7 Scary Prehistoric Marine Dinosaurs from the Ocean

by Gaute H. February 01, 2023 4 min read

Megalodon and Livyatan Melvillei

I got to say, I truly enjoyed writing this article as it covers some seriously fascinating and freaky animals that used to roam the oceans. As it's millions of years ago since they went extinct, we can only imagine what it would be like if these animals still existed today.

Below you can read about our top 7 scary prehistoric marine dinosaurs. T-Rex got nothing on these guys!

1. Helicoprion

Image credit: James St. John / Wikipedia

Helicoprion is a prehistoric shark that used to roam the oceans about 290 million years ago. It could grow to about 5 meters (15 feet) long, but its most disturbing feature was definitely its spiral of serrated teeth on its lower jaw. What's scarier than what seems like a cross between a shark and a chainsaw?

Although scientists aren't entirely certain about the structure of this unique shark, it's generally accepted that when it consumed prey, the jaw closed. Then, unbelieveably, the teeth rotated backwards in a sawing motion.

2. Megalodon

MegalodonImage credit: Herschel Hoffmeyer / Shutterstock

No list of prehistoric marine animals are complete without mentioning the legendary Megalodon. Like something out of a monster movie, the Megalodon was a terrifying creature that lived 23 to 3.6 million years ago.

The prehistoric shark was the size of a bus, and weighing the same as 10 elephants! In fact, the Magalodon weighted up to 30 times more than the largest great white shark ever recorded. Its name translates to "big tooth", and indeed the Megalodon had the largest teeth of any known shark, reaching 18 centimeters (7 inches) long.

Illustration of the Megalodon size

Megalodon size
Image credit: BBC

The Megalodon was the top predator of its age. Its bite force was more than three times that of the T-Rex with the ability to bite whales in half. At times, it would even snack on other sharks.

Sharks continually produce new teeth throughout their lives. Thus, fossil teeth found on the ocean floors enabled scientists to confirm the existence of the Megalodon.

So how did a gigantic shark like Megalodon, which was the size of a school bus, go extinct? It seems the only thing that could stand up to this gigantic predator was nature itself. At the time, the planet entered a phase of global cooling. Because the Megalodon survived in tropical waters, the drop in temperature was enough to wipe out the once-dominant species.

3. Livyatan Melvillei

Livyatan MelvilleiImage credit: Balazs Petheo

Named after the biblical sea monster, you know it's going to be a big badass animal! Imagine a sperm whale but with massive teeth and stellar hunting techniques, preying on basically any living thing in the oceans.

Skull and teeth of the Livyatan Melvillei

Livyatan Melvillei skullImage credit: Ghedoghedo / Wikipedia

The Livyatan Melvillei was a hypercarnivorous whale that ate other whales. It had the largest teeth of any animal ever recorded (excluding elephants tusks) reaching about 0.4 meters (1.2 feet). Combined with its echolocating abilities, other marine life didn't stand much chance. They lived in the same oceans and ate the same food as the Megalodon, so this whale actually had to compete with the largest predatory shark ever.

4. Plesiosaur

Image credit: Warpaint / Shutterstock

Plesiosaurs were prehistoric ocean animals that dominated the oceans during the Jurassic period about 203 million years ago. Resembling the mythical Loch Ness monster, these swimming reptiles had large bodies and small heads. In fact, they could reach lengths of up to 17 meters (56 foot). Some individuals had incredibly long necks, almost twice as long as the rest of their body. With limbs that had evolved into flippers, they could propell themselves through the water as they hunted for fish, turtles or other reptiles.

With a distinctive long neck and long, sharp teeth, the Plesiosaur is everything we'd expect from a prehistoric marine dinosaur. Despite its terrifying appearance, scientists believe that the Plesiosaur was a dedicated parent and a social animal forming strong bonds.

5. Basilosaurus

BasilosaurusImage credit: Roman Uchytel

This prehistoric sea creature was initially thought to be a giant marine reptile and was given the misleading name Basilosaurus, which means "king lizard". We can see why the confusion given its long, snake-like body that was up to 26 meters (85 feet) long. However, it turned out that the Basilosaurus in fact was a predatory whale that roamed the oceans 40 million years ago. Imagine swimming in the ocean with a 26 meter (85 feet) long alligator-snake-whale.

Scientists believe that the Basilosaurus was a top predator with a powerful bite that would tear its prey apart before eating it in pieces. However, it wasn't the smartest of species, and lacked both the cognitive ability of modern whales and the ability to echolocate. Moreover, it was also believed to only be able to navigate in two dimensions, which means no deep diving or breaching to show off.

6. Jaekelopterus

Image credit: Lucas Lima / Earth Archives

Did anyone say gigantic sea scorpions? Oh yes. About 400 million years ago, Jaekelopterus, the largest known sea scorpion (Eurypterid) lurked around in the waters. This was a 2.6 meter (9 feet) long sea scorpion, which means it was larger than a grown human. Most of us freak from centimeter long spiders, imagine 2.6 meters of armored, clawed terror.

Sea scorpions have been extinct since before the dinosaurs as they were wiped out in the Permian Triassic extinction event that wiped out 90% of all life on the planet at the time.

7. Dunkleosteus

Image credit: MR1805 / Getty Images

Dunkleosteus was a 9 meter (30 foot) long carnivorous tank that roamed the oceans 358 million years ago. According to Philip Anderson, at the Department of Geophysical Sciences at the University of Chicago, the Dunkleosteus was "able to devour anything in its environment."

Dunkleosteus skull recreation at Vienna Natural History Museum

Dunkleosteus skullImage credit: Zachi Evenor under Creative Commons / Wikipedia

Instead of teeth, Dunkleosteus had bony ridges, like a turtle, and a bite force similar to that of the T-Rex. The plates that made up the "teeth" changed as the fish aged, transforming into segments that allowed it to trap prey easier. Moreover, it made it more effective in biting through other armored fish. Dunkleosteus was truly a prehistoric predatory super tank.

Gaute H.
Gaute H.

Founder of Oceanness. Enjoys the ocean, hanging out with friends, and exploring the world. Favorite ocean animal: Dolphins.

Also in The Oceanness Blog | Ocean Conservation & Sustainable Fashion

Stop plastic pollution sign
7 Tips for Plastic Free July

by Gaute H. June 01, 2023 2 min read

July 1st marks the start of the global movement, Plastic Free July. Here are 7 tips for how to become plastic free and reduce single-use plastic.
Clownfish in the ocean
7 Facts About Diversity, Pride & Queerness in the Ocean

by Gaute H. May 01, 2023 2 min read

To celebrate diversity and queerness, we are sharing 7 fabulous facts about the rainbow diversity found in our oceans.
Ocean landscape
7 Endangered Ocean Species and Marine Animals

by Gaute H. April 02, 2023 4 min read

30% of all species are endangered and vulnerable, including blue whales, Hawaiian monk seals, whale sharks, vaquitas, hawksbill turtles, Galápagos penguins and sea otters.