The Biggest, Nastiest Sea Monsters of All-time

By Robert A. Vella

Throughout the history of life on Earth, real monsters terrorized all the creatures of the seas large enough to be preyed upon by them.  They were apex predators who dominated our watery world like the many tyrannical beasts who ruled over the land.  But, unlike their counterparts on terra firma, these sea monsters have reigned over Earth’s saltwater oceans and freshwater ecosystems for far longer and generally grew much larger.  Here is a representative list of the biggest and nastiest among them.  Are you scared?  You should be!

Paleozoic Era:  This was a time of rapid and experimental evolution which greatly diversified life on Earth.  The sea monsters of this extended time span took on many unique and bizarre forms.

Meet “Jumbo”

Anomalocaris was an ancestral arthropod or “abnormal shrimp” which swam the Cambrian seas 541 to 485 million years ago.  Having excellent vision and at up to 3 ½  feet in length, it was a giant in those early times which grabbed and crushed its prey with two protruding spiked appendages.  Trilobites were certainly on the menu, but it probably could eat anything it could catch.  If you could walk in those shallow waters back then, it would be wise to watch out for this critter.  It could hold onto and bite a big chunk out of your leg with its disk-shaped mouth.  Not a pleasant day at the beach, I can assure you.

Meet “Scorpio”

Although not a true scorpion, Pentecopterus was a similar looking arthropod during the Ordovician Period about 467 million years ago.  It inhabited the deep ocean as well as brackish zones along the ancient coastlines (and possibly freshwater habitats too) which may have allowed it to venture out of the water for short durations.  At 6 feet long, this was no “scorpion” you would want to mess around with.  Its eurypterid relatives (one of which exceeded 8 feet in length) thrived throughout the Silurian Period (443-419 million years ago) but disappeared after the horrific Permian–Triassic mass extinction event 250 million years ago (a.k.a. the “Great Dying”).

Meet “Cam”

Cameroceras was a large cephalopod that hunted the Ordovician seas 470 to 425 million years ago.  Akin to a cone-shaped nautilus, it could reach 20 feet in length.  Because it wasn’t a very good swimmer, this beast was probably an ambush predator.  Armed with a powerful beak like modern squid and octopus, Cameroceras wouldn’t be something you would want to see while scuba diving.  It’d give you a fatal hug, for sure.

Meet “Bones”

If those first three sea monsters weren’t scary enough for you, this one will be.  Dunkleosteus was a 20 foot long placoderm (i.e. bony armored fish) that made the Devonian and later oceans 382 to 258 million years ago a very dangerous place to be.  Instead of teeth, this frightening predator brandished sharp protrusions from its massive jawbones and an incredibly ferocious bite-force.  It would have no problem attacking prey larger than itself, and it could easily bite a human in half.  Want to go swimming?

Meet “Rizzo”

Another 20+ ft. monster was Rhizodus, a giant lobe-finned fish (i.e. rhizodont) which prowled the freshwater habitats of the Carboniferous Period 330 to 300 million years ago.  It probably preferred lowland swamps with heavy vegetation where it could stalk its prey under cover like crocodiles.  If these beasties – with their 8-inch fangs – were still around today, recreational activities in the Louisiana bayous would be drastically more hazardous.

Meet “Buzz”

The eugeneodont Helicoprion was a cartilaginous shark-like fish with a peculiar buzz-saw shaped tooth-whorl which savaged the Permian seas about 290 million years ago.  It grew to a length of 39 feet or more, and would’ve been really bad news to any unfortunate creature who stumbled upon it.  Power tools, anyone?

Mesozoic Era:  When the dinosaurs emerged and took over the land, reptiles took to the seas and evolved into increasingly large shark-eating monsters.  There were four basic types the first of which resembled the dolphins of today.

Meet “Thad”

Thalattoarchon may have looked like a dolphin, but a friendly little “Flipper” it was not.  At least 28 feet long, this ichthyosaur or “lizard-eating sea sovereign” was the apex predator of the Triassic Period 247 to 242 million years ago.  Believe me, a swimming excursion with this critter would neither relieve your depression nor would you be able to do it more than once.

Meet “Lilly”

Although initially believed to be much larger, the short-necked plesiosaur Liopleurodon was nonetheless an intimidating sea monster of the Jurassic Period 160 to 155 million years ago.  At around 21 feet in length, it could quickly catch-up to prey with powerful thrusts of its large flippers;  and, its huge jaws and teeth made escape highly unlikely.  The ichthyosaur in this image seems to be thinking:  “Oh, crap!”

Meet “Cronus”

The pliosaur Kronosaurus lived during the Cretaceous Period 120 to 100 million years ago.  It grew up to 34 feet long and had very strong conical teeth designed for grabbing and crushing rather than cutting.  This powerful sea monster was bigger and faster than the ambush predator Liopleurodon and was probably a more active hunter.  If you decide to time-travel back to this time, make sure you take along a much bigger boat than the one “Jaws” destroyed in the movie.

Meet “Moses”

Mosasaurus dwarfed the other marine reptiles of the Mesozoic including other mosasaurs and was the undisputed heavyweight champion of the Cretaceous seas 70 to 66 million years ago.  Up to 56 feet long, it was about the size of a large Right whale.  But, this living nightmare didn’t just eat tiny sea creatures.  No, no, no!  It ate whatever it wanted whenever it wanted.  If it saw you nearby, it’d eat you too – probably by swallowing you whole.  No “shaking” and no “tenderizing” necessary, down you’d go!

Cenozoic Era:  After a huge asteroid impact wiped out the dinosaurs along with three-quarters of all species on Earth (i.e. the Cretaceous–Tertiary extinction event), long-oppressed mammals rose to prominence and became sovereigns of the sea.  But, they quickly met a truly formidable sea monster which challenged their reign.  Sharks apparently had enough of being victims and second-rate predators, and an arms race ensued which continues to this very day.

Meet “Basil”

The primitive cetacean Basilosaurus was the whale from hell 40 to 35 million years ago during the Eocene Epoch.  It reached 59 feet in length, was eel-shaped, undulated in an odd way for whales, and stayed close to the sea surface where it fed on fish, sharks, and other cetaceans.  It wasn’t particularly intelligent nor did it socialize much with its own species.  This sea monster was a real bad-boy nobody would want any part of.

Meet “Meg”

Sharks finally rose to the very top of the food chain with the arrival of Megalodon, a carcharodon of monumental proportions.  59 feet long with a massive body and an enormous razor-sharp toothed mouth were more than enough for it to terrorize the Miocene-Pliocene oceans 23 to 3.6 million years ago.  Whales were its favorite food, but just about any large prey item was on the menu.  However, its own incredible success may have doomed it to extinction.  Baleen whales migrated to the polar regions to escape them, a cooler climate ensued, and competition from predatory cetaceans and other sharks (e.g. the Great White) added to their survival problem.  Before it completely disappeared, our earliest ancestors were emerging in Africa so it’s possible that our ancient relatives knew of this terrifying sea monster.

Meet “Liv”

One of the predatory cetaceans that directly competed with Megalodon was the “Leviathan Sperm Whale” Livyatan which lived 9.9 to 8.9 million years ago although a close relative survived until 5 million years ago.  It grew to 57 feet in length, had a gigantic skull, and sturdy 12 inch long teeth.  It preyed on the same creatures as did “Meg” and the interactions between them must have been titanic.  It also probably went extinct for the same reasons.

And, you already know “Willy”

The popular misconception of the Great White shark as today’s top oceanic predator is betrayed by the facts.  It is the Orca or “Killer Whale” which reigns supreme.  Orcas have been documented in recent years preying on Great Whites apparently just to eat their mineral-rich livers.  Or, maybe they just like the taste of shark foie gras.  Orcas, as pack hunters, could easily overpower a solitary Great White as a group;  but, most of the recorded incidents involved very few or a single attacking Orca.  In one case off the coast of central California, a mother Orca killed a large Great White and fed it to her baby.  Orcas are cetaceans but not whales, and are actually members of the dolphin family.  They live in highly social groups called “pods” which can have as many as 100 individuals.  These remarkable and intelligent creatures can get up to 26 feet long.  Each pod specializes in hunting different prey items.  Some live entirely on fish while others hunt marine mammals including the largest of whales.  Both the Great White and the Orca emerged in the Miocene Epoch, 16 million years ago for the former and 11 million years ago for the latter.  Both were alive during the time of Megalodon.  But, unlike the Great White, Orcas don’t seem to be interested in eating us puny humans.  Hopefully, they won’t change their minds!

Which is the baddest sea monster of all time?  That is a matter of personal choice, but it’s hard to argue against the Orca.  It is the only species among these which hunted in coordinated groups.  A pod of Orcas could pose a grave danger to any individual creature no matter its size and capabilities.  Strength in numbers would be the determining factor.