Phorusrhacids, or the Terror Birds

Phorusrhacidae is a clade of large, flightless birds that existed in South America from 62 million years to 2 million years ago. They were largely carnivorous and ranged from approximately 1 meter tall to 3 meters tall (3.3-9.8 feet). Because of their size and eating habits, they have been dubbed as the Terror Birds.

Reconstructions of some phorusrhacids compared to the extant Cariama. (A)Cariama cristata; (B) Mesembriornis milneedwardsi; (C) Psilopterus bachmanni; (D) Andalgalornis steuletti; (E) Phorusrhacus longissimus; (F)Paraphysornis brasiliensis; and (G) Brontornis burmeiteri. A man’s silhouette (1.75 m) is used as scale. (Drawing by Eduardo Brettas.)
From Living Dinosaurs: The Evolutionary History of Modern Birds
Note: A, the Red-legged Seriema, is not a Phorusrhacid. It is the closest living relative.

Phorusrhacids expanded in South America because of its separation from all other continents (it was not yet connected to North America for the majority of the clade's existence), and the savannah environment that existed. They coexisted with ratites (ratites are the group of birds that include ostriches, emus, cassowaries, and kiwis, among others). Ratites currently exist in South America with the rhea, a flightless bird which is approximately 4.6 feet tall and 33-88 pounds. The Terror Birds, unfortunately, are all extinct.

The first phorusrhacid skull was discovered by Ameghino, which he described as the skull of a "probable toothless mammal" in a 1887 publication. Moreno described a partial skeleton of a large bird in a 1889 paper. Phorusrhacus longissimus, the first described phorusrhacid, was described by in two papers by Ameghino (who admitted it was not a mammal skull) and  a paper by Moreno and Mercerat, all published in 1891. Phorusrhacids currently contains 14 genera and 18 species, but as with all groups that rely on fossils to define species probably contain more. An interesting feature of phorusrhacids is a lack of uncinate processes, which are extensions of the rib cage found in birds which connect the ribs in order to strengthen them, although this may due to the fact that they may not be fossilized rather than actually be lacking.

Artist restoration of Phorusrhacus longissimus by Charles R. Knight

There has been discussions upon the feeding habits of phorusrhacids. They were the apex predator of the savannah ecosystem, and as such were quite carnivorous. At first some researches believed that they killed their prey by breaking their spines with a lateral shaking of their heads. However, the structure of their necks don't support this movement. A study by Degrange, Tambussi, Moreno, Witmer, and Wroe examined the skulls of several species of phorusrhacids. CT scans of skulls and stress points revealed that multiple downward strikes would cause the least stress when compared with direct bites or head thrashing. Thus the feeding behavior is probably most like the extant Seriema, a three feet tall terrestrial South American bird which is related to the Terror Birds. The Seriema feed by grabbing their prey with their beaks and beating it against the ground. Now imagine a ten foot tall bird doing this. They could have also used a fight and retreat model, striking the prey repeatedly until it dies and then tearing its flesh with its eagle-like beak. Some researchers have also suggested that they killed their prey with "crushing kicks." Different species probably utilized different feeding strategies or a combination of strategies. They were also very fast. The average top speed among all the species is believed to be about 45 miles per hour, but some specific species are thought to be much faster. The two known species of the genus Mesembriornis are estimated from the average top speed of 45 miles to up to even to 56 miles per hour.

Stress (Von Mises) distribution of FE models.
(A-C) Andalgalornis steulleti, (D-F) Haliaeetus albicilla, and (G-I) Cariama cristata under three load cases:
(A, D, G) lateral shake, (B, E, H) Normal Bite and (C, F, I) Pullback. VM= Von Mises; MPa, mega pascals. White areas indicate VM exceeds the scale maximum (2 MP) at those areas)
My caption: First row is sideways shaking, second row biting, and third row downward striking. Blue means the least stress, red means a lot of stress, and white means stress exceeding the scale used. This shows that the downward striking, being the least stressful, is the most plausible hunting tactic.
Taken from Derange, et al., 2010.

Unfortunately, around the beginning of the Pleistocene era the Terror Birds went extinct. The cause of this was probably the Panama land bridge which connected North America to South America. With this land bridge species of cats, dogs, and other predators invaded the phorusrhacids' territory and provided competition which led to their eventual demise. In the time before they went extinct, however, a species of phorusrhacid, Titanis walleri, made it as far north as Texas and Florida.

Titus walleri skeleton

If you want to know what a phorusrhacid may (again, MAY) have looked like with something other than their bones, you can look at its closest living relatives. Seriema, which I mentioned before in the feeding habits section, is the closest and currently lives in South America. The next closest, the Secretary Bird, is my favorite bird and looks pretty spectacular. In other words, the phorusrhacids were probably epic, and its a shame that they went extinct some 2 million and a few thousand years before I could see them.

Red-legged Seriema

Secretary Bird