25 of the strangest ancient sea monsters

From the creepiest Cambrian critters to massive marine reptiles, wonderfully weird sea creatures have inhabited our oceans for over half a billion years. We've put together a list of 25 of the strangest ancient sea monsters ever to have lived, all of which went extinct long before humans came along.

The only reason we know that these evolutionary marvels existed is because some left behind fossilized remains in rocks. Modern researchers are still interpreting these fossils and making fresh discoveries all the time, so be sure to keep up with the latest Live Science fossil news.


An artist's depiction of a short-necked plesiosaur attacking a juvenile long-necked plesiosaur. (Image credit: Mark Garlick/Science Photo Library via Getty Images)

Plesiosaurs were a group of marine reptiles with boat-like bodies and four flippers. There were long-necked plesiosaurs (think ancient Loch Ness monster) and short-necked plesiosaurs (imagine a Loch Ness monster with a short neck and a massive head). Plesiosaurs lived from the Triassic period (251.9 million to 201.4 million years ago) until they went extinct alongside the non-avian dinosaurs at the end of the Cretaceous period (145 million to 66 million years ago). They lived across the world's oceans. 

"Not only were these animals odd compared to things that we have alive today, but they were also globally distributed and very, very diverse," Michael Caldwell, a vertebrate paleontologist at the University of Alberta in Canada, told Live Science.  

Related: Newfound 'snaky croc-face' sea monster unearthed in Wyoming

Tanystropheus hydroides

The long neck of Tanystropheus hydroides may have helped the species sneak up on ocean prey. (Image credit: Spiekman et al., Current Biology (2020))
Michael Caldwell

Michael Caldwell is a professor in the departments of Biological Sciences and Earth & Atmospheric Sciences at the University of Alberta. His research career has broadly focussed on marine reptile evolution, and includes studies on mosasaurs, dolichosaurs, ichthyosaurs, plesiosaurs and extinct snakes.

Tanystropheus hydroides lived in the Tethys Sea off the ancient supercontinent Pangaea, when all of the continents were joined together, during the Triassic period around 242 million years ago. Researchers identified these ancient marine reptiles from bizarre fossils located on what is now the border between Switzerland and Italy. They had weird, broomstick-like necks that stretched to 10 feet (3 meters) in length — three times the length of their torsos.

"Like [long-necked] plesiosaurs, tanystropheids have small heads on the front and these tiny, weird little bodies way behind this gigantic neck," Caldwell said. "They are ungainly and awkward."


An illustration of two Helicoprion individuals. (Image credit: HYPERSPHERE/SCIENCE PHOTO LIBRARY via Getty Images)

Helicoprion, or the "buzz saw sharks," was a group of shark-like fish with a spiral jaw that made their teeth resemble the edge of a buzz saw. They inhabited Earth's oceans from the Devonian period (419.2 million to 358.9 million years ago) to the Triassic period, according to the Australian Museum. Fossil records indicate that these fish grew up to around 25 feet (7.7 m) long, making them 5 feet (1.5 m) longer than the largest known modern great white sharks (Carcharodon carcharias).   

Habelia optata 

 An artistic reconstruction of the tiny sea predator Habelia optata. (Image credit: Joanna Liang/Royal Ontario Museum)

Habelia optata was more of a mini monster, with a body length of up to 1.6 inches (4.1 centimeters). These tiny sea predators had helmet-like heads and creepy mouth appendages for catching and ripping apart their prey. H. optata fossils can be found in British Columbia, Canada, and date back around 505 million years to the Cambrian period (538.8 million to 485.4 million years ago), according to the Royal Ontario Museum

Lyrarapax unguispinus

An artist's rendering shows a baby (foreground) and adult Lyrarapax unguispinus hunting the Cambrian seas like the creepy predators they were. (Image credit: Science China Press)

The Cambrian period also saw the reign of a claw-faced sea monster that was totally unlike anything swimming in our oceans today. Lyrarapax unguispinus was one of many bizarre arthropods that lived during the Cambrian period, but even for its time, this species was strange. It grew up to 3.2 feet (1 m) long and had a claw-shaped appendage on the front of its head to grasp prey. This killer arthropod was one of the world's first apex predators. 


An illustration of two mosasaurs fighting for territory. (Image credit: Mohamad Haghani/Stocktrek Images via Getty Images)

Mosasaurs may not be the strangest animals on this list, but they are certainly worthy of the name "sea monster." Before they fell to the same fate as the nonavian dinosaurs, this group of marine reptiles roamed the world's oceans, chowing down on almost anything that moved, including other mosasaurs. A 2014 study in the journal Proceedings of the Zoological Institute RAS estimated that the mosasaur Mosasaurus hoffmanni grew to be around 56 feet (17 m) long.

Related: This ancient sea monster could do the breaststroke


An illustration of a Placodus species from the placodontid family. (Image credit: Corey Ford/Stocktrek Images via Getty Images)

Placodonts were an order of turtle-like Triassic marine reptiles that lived in what is now Europe, the Middle East and China. Caldwell told Live Science that placodonts "had incredibly bad buck teeth that they could have picked apples through a picket fence with." They used their front teeth to pluck shells and mollusks off reefs or the ocean floor, and they had flat crushing plates at the backs of their mouths for munching. 

Sea scorpions 

An illustration of a Eurypterid on the seafloor. (Image credit: Aunt_Spray via Getty Images)

Sea scorpions, or eurypterids, were a group of ocean-dwelling arthropods that resembled modern-day scorpions. What made them strange? Well, some were enormous compared with scorpions living today. For example, one eurypterid fossil found in New York is estimated to have come from a sea scorpion larger than a human. Members of this group could exceed 8 feet (2.5 m) in length, according to the Yale Peabody Museum of Natural History in Connecticut. Sea scorpions terrorized the seas for more than 200 million years, until they went extinct at the end of the Permian period (298.9 million to 251.9 million years ago). 

 Saccorhytus coronarius

A 3D digital model of Saccorhytus coronarius(Image credit: Philip Donoghue et al)

Saccorhytus coronarius was essentially a wrinkly sac with no anus. These weirdos lived during the Cambrian period around 500 million years ago and are known from microfossils discovered in China. The Minion-like creatures may have spent their days catching prey in seafloor sediment, but researchers' understanding of the animals' lives is limited. They are believed to be related to penis worms and mud dragons. 


 A 3D science rendering of ichthyosaurs in the Stenopterygius genus. (Image credit: Dotted Yeti via Shutterstock)

Try to picture a reptilian version of a dolphin, and you won't be far off the appearance of an ichthyosaur. This diverse group of pointed-nose predators evolved to have dolphin- or fish-like bodies, but they looked far more menacing. Ichthyosaurs evolved around 250 million years ago and went extinct around 90 million years ago. While there were ichthyosaur species as small as 1 foot (0.3 m) long, the group was home to several giants in the late Triassic period. In 2018, researchers estimated that a fossilized jawbone from the U.K. belonged to an ichthyosaur that was more than 85 feet (26 m) long, which is nearly the size of a blue whale (Balaenoptera musculus). 

Tully monsters

An illustration of a Tully monster (Tullimonstrum gregarium). (Image credit: Stocktrek Images/Getty Images)

The Tully monster (Tullimonstrum gregarium) was a soft-bodied species with primitive eyes on stalks and a long, thin appendage that ended in a claw-like feature. These mysterious creatures were so strange that researchers today have had trouble agreeing on the animals' place on the tree of life. Whatever they were, these monsters hunted in marine coastal environments 300 million years ago and are found only in fossils from Illinois, according to the Illinois State Museum.  

Related: The mysterious 'Tully monster' just got more mysterious

Odontochelys semitestacea 

An illustration of two members of Odontochelys semitestacea. (Image credit: Marlene Hill Donnelley, Field Museum)

Odontochelys semitestacea swam in the Triassic coastal waters of what is now China 220 million years ago. The species was one of the first known turtles, but it looked very different from its modern relatives.  

"These most ancient turtles have got the chest piece, or the plastron, but they don't have the carapace on the back," Caldwell said. "So, here we have early versions of turtles that are lacking the turtle shell, the carapace, and are still toothed."

Typhloesus wellsi

An artistic representation of the "alien goldfish" Typhloesus wellsi hunting prey. (Image credit: Drawing by Joschua Knüppe © Royal Ontario Museum.)

Typhloesus wellsi left behind such strange fossils that Simon Conway Morris, an emeritus professor of paleobiology at the University of Cambridge in the U.K., gave them the nickname "alien goldfish" in a 2005 article published in the journal Astronomy & Geophysics. Morris joked that they could have been brought to Earth by a visiting intergalactic commodore who grew tired of keeping them as pets and dumped them here during the Carboniferous period (358.9 million to 298.9 million years ago). The species shot a toothy "tongue" out of its gut to catch prey and may have been an early gastropod. 


An illustration of Basilosaurus. (Image credit: Sebastian Kaulitzki/Science Photo Library via Getty Images)

Basilosaurus swam through the ocean like a giant sea serpent from 37.8 million to 33.9 million years ago, with a slender body that stretched up to 59 feet (18 m) in length. The name Basilosaurus translates to "king lizard" because the researchers who named it mistook the gigantic life-form for a marine reptile, like a mosasaur or ichthyosaur. But the species wasn't a serpent or a lizard; it was a mammal, and a relative of modern whales, according to the University of Michigan's Museum of Paleontology.

Fanjingshania renovata

A reconstruction of Fanjingshania renovata(Image credit: ZHANG Heming)

This shark-like fish was heralded as being unlike any vertebrate ever discovered when it was unveiled in 2022. Covered in spiny fins with teeth-like scales and bony armor, Fanjingshania renovata is somewhere between a bony fish and a shark on the fish family tree. It lived in what is now southern China during the Silurian period (443.8 million to 419.2 million years ago). 

Opabinia regalis

An illustration of Opabinia regalis(Image credit: Nobumichi Tamura/Stocktrek Images via Getty Images)

When paleontologist Harry Blackmore Whittington presented an early reconstruction of Opabinia regalis to a meeting of fellow paleontologists in 1972, everyone in the room laughed, according to the Royal Ontario Museum. Another small, British Columbian beasty from the middle Cambrian, O. regalis had five eyes and claws on its long, flexible snout to catch prey. The species swam through ancient oceans around 505 million years ago using lateral lobes and a tail fan to steer.

Related: The 'weirdest wonder' of evolution had an even weirder cousin, new study finds 

Archelon ischyros

An illustration of Archelon, the largest turtle ever to have lived.   (Image credit: Sciepro/Science Photo Library via Getty Images.)

There's nothing particularly strange about the sea turtles we see today, but what if they were bigger — like, much bigger? That would be a little odd, right? Turn back the clock 65 million years, and the ocean featured 15-foot-long (4.6 m) supersize turtles named Archelon ischyros. They would have dwarfed the biggest turtles alive today — leatherback turtles (Dermochelys coriacea), which max out at around 5.9 feet (1.8 m) long.


A computer-generated image of a megalodon with its mouth open. (Image credit: Gil Cohiba/Shutterstock)

Megalodon (Otodus megalodon) was another supersize version of a modern animal. Fossilized teeth suggest that megalodon, which reigned over ocean ecosystems between around 23 million and 2.6 million years ago, was at least three times longer than a modern great white shark, and the biggest shark on record. The beast's exact size is disputed in scientific circles, but it could have been up to 60 feet (18 m), or even 80 feet (24 m), long. This shark was so big, it could have devoured a modern orca (Orcinus orca) in just a few bites.

Titanokorys gainesi

Titanokorys gaines viewed from underneath.  (Image credit: Illustration by Lars Fields, copyright Royal Ontario Museum)

Titanokorys gainesi may have been only 2 feet (0.6 m) long, but it was one of the largest predators during the Cambrian period. The early arthropod swam across the ocean floor, hoovering up prey like a Roomba and devouring it with a toothy, circular mouth. Half-a-billion-year-old fossils from British Columbia reveal that the creature's helmeted head was disproportionately large, making up around two-thirds of its total body length.

Websteroprion armstrongi

Head of a living marine worm (Eunice aphroditois), photographed in Indonesia. (Image credit: WaterFrame/Alamy Stock Photo)

Websteroprion armstrongi was a mighty worm of the Devonian period and extinct relative of modern marine worms. The carnivore dwarfed its fellow ancient worms, with an estimated body length of up to 6.6 feet (2 m). It was so large, in fact, that when researchers described the species from Canadian fossils in 2017, it immediately became the largest marine jawed worm on record. And if a giant worm weren't already metal enough, the researchers named its genus Websteroprion after death-metal guitarist Alex Webster from the band Cannibal Corpse.

Related: Giant worms terrorized the ancient seafloor from hidden death traps

Dunkleosteus terrelli

An illustration of the Devonian-period fish Dunkleosteus. (Image credit: MR1805 via Getty Images)

Dunkleosteus terrelli, or "Dunk" for short, was a bus-size armored fish that lived during the Devonian period. When researchers started discovering Dunk skulls in Cleveland 150 years ago, they estimated that the creature was 30 feet (9.1 m) long. However, a 2023 study published in the journal Diversity found that the creatures were actually more like 13 feet (4 m) long, but super chunky. D. terrelli was a superpredator, with blade-like jaws for slicing through any animal it could digest. 


A 3D illustration of a nothosaur. (Image credit: Warpaintcobra via Getty Images)

A 2014 study published in the journal Scientific Reports described a nothosaur species, Nothosaurus zhangi, that had a 26-inch-long (65 cm) lower jaw and an estimated total body length of up to 23 feet (7 m). These predators propelled themselves through the water with their forelimbs and snatched prey with fang-like teeth. N. zhangi lived around 245 million years ago in what is now southwestern China.


A fossilized skeleton of Dolichosaurus. (Image credit: The History Collection/Alamy Stock Photo)

Dolichosaurs were slender, serpent-like lizards with small limbs that snaked through the water, chasing prey. They lived during the Cretaceaous period and were discovered in English fossils in the mid 19th century. Caldwell said the largest dolichosaurs he encountered in the fossil record were only around 2 feet long, but their necks were longer than those of modern lizards, and they contained many more cervical vertebrae. "They had this fiendishly long neck, which is bizarre among lizards," Caldwell said. 

Diplocaulus magnicornis

An illustration of Diplocaulus. (Image credit: Dottedhippo via Getty Images)

Diplocaulus magnicornis stands out among even the strangest creatures of the ancient aquatics because of its boomerang-shaped skull. Researchers aren't sure why this amphibian evolved such a bizarre head, but it probably played a role in how the species swam. D. magnicornis lived about 275 million years ago, during the Permian period, according to the American Museum of Natural History. The fossils left behind by this species are found in modern-day Texas. 

Shell-dwelling penis worms

An illustration of a Cambrian penis worm inhabiting a hyolith shell. (Image credit: Zhang Xiguang)

And finally, there are the ferocious penis worms of the Cambrian period. Don't let their comical connotations fool you; these marine worms were mighty predators 500 million years ago, with teeth-lined mouths for devouring prey all across the ocean. To avoid becoming prey in the competitive Cambrian seas, penis worms suited up for protection. A 2021 study in the journal Current Biology found that these animals inhabited cone-shaped shells like hermit crabs do. The shell-dwelling penis worm fossils belong to the priapulida group, which includes their shell-less living descendants. The name of this group honors the well-endowed Greek god Priapus.  

Patrick Pester
Live Science Contributor

Patrick Pester is a freelance writer and previously a staff writer at Live Science. His background is in wildlife conservation and he has worked with endangered species around the world. Patrick holds a master's degree in international journalism from Cardiff University in the U.K.