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So-called “Terror Birds” are members of the family Phorusrhacidae, a group of large carnivorous flightless birds which were the dominant predators in South America from 62 to 2 million years ago, throughout the Cenozoic era. Standing between 1 and 3 meters (3-10 feet) tall, these birds had huge curved beaks made for killing, and would have dispatched mammals up to the size of dogs with ease. Most of the smaller species and a few of the larger species would have been very fast runners, with top speeds up to 60 km/h. Thankfully for our ancestors, the Terror Birds went extinct before the spread of humans to the Americas across the Bering Straight land bridge.
Around three million years ago, when the Isthmus of Panama land bridge formed, at least one species, Titanis, made it north into the area around Texas and Florida. The largest finds here point to Terror Birds with a height of 3 m and weight of 350 kg or more, making them among the largest birds in history. Although sometimes Terror Birds are incorrectly called the confirmed largest bird ever, the largest aepyornithids and dromornithids would have approached or exceeded their size. A new terror bird fossil discovered in 2006 in Argentina possesses the largest avian skull yet discovered, with a length of 71 cm.
Terror Birds are thought to have grasped their prey in their beaks, then dashed it against the ground, rendering it unconscious. After this, the bird swallowed its prey whole. This is a feeding technique also common to seriemas, roadrunners, and secretary birds. Seriemas are considered the closest living relatives of Terror Birds, and thankfully they are much smaller.
The oldest Terror Birds were called mesembriornithines. These birds were mid-sized (1-2 m) and gracile legged. Based on studies of their anatomy, it is thought these birds may have been the fastest running birds ever. The contrast of leg length among Terror Birds is unusual, as the mesembriornithines had very long legs, similar to emus, whereas another type, the brontornithines had among the stockiest bird legs. Altogether, fossil remains of 17 species have been found.
Terror Birds were the number one predator in South America for tens of millions of years. The duration of their survival is an indication of how successful they were. For some paleontologists, the niche, behavior, and anatomy — oversized heads, short forearms, long legs — of Terror Birds is reminiscent of their distant relatives, the Tyrannosaurs.
When the Isthmus of Panama came into existence three million years ago, species from North America and South America were allowed to intermix, in an event called the Great American Interchange. Within a million years or so, the Terror Birds lost their position of predatory dominance and went extinct.