Terror Birds like Phorusrhacos where for 26 million years the top carnivores in South-America. This flightless bird became up to 3 meters high, its head with hooked beak was as large as a horses head. With a lightly built body, hollow bones and long skinny legs, capable of fast running, Phorusrhacos was possibly the fastest big animal of its time, and few potential preys could escape from the Terror Bird.
Several kinds of Terror Birds have lived, for example Phorusrhacos inflatus, who became 1,5 meters high and lived during the Miocene. The largest kind was Phorusrhacos longissimus from the Pleistocene. From this animal a skull has been found of half a meter long. In North-America also lived Titanis walleri, although this one is perhaps the same as Phorusrhacos longissimus.
Although they were very look a like, this complete family is not very closely related with the birds like Diatryma, which made the forests unsafe during the Eocene. The ancestor of the Terror Birds, a small bird called Aenigmavis, has been found in the Messel shales. This one was as large as a chicken, and for a giant bird such as Gastornis it would have run away. Just many later, when Gastornis and Diatryma had disappeared the Terror Birds became enormous carnivores. Only in South-America as a matter of fact, in the other parts of the world the places of top carnivore had been already taken by mammals. When North-and South-America joined with each other in the late Pleistocene the Terror Birds moved into the north. Here they probably lost their position as top carnivores, because North-America was the place of the sabertooth cats. These cats also went to South-America, where they hunted on the preys of Phorusrhacos. The Terror Birds lived on as scavengers, in which they did also very well, they died out just before the first people appeared.