum of Natural History (FLMNH) and are designated with a UF catalogue number which are available in its database: www.flmnh.ufl.edu/databases/VP/intro.htm. The remaining specimens are designated with a CTPA or AT number and are part of the STRI collection.

Crown height (CH) and width (CW) (Figure S2) of all specimens were measured (Table 1, S1, S2, S3 and S4) using digital calipers. In order to calculate dimensions of incomplete specimens, CW vs. CH data were plotted and a line regression was calculated (Figure S3). Measurements were then compared with isolated teeth from geologically younger and older collections and with different tooth sets from individuals of different life stages. The specimens' total lengths (TL) were calculated based on their CH.

Temporal comparisons of similar faunas

Isolated teeth from the younger Bone Valley Formation, Florida (early Pliocene, ~5 Ma) [50]–[51], from the Vertebrate Paleontology Collection at the FLMNH in Gainesville, Florida, were measured (Table S1) and compared with the Gatun teeth. Additionally, isolated teeth from the older Calvert Formation, Maryland (middle Miocene, ~14 Ma) [52], from the Vertebrate Paleontology Collection at the U. S. National Museum of Natural History (NMNH), in Washington, D.C, were also measured (Table S2) and then compared with the Gatun Formation teeth.

Life stage comparisons

Tooth sizes of the Gatun isolated teeth were measured and compared with two C. megalodon associated tooth sets of different life stages from the Hubbell collection at Gainesville, FL. The adult tooth set is from the Yorktown Formation, North Carolina (early Pliocene) [52] (Table S3). And the juvenile tooth set is from the Bone Valley Formation, Florida (early Pliocene) [50]–[51] (Table S4).

Total length estimates

As described above, the extant white shark (Carcharodon carcharias), has been used as a general ecological analog to the extinct Carcharocles megalodon. Likewise, previous studies have asserted that teeth of C. carcharias can be used to estimate the total length of C. megalodon [14], [38]. Based on C. carcharias tooth height and total length ratios, we have measured C. megalodon tooth CH to extrapolate its TL estimates based on the work of Shimada (2003) [39] on the white shark, where every tooth position in the jaw corresponds to one regression equation that calculates its body size (Table S5). We assigned a range of possible positions to the Gatun teeth and estimated the TL of every specimen by calculating it from the average among the different positions where every tooth could have belonged (TL, Table 1). We then inferred the life stage of every C. megalodon, by extrapolating it from the relationship between body size and life stage in C. carcharias following Gottfried et al. (1996) [14]. We based our C. megalodon estimates on extrapolations from the extant C. carcharias given their similarities in body shape, feeding habits, and tooth and vertebral morphology. In addition, both species belong to the same order (Lamniformes), and in the absence of living members of the Otodontidae, C. carcharias is the most analogous species available.

Source: www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0010552

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