For more than half a billion years, evolution has run amok in our world’s oceans and created some truly bizarre creatures.
From the soft-bodied, alien lookalikes of the Cambrian to the murderous marine reptiles of the Triassic and Jurassic, our underwater realm has seen a lot of monsters come and go.
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A lot of these beasts were giants - unlike those that live on land, animals that live underwater aren’t as beholden to the force of gravity and can rely on the buoyancy of water to support a lot of their weight.
This means that they’re able to grow to gigantic sizes, sometimes larger than a double-decker bus. Not all prehistoric sea monsters were massive, however; some were no bigger than a shoebox yet were just as, if not even more terrifying than creatures ten times their size.
Prehistoric sea monsters
Megalodon
Thanks to several popular films and a wave of conspiratorial posts on social media, this extinct shark has become somewhat of a cultural icon - its fans claiming it still lives in the depths of the Mariana Trench. This is, of course, not true. In reality, Megalodon became extinct roughly 3.6 million years ago, during a time when our planet was plunged into a series of long ice ages.
While scientists are certain that Megalodon is long extinct, they’re still unsure as to why it died out. There are many different theories; the climatic changes associated with a colder period being the most likely killer. That said, recent studies have suggested that great white sharks and tiger sharks, which overlapped with Megalodon, may have outcompeted them and contributed to their eventual extinction.
Like their extant cousins, great whites, Megalodon were super predators that stalked the subtropical and temperate seas of Europe, Africa, the Americas, and Australia in search of their prey, predominantly seals, dolphins, and small whales, but also other sharks. They were more than three times the size of great whites, however, with the largest measuring in at ~20m.
Ichthyotitan
Discovered in Somerset in 2018 and described as a brand-new species in 2024, Ichthyotitan is widely considered the biggest marine reptile that ever lived, measuring ~25m from snout to tail. This makes it only a few metres shorter than a blue whale, the largest animal ever.
The first piece of Ichthyotitan’s skeleton to be found - a fragment of jaw bone - was so large that it was mistaken as a dinosaur bone. A later discovery, made by an avid, 11-year-old fossil collector and her father, shed more light on this mysterious creature and allowed a team led by expert Dean Lomax to identify it as a new species and estimate its immense length.
Ichthyotitan belongs to a diverse group of marine reptiles known as ichthyosaurs. These long-snouted, air-breathing sea creatures looked a lot like dolphins, only larger. It’s thought that Ichthyotitan, the largest amongst them, were the apex predators of the Triassic (~205 million years ago) and hunted a lot like orcas do today, using their six-foot-long jaws to subdue other smaller, yet equally ferocious marine reptiles.
Liopleurodon
This marine reptile was the major villain of the hit 2000s BBC TV series, Sea Monsters. While it was no doubt a super predator, it probably wasn’t the 25m-long, murdering menace this programme made it out to be. More recent studies of Liopleurodon have estimated its size at a more conservative 8-10m - similar to a large orca.
Still, Liopleurodon had some of the largest jaws in the animal kingdom relative to its body size, with roughly 20% of its total length made up by its toothy grin. These jaws were lined with characteristically smooth yet dagger-sharp teeth that were perfectly adapted for dismembering and deshelling the squids and ammonites it's believed to have preyed upon.
Liopleurodon lived in Western Europe from 166 to 155 million years ago, sharing its open ocean habitat with other marine reptiles, such as ichthyosaurs and plesiosaurs. Some anatomical studies have found that Liopleurodon was likely a very fast and agile swimmer, built for ambushing its prey from below.
Jaekelopterus
Just over 400 million years ago, during a time when scorpions, spiders, and insects ruled over the land, another type of invertebrate stalked the silty, estuarine environments that littered the coastline of the ancient supercontinent of Euramerica. These predatory invertebrates were known as eurypterids, or ‘sea scorpions’, and the largest amongst them was a 2.5m-long monstrosity known as Jaekelopterus.
While they may be known as ‘sea scorpions’, Jaekelopterus and their eurypterid relatives weren’t scorpions, nor did they all live in the sea - Jaekelopterus actually lived in brackish and freshwater habitats, swimming up and down rivers as it hunted armoured, jawless fish known as ostracoderms.
Why Jaekelopterus and other eurypterids grew so large is a bit of a mystery. Several theories have been put forward to try to explain their gigantism, including competition with fish and elevated oxygen levels during parts of the Paleozoic Era, but there’s no definitive answer. What we do know is that Jaekelopterus faced extinction in the Early Devonian, roughly 150 million years before the last eurypterids were wiped out at the end of the Permian.
Dunkleosteus
This heavily armoured fish was one of the world's first open ocean predators, filling a niche now occupied by great whites and orcas.
Dunkleosteus lived during the Late Devonian (from 382 to 358 million years ago) and inhabited the warm, shallow seas around Euramerica. Its size has been greatly debated over the years, with estimates ranging from 10m to 4m. The most recent estimates err on the smaller side, but that’s not to say they were any less ferocious than previously thought.
Interestingly, Dunkleosteus didn’t have any teeth. Instead, it had sharp jaw extensions that functioned just like teeth and were strong enough to puncture steel. There’s evidence of Dunkleosteus-shaped bite marks on other Dunkleosteus, suggesting they weren’t above cannibalism when their more commonly eaten prey, ammonites, were hard to come by.
Helicoprion
In the late 1800s, a strange fossil was found that resembled an octopus tentacle lined with many sharp, serrated teeth. Scientists were quick to note it came from some kind of extinct shark, but where it came from on the shark’s body - tail fin, dorsal fin, snout, or mouth - was a lot less clear.
By the mid 20th century, this so-called ‘tooth whorl’ was widely accepted as coming from the shark’s lower jaws where it sat vertically like a buzzsaw, ready to deshell the hard-shelled ammonites and nautiloids it’s thought to have preyed on. These tooth whorls make up the majority of Helicoprion fossils, in fact most specimens are known solely from the strange teeth they’ve left behind.
Helicoprion lived during the Permian (from 290 to 270 million years ago) and were truly cosmopolitan, inhabiting oceans across the world. This is evidenced by the fact that we’ve found their tooth whorls everywhere from Western Australia to Norway.
Basilosaurus
Known as ‘King Lizard’ in Ancient Greek and with a body shape reminiscent of Ichthyotitan and Liopleurodon discussed above, you’d think that Basilosaurus was another monstrous marine reptile. In fact, this 20m-long behemoth was a mammal and an early ancestor of today’s whales.
Basilosaurus evolved from a land-dwelling animal that looked a lot like a goat. These terrestrial roots can be seen on its body, which has tiny, 35cm-long hind limbs that would have been of little use in water. Its modern descendants, blue whales, orcas, and dolphins, also possess these evolutionary ‘leftovers’, though theirs are internalised and truly vestigial. Still, they’re throwbacks to a time when their ancestors were a lot smaller and ran rather than swam.
Analyses of Basilosaurus’ stomach contents suggest it fed on fish, sharks, and perhaps even other early whales, like Dorudon. Some bite marks on the skull of a juvenile Dorudon have led researchers to hypothesise that Basilosaurus killed its prey by biting down on their heads before tearing them apart.
Tullimonstrum
This strange-looking creature defied classification for decades and even now has scientists confused as to its origins. It’s unlike anything alive today, with the body of a squid, the fins of a cuttlefish, the stalked eyes of a snail, and a grasping, toothed ‘trunk’ known as a proboscis.
Since its discovery in 1955, Tullimonstrum’s evolutionary origins have been heavily debated. It has been likened to molluscs, arthropods, conodonts, worms, tunicates, and even vertebrates, but the jury is still out on where exactly it falls on the tree of life. It’s also unclear how and what Tullimonstrum ate. It’s thought it may have used its trunk to capture its prey, likely small fish, shrimp, and jellyfish, before using its tongue to ‘lick’ them to death.
Tullimonstrum is by far the smallest monster on this list, measuring just 30cm in length. It lived 300 million years ago in the shallow seas and estuaries that once covered the United States.
Dakosaurus
Today, crocodiles are largely confined to freshwater habitats, save for several species that live in brackish waters. In the Mesozoic Era and during the time of the dinosaurs, however, many of their extinct relatives, known as crocodylomorphs, lived in the open ocean. Dakosauruswas one of these seafaring crocodiles, inhabiting the warm, shallow seas that covered parts of Eurasia 157 to 137 million years ago.
Dakosaurus measured 5m in length and had the familiar, elongate body shape of a modern-day crocodile but the paddle-like fins of a sea turtle. It was perfectly adapted to a marine-based lifestyle and may have even spent its entire life at sea, hunting and mating underwater.
From its shortened tooth row and stout jaws, researchers have deduced that Dakosaurus was likely a suction feeder, sucking fish into its mouth before swallowing them whole. This way of hunting and taste for fish meant that Dakosaurus wouldn’t have directly competed with Plesiosuchus, another large-bodied crocodylomorph that shared its habitat and preyed on other marine reptiles.
Rhizodus
This 5m-long fish was more of a ‘river monster’ than a ‘sea monster’, living in freshwater environments across Eurasia and North America during the Carboniferous, roughly 335 million years ago.
Rhizodus, meaning ‘root tooth’, is characterised by the two 20cm-long fangs that protrude from the front of its jaws. It no doubt used these fangs to tear apart its prey, which included other fish and large amphibians. Some studies have suggested Rhizodus may have hunted like today’s crocodiles, lunging at prey that lived in the shallows and/or on the shores of lakes, rivers, and swamps.
While it was a fish, Rhizodus belonged to a group of early tetrapodomorphs that gave rise to all living terrestrial vertebrates, including amphibians, reptiles, birds, and mammals. This means that you’re more closely related to Rhizodus than a goldfish is.