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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, these enormous predators made pre-historic oceans a truly 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.

What was the Jurassic period?

The Jurassic Period is a prehistoric  period of time that ranges from between 201.3 million years ago and 145 million years ago.

The Jurassic period came after the Triassic Period and before the Crustaceous period, with dinosaurs being found in all three of those periods, but the Jurassic in particular was dominated by lizards!

 During this time, the earth’s climate changed from being dry and humid to subtropical, although for a huge variety of wildlife.

The continents continued to split off from each other, going from one land mass of Pangea to closer resemble the continents we know today. As well as this, there were tectonic shifts, leading to underwater mountains, displacing water onto land to give us the oceans that we see in the modern era.

Dinosaur Prehistoric Era

During this period, several aquatic dinosaurs called the earth home, including:


Around 252 million years ago, there was a mass extinction event. This event 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.

One species that did survive, originally a land creature that adapted to live in the sea became the Ichthyosaur. Ichthyosaur 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 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. Like whales and porpoises today, they gave birth to their young at sea. Ichthyosaurs weren’t much use on land and would be helpless if stranded ashore.


The Ichthyosaur’s Nemesis. Plesiosaurs were large, carnivorous reptiles. Thought to prey on Ichthyosaurs. 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 skull at the wrong end! In fact, their necks were so big, that they usually accounted for over half of its 40-foot length.

Because of its body, it swam by flapping its fins in the water. This gave it the appearance of flying when submerged, much like sea lions do today. 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 in at the same as 10 elephants (which is about 70 tonnes!). The megalodon makes the great white shark look like a clownfish in comparison. Its name 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 and dolphins. It would even snack on other sharks when it felt especially peckish.

Evidence of these monstrous creatures feeding frenzies exists in the number of 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; meaning, 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 seems 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! At 50 to 80 feet long, they were fearsome, sharp-toothed beasts. Existing 35-40 million years ago.

Preying on animals considered too big by other predators of the age. Which goes a long way to show just how big this beast really was. That said, despite the Basilosaurus’ reputation as one of the ocean’s biggest predators. Studies revealed its movement was quite restricted. 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. They listen to the echoes of returning calls from objects near them.

Livyatan melvillei

Named after the biblical sea monster, you know it’s going to be big! Able to compete with the megalodon, it has a reputation as one of the ocean’s biggest predators. Living in the same oceans, consuming the same prey as the Megalodon. Livyatan was a hyper-carnivorous whale that ate other whales. It had the largest teeth of any animal, aside from elephants’ tusks.

Though no bigger than today’s sperm whales, which also rely on suction to carry prey into their open mouths. 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, 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 consumed prey, the jaw closed. Then, 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 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. However, we do know that it had an incredibly strong head to match its powerful bite.

What happened to the dinosaurs?

The dinosaurs, despite surviving for a heck of a long-time were absolutely no match for the asteroid that hit earth, marking the end of the Jurassic period and the start of the Crustaceous period. It wasn’t just the initial impact that affected the dinosaurs, but mainly the wider effect that the asteroid had on the ecosystem.

Ash covered huge parts of the earth for up to two years, meaning that plant life could not thrive, killing off many animals that relied on them in a chain reaction.

What happened after the dinosaurs?

Just because the dinosaurs dominated the landscape, it doesn’t mean they were the only animals to exist during this time. Many animals coexisted with the dinosaurs, some of them are still thriving today.

Once the dinosaurs were gone, it allowed for other types of animals to expand across the spaces that had been previously filled by dinosaurs in the ecosystem, such as birds, mammals, lizards and amphibians.

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.

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