Phorusrhacids, colloquially known as terror birds, are an extinct family of large carnivorous flightless birds that were among the largest apex predators in South America during the Cenozoic era; their conventionally accepted temporal range covers from 53 to 0.1 million years (Ma) ago.[1]
They ranged in height from 1 to 3 m (3 to 10 ft). Their closest modern-day relatives are believed to be the 80-centimetre-tall (31 in) seriemas. Titanis walleri, one of the larger species, is known from Texas and Florida in North America. This makes the phorusrhacids the only known large South American predator to migrate north in the Great American Interchange that followed the formation of the Isthmus of Panama land bridge (the main pulse of the interchange began about 2.6 Ma ago; Titanis at 5 Ma was an early northward migrant).[3]
It was once believed that T. walleri became extinct in North America around the time of the arrival of humans,[4] but subsequent datings of Titanis fossils provided no evidence for their survival after 1.8 Ma.[5] However, reports from Uruguay of new findings of a relatively small form (Psilopterus) dating to 18,000[6] and 96,000[7] years ago would imply that phorusrhacids survived there until very recently (i.e., until the late Pleistocene); the initial report of such a recent date has been questioned.[8]
Phorusrhacids may have even made their way into Africa; the genus Lavocatavis was discovered in Algeria, but its status as a true phorusrhacid is questioned.[9] The possible European form (Eleutherornis) and possible Antarctic specimens have also been identified, suggesting that this group had a wider geographical range in the Paleogene.[10][11][12]
The closely related bathornithids occupied a similar ecological niche in North America across the Eocene to Early Miocene; some, like Paracrax, were similar in size to the largest phorusrhacids.[13][14] At least one analysis recovers Bathornis as sister taxa to phorusrhacids, on the basis of shared features in the jaws and coracoid,[15] though this has been seriously contested, as these might have evolved independently for the same carnivorous, flightless lifestyle.[16]
The neck can be divided into three main regions. In the higher regions of the neck, the phorusrhacid has bifurcate neural spines (BNS), while it has high neural spines in its lower regions. This suggests that the phorusrhacid had a highly flexible and developed neck allowing it to carry its heavy head and strike with terrifying speed and power. Although the phorusrhacid externally looks like it has a short neck, its flexible skeletal neck structure proves that it could expand farther beyond the expected reach and intimidate its prey using its height, allowing it to strike more easily. Once stretched out into its full length in preparation for a downward strike, its developed neck muscles and heavy head could produce enough momentum and power to cause fatal damage to the terror bird's prey.[17]