10 Extinct Creepy Sea Animals (With Photos)

kronosaurus vs mosasaurus
© Daniel Eskridge/Shutterstock.com

Written by Volia Schubiger

Updated: August 23, 2022

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A large portion of the ocean is largely unknown and unexplored. We know very little about more than eighty percent of the ocean due to the lack of mapping, observation, and exploration. Deep-sea exploration has much to teach us. Although we are still learning a lot about the ocean, there are a few things that we have learned that are interesting. One of these interesting discoveries is that the ocean used to be home to some prehistoric monsters that are extinct today. Let’s take a look at ten extinct creepy sea animals that you won’t believe ever existed!

1. Tylosaurus

Adult mosasaur tylosaurus swimming close to the sea bed

Tylosaurus was the largest of the marine reptiles called mosasaurs, growing up to 45 feet (14 meters) long.


Tylosaurus is the first extinct creepy sea animal on the list. It was an enormous marine reptile and a mosasaur. It was one of the largest mosasaurs to ever exist, measuring over 45 feet long. During the Cretaceous period, Tylosaurus was an apex predator and deadly hunter. The remains of some of these creatures have been found in extremely arid environments of today, suggesting that they once roamed the oceans 65 million years ago.

2. Plesiosaurs

Portrait of a plesiosaurus isolated on white background. Plesiosaurus was a swimmer dinosaur. Also known as Elasmosaurus.

Plesiosaurus fossils were discovered for the first time in 1821.


During the Jurassic period, Plesiosaurs dominated the oceans. These swimming reptiles had large bodies and small heads. You could almost compare them to the mythical Loch Ness monster. There are some who believe the Loch Ness Monster sightings are the remains of plesiosaurs surviving on the island of Loch Ness. However, this has yet to be proven. In search of fish and other food sources, they propelled themselves through the water with their limbs. It has been estimated that over 100 species of Plesiosaurs have been found since the discovery of the first plesiosaur bones.

3. Helicoprion


Helicoprions were prehistoric shark-like fish with curled-up teeth resembling circular saws.

© – License

An underwater predator, this prehistoric shark was best known for its circular, toothy saw mouth which was a natural defense tool. In fact, the Helicoprion looks like it has a circular saw wedged in its mouth. Others suggested it was connected to the tail of the creature, while others suspected it protruded from the upper or lower jaw of the animal. The species lived around North America, Eastern Europe, Asia, and Australia 290 million years ago.

4. Megalodon

Megalodon Shark Hunting some Dolphins.

Megalodon’s bite force was at least three times greater than that of T. Rex.

©Antonio Viesa/Shutterstock.com

Due to its prominent appearance in films and novels, the megalodon is perhaps the most famous extinct creepy sea animal. During the Early Miocene and Pliocene, it lived 23-3.6 million years ago. As one of the most powerful and dangerous predators to ever live in our oceans, it is a close relative of the great white shark. Despite some people’s desire to prove that this prehistoric sea creature still exists, scientists believe the species became extinct about 3.6 million years ago.

5. Frilled Shark

Frilled shark in museum

The frilled shark is named after the frilly appearance of its gills.

©saname777 from Tokyo, Japan / CC BY 2.0 – Original / License

The frilled shark is the only creepy sea animal on our list that isn’t fully extinct yet. There have been frilled sharks living in the deep ocean for around 80 million years. Although they’re related to what we consider to be more traditional sharks, they’re much smaller than the great white sharks because of the frilled shape of their six gills. Theya-z-animals.com/animals/megalodon-shark/ also look more like a snake or eel than a great white shark. An average frilled shark has 300 teeth. This shark can be found both in the Atlantic and the Pacific oceans.

6. Mosasaurus


As one of the scariest extinct creepy sea animals, the Mosasaurus was at the top of the food chain.

©Esteban De Armas/Shutterstock.com

During the prehistoric era, the Mosasaurus was a ferocious predator of the seas, related to the Tylosaurus. In terms of size, the Mosasaurus was the largest member of the Mosasaurs family, measuring more than 55 feet long. As a top predator, the mosasaurus probably ate anything it could find. It would more than likely feat on sharks, birds, reptiles, and even other mosasaurs. This is definitely one extinct creepy sea animal that we’re happy no longer preys in the waters!

7. Basilosaurus

Basilosaurus cetoides fossilized skull

The fossils of the Basilosaurus were once made into furniture in the Southeastern U.S.


Basilosaurus, which means “king lizard,” was originally thought to be a giant marine reptile, like the Mosasaurus. However, it turned out to be a prehistoric, predatory whale that roamed the oceans during the late Eocene period. The Gulf Coast is the first place fossils of the first prehistoric whale were ever discovered. In terms of length, the whale measured 49-66 feet (15-20 meters). This creature was at the top of its food chain, and probably ate sharks, big fish, and other marine mammals.

8. Kronosaurus

kronosaurus vs mosasaurus

Kronosaurus was one of the largest pliosaurs and was named after the leader of the Greek Titans, Kronos.

©Daniel Eskridge/Shutterstock.com

As part of the Plesiosaurian group, the Kronosaurus has a long neck, but a larger skull than other Plesiosaurs. Kronosaurus had teeth 10 inches long and a bite force comparable to a crocodile‘s. Their name comes from Kronus, a Greek mythological character who was alleged to have killed and eaten his own children. Although fossils of Kronosaurus have been found in South America and Australia, the dinosaur is thought to have been widespread throughout the world.

9. Leedsichthys

There was a giant fish called Leedsichthys problematicus (aka the Leeds fish) that lived during the Jurassic era. In the modern era, it is considered to be the largest ray-finned fish ever recorded. Alfred Nicholson Leeds made important finds of the remains in 1889, so the fish is named after him. There have been a lot of different debates on how big this fish was. It’s assumed to have been anywhere from thirty feet (nine meters) to 100 feet (30 meters). Scientists have finally settled on fifty feet (16 meters) as the maximum size of this creepy sea animal.

10. Ichthyosaur

Giant Ichthyosaurus 3D Rendered

The Ichthyosaurus was the first Ichthyosaur to be discovered by man.

©Giant Ichthyosaurus Aquatic Dinosaur 3D Rendered/Shutterstock.com

Several million years before the last mass extinction, these giant marine reptiles went extinct. We don’t know what killed it, but it was once a diverse and complex group. While some Ichthyosaurs looked like eels, others looked like sharks and powered through the water at high speeds to catch their prey. The ichthyosaur measured up to 85 feet long, an animal that ranks second only to the colossal blue whale in size, which measures up to 98 feet long. We know for a fact we wouldn’t want to be caught alone with this extinct creepy sea animal!

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About the Author

Volia Schubiger is a freelance copywriter and content editor with a passion and expertise in content creation, branding, and marketing. She has a background in Broadcast Journalism & Political Science from CUNY Brooklyn College. When she's not writing she loves traveling, perusing used book stores, and hanging out with her other half.

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