Published: October 31, 2016

Eight of the Most Nightmarish Prehistoric Animals

There's been life on earth for about four billion years, and a lot of it has been freaking terrifying. Great job, evolution, we’ll all be having bad dreams tonight.

1. Basilosaurus

Whales haven't always been ocean-dwellers—their ancestors lived on land, and they moved to the water about 50 million years ago. They had some awkward in-between years before becoming the whales we know and love today (and by awkward, we mean horrifying). Take Basilosaurus, whose three-foot skull you can see in our Evolving Planet exhibition. Recent studies have suggested that this 60-foot leviathan had a bite force to rival that of T. rex, which it put to good use cracking open the skulls of other, smaller whales (you can see Basilosaurus tooth marks on their skulls).

2. Therizinosaurus

Yes, we're home to the biggest, baddest T. rex ever, but SUE's not the only dinosaur that you wouldn't want to run into. Exhibit A: Therizinosaurus cheloniformis, the Freddy Kreuger of the late Cretaceous. It had three-foot long claws—no one's found its head yet, but scientists' best guesses are that it looked something like this. Oh, and T. cheloniformis is estimated to be around 33 feet long, so, approaching school-bus size. One bit of consolation: they were probably herbivorous and used those claws to tear down plant material to eat.

3. Meganeuropsis

Hold out your arm. The distance from your fingertips to your shoulder is probably in the neighborhood of 28 inches, which, it so happens, is also the wingspan of the biggest insects ever to fly. Meganeuropsis lived 250-300 million years ago, and they look like enormous dragonflies. (Technically, they're griffinflies, not dragonflies.) Scientists are still debating why these insects evolved to be so huge—some think insects are limited in size by the amount of oxygen that they're able to take in, so the concentration of oxygen in the atmosphere controls how big they can grow. During the Permian, atmospheric oxygen levels were higher, which allowed these insects to grow to enormous sizes. Other scientists think that they were able to grow bigger simply because there weren’t airborne predators like birds that could pick on them. Either way, you'd definitely want a flyswatter the size of a mattress if these critters were buzzing around.

4. Arctodus smilus

And you thought today's grizzly bears were something to avoid. Come check out the Arctodus smilus in our Evolving Planet exhibition (it's one of the best-preserved and most complete specimens in the world). A big Arctodus smilus, also called  the short-faced bear, could reach a height of 12 feet tall when standing on its hind limbs and could hit a top speed of 40 miles per hour. Some studies suggest it was an omnivore like many of the bears alive today, but in another study, chemical analysis of its bones suggested that it ate meat almost exclusively, and it probably would have needed to eat 35 pounds of flesh every day. And, since they only went extinct 11,000 years ago, there's a solid chance that humans had to deal with these guys. And by deal with, we mean, "get eaten by."

5. Tully monster

Not quite the scariest animal of the bunch, but A. it's got "monster" in its name, and B. it's Field Museum bragging rights. Illinois used to be covered by a tropical sea, and the Mazon Creek area outside of Chicago is a motherlode of soft-bodied animals fossils. The Field has the world's best collection of these fossils, including some bizarre ones called Tully monsters that flummoxed scientists trying to identify it for decades. The foot-long "monsters" have eyes on stalks, a jaw attached to a noodly proboscis—decidedly weird. But earlier this year, Field Museum scientists finally figured out what kind of animals they are—they're jawless fish, distant relatives of the horrifying lampreys that are alive today and currently invading the Great Lakes. That sounds pretty monstrous to us.

6. Eurypterid

The biggest arthropods (the group that contains insects, spiders, crabs, and lobsters) that ever existed were eight-foot sea scorpions called eurypterids. They were the top predators of their watery environments 390 million years ago, feasting on whatever they could get their claws on. Those claws, by the way, are actually mouthparts, and they could be up to a foot and a half long. With their claw-mouthparts fully extended, the biggest eight-foot eurypterids could reach lengths of 11 feet. Not something you'd want to run into while swimming.

7. Thylacosmilus

These days, the scariest marsupials are…we guess, opossums? But back in the good old days of the Upper Miocene (~five million years ago), that title went to Thylacosmilus. These ferocious predators look like saber-tooth cats, but actually belonged to an extinct group called sparassodonts, closely related to the marsupials alive today. They lived in what's now South America and were discovered by Field Museum paleontologist Elmer Riggs in the 1920s.

8. Archaeotherium

Most modern-day artriodactyls aren't that scary—the group contains deer, sheep, and llamas. But some of their ancient relatives were downright ferocious. Take Archaeotherium. They're sometimes referred to as hell pigs, and while the name fits their appearance, they're not actually pigs—they're more closely related to hippos and whales. Archaeotherium was a cow-sized predatory omnivore with huge jaws that it used to hunt animals including prehistoric rhinos—we've got some of these giant jaws behind the scenes in our collections.