10 Extinct Sea Creatures

10 Extinct Sea Creatures: That Once Ruled Earth’s Oceans

Earth’s oceans have always been home to extraordinary life.10 Extinct Sea Creatures from giant predators to armored swimmers and bizarre early experiments in evolution. While today’s seas are full of wonder, many of the most remarkable marine creatures disappeared millions of years ago.

Here are extinct sea creatures that once dominated Earth’s waters, each leaving behind clues to our planet’s ancient story.

1. Megalodon (Otodus megalodon) – 10 Extinct Sea Creatures

The most famous extinct predator on Earth, Megalodon ruled the oceans between 23–3.6 million years ago.

• Size: up to 60 feet long

• Bite force: strongest of any known animal

• Notable for: enormous triangular teeth, some over 7 inches long

It hunted whales, dolphins, and giant fish — a true apex predator.

2. Dunkleosteus

A heavily armored fish from 358 million years ago, Dunkleosteus was one of the largest placoderms ever discovered.

• Size: 20–30 feet

• Features: armored skull plates instead of teeth

• Era: Devonian Period (“Age of Fishes”)

With powerful jaw mechanics, it could slice prey with scissor-like precision.

3. Mosasaurus – 10 Extinct Sea Creatures

A fierce marine reptile from 82–66 million years ago, Mosasaurus swam the oceans while dinosaurs roamed the land.

• Size: 30–50+ feet

• Relation: distant relative of modern snakes and monitor lizards

• Notable for: long body, double-hinged jaws, and razor-sharp teeth

It became one of the ocean’s top predators before the dinosaur extinction.

4. Plesiosaurus

Known for its long neck and small head, Plesiosaurus lived about 200 million years ago.

• Era: Early Jurassic

• Size: ~11 feet

• Features: four powerful flipper-like limbs for “underwater flight”

Its graceful shape inspired countless sea serpent legends.

5. Ichthyosaurus – 10 Extinct Sea Creatures

Shaped like a modern dolphin, Ichthyosaurus thrived during the Jurassic period.

• Era: 190–180 million years ago

• Size: 6–13 feet

• Features: large eyes for deep diving

• Claim to fame: one of the first marine reptiles adapted fully to aquatic life

They even gave birth to live young at sea.

6. Helicoprion – 10 Extinct Sea Creatures

One of the strangest prehistoric fish ever found, Helicoprion had a spiral “tooth whorl” resembling a circular saw.

• Era: 290–250 million years ago

• Size: up to 25 feet

• Mystery: paleontologists only recently discovered the “whorl” sat inside its lower jaw

Its appearance remains one of paleontology’s biggest puzzles.

7. Ammonites

These coil-shelled cephalopods thrived for over 300 million years before disappearing with the dinosaurs.

• Era: Devonian to Cretaceous

• Related to: modern squid and octopuses

• Size range: tiny spirals to giants over 6 feet in diameter

• Notable for: ornate shells, excellent fossils

They’re one of the most iconic extinct marine species.

8. Basilosaurus – 10 Extinct Sea Creatures

Despite its name (“king lizard”), Basilosaurus was an early whale.

• Era: 40–34 million years ago

• Size: 50–66 feet

• Features: long eel-like body, small hind limbs

• Notable: one of the first fully ocean-dwelling whales

It represents a key transition from land mammals to modern cetaceans.

9. Trilobites

Among the earliest complex sea creatures, trilobites lived for nearly 270 million years.

• Era: Cambrian to Permian

• Features: segmented bodies, hard exoskeletons

• Notable for: incredible variety — over 20,000 species

They vanished during the Permian extinction, Earth’s largest die-off.

10. Stethacanthus – 10 Extinct Sea Creatures

This ancient shark (360 million years ago) is known for the bizarre “anvil-shaped” spine on its back.

• Size: about 3 feet

• Era: Late Devonian

• Mystery: structure may have been used for mating display or intimidation

Its unusual silhouette made it one of the strangest early sharks.

Our Take: Ghosts of Oceans Past

From titanic hunters like Megalodon to strange evolutionary experiments like Helicoprion and Stethacanthus, Earth’s ancient oceans were filled with creatures far more varied and dramatic than the ones we know today. Their fossils tell a story of adaptation, extinction, and the ever-changing pulse of life beneath the waves.