Kinds of dinosaurs

Water dinosaurs: were there dinosaurs in the sea?

Giant animals with flippers, long necks and jaws full of teeth really did swim in the seas when the dinosaurs were alive. People call them water dinosaurs or sea dinosaurs.

The surprise is that they weren't dinosaurs. No dinosaur lived in the sea. But one dinosaur was a real swimmer, and some dinosaurs are swimming in the sea right now.

An old watercolor of ancient sea life: long-necked plesiosaurs, fish-shaped ichthyosaurs and flying pterosaurs fighting in and above the water
“Duria Antiquior”, painted by Henry De la Beche in 1830.

The short answer

  • No dinosaur lived in the sea. The big sea animals were reptiles called ichthyosaurs, plesiosaurs and mosasaurs. They weren't dinosaurs.
  • Some dinosaurs could swim. Spinosaurus hunted fish in lakes and rivers and could swim like a crocodile.
  • Dinosaurs swim in the sea today. Birds are dinosaurs, and a penguin is a bird!

Marine reptiles

The three great sea reptiles

Scientists call them marine reptiles, which just means reptiles of the sea. There were three famous kinds. They aren't close relatives of each other: different reptiles went back to the water at different times.

  • Ichthyosaurs, an outline

    Ichthyosaurs

    Say it: ICK-thee-oh-sores

    Look for: A body like a dolphin's, a long snout full of sharp teeth, and huge eyes.

    One of them, Ophthalmosaurus, had eyes more than 22 centimeters (9 inches) across. For its size, it had the biggest eyes of any animal that ever lived.

  • Plesiosaurs, an outline

    Plesiosaurs

    Look for: Four flippers. Some had a very long neck and a small head. Others had a short neck and enormous jaws.

    The long-necked Elasmosaurus had 72 bones in its neck. The neck alone was more than seven meters (23 feet) long.

  • Mosasaurs, an outline

    Mosasaurs

    Say it: MOH-sah-sores

    Look for: A long body, four flippers and a long tail with a fin: a giant reptile built like a sea monster.

    They were close cousins of today's snakes and lizards. The most famous one is Mosasaurus, the giant from the dinosaur films.

The outlines show Ichthyosaurus, the plesiosaur Plesiopterys and the mosasaur Tylosaurus. They are drawn in blue because they aren't dinosaurs.

They breathed air

Sea reptiles had no gills. Like whales today, they had to come up to the surface to breathe.

Babies born in the water

No eggs on the beach for them. Ichthyosaurs, plesiosaurs and mosasaurs all gave birth to live babies out in the open sea.

Some ate dinosaurs

Short-necked plesiosaurs with giant jaws are called pliosaurs. Some of them ate dinosaurs whose bodies had been washed out to sea.

Drawn to scale

How big were the sea reptiles?

Some were small. Plesiosaurus was about 3.5 meters (11 feet) long. Some were giants. The biggest found so far is an ichthyosaur called Ichthyotitan, which may have been 25 meters (82 feet) long. That's about as long as a blue whale, the biggest animal alive today.

  1. Ichthyotitanmaybe 25 m (82 ft)the biggest sea reptile found so far
  2. A blue whaleabout 25 m (82 ft)a mammal: the skeleton at the Natural History Museum, London
  3. Spinosaurusup to 15 m (49 ft)a dinosaur that could swim
  4. Tylosaurus14 m (46 ft)a mosasaur skeleton from Kansas
  5. A school busabout 12 m (40 ft)
  6. Liopleurodonabout 10 m (33 ft)a short-necked plesiosaur
  7. Plesiosaurusabout 3.5 m (11 ft)
  8. You, lying downabout 1.2 m (4 ft)
Sizes from the Natural History Museum, London; Spinosaurus from Katia's fact in Dinosaurs Are Back. Only the jaw bones of Ichthyotitan have been found, so the outline is its smaller cousin Shonisaurus, and the Plesiosaurus is drawn with the outline of its cousin Plesiopterys.

Is Mosasaurus a dinosaur? Is a plesiosaur?

Why weren't they dinosaurs?

Big, ancient and gone doesn't make an animal a dinosaur. Dinosaurs were land animals, and you can tell one by its legs: they stood straight underneath the body, like the legs of a table. The sea reptiles had flippers, and they belong to other branches of the reptile family.

Scientists also check the hip. A dinosaur has a hole in its hip socket where the leg bone fits. Other reptiles don't.

The same goes for the fliers: Pteranodon wasn't a dinosaur either. All the kinds that really were: types of dinosaurs.

Who went first?

The ichthyosaurs died out about 90 million years ago. The plesiosaurs and mosasaurs lasted until the asteroid hit, 66 million years ago, and vanished along with the big dinosaurs.

Is Nessie a plesiosaur?

People say the Loch Ness Monster looks like one. But no plesiosaur has ever turned up there. A test of the water in 2019 suggests that, if anything big lives in the loch, it's likely to be a giant eel.
A page of an old handwritten letter with an ink drawing of a plesiosaur skeleton: a tiny head, a very long neck, a round body and four flippers
Mary Anning's own drawing of her plesiosaur, in a letter she wrote on 26 December 1823.

Who found them?

The girl who found sea monsters

Mary Anning lived in Lyme Regis, by the sea in the south of England. She hunted for fossils in the cliffs and sold what she found.

In 1811, when Mary was 12, her brother Joseph found a strange skull. About a year later Mary found the rest and dug it out: the skeleton of an ichthyosaur, five meters long. It was the first one scientists ever studied properly.

In December 1823 she found the first complete Plesiosaurus. It looked so odd that some people said it must be a fake. It wasn't.

The painting at the top of this page was made in 1830 by her friend Henry De la Beche. Her discoveries inspired it, and he sold prints of it to raise money for her.

The fossil skeleton of an ichthyosaur in a slab of dark rock: a long pointed snout, a big round eye socket, flippers and a long tail
An ichthyosaur at the Natural History Museum in London. Look at the size of the eye! Photo: Gary Todd.

Dinosaurs that swim

The dinosaur that could swim

One dinosaur comes close to being a water dinosaur: Spinosaurus (say it: SPINE-oh-SORE-us). It lived by lakes, swamps and rivers and hunted fish.

Spinosaurus was even bigger than Tyrannosaurus rex! It could grow up to 15 meters (49 feet) long, weigh 20 tons, and swim like a crocodile!
Katia, in Dinosaurs Are Back

Its cousin Baryonyx was a fisher too. One was found with what looks like its last meal: fish scales. And other dinosaurs paddled now and then: scientists have found swim tracks, scratches made by the toes of a floating dinosaur as they touched the bottom.

But rivers and lakes aren't the sea. And every dinosaur had to come back to land to lay its eggs.

Spinosaurus facts for kids

The Spinosaurus from Dinosaurs Are Back, a paper cut-out
A penguin, a paper cut-out

Alive right now

Dinosaurs in the sea today

Birds are dinosaurs. So penguins, ducks and cormorants are dinosaurs that swim! The Natural History Museum in London puts it like this: “With the exception of some birds, for example penguins, dinosaurs lived on land, not in the sea.”

And what swimmers they are. An emperor penguin can dive to about 518 meters (1,700 feet).

There are sea reptiles today, too: sea turtles, the marine iguana of the Galápagos Islands, and the saltwater crocodile, the largest reptile alive.

Riddle · who am I?

A dinosaur in a dinner suit

I'm a bird, but I don't fly in the sky.I fly underwater instead. Oh my!
Need another clue?
I wear a tuxedo everywhere I go.I waddle, then slide on my tummy in the snow.
Who am I? Tap to see
The penguin, a paper cut-out

penguin!

The true bit: Penguins “fly” underwater, using their wings like flippers. (SeaWorld)

Riddle · who am I?

A sea reptile of today

I was born on a beach, then I ran to the sea,and one day I'll come back where the sand welcomed me.
Need another clue?
My shell is my house, I never move out.I'm slow on the sand, but I swim all about.
Who am I? Tap to see
The sea turtle, a paper cut-out

sea turtle!

The true bit: Mother sea turtles swim back to the beach where they hatched to lay their own eggs. (NOAA National Ocean Service)

More: ocean animal riddles.

Questions kids ask

Quick answers

Were there dinosaurs that lived in the water?
No dinosaur lived under the water. Some lived beside lakes, rivers and the sea, and went for a swim. The big animals of the ancient seas were reptiles, but they weren't dinosaurs.
Is Mosasaurus a dinosaur?
No. Mosasaurus was a mosasaur, a giant reptile of the sea and a close cousin of today's snakes and lizards.
Is a plesiosaur a dinosaur?
No. Plesiosaurs were sea reptiles with four flippers. They were only distant relatives of the dinosaurs, which lived on land.
Could any dinosaur swim?
Yes, Spinosaurus could. Katia's fact from Dinosaurs Are Back: "It could grow up to 15 meters (49 feet) long, weigh 20 tons, and swim like a crocodile!" And fossil swim tracks, scratches left by the toes of a floating dinosaur, show that other dinosaurs paddled too.
What was the biggest sea reptile?
An ichthyosaur called Ichthyotitan. It may have been as much as 25 meters (82 feet) long, about as long as a blue whale. Only its jaw bones have been found so far.
Did sea reptiles lay eggs?
No. Ichthyosaurs, plesiosaurs and mosasaurs all gave birth to live babies in the water.
Who found the first plesiosaur?
Mary Anning, a fossil hunter from Lyme Regis in England. She found the first complete Plesiosaurus skeleton in December 1823.
Are there dinosaurs in the sea today?
Yes: sea birds. Birds are dinosaurs, so a penguin diving for fish is a dinosaur in the sea.
Is the Loch Ness Monster a plesiosaur?
No plesiosaur has ever turned up in Loch Ness. A test of the water in 2019 suggests that, if anything big lives there, it's likely to be a giant eel.

Keep sorting

More kinds of dinosaurs

Got a big question instead? The biggest, the fastest, the fliers and the long necks. Or meet ten dinosaurs, one page each, with a fun fact and a riddle.

Cover of Dinosaurs Are Back, an interactive 3D dinosaur book in augmented reality

Our own bookA picture book · ages 6 to 10

Dinosaurs Are Back

One of the ten dinosaurs in this story is a swimmer: a Spinosaurus who spends all day splashing in the lake. One look at Spike, and out come his huge teeth! All ten step off the page with a phone or tablet.

Buy on Amazon · $14.99Look inside

Where these facts come from

Katia's facts are from the fact page near the end of Dinosaurs Are Back. Everything else was checked on these pages on October 1, 2026.

Picture credits