Watch Monsters Resurrected

For 65 million years, fantastic creatures roamed the prehistoric world. At the end of the last Ice Age, hundreds of species disappeared. Learn what caused the demise of the 17-foot-tall sloths, the woolly mammoths and the Australian marsupial lion.

The mega beast Megalania seems like science fiction -- an 18-foot lizard with a mouthful of dagger teeth and venom -- but it was in fact the biggest reptilian predator since T-Rex. Some say they still survive in the outback of Australia.

Acrocanthosaurus was a prehistoric weapon of mass destruction that lived 50 million years before T-Rex. With the most powerful arms in dinosaur history, a scissorlike bite and a hi-tech neck brace, this beast could kill prey 10 times its size.

Meet Spinosaurus, a carnivore twice the size of T-Rex with a six-foot-high sail, long crocodilian skull and massive claws. It may have been king of the dinosaurs 100 million years ago, but it wasn't without rivals, such as prehistoric alligators.

With six-foot jaws and a nasty disposition, the Mosasaur dined on giant sharks and Plesiosaurs. An extra set of teeth on the roof of its mouth guaranteed that when it sunk its teeth into something, the only way for it to go was down its gullet.

The biggest, baddest bird to ever stalk the planet. With a massive 18-inch beak, Terror Bird bashed its victims' brains in and swallowed giant rodents in one gulp. Four million years ago it dominated a continent, battling wolves and saber tooth cats.