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Dinosaurs of the Sea: Ocean Life in the Prehistoric Era


Back in the Jurassic Era, our oceans were teeming with all sorts of reptiles and marine life that were both amazing and terrifying. Although we tend to think of land-dwelling dinosaurs during this period; oceans, rivers, lakes and seas also had their fair share of spectacular marine-based creatures – the like of which would be right at home in Jurassic Park.

From sharks as big as buses to carnivorous whales and giant sea-lizards, these enormous predators made pre-historic oceans a truly treacherous, dangerous place to venture in. Here, we’ll take a look at some of the most amazing extinct creatures that once roamed the world’s oceans.


Around 252 million years ago, there was a mass extinction that led to the disappearance of a large number of animals, plants and other species. In fact, about 90% of ocean species and 70% of land species vanished as a result. The few species that did survive, evolved to better fit in with the new environmental conditions.

Originally a land creature that adapted to live in the sea, one of the species that did survive became the Ichthyosaur. With a name that means “fish lizard” in Greek, they were fish-like in appearance, with a tail and paddle-style appendages used to propel and steer themselves. Though generally said to be about 10 feet in length, certain fossils indicate that some ichthyosaurs could be over 40 feet!

Some lived and foraged in coastal waters at the edge of continents, while others swam in the open ocean, far from land. And like today’s whales and porpoises, they gave birth to their young at sea – Ichthyosaurs weren’t much use on land and would be helpless if stranded ashore.


The nemesis of the ichthyosaur, Plesiosaurs were large carnivorous reptiles thought to prey on the above. Their name came from the Greek for “near to lizards”, and they typically had broad bodies, flippers and short tails. However, their most distinctive feature was their huge neck – when scientists first assembled their fossils, they actually mixed up their long neck and short tails, putting their skulls at the wrong end! In fact, their necks were so big, they usually accounted for over half of its 40-foot length.

Because of its body, it swam by flapping its fins in the water, which gave it the appearance of flying when submerged, much like sea lions do today. And when it came to eating, the Plesiosaur is said to have fed by swinging its head from side to side through schools of fish, capturing prey in its long, sharp teeth.


Like something out of a monster movie, the Megalodon was a terrifying spectacle. As big as a bus, and weighing as much as 10 elephants (which is about 70 tonnes!), the megalodon makes the great white shark look like a clownfish in comparison. With a name that translates to “big tooth”, its teeth measured over seven inches long, the largest of any known shark!

You wouldn’t want to be in the water at the same time as the Megalodon. With the ability to bite whales in half with its enormous jaws and teeth, it was the top predator of its age, eating fish, dolphins and even taking on other sharks when it felt especially peckish.

Evidence of this monstrous shark’s feeding frenzy can be seen in the number of its fossilised teeth found on the ocean floor, which have appeared on every continent except Antarctica. This is because sharks continually produce new teeth throughout their entire lives; that means, depending on what they eat, the megalodon and other sharks may get up to 40,000 teeth in their lifetime.

Though these gigantic predators once ruled the oceans, it seemed the only thing that could stand up to them was nature itself. Roughly 2.6 million years ago, the planet entered a phase of global cooling. Because they survived in tropical waters, this drop in temperature was enough to wipe out the once-dominant species.


Although this scary-looking predator looked like a reptile, the Basilosaurus was actually a whale, and a big one at that! Measuring 50 to 80 feet long, they were a fearsome, sharp-toothed beast that lived 35-40 million years ago.

They tended to dine on smaller whales or large fish, prey that was considered too big by other predators of the age, which goes to show just how big this beast really was. That said, despite the Basilosaurus’ reputation as one of the ocean’s biggest predators, studies have revealed that it was actually quite restricted in the way it moved. It had relatively weak muscles and couldn’t dive or swim for extended periods, which means it probably only swam and hunted in waters near the surface.

Unlike today’s whales, they also lacked the ability to do a thing called echolocate, which is when animals send out calls to the environment and listen to the echoes of returning calls from objects near them.

Livyatan melvillei

When you’re named after the biblical sea monster and can compete with the megalodon, then you know you’ve got a reputation as one of the ocean’s biggest predators. Living in the same oceans and dining on the same prey as the huge shark we mentioned above, Livyatan was a hyper-carnivorous whale that ate other whales and had the largest teeth of any animal, aside from elephants’ tusks.

Though it was no bigger than today’s sperm whales, who also rely on suction to carry prey into its open mouth, Livyatan’s enormous teeth grabbed prey with huge chomps, aided by its massive 10-foot-long skull that helped to capture and trap food inside. And with its echolocating abilities, other marine life didn’t stand much chance, even in the murkiest depths of the ocean.


At 15 feet long, and with a lower jaw made of a spiral of serrated teeth, the Helicoprion seems like a cross between a shark and a chainsaw. Though researchers aren’t entirely certain about the structure of this unique-looking shark, it’s generally accepted that when it caught and consumed prey, the jaw closed and, unbelievably, the teeth rotated backwards in a sawing motion.


A huge fish that could grow up to 30 feet long, the Dunkleosteus has been compared to an armoured tank. Rather than having teeth, it ate its food using bony ridges and a bite force of 8,000 pounds per square inch. Evidence suggests that its jaw muscles were so powerful, it could have opened its mouth in one-fiftieth of a second, vacuuming food inside in no time at all! And because these ridges were so sharp, they had no problem biting through the bony bodies of other armoured fish.

Scientists have had to make educated guesses as to what the entire creature looked like, since only the armoured skulls of the Dunkleosteus are left as fossils, but we do know that it had an incredibly strong head to match its powerful bite.

Deep Blue Planet is home to the modern-day giants of the ocean, and you can get up-close-and-personal with these incredible creatures. Click here for more details on how to book a shark dive experience at Blue Planet Aquarium.