Phorurhacoids lived from 62 million to 2.5 million years ago in South America (some have also been found in Florida) and were the dominant carnivores in the region. They are popularly referred to as "terror birds" because of their killing efficiency. They are related to the modern seriema, which is capable of short-distance flights when necessary and runs at speeds greater than 60 km per hour.

These birds comprise about a dozen genera and 25 species. In 1960 Bryan Patterson (Harvard University) and Jorge Kraglievich (Mar del Plata, Argentina) classified them into three families. Brontornithidae, the largest, had a heavy build with short legs and huge beaks, suggesting they were relatively slow runners. Fossils have been found dating from 27 to 17 million years old. Phorusrhacidae were medium-sized, standing two to three metres tall. They range in age from 27 to 3 million years. The smallest, Psilopteridae, stood about one metre tall. Their fossils range in age from 62 to 2 million years. The two smaller families were lightly built and fast runners. A gigantic species, Titanis walleri, was found in rocks 1.5 to 2.5 million years old in northen Florida. This beast stood over three metres tall.

Phorusrhacoids took over the niche vacated by the small, bipedal dinosaurs called coelurosaurs. (The transition is called an evolutionary relay.) They had similar body forms and hunting styles.

Terror birds hunted small mammals. They grasped their prey in their beaks and dashed it against the ground to render it unconscious, then swallowed it whole. Seriemas, roadrunners, and secretary birds still practice this feeding technique.

Phorusrhacoids were able to achieve dominance in South America because that continent was isolated for most of the Cenozoic. When the Panamanian land bridge arose 2.5 million years ago, fauna from North and South America were able to intermingle (the Great American Interchange). Shortly after, the terror birds lost their dominant position and disappeared.