Phorusrhacidae

, Devincenzia, Phorusrhacos, and Titanis'' 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 The structure of the fossils also suggest that these birds may have been swifter than originally thought. == 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. , named Rionegrina, which confirms that they held their second toe off the ground like seriemas and dromaeosaurs 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. == 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. , the closest living relative of phorusrhacids Phylogenetic analysis of Cariamiformes and their relatives according to Mayr (2016) in his redescription of Bathornis: 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): '' at the Museu Nacional, Rio de Janeiro of the skull of P 14357, holotype of Andalgalornis ferox in the collections of the Field Museum of Natural History Family Phorusrhacidae • Incertae sedis • Genus ?Lavocatavis – Middle Eocene Glib Zegdou Formation of Algeria (likely more related to a possible paleognath Eremopezus) • Genus ?Patagorhacos – Early Miocene Chichinales Formation of Rio Negro Province, Argentina. • Genus ?Paleopsilopterus – Lower Eocene (Itaboraian) Itaboraí Formation of Itaboraí, Brazil (identity as a phorusrhacid dubious) • Genus ?Brontornis – Early to Middle Miocene (Santacrucian–Laventan) Santa Cruz and Monte León Formations, Argentina – gigantic species, standing on average high. Placement in Phorusrhacidae and/or monophyly disputed. • Genus ?Eleutherornis – Middle Eocene (Bartonian) of Rhône, France and Baselland, Switzerland (a cariamiform, probably more related to Strigogyps) • Subfamily Physornithinae — equivalent to Brontornithinae, if Brontornis is included within the family • Genus Paraphysornis (Late Oligocene to Early Miocene (Deseadan) Tremembé Formation of São Paulo State, Brazil) • Genus Physornis (Middle to Late Oligocene (Deseadan) Sarmiento Formation of Santa Cruz Province, Argentina) • Subfamily Phorusrhacinae — giant species high (Kelenken up to high), but somewhat slender and decidedly more nimble than the Brontornithinae • Genus Devincenzia – Miocene to Early Pliocene, possibly up to Early Pleistocene • Subfamily Patagornithinae — intermediate sized and very nimble species, standing around high • Genus Patagornis – Early to Middle Miocene (Santacrucian–Laventan) Santa Cruz Formation of Santa Cruz Province, Argentina – includes Morenomerceraria, Palaeociconia, Tolmodus • Genus Andrewsornis – Middle to Late Oligocene (Deseadan) Agua de la Piedra Formation of southern Argentina • Genus Andalgalornis – Late Miocene to Early Pliocene (Huayquerian) Ituzaingó Formation of northwestern Argentina • Subfamily Psilopterinae — small species, standing high • Genus Eschatornis - Late Pleistocene of Brazil • Genus Psilopterus – Middle Oligocene (Deseadan) Santa Cruz Formation and Late Miocene (Chasicoan) Arroyo Chasicó Formation of southern and eastern Argentina respectively (Possible Late Pleistocene (Lujanian) records from Uruguay) • Genus Procariama – Late Miocene to Early Pliocene (Huayquerian–Montehermosan) Cerro Azul and Andalhualá Formations of Catamarca Province, Argentina • Subfamily Mesembriornithinae — medium-sized species, standing high • Genus Mesembriornis – Late Miocene to Late Pliocene (Montehermosan) Monte Hermoso Formation of Argentina • Genus Llallawavis – Late Pliocene (Chapadmalalan) Playa Los Lobos Allo Formation of northeastern Argentina Alvarenga and Höfling did not include the Ameghinornithidae from Europe in the phorusrhacoids; these have meanwhile turned out to be more basal members of Cariamae. Though traditionally considered as members of the Gruiformes, based on both morphological and genetic studies (the latter being based on the seriema) Cariamiformes may belong to a separate group of birds, Australaves, and their closest living relatives, according to nuclear sequence studies, are a clade consisting of Falconidae, Psittaciformes and Passeriformes. The following cladogram follows the analysis of Degrange and colleagues, 2015: |label1="psilopterines" |1= |label2=true "terror birds" |2= }} }} == Extinction ==

'' in paleoenvironment During the Miocene and early Pliocene epochs, there was an increase in the phorusrhacid population size in South America, suggesting that, in that time frame, the various species flourished as predators in the savanna environment. With the emergence of the Isthmus of Panama 2.7 million years ago, carnivorous dogs, bears, and cats from North America were able to cross into South America, increasing competition. (They had been preceded by procyonids as early as 7.3 million years ago. Similar ideas have been considered for sparassodonts and for South America's terrestrial sebecid crocodilians. However, the role of competitive displacement in the extinction of South American predator lineages has been questioned in more recent years, There were some suggestions that phorusrhacids, like the majority of Pleistocene megafauna, were killed off by human activity such as hunting or habitat change. This idea is no longer considered valid, as improved dating on Titanis specimens show that the last large phorusrhacids went extinct well over one and a half million years before humans arrived. but the validity of this previous radiocarbon dating has been considered highly questionable due to the enamel's lack of collagen; the tibia of Macrauchenia patachonica from the same site has been more precisely dated to a mean value of approximately 21,600 ± 1,000 years ago based on gamma spectrometry and radiocarbon dating. == Notes ==