Most people think of prehistoric times as the Time of the Dinosaurs. And it was…from about 250 million years ago to around 65 million years ago, dinosaurs did run the show…on land, that is. The air and the sea were quite a different story. The world’s skies, oceans, seas, rivers and lakes were teeming with prehistoric life, and if it lived its life in the water or flew through the air, chances are it was not actually a dinosaur.

But why should dinosaurs have all the fun, anyway? Today we’re going to look at some super cool creatures from ancient times who called the ocean their home, because they deserve the spotlight every now and then too. Let’s check ‘em out!

1. Ammonite

What's an Ammonite?

Way back before the time of the dinosaurs, over 400 million years ago, you’ll find the first entry in our list: the ammonite. Ammonites were molluscs, or more specifically, cephalopods, meaning they’re related to the octopus and squid of today.

However, unlike squids and octopuses, ammonites had a hard spiral shell. While they first appeared long before the dinosaurs, they died out around the same time, about 65 million years ago. Ammonites are known as “index fossils”, which means they can help paleontologists determine the age of the rocks around them depending on which species of ammonite they find there.



Not too much is known about ammonites since only their shells are usually left behind, but it is believed they had tentacles like their present-day relatives. They also may have squirted ink to evade predators, as preserved ink is sometimes found in fossil ammonites.

The name “ammonite”, if you were wondering, comes from the Egyptian god Ammon, who had horns like a ram. The spiral shape of their shells resembles ram’s horns, leading Pliny the Elder, a naturalist of Ancient Rome, to liken them to the horns of Ammon.


2. Dunkleosteus

What's a Dunkleosteus?

Let’s jump ahead a bit now about 50 million years to the Late Devonian Period. If you were to go for a swim during this time period, you might want to watch out for Dunkleosteus, a huge fish that could grow up to 20 feet long!

Dunkleosteus was one of the largest of a group of fish known as placoderms, which had armored bony heads. Dunkleosteus didn’t have teeth, but it did have bony plates that ended in very sharp edges, and very strong jaws that it could use to clamp down on just about anything in its path, including our old friend the ammonite (sorry, little guy!).



Because only the armored skull areas of Dunkleosteus are usually left as fossils, scientists have had to make educated guesses about what the rest of this creature looked like, based on its more well-known relatives and its feeding habits.

What does “Dunkleosteus” mean? Well, the second part “-osteus” comes from the Greek word for “bone”, referring to its very bony armored head. “Dunkle” refers to David Dunkle, a paleontologist at the Cleveland Museum of Natural History, who studied prehistoric fish.


3. Liopleurodon

What's a Liopleurodon?

Jump way ahead to around 150 million years ago, and we find ourselves in the seas of the Late Jurassic Period. Here, you’ll want to be very careful you don’t end up in the jaws of the pliosaurs, a group of meat-eating marine reptiles with huge heads full of lots of sharp teeth!

Liopleurodon was one such pliosaur. It could grow over 20 feet long, and its skull alone was over five feet in length. It used its four paddle-like flippers to propel itself powerfully through the sea, using its strong sense of smell to sniff out its prey. As an apex predator, it could eat just about anything it wanted.

“Liopleurodon” means “smooth-sided teeth”, which, if we’re being honest, seems to sell this prehistoric monster a bit short. However, when it was named, its teeth were the only remains anyone had found so far!



4. Kronosaurus

What's a Kronosaurus?

Liopleurodon wasn’t the only pliosaur patrolling the prehistoric seas, and it certainly wasn’t the biggest. That honor goes to Kronosaurus, which may have grown as long as 40 feet! Its teeth could grow up to 12 inches long.

Kronosaurus lived a bit later than Liopleurodon, in the Early Cretaceous Period, around 120-100 million years ago. It preyed on large turtles, other marine reptiles, and possibly even giant squids!

“Kronosaurus” was named after Kronos, a Titan of Ancient Greek mythology, who was the father of the god Zeus.



5. Elasmosaurus

What's an Elasmosaurus?

Pliosaurs like Liopleurodon and Kronosaurus belong to a group of reptiles known as “plesiosaurs”. While pliosaurs had short necks, there were also long-necked plesiosaurs, like Elasmosaurus. Elasmosaurus could grow up to 35 feet long, and over 20 feet of that was neck! It has the largest number of neck bones of any known animal, with 72 vertebrae in all.

Elasmosaurus is often depicted in older artwork lifting its head high above water, or turning its neck in a curvy “S” shape, but the truth is plesiosaur necks weren’t flexible enough to do this. It isn’t known why they had such huge necks, but scientists believe it may have aided in their feeding techniques.



Like pliosaurs, Elasmosaurus had large, paddle-like flippers to help it swim through the sea, but unlike its short-necked relatives, its head was very small. Still, it was full of sharp teeth for snatching prey, which likely included a wide variety of fish.

Elasmosaurus lived in the late Cretaceous Period, about 80 million years ago. Its name means “thin-plated lizard”.


6 and 7. Tylosaurus and Mosasaurus

How Big Where the Tylosaurus and Mosasaurus?

Plesiosaurs and pliosaurs weren’t the only reptiles swimming in the prehistoric oceans. One group, the mosasaurs of the Cretaceous Period (100-65 million years ago), could grow even larger. Mosasaurs resembled pliosaurs, with large toothy heads and paddle-like flippers, but they weren’t related. Unlike pliosaurs, who relied on their paddle-like flippers to swim, mosasaurs had long tails with fin-like structures that helped propel them through the water.

One of the most famous mosasaurs was Tylosaurus (which means “protuberance lizard”, referring to its large snout). Tylosaurus could grow over 40 feet long and ate plesiosaurs for breakfast, lunch and dinner. In fact, a Tylosaurus skeleton was actually found with a plesiosaur still inside of it! It also ate fish, sharks, sea birds and even other mosasaurs.



Another, even larger mosasaur was Mosasaurus, the sea reptile for which the family was named. Mosasaurus means “lizard of the Meuse River”. This great beast grew to 56 feet in length, and was every bit as deadly as its cousin Tylosaurus, if not more so. The first Mosasaurus remains were found in the mid-1700s, and for a long time were believed to belong to a whale, and after that, a crocodile. It wasn’t until 1798 that it was proposed the remains were from a type of large monitor lizard (like the Komodo Dragon).

That analysis would prove to be the most accurate, as it’s now believed that mosasaurs were giant marine lizards, closely related to the monitor lizards of today. The largest living lizard is the Komodo Dragon, which can reach about 10 feet long, but mosasaurs could grow more than five times as large!


8. Spinosaurus

What’s this? We snuck a dinosaur in here?

It’s true, we did say that no dinosaurs were fully aquatic, meaning fully adapted to a watery lifestyle. However, recent evidence supports that some, like Spinosaurus, were at least semi-aquatic and may have spent much of their time in the water.

Spinosaurus (“spine lizard”) is known mainly for the large spines on its back which were believed to support a large sail-like structure. For a long time, not much else was known about this dinosaur, and it was often depicted looking like a typical theropod dinosaur (think Tyrannosaurus rex) with a big sail on its back.



However, as more skull material was found, it became clear that Spinosaurus was quite different from many other meat-eating dinosaurs. Its skull was long and thin, like that of a crocodile, and seemed uniquely suited for snapping up fish.

Even more recently, as more remains of Spinosaurus were discovered and described, our idea of what this giant, 50 foot long Cretaceous dinosaur may have looked like changed drastically. It had much smaller hind limbs than previously believed, and its body proportions seemed to indicate a creature that favored a life more suited to shallow water than dry land. It’s now believed by many scientists that Spinosaurus divided its time between the shore and the shallow coastal waters of its habitat, hunting prey that included giant prehistoric sawfish.


9. Megalodon

How Big was the Megalodon?

Sharks have been around for an eye-popping 425 million years, and over that immense stretch of time, they have changed very little, relatively. What we would recognize as modern sharks first appeared around 100 million years ago. Despite threats from humans today, sharks are still going strong, following the old “if it ain’t broke, don’t fix it” style of evolution. They’re very good at what they do, and well designed for it.

You may think the great white shark is the biggest and baddest of them all, but about 25 million years ago there was a shark that would make short work of the great white: the Megalodon. Megalodon means “big tooth” and that is a very accurate description: their teeth can measure over seven inches long, the largest of any known shark.



Megalodon was believed to grow as long as 50 feet, and could bite whales clean in half with its immense jaws and massive teeth. Megalodon was the top predator of its age, and even great white sharks, which lived at the same time, knew to stay out of its way.

For a long time it was thought that Megalodon was closely related to the modern great white due to similarities in their teeth, and they were even thought to be in the same genus (Carcharodon) until recently. However, now some scientists put Megalodon in its own genus (Carcharocles) and believed it is more closely related to the sand tiger shark than to the modern great white.

Either way, this was certainly not a fish that anyone wanted to mess with, and no one was safe while this monster shark was loose in ancient oceans.


10. Coelacanth

Is the Coelacanth Fish Still Alive?

The final entry on this list might seem a bit lackluster. After all, it’s just a fish, right? After marine reptiles, dinosaurs, armored fish and giant sharks, the coelacanth seems a bit…ordinary. But that’s kind of what makes it so special!

Coelacanths were a family of fishes that first appeared during the Devonian Period, back when our old friends the ammonites were first popping up. Coelacanths resemble many other kinds of fish, but have a few unique features, including a small “secondary” tail that extends past the fins of their primary tail. They’re believed to be a transition between fish and four-legged animals that would one day leave the water and pursue a life on land.



These ancient fish were thought to have gone extinct around the end of the Cretaceous Period, around 65 million years ago. And that’s where it gets interesting. In 1938, a fisherman in South Africa named Hendrik Goosen caught a strange fish, and invited his friend Marjorie Courtenay Latimer, a museum curator, to come see it. She had never seen anything like it before, so she reached out to her friend, Professor J. L. B. Smith. Upon seeing the creature, Smith immediately realized it was a coelacanth, a fish thought to have been extinct for millions of years.

After that, another specimen was caught off the coast of South Africa in 1952. And in 1997, a couple on their honeymoon in Indonesia noticed a strange fish on display in a local market. This was found to be yet another species of coelacanth! It turns out this fish wasn’t extinct at all, but had been living deep in the depths of the ocean, waiting to be found.

And that’s what makes it so fascinating. Because if something like the coelacanth is still out there after all this time, what else might be as well? The ocean is a huge place, and largely remains unexplored. You never know when the next extinct animal may turn out to be more alive than we thought!

For even more fun with prehistoric sea creatures, check out our Marine Monsters of the Mesozoic set!


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Marine Monsters of the Mesozoic

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