They ranged in height from 1 to 3 m (3 to 10 ft). One of the largest specimens from theEarly Pleistocene ofUruguay, possibly belonging toDevincenzia, would have weighed up to 350 kilograms (770 lb).[5][6] Their closest modern-day relatives are believed to be the 80-centimetre-tall (31 in)seriemas.Titanis walleri, one of the larger species, is known fromTexas andFlorida inNorth America. This makes the phorusrhacids the only known large South American predator to migrate north in theGreat American Interchange that followed the formation of theIsthmus of Panamaland bridge (the main pulse of the interchange began about 2.6 Ma ago;Titanis at 5 Ma was an early northward migrant).[7]
It was once believed thatT. walleri became extinct in North America around the time of the arrival of humans,[8] but subsequent datings ofTitanis fossils provided no evidence for their survival after 1.8 Ma.[9] However, reports fromUruguay of new findings of phorusrachids such as a specimen ofPsilopterus dating to 96,040 ± 6,300 years ago would imply that phorusrhacids survived in South America until thelate Pleistocene.[b]
The closely relatedbathornithids occupied a similarecological niche inNorth America across theEocene toEarly Miocene; some, likeParacrax, were similar in size to the largest phorusrhacids.[16][17] At least one analysis recoversBathornis as sister taxa to phorusrhacids, on the basis of shared features in the jaws andcoracoid,[18] though this has been seriously contested, as these might have evolved independently for the same carnivorous, flightless lifestyle.[19]
The neck can be divided into three main regions. In the higher regions of the neck, the phorusrhacid has bifurcateneural 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.[20]
Kelenken guillermoi, from theLanghian stage of theMiocene epoch, some 15 million years ago, discovered in theCollón Curá Formation inPatagonia in 2006, represents the largest birdskull 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).[21] Examination of phorusrhacid habitats also indicates that phorusrhacids may have presented intense competition to predatory metatheriansparassodonts such asborhyaenids andthylacosmilids, causing the mammalian predators to choose forested habitats to avoid the more successful and aggressive avian predators on the open plains.[22]
Phorusrhacinae skulls compared
The feet of the phorusrhacids had four toes, the first of which, known as thehallux, 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 thandromaeosaurid dinosaurs.[23] This is further supported by footprints from theLate Miocene of theRío 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.[24]
In the past, these birds were thought to have high beaks, roundorbits, and vaultedbraincases[25] 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, arostrum that is hooked and more than half the length of the actual skull, and a more compactcaudal portion. The externalnares and antorbitalfenestras (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.[25] The structure of the fossils also suggest that these birds may have been swifter than originally thought.[25]
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-walledtrabeculae. There is also an absence of both zona flexoria palatina and zona flexoria arcus jugalis, which are key features that relate to the evolution ofcranial 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.[26]
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.[27]
Some phorusrhacids likeAndalgalornis, 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.[28]
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.[29]Florentino Ameghino claimed in a letter toÉdouard Trouessart that he had specimens fromArgentina of "petrified masses preserving skeletons of large rodents,Interatheriidae [smallnotoungulates] and evenProterotheriidae [deer-sizedlitopterns], 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 giantowl pellets, suggesting that phorusrhacids may have swallowed their prey whole and regurgitated the indigestible parts similar toowls.[30][31] However, Ameghino never formally described these specimens and they have not yet been relocated, making it difficult to determine if they are phorusrhacid pellets.[31] Fossilizedpellets from northwesternArgentina have also been suggested to pertain to small phorusrhacids likeProcariama.[32]
The etymology of the name Phorusrhacidae is based on the type genusPhorusrhacos. When first described byFlorentino Ameghino in 1887, the etymology ofPhorusrhacos was not given. Current thinking is that the name is derived from a combination of theGreek words "phoros", which meansbearer orbearing, and "rhakos", which translates towrinkles,scars orrents.[33] Researchers have compared Phorusrhacidae with the living families ofCariamidae andSagittariidae, but their differences in body mass are too drastic and, thus, one cannot overly depend on these living families for answers.
During the earlyCenozoic, after theextinction of the non-bird dinosaurs,mammals underwent anevolutionary diversification, and some bird groups around the world developed a tendency towardsgigantism; this included theGastornithidae, theDromornithidae, thePalaeognathae, and the Phorusrhacidae.[34] Phorusrhacids are an extinct group withinCariamiformes, the only living members of which are the two species ofseriemas 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.[35] A lineage of related predatory birds, thebathornithids, occupied North America prior to the arrival of phorusrhacids, living from the Eocene to Miocene and filled a similar niche to phorusrhacids.[36] Only one genus belongs in the family,Bathornis, according to a 2016 analysis by paleontologist Gerald Mayr, who noted thatBathornis was more lightly built, with longer limbs proportionally and skulls more akin to those ofCariama.[37]
Phylogenetic analysis of Cariamiformes and their relatives according to Mayr (2016) in his redescription ofBathornis:[37] A 2024 study findsBathornis as closer to seriemas than phorusrhacids were.[14]
Following the revision by Alvarenga and Höfling (2003), there are now 5subfamilies, containing 14genera and 18species:[38] These species were the product of adaptive radiation.[39] The following classification is based on LaBarge, Garderner & Organ (2024), and taxa identified asincertae sedis were all excluded from phylogenetic analysis in their study (except forBrontornis):[14]
Subfamily Phorusrhacinae — giant species 8.3 feet (2.5 m) high (Kelenken up to 9.8 feet (3.0 m) high[42]), but somewhat slender and decidedly more nimble than the Brontornithinae
Alvarenga and Höfling did not include theAmeghinornithidae from Europe in the phorusrhacoids; these have meanwhile turned out to be more basal members of Cariamae.[46] Though traditionally considered as members of theGruiformes, based on both morphological and genetic studies (the latter being based on the seriema[47])Cariamiformes may belong to a separate group of birds,Australaves, and their closest living relatives, according to nuclear sequence studies, are aclade consisting ofFalconidae,Psittaciformes andPasseriformes.[48][49]
The following cladogram follows the analysis of Degrange and colleagues, 2015:[45]
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 theIsthmus of Panama 2.7 million years ago, carnivorous dogs, bears, and cats from North America were able to cross into South America, increasing competition.[50] (They had been preceded byprocyonids as early as 7.3 million years ago.[7]) The population of phorusrhacids declined thereafter according to older hypotheses, suggesting that competition with newly arrived predators was a major contributor to their extinction.[51] Similar ideas have been considered forsparassodonts and for South America's terrestrialsebecid crocodilians.[52]
However, the role of competitive displacement in South American predator lineages has been questioned by some researchers.[53] The timing of turnover events and the decline of South American predators do not correlate well with the arrival of large carnivores like canids or sabretooths (although they do correlate well with the earlier-arriving procyonids, which evolved tolarge body size in South America, but these were omnivorous[54]), with native South American predator lineages (including most phorusrhacids and all sparassodonts and sebecids) dying out well before the arrival of most larger placental carnivores.[55]Bathornithids, which were similar in ecology and are likely close relatives of phorusrhacids, existed entirely within North America during part of the Cenozoic and competed successfully for a time with large carnivorans such asnimravids,[17] before becoming extinct in the Early Miocene, about 20 million years ago. The phorusrhacidTitanis expanded northward into southern North America during the Interchange and coexisted for several million years with large canids and big cats likeXenosmilus, before its extinction about 1.8 million years ago. Paleohistological analysis further refutes competitive replacement, as their uninterrupted growth patterns contrasts that of birds that inhabit islands or well adapted, stable ecosystems with a lack of strong predation pressure. So the authors concluded that their extinction due to environmental conditions.[56]
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 onTitanis specimens show that the last phorusrhacids went extinct over one million years before humans arrived.[9] However, several fossil finds of smaller forms have been described from the late Pleistocene ofUruguay in South America.Psilopterus may have been present until 96,040 ± 6,300 years ago (maximum age obtained from the bottom of the fossil-containing stratum), which would extend the existence of the smaller members of this group of avian predators considerably.[2] Another unidentified smaller type which may be a possible psilopterine[2] from the La Paz Local Fauna ofUruguay has also been dated to the late Pleistocene, perhaps 17,620 ± 100 years ago based on radiocarbon analysis usingaccelerator mass spectrometry (AMS) for the molar enamel samples of a proboscidean from the same site,[57] but the validity of this previous radiocarbon dating has been considered highly questionable due to the enamel's lack of collagen;[58] the tibia ofMacrauchenia 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.[59]
^It has been suggested that psilopterines likePsilopterus may have been able to fly briefly in a clumsy manner, primarily to reach the treetops for nesting and protection, on the basis of body mass estimates and hindlimb proportions being similar to those of certain birds likePsophia andOtis which often walk but are able to run and fly.[4][1]
^To be specific, this is the maximum age obtained from the bottom of the fossil-containing stratum.[2]
^abcAcosta Hospitaleche, C.; Jones, W. (2024). "Insights on the oldest terror bird (Aves, Phorusrhacidae) from the Eocene of Argentina".Historical Biology: An International Journal of Paleobiology:1–9.doi:10.1080/08912963.2024.2304592.S2CID267475903.
^Ameghino, F (1889). "Contribuición al conocimiento de los mamíferos fósiles de la República Argentina" [Contribution to the knowledge of fossil mammals in the Argentine Republic].Actas Academia Nacional Ciencias de Córdoba (in Spanish).6:1–1028.
^Degrange, F.J. (2015). "Hind limb morphometry of terror birds (Aves, Cariamiformes, Phorusrhacidae): functional implications for substrate preferences and locomotor lifestyle".Earth and Environmental Science Transactions of the Royal Society of Edinburgh.106 (4):257–276.Bibcode:2015EESTR.106..257D.doi:10.1017/S1755691016000256.hdl:11336/44728.
^abcMacFadden, Bruce J.; Labs-Hochstein, Joann; Hulbert, Richard C.; Baskin, Jon A. (2007). "Revised age of the late Neogene terror bird (Titanis) in North America during the Great American Interchange".Geology.35 (2):123–126.Bibcode:2007Geo....35..123M.doi:10.1130/G23186A.1.S2CID67762754.
^abcMayr, G. (2017). "Cariamiforms and Diurnal Birds of Prey".Avian Evolution: The Fossil Record of Birds and its Paleobiological Significance. Chichester: Wiley-Blackwell. pp. 189–204.ISBN978-1119020769.
^abMayr, G. (2022). "Accipitriformes (New World Vultures, Hawks, and Allies), Falconiformes (Falcons), and Cariamiformes (Seriemas and Allies)".Paleogene Fossil Birds. Fascinating Life Sciences. Springer Cham. pp. 153–176.doi:10.1007/978-3-030-87645-6_8.ISBN978-3-030-87644-9.
^abcLaBarge, T. W.; Gardner, J. D.; Organ, C. L. (2024). "The evolution and ecology of gigantism in terror birds (Aves, Phorusrhacidae)".Proceedings of the Royal Society B: Biological Sciences.291 (2021). 20240235.doi:10.1098/rspb.2024.0235.PMC 11040249.PMID38654650.Supplementary Information
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^Bertelli, Sara; Chiappe, Luis M; Tambussi, Claudia (2007). "A New Phorusrhacid (Aves: Cariamae) from the Middle Miocene of Patagonia, Argentina".Journal of Vertebrate Paleontology.27 (2):409–419.doi:10.1671/0272-4634(2007)27[409:ANPACF]2.0.CO;2.S2CID85693135.
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^King, Logan; Barrick, Reese (October 2016).Semicircular canal shape within Aves and non-avian Theropoda: Utilizing geometric morphometrics to correlate life history with canal cross-sectional shape. Society of Vertebrate Paleontology 76th Annual Meeting At: Salt Lake City, Utah, United States.
^Ameghino, Florentino (1936). Torcelli, A.J. (ed.).Obras completas y correspondencia cientifica de Florentino Ameghino. Vol. 21. La Plata: Taller de Impresiones Oficiales. p. 573.
^Nasif, Norma L.; Esteban, Graciela I.; Ortiz, Pablo E. (2009). "Novedoso hallazgo de egagrópilas en el Mioceno tardío, Formación Andalhuala, provincia de Catamarca, Argentina".Serie Correlación Geológica.25 (105–114).
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^abFederico J. Degrange; Claudia P. Tambussi; Matías L. Taglioretti; Alejandro Dondas; Fernando Scaglia (2015). "A new Mesembriornithinae (Aves, Phorusrhacidae) provides new insights into the phylogeny and sensory capabilities of terror birds".Journal of Vertebrate Paleontology.35 (2): e912656.Bibcode:2015JVPal..35E2656D.doi:10.1080/02724634.2014.912656.hdl:11336/38650.S2CID85212917.
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