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We know what the dinosaurs looked like thanks to fossils, but have you seen some of these bizarre prehistoric creatures? Join Michael Aranda for a new episode of SciShow and take a closer look at some of the strangest-looking animals evolution has created.

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[SciShow intro plays]   Michael: Evolution leads to the darnedest things sometimes! Animals like stegosaurus and dimetrodon are really very strange when you think about them, and if we only knew the platypus from the fossil record, we'd probably think it was some kind of joke. But these adaptations didn't happen just for us to be amused by them millions of years later. Even the strangest adaptations help an animal fit into its environment to get food, make babies, and survive.

You might think a platypus looks weird, but its mom probably think it's very handsome. The same was true for these ten prehistoric animals; they might have been incredibly odd, but they were also incredibly well-adapted to their environments.

Helicoprion is not only a candidate for weirdest fossil ever, but also for most enduring scientific mystery. The 270 million year old fish was first described in 1899 based on its buzzsaw-shaped whorl of teeth and nothing else. Paleontologists figured the teeth belonged to some kind of shark, but beyond that they were at a loss. They didn't even know where to stick the tooth whorl on the fish's body... the end of its nose? The dorsal fin? The tail? Eventually they came to the conclusion that the teeth belonged in the jaw, which is where you generally find teeth, but they still had no idea how the weird looking tooth spiral was oriented or how the helicoprion used it.

Then in 2013, researchers at Idaho State University turned their attention to a helicoprion tooth whorl with some of the animal's jaw cartilage preserved around it. The fossil was discovered in 1950 and left sitting in a museum for a while. Scientists had waited until 1966 to formally describe it, but even then the cartilage seemed too mangled to be informative. Then CT scanners and computer modeling came along. Using this technology the researchers decided the whorl must have fit at the base of the creature's jaw. They also found that helicoprion isn't quite a shark: it's a closer relative to ratfish and chimaeras (the type of fish not the mythological creature).

But how on earth would a wheel of teeth be useful? Well, probably the same way a saw would be useful. Helicoprion probably fed on soft-bodied organisms like squid. That round jaw wouldn't exactly spin like an electric saw, but it would rotate a little bit as the animal closed its mouth, catching prey on the teeth and dragging them inside.

Quetzalcoatlus was a giraffe-sized pterosaur named after an Aztec god. This huge flying reptile from the Cretaceous period, between 65 and 145 million years ago, had a wingspan of more than 10 meters! It seems to have evolved the ability to walk on its front legs again, even after its pterosaur ancestors evolved flight. So it walked on his front feet but its last digit stuck out with the tip of the wing hanging off of it.

Fossil remains of Quetzalcoatlus are pretty fragmentary. We know that it was a pterosaur and we know that it was big. But how big it was, what it ate, and whether it could fly are still open questions in the pterosaur world. It's hard to know how heavy a pterosaur was, because their bodies would have contained air sacks to make them lighter. That means without more complete remains, we don't know how much of it was tissue and how much of it was air, which would have affected whether or not it could fly or if it had lost the ability to fly, like an ostrich.

It might have used its walking ability as a terrestrial predator and scavenger, or it might have skimmed along the water and caught fish in its jaw like a pelican. It had a very long neck, but it wasn't very flexible. It was probably best suited for sweeping along the ground and snatching prey. Without more fossils, it's hard to know much about the ecology of this thing and why a carnivorous flying reptile would have had the approximate proportions of a giraffe.

Imagine a toothless dinosaur covered in feathers... now give it a huge pot belly... and now imagine it with burly forearms and meter-long claws that make it look like Edward Scissorhands. You might think this could not possibly be real but what you're imagining right now is therizinosaurus. Therizinosaurus belongs to the group of dinosaurs called theropods, which also includes ferocious meat-eaters like T. rex and deinonychus.

But this Cretaceous misfit was a plant-eater. It was big enough that it could probably sit on the ground next to trees and use its claws to reach up and strip off vegetation. The big gut might seem out of place but it's the best practical way to digest plant matter -- that's why cows and manatees have a similar big-bellied look. We don't know for sure why its claws were so long, but therizinosaurus's odd proportions would have made it very slow. So in addition to using them to slice off vegetation, it may have needed them to fend off predators and the giant arms, so different from a T. rex's, probably for holding up the claws.

Microraptor was a very small dinosaur, as you might have guessed from the name. Evolution didn't take a straight or simple path from dinosaurs to birds, and microraptor, another Cretaceous dinosaur that lived about 125 million years ago, was one of the more interesting detours. Tiny dinosaur, the size of a crow, microraptor had glossy black feathers, it couldn't fly, but it could probably glide using its four wings.

Dinosaurs evolved feathers before flight. At first these feathers were simple fuzz. Later on, they became asymmetrical in shape to accommodate flight and microraptor had these asymmetrical flight feathers on both its arms and legs, effectively giving it a second pair of wings, which seems like a lot of wings by the standard of today's birds. But microraptor probably used the second set of wings as stabilizers, spreading them out to make a kind of second airfoil. Other lineages of birds evolved more efficient two-wing designs, so the four-wing models became obsolete.

Glyptodon was a giant relative of armadillos, except unlike the small, flexible armadillos of today, you can be excused for mistaking a glyptodon for a Volkswagen Beetle. These animals from the Pleistocene, about 12,000 to 3 million years ago, had a huge dome of armor covering most of their bodies, as well as additional plates on their heads and tails. Some species even had tail clubs. So much protective armor might seem like overkill, but fossils of glyptodon have been found with holes in their skulls, holes delivered by predators with giant teeth. So when your neighbors are saber-toothed cats it seems you can't have too much armor.

We often refer to glyptodon as a giant armadillo, and they were related, but glyptodons and armadillos, as well as their relatives the Pampatheres, are distinct evolutionary lineages, so glyptodon wasn't really an armadillo -- more of a separate sort of car-tank thing.

It looks like a camel with a trunk, but macrauchenia isn't closely related to camels or elephants. It belongs to an extinct group of South American mammals called litopterns which died out at the end of the last Ice Age twelve thousand years ago. Litopterns are technically ungulates, meaning they're related to things like antelope and camels, but distantly. It might look a little silly but macrauchenia's adaptations allowed it to take advantage of any plant food it came across. The trunk was good for plucking leaves and branches, and its teeth were shaped for grinding tough grasses. These adaptations allowed it to spread throughout South America and outlive all the other litopterns, possibly even outlasting the Ice Age. I guess it was worth looking a little strange if it meant surviving.

This next animal actually is an elephant and it's a pretty outlandish variation on the elephant body plan, but platybelodon, an elephant from around eight to twenty million years ago takes the cake. It had tusks and a trunk like a modern elephant, but it also had this huge scoop- shaped lower jaw with a second pair of tusks. This animal has suffered from bad reconstructions over the years. Paleontologists used to give it a wide flat trunk to match its shovel shaped lower jaw. But according to more recent research, it had a relatively normal-looking trunk, and that's the secret to what its weird mouth was actually for. That lower jaw wasn't used for chewing. The broad teeth show signs of wear right on the front edge, so scientists now believe it would grab hold of grasses with its trunk and use the teeth to saw it off. Voila! Instant bulk grass meal, no dirt included.

The internet loves sloths -- they're just so slow and cute and small -- but they weren't always small. Those cute little guys that live in the treetops used to be earthbound grazers. Megatherium was a ground sloth the size of an elephant. It lived between 12,000 and five million years ago. Like the therizinosaurus we talked about earlier, it used its large claws to reach up and pull down vegetation that would have been out of reach to other animals. It may also have used them to dig up roots. The claws meant megatherium couldn't walk very fast on its front feet, so like therizinosaurus, it would have been pretty slow, but it was also so big that most predators couldn't get their teeth into it, so it didn't matter. You can just plod along and feed as slowly as you like. I guess that's one thing megatherium and modern sloths have in common.

If you're not a fan of creepy crawlies, you're not gonna like this one. Arthropleura was a millipede that could grow up to two meters long and half a meter wide. That is longer than I am tall. Luckily, we know from its fossilized poo it was probably a vegetarian. If you're wondering what possible reason a millipede could have to grow that big, well, it was probably so big it didn't have any natural predators, and like other arthropods, a category that includes insects and spiders and crustaceans, it was able to grow bigger during the Carboniferous period 302 to 360 million years ago, because back then Earth had a higher percentage of oxygen in its atmosphere.

Arthropods don't have circulatory systems like we vertebrates do. They need oxygen to diffuse through their bodies instead. When oxygen is scarce, that requirement keeps them relatively small, but when there was a lot of it, here come the giant B-movie bugs!

Animal life was just starting to get a leg up in the Cambrian, 485 to 540 million years ago. That meant some seriously strange body plans and new predators at the top of the food chain. Anomalocaris, a name that means "weird shrimp," fit both of those descriptions. It's related to modern arthropods, though it's hard to tell how closely. To give you an idea of how strange this thing is, paleontologists saw different parts of this creature and described them as a shrimp, a sea cucumber, a jellyfish, or a jellyfish on top of a sponge, but all those fossils belong to just two species of arthropod, not half a dozen kinds of smaller species.

With its big eyes and grasping mouthparts, they now figure anomalocaris was a major predator in the Cambrian seas. It probably snacked on trilobites. It could reach a meter in length, which was gigantic by Cambrian standards. In fact, it's the biggest predator known from its time. You don't really see these adaptations anymore, because the environment has changed over time. They helped these animals survive in a situation that no longer exists, but the fossil record has preserved them so we can now see how animals used to get by in all these useful ways, even if they do seem very strange.

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