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Jaws of the Deep on Discovery Channel

See how sharks have evolved.

1. EARLY SHARKS HAD SMOOTH SKIN AND COULDN’T THRUST THEIR JAW FORWARD 
 
When sharks first appeared, at least 420 million years ago, they weren’t quite like the sharks we know today. Like modern sharks, they had a skeleton made of cartilage. They had triangular fins, gills and pointed teeth. But they lacked a few key shark traits.
One of the most well-known early sharks, Cladoselache, lived 370 million years ago. Unlike today’s sharks, it couldn’t thrust its upper jaw forward when attacking. Like other sharks of its time – and most animals – Cladoselache’s jaw was entirely fused to its head. This restricted the size of the food it could suck in and eat. It wasn’t until sharks evolved the ability to unhinge their upper jaw – sometime during the Jurassic period, 201 to 145 million years ago – that they started to take on larger prey, eventually growing to massive size.
Cladoselache had another odd early shark trait: smooth skin. Unlike modern sharks, which are covered in tooth-like scales called dermal denticles, Cladoselache was virtually naked. A shark’s denticles not only protect it from injury, they reduce drag in the water and strengthen the skin to provide firmer attachment for swimming muscles. But this didn’t seem to slow down Cladoselache; it was a high-speed predator with a keeled, crescent-shaped tail fin similar to today’s turbo-charged mako sharks.

 

2. A WEIRD ANCIENT SHARK LOOKED LIKE IT HAD A HAIRBRUSH STICKING OUT OF ITS BACK 
Stethacanthus – a fast-swimming shark that lived around 320 million years ago – was a very odd-looking fish. It had an enormous, flat-topped dorsal fin bristling with enlarged scales; together, they gave Stethacanthus the appearance of having a hairbrush sticking out of its back. Even the top of its head was covered in bristle-toothed scales.
 
But what were these for? Perhaps they were defensive, the brushy fin and cranial bristles looking like the jaws of a far larger predator. It seems more likely though that they played some role in courtship rituals. Females may have chosen males with the biggest dorsal brush, or perhaps males used these to push against one another in vigorous battles over mating rights.
Stethacanthus is also one of the earliest examples of a shark with dermal denticles – toothlike scales that cover the skin. These scales prevent injury, reduce drag in the water and strengthen the skin to provide firmer attachment for swimming muscles. Earlier sharks had smooth skin, but all modern sharks have denticles like Stethacanthus.

 

3. MOST OF WHAT WE KNOW ABOUT ANCIENT SHARKS COMES FROM THEIR TEETH 
 
Fossil shark’s teeth can tell us a surprising amount of information about prehistoric sharks, from where they lived to what they ate to how large they were. Shark’s teeth are the most abundant type of shark fossil, in part because sharks shed thousands throughout their lifetime, and in part because they fossilize comparatively easily.
To become a fossil, a tooth must be buried in sediment rather quickly. This protects it from weathering and abrasion, and limits its exposure to oxygen and bacteria – the agents of decay. In a process called permineralization, water seeps through sediments and over the tooth, depositing minerals in the tooth’s pores. This slow process, which can take many thousands of years, transforms the tooth into a fossil.
Why don’t we find shark skeletons in the fossil record? We do find bits and pieces, but rarely. A shark’s skeleton is made of cartilage, which doesn’t mineralize to the extent that bone does and breaks down much quicker. Shark scales and vertebrae are more commonly found – both are prone to calcification – but in nowhere near the same numbers as teeth.
Luckily, shark’s teeth can tell us lots about the past. A recent discovery, for example, indicates that sharks may be able to adapt to climate change. Thousands of shark’s teeth dating back to the Eocene (56 to 34 million years ago) were found in the Canadian Arctic. An analysis showed that these sharks (extinct sand tigers) lived in a relatively warm Arctic sea with very low salinity – practically freshwater. Meanwhile, their modern-day relatives prefer saltwater. This finding suggests that at least some sharks could potentially adapt to the rising temperatures and decreased salinity that scientists expect to see in the Arctic of the future.

 

4. MEGALODON, THE LARGEST SHARK THAT EVER LIVED, FED ON PREHISTORIC WHALES 
 
The king of all sharks, Megalodon, first appeared 15.9 million years ago. One of the largest and most powerful predators in vertebrate history, Megalodon specialized in eating whales and other large sea creatures. Over the years, many fossilized whale bones have been found with signs of bite marks from teeth that match Megalodon’s massive choppers, which could grow to over 7 inches in length.
Recent findings are shedding light on Megalodon’s whale hunting strategies. In the late 1990s, paleontologist Dr. Bretton Kent examined the remains of a 30-foot long prehistoric baleen whale that was attacked by a Megalodon, discovering that the “monster shark” was far more aggressive in its attack style than the great white.
Megalodon’s attack focused on the tough bony portions of its prey, such as the shoulders, front flippers, rib cage and upper spine, which great whites tend to avoid. Its teeth were large and robust and its jaws were massive; together, they packed a 24,400 to 41,000-pound bite force – the strongest ever recorded. While great whites prefer to hit their victims from below and then retreat while they bleed out, Megalodon clamped down and crushed its victim’s bones and the delicate organs harbored within. An ancient whale – its rib cage pulverized and its heart and lungs ruptured from the powerful bite – would die quickly from its injuries.
But this wasn’t Megalodon’s only attack strategy. Fossil evidence shows a variety of hunting techniques, from ripping apart and biting off fins in order to immobilize prey, to attacking small whales from below, perhaps similarly to the Polaris attacks displayed by the great whites of South Africa.
When it wasn’t chowing down on whales, Megalodon also fed on seals, sea lions, giant sea turtles, sea cows, dolphins, porpoises, and other large creatures before going extinct 2.6 million years ago.