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 , though some specimens suggest that they were present since the Early Eocene.
Phorusrhacids ranged widely in size, with the smallest members like Psilopterus measuring tall whereas giants like Devincenzia possibly reached tall. Their closest modern-day relatives are believed to be the 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 , nearly intact skull. The beak is roughly long and curves in a hook shape that resembles an eagle's beak. Most species described as phorusrhacid birds were smaller, tall, but the new fossil belongs to a bird that probably stood about tall. Scientists theorize that the large terror birds were extremely nimble and quick runners, able to reach speeds of . 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
In the past, these birds were thought to have high beaks, round orbits, and vaulted braincases though there was never enough empirical evidence to support this. However, new fossils have been discovered in Comallo, Argentina. These skulls reveal that the terror bird has a triangular dorsal view, a rostrum that is hooked and more than half the length of the actual skull, and a more compact caudal portion. The external nares and antorbital fenestras (areas found in the nose) were found to be more square than triangular. These all contribute to a skull that is more rectangular in view rather than triangular. The structure of the fossils also suggest that these birds may have been swifter than originally thought.
A skull of Psilopterus shows the internal structure of the beak was hollow and reinforced with thin-walled trabeculae. There is also an absence of both zona flexoria palatina and zona flexoria arcus jugalis, which are key features that relate to the evolution of cranial akinesis. The discovery of this skull allows for the establishment of primary osteological homologies, which are useful in comparative anatomy, functional morphology, and phylogenetic studies.
Palaeobiology
All phorusrhacids possessed a large hooked beak and a relatively large skull. The bones of the beak were tightly fused together, making the beak more resilient to force from the front to back direction, thus suggesting that it could cause a great amount of harm through pecking, but earlier studies indicated relatively weak bite force quotients. This has led some to argue phorusrachids were poorly suited to tackle larger prey due to them being unable to effectively inflict damage without repeated blows, forcing them to rely largely on relatively tiny prey like small rodents; however, similar cranial and cervical adaptations combining relatively weak bite forces with resistance to vertical and front-to-back stresses also exist in machaidorontine felids, other sabre-toothed mammalian carnivores, and various allosauroid non-avian theropods, animals generally considered predators of larger herbivores, and later studies have found phorusrhacids to have had far greater bite forces than assumed previously. Both of these factors have led to larger phorusrhacids being once again viewed as predators of larger mammals in most recent research, while small phorusrhacids with much less robust skulls were likely small-prey specialists.
Some phorusrhacids like Andalgalornis, while very fast runners in a straight line, were poor at tight turns at speed, which contradicts the idea of phorusrhacids being agile predators of small prey and indicates a greater focus on larger prey.
Diet All phorusrhacids are thought to have been carnivorous. The strong downwards curve from the tip of this beak suggests that it ripped the flesh from the body of other animals; many extant bird species with this feature are carnivorous. CT scans performed on the skull of a phorusrhacid reveal that the species would not have been able to shake its prey side to side, but rather exert significant downward force.
Florentino Ameghino claimed in a letter to Édouard Trouessart that he had specimens from Argentina of "petrified masses preserving skeletons of large rodents, Interatheriidae [small notoungulates] and even Proterotheriidae [deer-sized litopterns], with all their bones crushed and corroded, piled on with no apparent order and forming a nearly spherical mass with the skull in the center" that resembled giant owl pellets, suggesting that phorusrhacids may have swallowed their prey whole and regurgitated the indigestible parts similar to owls. However, Ameghino never formally described these specimens and they have not yet been relocated, making it difficult to determine if they are phorusrhacid pellets. Fossilized pellets from northwestern Argentina have also been suggested to pertain to small phorusrhacids like Procariama.
Classification The etymology of the name Phorusrhacidae is based on the type genus Phorusrhacos. When first described by Florentino Ameghino in 1887, the etymology of Phorusrhacos was not given. Current thinking is that the name is derived from a combination of the Greek words "phoros", which means bearer or bearing, and "rhakos", which translates to wrinkles, scars or rents. Researchers have compared Phorusrhacidae with the living families of Cariamidae and Sagittariidae, but their differences in body mass are too drastic and, thus, one cannot overly depend on these living families for answers.
During the early Cenozoic, after the extinction of the non-bird dinosaurs, mammals underwent an evolutionary diversification, and some bird groups around the world developed a tendency towards gigantism; this included the Gastornithidae, the Dromornithidae, the Palaeognathae, and the Phorusrhacidae. Phorusrhacids are an extinct group within Cariamiformes, the only living members of which are the two species of seriemas in the family Cariamidae. While they are the most taxon-rich group within Cariamiformes, the interrelationships between phorusrhacids are unclear due to the incompleteness of their remains. A lineage of related predatory birds, the bathornithids, occupied North America prior to the arrival of phorusrhacids, living from the Eocene to Miocene and filled a similar niche to phorusrhacids. Only one genus belongs in the family, Bathornis, according to a 2016 analysis by paleontologist Gerald Mayr, who noted that Bathornis was more lightly built, with longer limbs proportionally and skulls more akin to those of Cariama.
Phylogenetic analysis of Cariamiformes and their relatives according to Mayr (2016) in his redescription of Bathornis: A 2024 study finds Bathornis as closer to seriemas than phorusrhacids were.
Following the revision by Alvarenga and Höfling (2003), there are now 5 subfamilies, containing 13 genera and 22 species: These species were the product of adaptive radiation. The following classification is based on LaBarge, Garderner & Organ (2024), and taxa identified as incertae sedis were all excluded from phylogenetic analysis in their study (except for Brontornis): Supplementary information
Family Phorusrhacidae Incertae sedis Genus ?Lavocatavis – Middle Eocene Glib Zegdou Formation of Algeria (likely more related to a possible paleognath Eremopezus)