Think T. rex must've been scary? Plenty of other extinct animals would've been unnerving as all get-out to an unsuspecting homo sapiens.
Like, for example, Helicoprion here. Helicoprion was a predator whose teeth did not shed but instead grew into a wheel-like pattern. That deadly whorl sat somewhere in Helicoprion's lower jaw. Some scientists have dubbed Helicoprion a "buzzsaw killer," though exactly what this Permian-Triassic-era beast looked like is still uncertain. All we know is: Teeth.
Like Helicoprion, Edestus is another extinct animal whose exact form is still a matter of debate. And like Helicoprion, Edestus didn't shed teeth, either. Instead it just grew new teeth and gums near the back of the mouth, pushing the older teeth and gums forward and creating some manner of tooth-tastic oral display.
A scientific journal from around 1900 speculates that Edestus looked like this. Others have wondered whether the mouth looked like a "monstrous pair of pinking shears."
Just after the dinosaurs went extinct around 60 million years ago, a massive snake called the Titanoboa took their place as the biggest, baddest predator on earth. They were 50 feet long, weighed 2,500 pounds and killed their prey via constriction.
Imagine a scorpion with a paddle-shaped body meant for swimming. Now imagine it's 5 feet long. The Pentecopterus decorahensis, just discovered in 2015, swam around what is now Iowa roughly 460 million years ago.
Echoes of the ancient critter live on in today's ticks, spiders and lobsters.
A worm. That walks on legs. Which may actually be tentacles. That may or may not have had mouths on the ends of them.
No wonder this species is called Hallucigenia fortis. That's unnerving, all right, but there's good news: Hallucigenia was less than 2 inches long.
Dragonflies are harmless and nice to look at. Then again, their wingspans top out at around a couple of inches. Their ancestor, the Meganeura, however, had a wingspan roughly 2 feet wide ... considerably a little less adorable.
The Dunkleosteus lived 360 million years ago. It was a heavily armored fish that grew more than 30 feet long. Its beak-like mouth was specifically made for biting through armor like its own, indicating it may have been either a cannibalistic species or one that fought over territory.
Their jaws were so efficient they could hinge them open and snap them shut in a matter of milliseconds.
It's still unclear what the entire body of a Dunkleosteus looked like, but we're guessing that very few other ancient animals ever lived after finding out.
Deinosuchus means "terrible crocodile," and these creatures lived up to the name. Fossils from T. rexes and other dangerous dinosaurs show teeth marks from the deinosuchus. That means the ancient crocs regularly did battle with the king of the dinosaurs. Deinosuchus grew to be 35 feet long and was found in North America ... in the same places where modern-day humans flip out over crocodiles one-fourth that size.
Scientists have lovingly dubbed these guys "ratzillas." Fossil records suggest they were 10 feet long, with another 5 feet of tail. While the Phoberomys pattersoni was herbivorous, these tremendous rodents would have had foot-long incisors. Imagine what that bite would look like.
It's a shrimp! It's a squid! No, it's a hideous 6-foot-long carnivorous creature called Anomalocaris, and it had tentacles with teeth on them. Their species name literally translates to "abnormal shrimp." Here's a terrifying video of what they would have looked like in action.
A name like "short-faced bear" sounds cute. Arcdotus was anything but. Natives of California until about 11,000 years ago, they were almost entirely carnivorous and would have needed about 35 pounds of meat a day to get by. They were 50 percent larger than the biggest polar bears in recorded history.
These piranhas were so huge that scientists formally dubbed their species Megapiranha. They lived 10 million years ago, were at least 3 feet long, and had two rows of teeth. It's a wonder any sea creature survived back then.
The Megalodon makes the great white shark look like your goldfish. (In this photo, the scientist is holding up a modern shark's jawbone for comparison.) At nearly 60 feet long, these extinct ocean predators are still regarded as "the most formidable carnivore to ever have existed."
The Gigantopithecus roamed Asia up until about a hundred thousand years ago. These ancestors to modern apes and orangutans measured nearly 10 feet tall. Their diets were likely vegetarian, but then again, males are thought to have had an armspan of around 12 feet. That's one terrifying bear hug.
The lack of competition in landlocked spots can lead some species to develop what's known as "island gigantism." That's what happened with the Haast's Eagle, a 30-pound raptor relative that terrorized the South Island of New Zealand. After the Maori people whittled down the megabirds' food source (a flightless and apparently delicious bird called the moa) around the year 1400, local legends say the Harpagornis moorei was known to swoop down and grab children to snack on.
The mega-eagle eventually died out. But it lives on ... in your nightmares.
If every myth is rooted somewhere in reality, tales of fire-breathing dragons almost certainly came from the Megalania (or Varanus priscus). These distant relatives of the Komodo dragon would have weighed more than 1,300 pounds and grown to be 18 feet long.
They vanished from Australia about 50,000 years ago, which means the earliest Aboriginal people may have made contact with them. Certainly sounds like the stuff of legend to us.
A prehistoric vulture called Argavantis was as tall as a human and preyed on cattle. Despite its size, it was still able to fly.
Scientists call the Phorusrhacidae "terror birds," which should tell you all you need to know about these things.
No? OK: They lived in North and South America until about 2 million years ago; they could grow to be 10 feet tall; and their beaks were so strong and sharp that they could kill other animals by striking their heads downward and fatally cracking their skulls.