The deeper you descend into the ocean, the stranger its inhabitants. Over the past few million years, a wide array of creepy sea creatures have thrived in our seas. Sadly, many of these species are now extinct and many more critically endangered.
In some ways, we can be thankful that our extinct sea creatures include terrifying species like the Megalodon and the Mosasaurus. Who, in their right mind, would want to encounter these scary water dinosaurs?
On the other hand, we should be concerned about the vast swath of species that are seeing “a major collapse in their populations.”
Before we discover which ancient sea monsters still exist, let’s explore some of the largest and scariest prehistoric sea creatures that no longer inhabit our world.
Top 10 Creepy Extinct Sea Creatures & 1 Still Alive
#1 Plesiosaur – 203 million years ago
Plesiosaurs were prehistoric ocean animals that dominated the oceans during the Jurassic period. Resembling the mythical Loch Ness monster, these swimming reptiles had large bodies and small heads. They propelled themselves through the water with their limbs, hunting for fish, other reptiles, and turtles.
There were two basic types of Plesiosaurs. The Plesiosauromorphs were slow-moving creatures with extremely long necks, while the Pliosauromorph was a short-necked apex predator capable of pursuing high-speed prey.
Measuring up to 40-feet long, with long, sharp teeth, the Plesiosaur is everything we’d expect from an ancient sea monster. Despite its appearance, scientists believe the Plesiosaur was a dedicated parent that formed strong social bonds.
Reading Suggestion: The 15 Strangest Mariana Trench Animals You Ever Saw
#2 Helicoprion – 290 million years ago
The most distinctive and disturbing feature of this extinct ocean animal is the whorl of elongated teeth, or spines, located at the tip of its bottom jaw. This peculiar development makes the Helicoprion look as though it has a 290-million-year-old circular saw wedged in its mouth.
Although shark-like in appearance, the Heliocoprion belonged to the wider ratfish group and was thought to prey on squid and “other soft-bodied cephalopods.” Without any teeth on its upper jaw, it relied on the rotational movement of its tooth whorl to drag its prey into its mouth.
The Heliocoprion grew over 26-feet long and had a two-foot-wide jaw complete the monstrous saw-like teeth. In my mind, that makes it one of the creepiest prehistoric ocean animals ever discovered.
#3 Megalodon – 3.6 million years ago
While few of us have heard of a Heliocoprion, most of us are familiar with the giant shark that once dominated our oceans, even if it’s mainly due to movies like the 2018 horror flick, The Meg.
The Megalodon is often depicted as a larger version of the great white, but scientists admit that they know very little about its appearance beyond the fact that it was “a big, big shark with a lot of big teeth.”
Some people seem desperate to prove that this prehistoric sea creature still exists, but scientists are confident that the species became extinct around 3.6 million years ago.
The Megalodon was a cartilaginous species, so no skeletons or complete fossils exist. Based on the size of the teeth, scientists estimate it to have been between 49′ and 60’ long. That’s three times the size of the largest great white ever recorded!
Reading Suggestion: The Top 10 Most Dangerous Sea Creatures In The World
#4 Tylosaurus – 66 million years ago
At over 45 feet long, the Tylosaurus was one of the largest mosasaurs, or marine reptiles, ever to exist. The Tylosaurus was a deadly hunter and apex predator throughout the Cretaceous period.
Reading Suggestion: Blue Whale vs Humpback Whale Comparison
At some point in its evolution, the Tylosaurus developed an unusual hunting technique and used “brute force to ram prey at high speeds.”
Remains of their stomach contents show these sea monsters had a widely varied diet that included fish, sharks, other mosasaurs, and flightless birds.
Along with many other reptiles and dinosaur species, all three known types of Tylosaurus, including the Tylosaurus proriger, died out during the mass extinction at the end of the Cretaceous period, some 65 million years ago.
#5 Mosasaurus – 66 million years ago
A close relative of the Tylosarus, the Mosasaurus was a ferocious predator of the prehistoric seas. The Mosasaurus were the largest members of the Mosasaur family, measuring over 55’ long.
Mosasaurs dominated the seas while the dinosaurs were roaming Earth, hunting down large prey and tearing them into pieces before devouring them. Like other reptiles, they had a second set of palatal teeth in the roof of the mouth that they used to prevent their prey from escaping.
It’s thought that the mosasaurs initially used an eel-like motion to move through the water but subsequently evolved a shark-like tail and powerful “paddle-like forelimbs” that enabled them to catch high-speed prey species.
The Mosasaurus disappeared, along with many other species, during the last of the world’s mass extinctions.
Reading Suggestion: 10 Big Differences Between a Swordfish vs Marlin
#6 Basilosaurus – 40 million years ago
This prehistoric sea creature was initially thought to be a giant marine reptile, like the Mosasaurus, and was given the misleading name Basilosaurus, which means “king lizard.” It later turned out that it was a rare species of prehistoric, predatory whale that roamed the oceans during the late Eocene epoch.
A predecessor of the giant sperm whale, the Basilosaurus measured around 65’ long and had an unusually sleek appearance. That might sound big, but it’s nothing compared to the 50-ton Leviathan killer whale that presided over the oceans some 25 million years later.
Scientists believe the Basilorous was an active predator that killed with a powerful bite to its victim’s skull before tearing its prey apart and consuming it in pieces.
#7 Kronosaurus – 66 million years ago
A member of the Plesiosaurian group, the Kronosaurus resembles the shorter necked Plesiosaurs but with an even larger skull. With teeth 10 inches long and a bite force comparable to that of the crocodile, there would have been little the Kronosaurus didn’t predate.
Indeed, these sea monsters get their name from Cronus, a character from Greek mythology who allegedly killed and ate his own children.
Kronosaurus fossils have been found in South America and Australia, but it’s thought to have had worldwide distribution.
The size of the Kronosaurus is often exaggerated, partly because the skeleton on display at Harvard Museums has too many vertebrae. Despite having a skull nearly 8’ long, the Kronosaurus only measured around 30’ altogether, making it one of the smaller sea monsters off the Cretaceous period.
Reading Suggestion: The Different Types of Coral in The Sea
#8 Eurypterid – 252 million years ago
The Eurypterid was an ancient sea scorpion that lived between 467 and 250 million years ago. Although the earliest eurypterids lived in the sea, the species later evolved to exist in freshwater environments, with some growing over 8 feet long.
The last known Eurypterid was discovered in Australia earlier this year, and its remains have now been officially dated as 252 million years old. This so-called sea scorpion would have been “among the largest predators in the area at that time.”
#9 Leedsichthys – 150 million years ago
The last name of this prehistoric fish is “problematicus” as scientists can’t seem to agree on very much with regards to this now-extinct species. It could have been anywhere between 30’ and 70’ feet long, and weighed as much as 50 tons.
The only thing we’re really sure of is that the Leedsichthys is the earliest known filter-feeding marine animal.
Scientists suspect that this prehistoric fish thrived when plankton populations exploded at the beginning of the early Jurassic period, and then met their demise when those populations mysteriously plunged at the start of the Cretaceous period.
Reading Suggestion: What Are The Smallest Shark Species In The World?
#10 Ichthyosaur – 90 million years ago
These giant marine reptiles, or fish lizards, became extinct several million years before the last of the mass extinctions.
What caused its demise is unknown but, before that, it was a diverse and complex group of creatures. Some types of Ichthyosaur were more eel-like in appearance, while others resembled the modern-day shark, powering through the water at high-speed in pursuit of its fast-moving prey.
Ichthyosaurs measured up to 85 feet long, making it second only to the colossal blue whale that, as the largest animal on Earth, can reach lengths of up to 98’.
Reading Suggestion: 18 Types of jellyfish: From Pretty To Pretty Deadly
#11 Dunkleosteus – still alive
For many years, the Dunkleosteus was thought to be extinct. It had dominated seas in North America some 358 million years ago, so the chances of it surviving were remote. That was before 2019 when a visitor to Rockport State Recreation Area discovered a small specimen “flourishing in the rocky environment.”
The modern-day Dunkleosteus bears little resemblance to the 20’ long, armored fish that once feasted on prehistoric sharks, leading paleontologists to speculate that “this particular Dunkleosteus adapted to the extreme.”
The original Dunkleosteus was an ancient fish species that, according to Philip Anderson, at the Department of Geophysical Sciences at the University of Chicago, was “able to devour anything in its environment.”
What Ocean Animals Went Extinct in the Past 1000 Years?
We’ve lost several species in more recent years, and scientists fear we’ll see many more vanish as the next mass extinction looms. Many shark species are endangered, and some, like the lost shark, or Carcharhinus Obsoletus, haven’t been seen for so long they could well be extinct.
Among the extinct ocean animals that have disappeared in my lifetime, the short-nosed sea snake is one of the most recent. Last seen in 2000, the population of short-nosed sea snakes was severely impacted by global warming, coral bleaching, and commercial prawn fishing.
Reading Suggestion: The Most Dangerous Animals in Hawaii
We also stand to lose both the angel shark and the seriously creepy goblin shark within the next few years, due to overfishing and habitat loss.
Other species that have disappeared in the past 1000 years include:
#1 Stellar’s Sea Cow
The curious, but not particularly creepy, Stellar’s sea cow disappeared shortly after it was discovered, illustrating the dramatic impact of humans on our oceans.
The demand for fur drew traders to the sea otters in the Bering Sea, where the small population of Stellar’s sea cow also happened to be. As the traders hunted for lucrative sea otter pelts, they stocked up on the delicious meat of the sea cow.
By 1768, the sea cow that had been discovered just 27 years earlier was no more.
#2 Sea Mink
The sea mink once frequented the rocky coastline of New England. Measuring 36” from head to tail the sea mink was over twice the size of the American mink. For fur traders, this meant larger, more lucrative pelts and, by 1894, they had hunted it into extinction.
#3 Caribbean Monk Seal
The Caribbean Monk Seal, or sea wolf, was last seen in 1952. It was only officially declared extinct in 2008, however, after an exhaustive search for any survivors. A combination of overfishing the seals’ food resources and excessive hunting of the seals themselves caused their extinction.
#4 Japanese Sea Lion
The Japanese Sea Lion was closely related to the Californian Sea Lion and inhabited the Sea of Japan until the 1940s.
The sea lion was harvested excessively, with Japanese trawlers taking a total of 16,500 – enough to decimate the population. The last confirmed sighting of the Japanese sea lion was in 1951.
Reading Suggestion: Are Mermaids Real? 12 Mermaid Sightings in History
FAQs About the Strangest Extinct and Endangered Sea Creatures
What is the Biggest Extinct Sea Creature?
At 85-feet, the Ichthyosaur is the largest extinct sea monster so far. With science and technology advancing every day, there’s always the chance of finding something even bigger.
Are Sea Monsters Extinct?
There are still a couple of sea monsters in existence, including the diminutive Dunkleosteus. The frilled shark has survived some 88 million years in our oceans and appears to have changed very little in that time, unlike the Dunkleosteus.
What is the Rarest Sea Creature?
According to the World Wildlife Fund (WWF), the most endangered ocean animal is the vaquita. This tiny porpoise measures between 4’6” and 4’9” long is found only in the Gulf of California in Baja California, Mexico.
Vaquitas are severely threatened by illegal fishing operations in the area. Their only real chance of survival is, the WWF believes, “a fully enforced gillnet ban throughout their entire habitat.”
There are some pretty creepy creatures in our oceans, but few compare to those that are already extinct. From the Megalodon to the giant Eurypterid, there are at least 10 species of sea monsters I’m relieved I won’t ever encounter.
Nevertheless, it’s troubling to see so many species disappearing and so many more under threat. Hopefully, more of our theoretically extinct creepy sea animals will start turning up in unusual places, like the Rockport Dunkleosteus.
Nicky is a British adventurer and animal lover who spends her time exploring the natural world and writing about her experiences. Whether on horseback, underwater, running, hiking or just standing with a fishing rod in hand, she embraces everything her adopted home of South Africa has to offer.