Phorusrhacidae, colloquially known as terror birds, is a family of large carnivorous, mostly flightless birds that includes possibly several subfamilies containing many genera, including the eponymous Phorusrhacos. The exact number of genera is uncertain, as some taxa such as Brontornis may be different forms of bird instead. All phorusrhacids lived during the Cenozoic era in the Americas. However, fossils have been recorded from Africa and Europe that may belong to phorusrhacids. Possible specimens have also been discovered from the La Meseta Formation of Seymour Island, Antarctica, suggesting that this group had a wider geographical range in the Paleogene. While a majority of phorusrhacids are known from South America, Titanis is known from the southern United States. Their definitive fossil records range from the Middle Eocene to the Late Pleistocene around 43 to 0.025 million years ago, though some specimens suggest that they were present since the Early Eocene. Phorusrhacids ranged widely in size, with the smallest members like Psilopterus measuring 1 meter (3.3 ft) tall whereas giants like Devincenzia possibly reached 3 meters (9.8 ft) tall. Their closest modern-day relatives are believed to be the 80-centimetre-tall (31 in) seriemas. Phorusrhacids were bipedal carnivores with large skulls that terminated in elongated, sharp beaks. Although it varies between genera, most phorusrhacids had elongated, lithe hindlimbs with a sickle claw on their innermost toe. In contrast, their arms were relatively small and bore no functional digits. Phorusrhacidae was coined by Argentine paleontologist Florentino Ameghino in 1889 and it initially included just two genera: Phorusrhacos and Tolmodus (a synonym of Patagornis). At the time, Ameghino believed it was a family of giant, edentulous xenarthran mammals possibly related to anteaters. In 1891, Argentine paleontologist Francisco Moreno pointed out that phorusrhacids were giant birds instead. The two scientists engaged in a heated scientific rivalry for decades, during which they named dozens of new phorusrhacid genera and species. Following this, the taxonomy of the group was poorly understood and many species named were later interpreted as nomen dubia or synonyms of previously named phorusrhacids. Although genera like Llallawavis and Patagornis are represented by complete remains, most phorusrhacids are only known from fragmentary or isolated material. This has caused debate and confusion over the interrelationships, anatomy, and size of the family.
Description
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. Kelenken guillermoi, from the Langhian stage of the Miocene epoch, some 15 million years ago, discovered in the Collón Curá Formation in Patagonia in 2006, represents the largest bird skull yet found. The fossil has been described as being a 71-centimetre (28 in), nearly intact skull. The beak is roughly 46 cm (18 in) long and curves in a hook shape that resembles an eagle's beak. Most species described as phorusrhacid birds were smaller, 60–90 cm (2.0–3.0 ft) tall, but the new fossil belongs to a bird that probably stood about 3 m (9.8 ft) tall. Scientists theorize that the large terror birds were extremely nimble and quick runners, able to reach speeds of 48 km/h (30 mph). Examination of phorusrhacid habitats also indicates that phorusrhacids may have presented intense competition to predatory metatherian sparassodonts such as borhyaenids and thylacosmilids, causing the mammalian predators to choose forested habitats to avoid the more successful and aggressive avian predators on the open plains.
The feet of the phorusrhacids had four toes, the first of which, known as the hallux, was reduced and did not touch the ground, while the others, corresponding to the second, third and fourth toes, were kept on the ground. Analysis of the resistance of the toes based on biomechanical models of curved beams, in particular of the second toe and its nail claw, indicate that it was modified into a "sickle claw" and was relatively uniform in various species and said claw would be relatively curved and large, which implies the need to keep it elevated to avoid wear or breakage due to contact with the ground, which would be achieved with a well-developed extensor tubercle and soft tissue pads on the fingers. The second toe, which was shorter and had fewer phalanges, also had more resistance and would make it easier to hold the claw off the ground and retain prey, a compromise with its predatory function and movement on the run, as occurs with modern seriemas, although to a lesser degree of specialization than dromaeosaurid dinosaurs. This is further supported by footprints from the Late Miocene of the Río Negro Formation, showcasing a trackway made by a mid-to-large sized terror bird with functionally didactyl footprints, the inner toe with the sickle claw raised mostly off the ground akin to their Mesozoic counterparts.
Skull structure
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