Sharks first appeared on Earth at least 420 million years ago. That’s nearly 200 million years before the first dinosaurs. All these years of evolution have given the many shark species we see today extraordinary abilities, ones that have enabled some to become the ultimate marine predators. Yet the sharks that no longer roam the oceans were in many respects even more impressive—here are just a few examples of these mighty monsters.
Megalodon was a huge shark, weighing 50-100 tons and measuring at least 59 feet—three times as long as the Great White Shark which evolved from the same line. Like the Great White, it had serrated teeth that were able to bite, taste-test and tear apart large animals, but the Megalodon’s teeth were seven inches long, hence its name which means ‘big tooth’.
Megalodon and the Great White actually co-existed for about 10 million years but it is thought that they kept out of each other’s way by feeding on different prey (the Great White on seals, Megalodon on whales) and by living in different areas (the Great White in cooler water and Megalodon in warm water). Megalodon died out around 1.6 million years ago, but left a legacy of being one of the most powerful predators in vertebrate history.
Living about 100-82 million years ago in the late Cretaceous period, Cretoxyrhina was the largest shark in its time at up to 24-foot-long. Its name is Greek for ‘Cretaceous jaws’ because its jaws contained an impressive seven rows of teeth. Its teeth were up to 7cm long, curved, smooth-edged and with a thick enamel coating which means that this shark could just as easily cut through bones and shells as they could flesh.
It has been given the nickname of the Ginsu shark in reference to the Ginsu knife because it fed by using those razor-sharp teeth to slice into its prey. Cretoxyrhina was first identified by the famous Swiss naturalist Louis Agassiz in 1843 and has since become one of the best understood fossil sharks. It is known that had a keen sense of smell and a tail adapted for fast swimming, both contributing to its predatory capabilities.
This genus of shark lived from the late Devonian to early Carbiniferous epoch, and died out around 320 million years ago. They were slow-moving and small in size, around 2.3 feet long, so probably fed on small fish, crustaceans and cephalopods. Due to their small fins and teeth compared to other sharks of the same size, they are believed to be bottom-dwellers.
It is also possible that they were migratory. Their name derives from the Greek word ‘steth’ meaning chest and ‘akanthos’ meaning spine or thorn. This refers to their distinctive anvil or ironing-board shaped first dorsal fin and spine on mature males. This was covered in small spikes, which also appeared on their head, thought to either play a role in mating rituals, clamping to the belly of larger marine animals, or used to frighten potential predators.
Also in the late Devonian lived this genus of fast moving and agile sharks, thanks to their streamlined bodies and deep forked tails. Growing up to six-feet-long, they lived in the oceans of North America. A lot is known of this genus because well preserved fossils have been discovered on the ‘Cleveland Shale’ on the south shore of Lake Erie, not just the skeleton but also traces of skin, muscle fibres and internal organs such as the kidneys.
Fossil remains of stomach contents indicate that these sharks seized prey by the tail and swallowed them whole, also likely from their teeth which were multi-cusped and smooth-edged so suitable for grasping rather than tearing or chewing. What makes this genus unusual is their almost complete lack of the scales that cover the body of most sharks—Cladoselache only had scales on their mouth, around their eyes and on the edges of their fins. It is likely this adaptation aided speed and agility when escaping predators.
The name of this genus is Greek for ‘spiral saw’, an apt name because almost all of their fossil remains have been of spirally arranged clusters of teeth called ‘tooth whorls’ which resemble a circular saw. When they were first discovered they were thought to be the spiral shells of ammonites but they actually represent all the teeth produced by an individual in the lower jaw in their lifetime.
As the shark grew, the older, smaller teeth were moved into the centre of the whorl by newer, larger teeth appearing. This feature is not known from any other vertebrate animal. Attempts to understand where the spiral was located have lasted many years and it has been placed on various parts of the body, such as the dorsal fin and tail, but it is now accepted to be part of the jaw. Individuals would have grown to up to 39 feet in length and lived between 290 and 250 million years ago.
The largest member of this genus grew up to 20 feet in length, so were some of the top predators when they lived, around 300 million years ago in the late Carboniferous. Their name is derived from the Greek word ‘edeste’ which means ‘to devour’. They are also known as the scissor-tooth sharks because, like their relatives Helicoprion, they grew teeth in curved brackets and did not shed their teeth as they became worn.
Instead, Edestus continued growing new teeth and gums near the back of their mouths, pushing older gums and teeth forward until they protruded from the mouth. With only a single row of teeth in each jaw, their mouths would have resembled a massive pair of pinking shears. These sharks may have used their teeth to vertically thrash their prey and create slashing wounds. If proven, this would be a unique method of killing never before seen in the animal kingdom.
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