Prehistoric Earth was filled with all kinds of impressively scary animals. But one category of avians, an extinct clade called Phorusrhacidae, was so fierce that they're today nicknamed "terror birds," as the latest video from PBS Eons explains below.

Phorusrhacidae was a group of large, flightless, carnivorous birds that roamed Earth for about 60 million years, beginning in the early Cretaceous Period. Over time, they evolved into 25 different species. Some of these birds were likely scavengers, but others lusted for blood, judging from fossils that revealed their physical makeup.

Sharp beaks allowed some terror birds to rip their prey's flesh straight from the bone, and their curved claws were ideal for stabbing. Even their skeletons were natural weapons, with strong legs powerful enough to crack bone. If these features weren't terrifying enough, experts think that the birds may have also subdued their still-living meals by lifting them up and repetitively smashing them against the ground.

One of the largest species among these birds was Titanis walleri, which once roamed the coastal plains of what's today Texas and Florida after crossing a land bridge from South America around 5 million years ago. Learn how it came to America, and why it went extinct, by watching the video below.