Only partial fossils of Phorusrhacos have been found at Monte Hermoso in Argentina. Recent finds of its North American form, Titanis, in Texas and Florida are completing the picture.
Phorusrhacos was a huge, yet lightly built bird with a fearsome beak. A recent discovery in North America suggests that they also had claws on their wings, which is unusual, although it is not clear how these would have been used. Among the bones found in the stratum of the Santa Cruz Formation (now considered as mainly of mid-Miocene date) was the piece of a mandible which Florentino Ameghino (1887) at first described as that of an edentate mammal. In 1891, it was recognized to be a bird. Remains are known from several localities in the Santa Cruz Province, of Argentina.
Phorusrhacos stood around 2.5 meters (8 ft) tall and weighed approximately 130 kilograms (280 lbs) (Alvarenga & Höfling, 2003). It was nicknamed the "Terror Bird" for obvious reasons: it was one of the largest carnivorous birds to have ever existed, along with its relatives Titanis, Kelenken and Brontornis, and its rudimentary wings formed arm-like structures with claws shaped like a meat hook, possibly for tackling prey, which was killed with the massive beak. It ate small mammals and carrion. It is speculated in the Discovery Channel series "Monsters Resurrected" that the bird killed in two ways: first by grasping its prey with its beak and smashing it to the ground repeatedly like its modern relatives, the seriemas do. Secondly, and more dramatically, by striking downward with the sharp downward spike at the front of its upper beak. If in the right position, the series speculated, the spike could penetrate the braincase and kill instantly.In particular, Phorusrhacos had an enormous skull up to sixty centimeters long, armed with a powerful, hook-tipped beak. The structure of the beak and the large claws on the toes show that this was a bird of prey. It raced over the grassy plateaus and hills of Patagonia, catching small reptiles and mammals and leaving larger prey to its far more massive relatives such as Brontornis.