Phorusrhacids ("Rag-Bearers"), or terror birds, were a family of large carnivorous flightless birds that were the dominant predators in South America during the Cenozoic, 62–2 million years (Ma) ago. They were roughly 1–3 meters (3–10 feet) tall. Their closest modern-day relatives are believed to be the 80 cm-tall seriemas. Titanis walleri, one of the larger species, is known from Texas and Florida in North America. This makes the phorusrhacids the only known example of large South American predators migrating north during the Great American Interchange (which occurred after the volcanic Isthmus of Panama land bridge rose ca. 3 Ma ago). It was once believed that T. walleri only became extinct around the time of the arrival of humans in North America, but subsequent datings of Titanis fossils have failed to provide evidence for their survival more recently than 1.8 Ma ago.
A recently discovered species, Kelenken guillermoi from Middle Miocene some 15 million years ago, discovered in Patagonia in 2006, represents the largest bird skull yet found. The fossil has been described as being a 71 cm (28 inch), nearly intact skull. The beak is roughly 46 cm (18 inch) long and curves in a hook shape that resembles an eagle's beak. It is thought that this new species would easily be able to swallow dog-sized prey. Most species described as Phorusrhacidae birds were smaller, 60-90 cm (2-3 feet) tall, but the new fossil belongs to a bird that probably stood about 3 m (10 feet) tall. Although scientists cannot be sure, they predict that the large terror birds were extremely nimble and quick runners able to reach speeds of 48 km/h (30 mi/h).
Phorusrhacids are colloquially known as "terror birds", as the larger species were apex predators and the most fearsome carnivores of their habitat, especially during the Miocene. Most of the smaller and some of the larger species are believed to have been fast runners.