Phorusrhacids, colloquially known as terror birds, are a clade of large carnivorous flightless birds that were the largest species of apex predators in South America during the Cenozoic era; their temporal range covers from 62 to 1.8 million years (Ma) ago.
They ranged in height from 1–3 metres (3.3–9.8 ft) tall. Their closest modern-day relatives are believed to be the 80 cm-tall seriemas. Titanis walleri, one of the larger species, is also known in North America from Texas and Florida. This makes the phorusrhacids the only known large South American predator to migrate north during the Great American Interchange, which commenced after the Isthmus of Panama land bridge rose about 10 to 15 Ma.
It was once believed that T. walleri became extinct in North America around the time of the arrival of humans, but subsequent datings of Titanis fossils provided no evidence for their survival after 1.8 Ma. Still, reports from Uruguay of new findings dating to 450,000 and 17,000 years ago, would imply that some phorusrhacids survived there until very recently (i.e., until the late Pleistocene); but this claim is debated.
Phorusrhacids may have even made their way into Africa; the genus Lavocatavis was recently discovered in Algeria, but its status as a true phorusrhacid is questioned. A possible European form, Eleutherornis, has also been identified, suggesting that this group had a wider geographical range in the Paleogene.
The closely related bathornithids occupied a similar ecological niche in North America across the Eocene to Miocene; some, like Paracrax, reached similar sizes to the largest phorusrhacids. At least one analysis recovers Bathornis as sister taxa to phorusrhacids, on the basis of shared features in the jaws and coracoid, though this has been seriously contested, as these might have evolved independently for the same carnivorous, flightless lifestyle.