Phorusrhacids

Phorusrhacids, colloquially known as terror birds, are an extinct family of large carnivorous, mostly flightless birds that were among the largest apex predators in South America during the Cenozoic era. 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.

They ranged in height from . One of the largest specimens from the Early Pleistocene of Uruguay, possibly belonging to '' Devincenzia'', would have weighed up to . Their closest modern-day relatives are believed to be the seriemas. '' Titanis walleri'', one of the larger species, is known from Texas and Florida in North America. This makes the phorusrhacids the only known large South American predator to migrate north in the Great American Interchange that followed the formation of the Isthmus of Panama land bridge (the main pulse of the interchange began about 2.6 Ma ago; ''Titanis'' at 5 Ma was an early northward migrant).

It was once believed that ''T. walleri'' became extinct in North America around the time of the arrival of humans, but subsequent datings of ''Titanis'' fossils provided no evidence for their survival after 1.8 Ma. However, reports from Uruguay of new findings of phorusrachids such as a specimen of '' Psilopterus'' dating to 96,040 ± 6,300 years ago would imply that phorusrhacids survived in South America until the late Pleistocene.

Phorusrhacids may have even made their way into Africa and Europe, if the genus '' Lavocatavis'' from Algeria and '' Eleutherornis'' from France and Switzerland are included. However, the taxonomic placement of both taxa within phorusrhacids are considered highly questionable, and their remains are too fragmentary to be included in phylogenetic analyses. 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.

The closely related bathornithids occupied a similar ecological niche in North America across the Eocene to Early Miocene; some, like ''Paracrax'', were similar in size to the largest phorusrhacids. At least one analysis recovers ''Bathornis'' as sister taxa to phorusrhacids, on the basis of shared features in the jaws and coracoid, though this has been seriously contested, as these might have evolved independently for the same carnivorous, flightless lifestyle.

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 from a smaller subspecies of this bird was also found recently. With this fossil, it was found that the internal structure of the beak is 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

Most phorusrhacids were very fast and agile. All members possessed a large hooked beak and a relatively large skull. Phorusrhacids are assumed to have preyed on smaller animals that could both be killed more safely and be swallowed whole. This is due to the fact that with the phorusrhacids' beak proportions, the jaw could not generate a great deal of bite force with which to kill the prey. However, 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. If larger prey were to have been hunted, it would require multiple precise pecks. Despite a lack of sharp talons, struggling prey could also have been restrained by foot.

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.

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 mall notoungulates">notoungulates.html" ;"title="mall notoungulates">mall notoungulatesand 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 Pellet (ornithology), 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 14 genera and 18 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
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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
** Genus '' Kelenken'' – Middle Miocene ( Colloncuran) Collón Curá Formation of Río Negro Province, Argentina; largest known phorusrhacid
** Genus ''Phorusrhacos'' – Early to Middle Miocene (Santacrucian) Santa Cruz Formation of Argentina
** Genus '' Titanis'' – Early Pliocene to Early Pleistocene (Blancan

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