Phorusrhacos is an extinct genus of phorusrhacid, an extinct group of predatory, typically large and flightless birds. It lived during the Early to Middle Miocene epoch of the Neogene in what is now southern Argentina. The first fossil known to science, an isolated mandible fragment, was unearthed in 1887. Argentine paleontologist Carlos Ameghino discovered it in sediments of the Santa Cruz Formation in Santa Cruz Province, Argentina. Later that year, Carlos' brother Florentino scientifically described the fossil as a new genus and species, Phorusrhacos longissimus. Florentino stated it was a type of giant, toothless sloth that belonged to its own family, Phorusrhacidae. However, in 1891 Argentine naturalist Francisco Moreno pointed out that the fossil actually came from a bird. Since its 1887 description, Phorusrhacos has become one of the most well-known and common phorusrhacids, with fossils known from sites throughout southern Argentina.