1. Helicoproin existed 290 million years ago. They had 360 degree spiraling serrated teeth.

Helicoproin = Texas Chainsaw Massacre: Ocean Edition.

2. Dunkleosteus existed 358 million years ago. They were made of armored plating and their teeth were sharp bony plates.

Dunkleosteus size comparison.

This is what it'd look like if you were in a sea cage and a Dunkleosteus swam by.

3. Mosasaur existed more than 66 million years ago. They were incredibly fast despite their 13 feet of length.

Mosasaur: "She tastes like chicken".

4. Giant Crabs existed more than 50 million years ago and had a hammer-proof body and could move faster than Bdoubled.

Here's how the Giant Crab looks in a video game.

5. Plesiosaurus went extinct 66 million years ago despite being very adept at survival.

Would you want to swim with a Plesiosaurus? I'm afraid of seaweed, so no thanks.

6. Megaladon were the biggest sharks to ever roam the ocean, and went extinct only 23 million years ago.

Megaladon teeth indicate just how big the shark really was.