15 Extinct Creepy Sea Animals That Once Roamed the Deep

Leave a comment / / Updated on: 25th September 2023

extinct creepy sea animals

The ocean is one part of our planet that has remained relatively unexplored due to the challenging nature of deep-sea exploration. 

It is home to several creepy and weird-looking creatures demonstrating a wide range of adaptations to survive in harsh undersea conditions. 

A vast majority of these deep-sea species are still unknown to us today. 

Earth’s sea was even scarier in the prehistoric past than it is today. 

It was home to various creepy animals that were significantly bigger and weirder compared to some of the most notable ocean-dwellers today. 

In this article, we’ll list 15 of these prehistoric creeps that once roamed the deep ocean, explaining some of their unique adaptations and features that make them so intriguing. 

Gage Beasley's Prehistoric Shirt Collection
Gage Beasley’s Prehistoric Shirt Collection
Gage Beasley's Prehistoric Plush Collection
Gage Beasley’s Prehistoric Plush Collection

15. Basilosaurus 

Being the first proto-whale, Basilosaurus a lot of characteristics with its modern day counterpart
Being the first proto-whale, Basilosaurus a lot of characteristics with its modern day counterpart | Lythronax via Walking With Wikis
Name Meaning“The king lizard”
EraCenozoic — Paleogene
ClassificationMammalia, Artiodactyla, Basilosauridae
DietCarnivorous (Piscivorous)
Length15 to 20 meters (49 to 66 feet)
Weight5.8 tons (11,600 pounds)
LocationNorth America, Africa and Asia

What looks like a sea serpent but really isn’t one? 

The Basilosaurus is a possible answer to that question. 

It was named the “king lizard” because the early researchers who studied it thought it was a gigantic marine reptile

This was later found to be inaccurate.

Basilosaurus was a mammal closely related to modern whales. 

It lived during the Late Eocene, about 41 million years ago. 

As one of the largest animals in the Tethys Sea where it lived, Basilosaurus was an apex predator of various animals within its environment, including sharks, large fish, and marine reptiles. 

14. Megalodon

Megalodon 3d render
Megalodon 3d render | racksuz via Istock
Name Meaning“Giant Tooth”
EraCenozoic – Neogene
ClassificationChondrichthyes, Lamniformes, Otodontidae
Length15-18 meters (50-60 feet)
Weight50-100 tons (110,000-220,000 pounds) 

The Megalodon is one of the most well-known prehistoric aquatic animals. 

It is the largest shark ever discovered and is also considered the largest fish that has ever lived. 

The Megalodon was the apex predator of the Miocene Epoch, ruling the Earth’s oceans between 23 and 3.6 million years ago. 

With an estimated length of up to 60 feet, this giant shark was up to three times as long as a modern-day great white shark. 

Like most sharks, the megalodon is known from fragmentary remains. 

The most commonly preserved fossil of this giant shark is its teeth, each one with a diagonal length of about 180 millimeters (7.1 inches). 

Expectedly, this tooth is the largest of any known shark species, 

The massive jaws of this fish could deliver a bite force of up to 18,100 kilograms, strong enough to crush steel. 

13. Jaekelopterus

Scary Extinct Animals
Jaekelopterus rhenaniae | Junnn11 – License
Name MeaningReferences German Paleontologist Otto Jaekel and the Greek word “pteron,” which means wing or fin.
EraPaleozoic – Silurian Period
ClassificationEurypterida, Pterygotioidea, Pterygotidae
Height6 meters (20 feet)
Length2.5 meters (8.2 feet)
Weight272 kg (600 pounds)

Jaekelopterus was a predatory arthropod that lived during the Devonian Period. 

It was a eurypterid, a group of arthropods commonly referred to as sea scorpions due to their superficial similarity to modern scorpions. 

Interestingly, eurypterids are not related to scorpions at all. 

Jaekelopterus was one of the largest members of this group, with an average length of about 2.3 to 2.6 meters (7.5 to 8.5 feet). 

This makes it the largest arthropod ever discovered. 

Based on this estimate, this sea scorpion was larger than an average human. 

Despite their name, Jaekelopterus and other sea scorpions didn’t have stingers like modern scorpions. 

But it was an active predator that hunted prey with its giant claws. 

It was the apex predator that hunted fish, crustaceans, other arthropods, and even early vertebrates until it went extinct about 250 million years ago. 

12. Liopleurodon

Liopleurodon 3D Render
Liopleurodon 3D Render / CoreyFord via Istock
Name Meaning“Smooth-sided tooth”
EraMesozoic – Early Jurassic
ClassificationReptilia, Plesiosauria, Pliosauridae
Length6.6 meters (22 feet)
Weight3.3 tons (7275 pounds)
LocationEurope (England and France)

Liopleurodon was one of the largest marine reptiles of all time. 

With an estimated length of more than feet, this short-necked plesiosaur was the apex predator of Europe’s seas during the Late Jurassic Period

Liopleurodon preyed on fish, squid, and other marine animals.

It was a powerful swimmer with four giant flippers that provided good propulsion in the water. 

The skull of this prehistoric reptile takes up more than one-fifth of its total body length, and it had a massive jaw that gave it a powerful bite. 

It was an apex predator that preyed mainly on fish and squids but was big enough to prey on pretty much any other marine animal. 

11. Archelon

Archelon with its massive shell
Archelon with its massive shell | MR1805 via iStock
Name Meaning“Ruling turtle‭”
EraMesozoic — Late Cretaceous 
ClassificationReptilia, Testudines, Cryptodira
Length4.6 meters (15 feet)
Weight2.2 to 3.2 tons (4,850 to 7,200 pounds)
LocationCanada (North America))

Archelon was a giant sea turtle that is commonly considered the largest ever discovered. 

This close relative of the leatherback turtle grew to an average length of more than 15 feet (4.6 meters).

It had a paddle-shaped shell and a long neck that it used to reach for food.

Unlike turtles today, which are typically herbivores or omnivores, this giant turtle was a prolific carnivore with a huge appetite. 

Given its size, this giant sea turtle had a huge, hooked beak effective for crushing hard-shelled crustaceans, mollusks, and other marine creatures in the ancient seas of the Cretaceous Period. 

It could also shear soft flesh with its beak, and it preyed on fish and other soft-bodied prey as well. 

10. Mosasaurus

3D rendering of Mosasaurus
3D rendering of Mosasaurus / Vac1 via Istock
Name Meaning“Meuse lizard”
EraMesozoic – Late Cretaceous
ClassificationSquamata, Mosasauria & Mosasauroidea
Length12–18 meters (39–59 feet)
Weight10 to 20 metric tons (22,046.23 to 44,092.45 lbs)
LocationNorth America, Europe, Africa

Mosasaurus was a giant marine lizard related to present-day lizards and snakes. 

It could grow to a length of over 50 feet (15 meters), making it one of the largest marine reptiles that has ever lived. 

Discovered in the 18th century, the Mosasarus was one of the first Mesozoic reptiles ever found. 

It was one of the apex predators of the Cretaceous Period, preying on virtually anything that swims, including fish, sharks, cephalopods, and even other marine reptiles. 

It was built like a typical ancient marine reptile, with giant flippers and a massive tail that helped with locomotion. 

But the massive size of this reptile makes it worthy of being referred to as a prehistoric sea monster. 

9. Dunkleosteus

Dunkleosteus | Warpaintcobra via Getty Images
Name Meaning“Named after David Dunkle, a former curator at the Cleveland Museum of Natural History”
EraPaleozoic – Late Devonian
ClassificationPlacoderm, Arthrodira, Dunkleosteidae
Length4.1–10 meters (13–33 feet)
Weight2-5 kilograms (4.4–11 pounds)
LocationNorth America, Europe, and Africa 

Most bony fish today are soft-bodied animals, but some of their prehistoric ancestors had a more fearsome appearance. 

One of the strangest of them all was the armored Dunkleosteus

This massive fish lived during the Devonian Period and had a massive head with jaws made of hard, bony plates. 

Even more intriguing is the fact that this fish could open and close its jaws very quickly (in less than 20 milliseconds!). 

It also had the highest bite force of any living or extinct fish species, high enough to cut through the dermal armor of pretty much any hard-bodied animal. 

8. Anomalocaris

3D illustration of an Anomalocaris
Anomalocaris | CoreyFord via iStock
Name Meaning“Unusual shrimp”
EraPaleozoic – Cambrian 
ClassificationDinocaridida, Radiodonta, Anomalocarididae
Length1 meter (3.3 feet)

Anomalocaris was a weird-looking shrimp-like arthropod. 

This name, which translates as “abnormal shrimp,” is a reference to the bizarre appearance of this creature. 

The Anomalocaris lived during the Cambrian. 

Although the exact diet of the Anomalocaris isn’t known, experts think it was a carnivore. 

This would make it one of the earliest examples of an apex predator, given its size and physical attributes. 

The entire body of this primitive arthropod was covered in armored plates. 

It had a large head with a pair of spiny claws, which were useful for catching and holding onto prey. 

Anomalocaris probably hunted soft-bodied prey. 

However, some experts think it preyed on hard-bodied animals such as trilobites. 

This would be more fitting for the badass hunting predator reputation of this arthropod than a diet of floating “mush.” 

7. Helicoprion

Helicoprion | Nobumichi Tamura via GettyImages
Name Meaning“Spiral saw”
EraPaleozoic — Permian to Triassic
ClassificationChondrichthyes, Eugeneodontida, Helicoprionidae
Length5 to 8 meters (16 to 26 feet) 
Weight500 to 700 kilograms (1,100 to 1,500 pounds)
LocationNorth America, Europe, and Asia

The Helicoprion is arguably one of the most weird-looking sharks ever discovered.

Although very little is known about the full appearance of this shark, its most notable feature is its spiral-shaped “tooth whorls,” which fit into the front of its jaws. 

The shark is also commonly referred to as the buzz-saw shark due to this unique dentition. 

The largest tooth whorl discovered so far had a diameter of up to 56 centimeters (22 inches). 

The shark itself was about five to eight meters (16–26 feet) long, which means it was about the same size as modern basking sharks.

The unique detention of this shark would have allowed it to prey on both hard and soft-bodied prey. 

6. Tullimonstrum

Gage Beasley Prehistoric's Tullimonstrum Concept
Tullimonstrum | Entelognathus via Wikimedia Commons CC A-SA 4.0
Name Meaning“Tully’s Monster “
EraCenozoic —  Pennsylvanian Period
ClassificationParaHoxozoa, Bilateria, Tullimonstrum
Length35 centimeters (14 inches) 
LocationNorth America

The Tullimonstrum is more commonly referred to as the Tully monster, a name that aptly describes the bizarre appearance of this prehistoric marine creature. 

This soft-bodied creature had a long cigar-like body, an elongated nose with teeth on its end, and two protruding eyes on stalks. 

The maximum body length of this prehistoric weirdo is about 35 centimeters (14 inches). 

The Tullimonstrum is so weird that scientists had a hard time classifying it. 

The Tully monster hunted in the seas of North America about 300 million years ago, but the exact diet of this prehistoric creature isn’t well-known. 

5. Odontochelys

Odontochelys | Nobu Tamura via Wikipedia CC BY 3.0
Name Meaning“‬Toothed turtle‭”
EraMesozoic — Triassic Period
Classification‬Anapsida,‭ ‬Testudines,‭ ‬Proganochelydia
DietUncertain but likely omnivorous
Length40 centimeters (16 inches) 
Weight2 kilograms (4.5 pounds)
LocationChina (Asia) 

Odontochelys was one of the earliest turtles known from the fossil record. 

It lived in the seas of ancient China during the Triassic Period. 

Expectedly, this primitive turtle looked considerably different from its modern offspring. 

Imagine a turtle with no shell and with teeth in its jaws instead of a horny beak. 

That’s exactly what this prehistoric turtle looked like. 

Only the chest-piece or plastron is present in the Odontochelys

This lack of carapace on the back is the reason why this turtle is referred to as the “toothed turtle with a half-shell.”

4. Websteroprion

Websteroprion | Stepan Vdovichenko via Extinct-Animals Fandom
Name Meaning“‬Webster’s saw‭”
EraPaleozoic — Cambrian Period
Classification‬Annelida, Polychaeta, Websteroprion
LengthOne centimeter (0.4 inches)
LocationCanada (North America)

Websteroprion was a gigantic monster worm that lived about 400 million years ago during the Devonian Period. 

This ancient worm is most famous for its terrifying snapping jaws, which is the largest of any fossil polychaete worm ever discovered. 

The jaws of the Websteroprion were over one centimeter (0.4 inches) long and are visible to the naked eye, compared to those of its relatives that have to be studied using microscopes.

Websteroprion was a fearsome opportunistic predator of the Devonian seas. 

It lived in burrows at the bottom of the sea, capturing any prey swimming nearby and dragging them into its burrows using its powerful jaws. 

It preyed on fish, squids, and octopuses. 

3. Eximipriapulus 

Eximipriapulus | rehistorica bia Wikipedia CC BY-SA 4.0
Name Meaning“Distinguished priapulus”
EraPaleozoic — Cambrian Period
ClassificationPriapulida, Eximipriapulus
Length10 centimeters (3.9 inches)
LocationChina (Asia) 

The priapulids (commonly known as penis worms) are among the weirdest sea-dwelling animals today. 

They’re known for their bizarre body shape. 

Before the modern members of this family evolved, prehistoric forms like the Eximipriapulus existed. 

It was a bottom-dwelling creature that lived about 500 million years ago. 

Fossils of this creature have mainly been recovered from Cambrian-aged rocks from China. 

Eximipriapulus lived in Hyolith shells. 

This is considered the earliest example of a hermit crab lifestyle in a sea creature, which involves living in the discarded shells of other animals as a protection against predators.

It was also an active burrower and had a carnivorous diet. 

2. Livyatan 

A restoration of Livyatan melvillei by Cameron Dillon portraying it as a large black toothed whale
A restoration of Livyatan melvillei by Cameron Dillon portraying it as a large black-toothed whale | Blueyyeah via Dinopedia Fandom
Name Meaning“Derived from Hebrew mythology”
EraCenozoic — Neogene
ClassificationMammalia, Artiodactyla, Physeteroidea
Length13 to 17 meters (42 to 56 feet)
Weight57 tonnes (125,663 pounds) 
LocationPeru (South America)

The Livyatan is named after the biblical sea monster, and this weird creature will haunt your dreams. 

It was a prehistoric whale, big enough to easily prey on other large whales. 

The Livyatan had the largest biting teeth of any animal that has ever been discovered. 

Each tooth had an average length of about 36.2 centimeters (1.19 feet). 

The prehistoric whale was about 13.5 to 17.5 meters (44 to 57 feet), which puts it around the same size as modern sperm whales. 

Livyatan was still alive until about nine million years ago, which means it probably lived alongside the famous megalodon. 

1. Saccorhytus 

Saccorhytus | Nobu Tamura via Wikipedia CC BY-SA 3.0
Name Meaning“Wrinkle bag”
EraPaleozoic — Cambrian Period
ClassificationEcdysozoa, Saccorhytida, Saccorhytidae
Length1.3 millimeters (0.05 inches)

Saccorhytus was a bizarre animal both in its appearance and physiology. 

It was basically a floating sac of skin with no sign of an anus. 

This suggests that the animal likely consumed its food and excreted from the same orifice. 

Its body also had about eight to ten openings or body cones, which may have served a sensory function or for expelling excess water it swallowed while feeding.  

This weird animal lived during the Cambrian Period, about 500 million years ago, and is believed to be related to modern-day penis worms.


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