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Phorusrhacos

Phorusrhacos (meaning "bearer of scars" or "bearer of wrinkles") is an extinct genus of phorusrhacid ("terror bird"), an extinct group of predatory, typically large and flightless birds. It lived during the Early to Middle Miocene epoch (20-13 mya) of the Neogene in what is now southern Argentina. The first fossil known to science, an isolated mandible (lower jaw) fragment, was unearthed in 1887 from sediments of the Santa Cruz Formation near the Santa Cruz River in Santa Cruz Province, Argentina by Argentine paleontologist Carlos Ameghino. Later that year, Florentino, Carlos' brother, named the new genus and species Phorusrhacos longissimus based on it. Ameghino stated it was a type of giant, toothless sloth that belonged to its own family, Phorusrhacidae. However, his scientific rival, Argentine naturalist Francisco Moreno, pointed out that it was actually a bird. Since its 1887 description, Phorusrhacos has become one of the most well-known and common phorusrhacids, with fossils known from sites throughout southern Argentina.

Phorusrhacos
Skeletal reconstruction, with known remains of MPEF-PV 11355-56 in white
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Cariamiformes
Family: †Phorusrhacidae
Subfamily: †Phorusrhacinae
Genus: †Phorusrhacos
Ameghino, 1887
Type species
†Phorusrhacos longissimus
Ameghino, 1887
Synonyms
Genus synonymy
  • Callornis Ameghino, 1895
  • Darwinornis Moreno & Mercerat, 1891
  • Eucallornis Ameghino, 1901
  • Liornis Ameghino, 1895
  • Owenornis Moreno & Mercerat, 1891
  • Phororhacos Ameghino, 1889
  • Stereornis Moreno & Mercerat, 1891
  • Titanornis? Mercerat, 1893
Species synonymy
  • Callornis giganteus Ameghino, 1895
  • Darwinornis copei Moreno & Mercerat, 1891
  • Darwinornis socialis Moreno & Mercerat, 1891
  • Darwinornis zittelli Moreno & Mercerat, 1891
  • Eucallornis giganteus Ameghino, 1901
  • Liornis minor Dolgopol de Saez, 1927 (in partim)
  • Mesembriornis quatrefragesi Moreno & Mercerat, 1891
  • Mesembriornis studeri Moreno & Mercerat, 1891
  • Owenornis affinis Moreno & Mercerat, 1891
  • Owenornis lydekkeri Moreno & Mercerat, 1891
  • Phororhacos longissimus Ameghino 1889 (lapsus calami)
  • Phororhacos platygnathus Ameghino 1891
  • Phororhacos sehuensis Ameghino, 1891
  • Phororhacos shenensis Ameghino, 1891
  • Stereornis gaundryi Moreno & Mercerat, 1891
  • Stereornis rollieri Moreno & Mercerat, 1891
  • Titanornis mirabilis? Mercerat, 1893

Phorusrhacos is estimated to have been 1.4–1.9 meters (4.6–6.2 ft) tall and weighed 93 kilograms (205 lb), making it among the largest phorusrhacids known. However, it was more lithe and curisorial-adapted (built for running) than its large, robust relatives Kelenken and Devincenzia. As a phorusrhacid, it had an elongated, tall skull with a sharp-tipped beak. Its cervical (neck) vertebrae are short, thick, and wide, likely an adaptation for striking prey. In anterior (front) view, the skull is laterally compressed (thin from left to right). Its hindlimbs are long and slender; the tarsometatarsus measures around 70% of the length of the tibiotarsus (shin bone) and has a thin shaft. During the Miocene, Santa Cruz Province was blanketed in savanna-like grasslands, shrublands, and woodlands. Due to its isolation from other continents, South America developed a unique mammalian, avian, and reptilian fauna prior to the Great American Interchange. Phorusrhacos filled the apex predator role in Santacrucian ecosystems, likely preying on mammals like ground sloths, notoungulates, and litopterns.

The taxonomy of Phorusrhacos bears a long, convoluted history. Many genera, like Owenornis and Stereornis, and species, like Mesembriornis studeri, that are synonyms of P. longissimus. Others, like P. affinis and P. pozzi, are now thought to represent other genera. Additionally, a scientific rivalry between Florentino Ameghino and Francisco Moreno in the 1880s-1900s led to many of these genera and/or species to be described on the basis of isolated, fragmentary remains. Most recent studies have reduced Phorusrhacos to a single species: P. longissimus.

Discovery and naming

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Initial description

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Illustration of the holotype mandible
Illustration of the holotype (MLP-118) from Ameghino (1891)
Photograph of the holotype
Photograph of the holotype from Degrange and colleagues (2019)

The taxonomic history of Phorusrhacos is muddled on account of the fragmentary nature of fossils, few of which overlap with others, little consensus between researchers, and competition between scientists.[1][2][3] In the 19th century, interest in the rich "Tertiary" (now Neogene) fossil sites of Argentina boomed.[2] This included exploration by two teams of Argentine paleontologists; Florentino and Carlos Ameghino who operated independently, and Francisco Pascasio Moreno and Alcides Mercerat of the Museo de La Plata (MLP).[4][5] Their discoveries comprised thousands of fossils from Cenozoic fossil deposits throughout Patagonia and central Argentina.[6][7][8] Initially, Moreno hired Florentino and Carlos Ameghino as staff at the MLP in 1887; Florentino operated as a researcher and Deputy Director, while his brother conducted expeditions and assisted with preparation.[9][10] However, the relationship between Moreno and Florentino Ameghino deteriorated, culminating in the Ameghinos' resignation from the MLP in 1888.[5][11] Operating independently of any institutions until 1902, the Ameghinos competed with Moreno and the Museo de La Plata to discover and describe as many new fossils as they could. This led to the description of many genera and species based on fragmentary, undiagnostic, or poorly described material.[1][4]

The first known discovery of Phorushracos was made in 1887 by Carlos Ameghino during an expedition to an outcrop of the Miocene-aged Santa Cruz Formation along the Santa Cruz River in Santa Cruz Province, Argentine Patagonia.[5][8][10] This formation dates to the Santacrucian South American Land Mammal Age (a series of mammalian fauna-based geologic ages), which lasted from 22 to 16 million years ago, and preserves a diverse array of endemic birds, mammals, reptiles, and more.[4][12] This trip alone produced hundreds of fossils, which Florentino described by 1887 as coming from over 120 novel species of fossil mammals.[8] The descriptions produced by Florentino were short, vague, and sought to establish priority for new taxa before his competitor, Moreno, could. Along the Santa Cruz River, Carlos unearthed an isolated, incomplete edentulous (toothless) mandible (lower jaw), which was later deposited at the MLP.[13] Florentino believed this fossil belonged to a new genus and species of edentate mammal, around the size of Mylodon, which he named Phorusrhacos longissimus.[5][8] This mandible (specimen number MLP-118) was chosen as the holotype (name-bearing) specimen of P. longissimus.[13] The etymology of Phorusrhacos is convoluted. Ameghino did not explain the etymology of the generic name Phorusrhacos or the specific name longissimus. The former is derived from the Greek -φόρος, (-phoros), a word meaning "bearer" in word combinations, and ῥάκος, (rhakos), "rag" or "wrinkle", probably in reference to the wrinkled jaw surface of the holotype. This creates the literal meaning "wrinkle bearer (jaw)" for Phorusrhacos.[14] When the original derivation was no longer understood, other translations were given, such as the literal translation of "Rag-Thief",[15][16] and "branch-holder" from the mistaken assumption that the name had been intended to be derived from a Greek rhakis, "branch".[14][17] The specific name means "very long" in Latin, again in reference to the holotype.[14]

Competition

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Portrait of Florentino Ameghino
Portrait of Florentino Ameghino, the describer of Phorusrhacos
Portrait of Francisco Moreno
Portrait of Francisco Moreno, Ameghino's competitor

In 1889, Ameghino published a detailed monograph on Argentine fossil mammals, including Phorusrhacos. Here, he went into greater detail on its anatomy and classification, as well as attempted to amend the generic name to Phororhracos.[7]: 659–660  However, the original name Phorusrhacos has priority.[18][19] Despite this, Phororhacos continued to be used for decades[18][20] until 1967, when American paleontologist Pierce Brodkorb corrected its use.[19] Ameghino (1891) also erected the family Phororhacosidae (now Phorusrhacidae) and classified it in Edentata. Not knowing it was avian, Ameghino theorized that Phorusrhacos had a bird or turtle-like keratin sheath on its jaws.[5][7]

At the same time as Ameghino's discoveries, Moreno and the MLP sent out its own teams to explore the Santacrucian deposits of Patagonia. In 1888, Moreno described the discoveries made in Monte Hermoso by these expeditions, where he briefly mentioned that "large birds" had been found there.[21] The next year, he classified them as belonging to two new genera of lamellirostral web-footed birds: Mesembriornis and Palaeociconia.[10][22] Though these fossils do not belong to Phorusrhacos.[5][21] This led Carlos Ameghino to reconsider the classification of Phorusrhacos; in a 1890 letter to his brother, he proposed that Phorusrhacos is a genus of bird based on Moreno's discoveries, making Carlos the first to recognize Phorusrhacos' avian affinities. However, Florentino reaffirmed that it was mammalian in his response.[5]

In 1891, Moreno and Alcides Mercerat published a monograph describing many avian fossils from the Tertiary of Argentina. They named several genera and species, such as Owenornis and Stereornis, based on fragmentary material. Furthermore, they grouped many bird taxa in a new order called Stereornithes, which is no longer in use.[3][23] Unlike Ameghino's work, this monograph features figures and illustrations, giving a better impression of the anatomy of "stereornithine" birds. This featured a figure of the P. longissimus holotype, which they misinterpreted as being a premaxilla (front jaw bone), and assigned to a bird rather than an edentate mammal.[6] Later studies have considered Stereornithes a synonym of Phorusrhacidae. Many of the genera and species described in the paper are synonyms of P. longissimus,[11][13] though some like Titanornis are of indeterminate status.[3]

Known elements of NHMUK-A529, including Carlos Ameghino's sketch of the destroyed skull

In 1891, Ameghino named the genus and species Tolmodus inflatus, which he believed was a ground sloth too. However, it is now in its own genus, Patagornis. That same year, Carlos unearthed a poorly preserved skull and mandible (NHMUK-A529) of Phorusrhacos from a locality in Santa Cruz. This skull was noted to measure 69 centimetres (27 in) in length in the field, it soon fragmented, leaving only the mandible, beak tip, and a fragment from the supraorbital (above orbital bone) process (an extension of bone) of the frontal (skull roof bone) to be recovered.[24] Following its discovery, Ameghino gave a comprehensive description of Phorusrhacos and corrected his previous error by classifying it as a bird. In this paper, Ameghino figured and described the holotype of P. longissimus in detail, assigned Tolmodus inflatus to Phorushracos, and named more Phorusrhacos species.[25] In 1895, Florentino described the mandible in detail and noted the discovery of the fragmented skull, including an illustration. Additionally, the paper had illustrations and detailed analysis of many P. longissimus fossils for the first time, including vertebrae and limb bones.[26]

In response to Moreno and Mercerat (1891), Ameghino published a criticism of their work in which he stated that many of the species they had named were synonyms of Phorusrhacos (including Mesembriornis and Patagornis), and were inadequately described. In the same paper, Ameghino classified Phorusrhacidae, as he did not recognize Stereornithes, within Ratites, a group including other giant fossil birds like Aepyornis and Dinornis.[27] Ameghino went on to claim that Carlos was the first to recognize the existence of giant birds in the Tertiary of South America,[2] when in actuality Moreno had in 1889,[22][5] two years before Carlos did.[5][28] In 1895 and 1896, Ameghino and Moreno sold much of their respective fossil bird collections to the Natural History Museum of London.[5] By 1911, Ameghino had died and Moreno had retired, leaving behind a legacy of convoluted phorusrhacid taxonomy.[2][29] Many European naturalists did not study Ameghino's or Moreno's taxa, especially their phorusrhacids, due to their confusing taxonomy.[1]

Later study

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American expeditions

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Illustration of the AMNH 9497, limb bones of Phorusrhacos
Phalanges of Phorusrhacos
Remains of P. longissimus unearthed by the Princeton University expeditions, including hindlimb (AMNH 9264; lef)) and pes (AMNH 9497; left) material

At the close of the 19th century, American universities and institutions began taking an interest in the Neogene fauna of Patagonia. Inspired by Ameghino's papers and descriptions, these universities and institutions sought to establish their own collections of Santacrucian mammals and birds, including those of Phorusrhacos.[10][30][31] Between 1896 and 1899, Princeton University students, under the leadership of American paleontologist John Bell Hatcher led a series of expeditions to the same outcrops in Santa Cruz Province. Princeton was joined by the American Museum of Natural History on the last of these expeditions, altogether collecting a trove of new fossil bird and mammal material. Furthermore, in 1902, American professor William B. Scott ventured to the MLP and Ameghino's collections to photograph and review the taxa named prior by Ameghino, Moreno, and Mercerat. His studies, along with those of Hatcher, American paleontologist William John Sinclair, and others, culminated in a series of monographs that were published between 1903 and 1912 by Princeton University.[32][33] Although they reviewed Ameghino's mammal taxa in detail, their description of some of the phorusrhacid remains was brief.[33] Meanwhile, in 1926, American paleontologist Marcus Farr traveled to the Natural History Museum of London in order to study the Ameghino collection, with the assistance of Dr. F. A. Bather, the museum's curator.[2]

In 1932, Sinclair and Farr described the newly found phorusrhacid material in detail, assigning much of it to Pelecyornis (a synonym of Psilopterus) and Phorusrhacos. In their study, they briefly touched on the convoluted taxonomy of the clade, stating that there were likely too many genera and species of stereornithes named. Furthermore, Sinclair and Farr noted that Phorusrhacidae may be a paraphyletic clade,[2] a conclusion not supported by later authors.[11][34] From a locality 10 miles north of Cape Fairweather, Santa Cruz, members of the expedition unearthed an associated postcranial skeleton from a phorusrhacid (AMNH 9264) consisting of an incomplete pelvis, right femur (thigh bone), tibiotarsus (shin bone), tarsometatarsus, and pes (foot). Sinclair and Farr (1932) did not assign it to a specific species, instead referring to it simply as Phororhacos sp.,[2] while Alvarenga and Höfling (2003) assigned it to Patagornis marshi.[11] Another specimen, AMNH 9497, was provisionally considered Phororhacos sp.[2] until Alvarenga and Höfling (2003) identified it as P. longissimus. Other remains collected by the expedition, such as a nearly complete left tarsometatarsus (AMNH 9146), have been assigned to P. longissimus as well.[11] One specimen, an incomplete left humerus (deposited at the Yale Peabody Museum under YPM-PVVU 15866) was initially assigned to P. sp. but is now the holotype of Dryornis hatcheri, a cathartid.[35]

Tarsometatarsus of Liornis floweri, a synonym of P. longissimus named by Dolgopol de Sáez (1927)

Revision

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In 1927, Argentine paleontologist Mathilde Dolgopol de Sáez published a revision of Argentine phorusrhacid taxa named by Ameghino, Moreno, and Mercerat. Here, she described a new genus and species of phorusrhacid Liornis floweri, along with Liornis minor. Additionally, she considered all other phorusrhacids named up to that point, besides Brontornis, to be synonyms of Phorusrhacos. Dolgopol de Sáez believed the genus had five valid species: P. longissimus, P. sehuensis, P. milne-edwarsi, P. inflatus, and P. delicatus.[23] A 1967 paper by American paleontologist Pierce Brodkorb greatly revised the taxonomy of Phorusrhacidae. Here, he recognized that Phorusrhacos, not Phororhacos, was the valid generic name. Furthermore, Brodkorb reduced the genus to P. longissimus, synonymizing fourteen species with it. Many of Brodkorb (1967)'s conclusions have been supported by later studies.[3][11]

In 2019, Argentine paleontologist Federico Degrange and colleagues described an associated skull roof, rostrum, and incomplete mandible (deposited at the Museum of Paleontology Egidio Feruglio under MPEF-PV 11356) that had been unearthed from an outcrop of the Santa Cruz Formation southeast of Lago Belgrano in Santa Cruz Province. This site lies near the Chilean border and constitutes the westernmost discovery of Phorusrhacos yet published. Nearby at another site, Puesto Estancia La Costa, an isolated dorsal (back) vertebra (MPEF-PV 11355) was collected as well. This vertebra was provisionally assigned to Phorusrhacinae indet., but likely comes from Phorusrhacos itself. MPEF-PV 11356 was the first discovery of relatively complete Phorusrhacos skull material since 1891 and has provided a detailed look at the anatomy of the taxon.[13] Two years later, Argentine paleontologist Sergio Vizcaíno and colleagues described several fossils, including an ungual phalanx from P. longissimus, from the early-mid Miocene-aged Cerro Boleadoras Formation in Meseta del Lago Buenos Aires in Santa Cruz Province, Argentina. This was the first discovery of Phorusrhacos remains from this formation and expanded the known distribution of the taxon.[36]

Species

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Image of fossils of Phorusrhacos sehuensis
Tarsometatarsus of Callornis giganteus
Tibia of Callornis giganteus
Fossils from the holotype specimens of Phororhacos sehuensis (top) and Callornis giganteus (bottom), junior synonyms of P. longissimus

Synonyms of P. longissimus

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  • Callornis giganteus was described by Ameghino (1895) on the basis of a fragment of the left tarsometatarsus (MLP 176) that had been found in the Santa Cruz Formation of Argentina.[26] It was reclassified as Eucallornis giganteus by Ameghino (1901) based on a false notion that Callornis was preoccupied.[37] Until a comprehensive 2016 study by Buffetaut, C. giganteus was thought to be a synonym of P. longissimus. The paper claimed that it was a chimera (a mix of fossils from multiple species) instead, with the tarsometatarsus belonging to P. longissimus while the tibiotarsus comes from a brontornithid.[38]
  • Darwinornis copei was described by Moreno and Mercerat (1891) on the basis of the distal end of a left tibia, a tarsometatarsus fragment, and unidentified fragments (MLP 20-171-173) that had been found in the Santa Cruz Formation of Argentina.[6] It was considered a synonym of P. longissimus by Brodkorb in 1967,[19] an assessment that other authors have followed.[13][11]
  • Darwinornis socialis was described by Moreno and Mercerat (1891) on the basis of a fragment of the left tarsometatarsus (MLP 20-176) that had been found in the Santa Cruz Formation.[6] It was considered a synonym of P. longissimus by Brodkorb in 1967,[19] an assessment that other authors have followed.[13][11]
  • Darwinornis zittelli was described by Moreno and Mercerat (1891) on the basis of right tibiotarsus and tarsometatarsus fragments (MLP 20-174-175) that were collected from the Santa Cruz Formation.[6] It was considered a synonym of P. longissimus by Brodkorb in 1967,[19] an assessment that other authors have followed.[13][11]
  • Liornis minor was described by Dolgopol de Sáez (1927) on the basis of the distal end of a tibiotarsus that had been found in the Santa Cruz Formation of Argentina. Dolgopol de Sáez considered Liornis to be related to Brontornis,[23] although, Alvarenga and Höfling (2003) stated that the species is a synonym of P. longissimus.[11] This conclusion has been supported by other studies.[38][19]
  • Mesembriornis quatrefragesi was described by Moreno and Mercerat (1891) on the basis of a humerus fragment, the proximal end of the left ulna, the distal end of the right tibia, the trochlea of the right tarsometatarsus, and an ungual phalanx (MLP 135-139) that had been found in the Santa Cruz Formation of Argentina.[6] It was considered a synonym of P. longissimus by Brodkorb in 1967,[19] an assessment that other authors have followed.[11][13]
  • Fossils of Owenornis and Darwinornis, synonyms of P. longissimus
    Skull of Kelenken
    Fossils of Darwinornis, Owenornis (top), Mesembriornis studeri, M. quatrefagesi, (left) and Stereornis (right), junior synonyms of P. longissimus, as figured in Moreno and Mercerat (1891)
    Mesembriornis studeri was described by Moreno and Mercerat (1891) on the basis of a fragmentary skull and skeleton (MLP 122-134) that had been found in the Monte Leon Formation of Argentina.[6] It was considered a synonym of P. longissimus by Brodkorb in 1967,[19] an assessment that other authors have followed.[11][13][39] Due to this, in 1914 Italian geologist Gaetano Rovereto erected the new genus name Hermosiornis for "M". milneedwardsi, while M. studeri, the type species of Mesembriornis, was left as a synonym of P. longissimus. However, Alvarenga and Hölfing (2003) argued for the continued use of the name,[11] which has seen some recognition.[40][41][42] However, other studies have pointed out this discrepancy and opted to use Hermosiornis instead.[34][43][44][29]
  • Owenornis affinis was described by Moreno and Mercerat (1891) on the basis of a fragment of the right tarsometatarsus (MLP 177) that had been found in the Santa Cruz Formation of Argentina.[6] It was considered a synonym of P. longissimus by Brodkorb in 1967,[19] an assessment that other authors have followed.[13][11]
  • Owenornis lydekkeri was described by Moreno and Mercerat (1891) on the basis of fragmentary vertebrae, a right tibia fragment, and fragments of both tarsometatarsi (MLP 178-182) that had been found in the Santa Cruz Formation of Argentina.[6] It was considered a synonym of P. longissimus by Brodkorb in 1967,[19] an assessment that other authors have followed.[13][11]
  • Phororhacos platygnathus was described by Ameghino (1891) on the basis of a fragment of the left tarsometatarsus and an incomplete mandibular symphysis (MLP 176) that had been found in the Santa Cruz Formation of Argentina.[27] It was considered a synonym of P. longissimus by Brodkorb in 1967,[19] an assessment that other authors have followed. An isolated mandibular symphysis fragment was assigned to P. platygnathus by Uruguayan naturalist Lucas Kraglievich in 1931; however, it has been assigned to the other phorusrhacid Devincenzia pozzi.[11]
  • Phororhacos shenensis was described by Ameghino (1891) on the basis of a mandibular symphysis fragment, portions of the femur, and parts of the tarsometatarsus that had been discovered in the Santa Cruz Formation of Argentina.[25] It was considered a synonym of P. longissimus by Brodkorb in 1967,[19] an assessment that other authors have followed.[11][13]
  • Phororhacos sehuenensis was described by Ameghino (1891) on the basis of a right mandible, incomplete vertebrae, a right tibia fragment, and fragments of both tarsometatarsi (MLP 178-182) that had been found in the Santa Cruz Formation of Argentina.[27] It was considered a synonym of P. longissimus by Brodkorb in 1967,[19] an assessment that other authors have followed.[11][13]
  • Stereornis gaudryi was described by Moreno and Mercerat (1891) on the basis of the proximal ends of a left tarsometatarsus (MLP 121) that had been found in the early Miocene-aged Monte Leon Formation of Argentina.[6] It was considered a synonym of P. longissimus by Brodkorb in 1967,[19] an assessment that other authors have followed.[13][11]
  • Stereornis rolleri was described by Moreno and Mercerat (1891) on the basis of the proximal ends of a left tibia and a left tarsometatarsus (MLP 119- 120) that had been found in the Santa Cruz Formation of Argentina.[6] It has been considered a synonym of P. longissimus by various authors.[26][19][11]
  • Titanornis mirabilis was described by Mercerat (1893) on the basis of a large, associated skeleton including much of the rostrum, the left tarsometatarsus, and the distal end of a tibiotarsus (MLP 176) that had been found in the Santa Cruz Formation of Argentina.[45] It was considered a synonym of P. longissimus by Brodkorb in 1967,[19] an assessment that other authors have followed.[11][13] However, Agnolín (2009) argued that it would be over a third larger than the typical P. longissimus and bears a rostrum more comparable to that of the other phorusrhacid Kelenken. Furthermore, he stated that it cannot be a synonym of P. longissimus on account of its elongated distal (away from body) tarsometatarsus, again reminiscent of Kelenken. As a result, Agnolín regarded it as a nomen dubium, partly due to the loss of its holotype.[3]

Reclassified species

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  • Liornis floweri was described by Ameghino (1895) on the basis of a distal tibiotarsus, distal tarsometatarsus, and two pedal phalanges of a juvenile individual. This material was unearthed on an expedition to a deposit of the Santa Cruz Formation in Santa Cruz Province, Argentina.[26] Dolgopol de Sáez (1927) argued that Liornis is related to Brontornis,[23] whereas Alvarenga and Höfling (2003) stated that it is a synonym of P. longissimus instead.[11] This was cleared up by Buffetaut (2016), who followed the opinion of Brodkorb (1967)[19] and synonymized L. floweri with Brontornis burmeisteri.[38]
  • Phororhacos affinis was described by Ameghino (1899) on the basis of an isolated tarsometatarsus (MACN-A-52-184) that was collected from an outcrop of the Oligocene-aged Guaranitica Formation in Chubut Province, Argentina.[46] It is now considered a species of Psilopterus.[11][3]
  • Phororhacos daeutieri was named by Kraglievich (1931) on the basis of an incomplete femur that was recovered from rock layers dating to the Miocene from the Ituzaingó Formation, in Entre Ríos Province, Argentina.[47] In their 1960 description of the phorusrhacid Andalgalornis, American paleontologist Bryan Patterson and Kraglievich provisionally moved P. daeutieri to Andalgalornis,[48] despite the former coming from a site 900 kilometers (560 mi) away.[34][43] Nonetheless, Alvarenga and Höfling (2003) stated that A. daeutieri is a synonym of A. steulleti. This has seen widespread usage,[13][49] however studies like Agnolín (2013) have stated that A. ferox is valid whereas A. daeutieri is a nomen dubium.[43]
  • Phororhacos delicatus was described by Ameghino (1891) on the basis of a dorsal vertebra and a few fragmentary limb elements that were recovered from several localities in the Santa Cruz Formation in Santa Cruz, Argentina. It was considered a synonym of Psilopterus communis by Brodkorb (1967)[19] and Alvarenga and Höfling (2003),[11] whereas Agnolin (2009) considered it a synonym of Psilopterus bachmanni.[3]
  • Image of fossils of Phorusrhacos delicatus from top view
    Tarsometatarsus of Liornis floweri
    Tibia of Callornis giganteus
    Fossils from the holotype specimens of Phororhacos sehuensis (top) and Callornis giganteus (bottom), junior synonyms of P. longissimus
    Phororhacos inflatus was initially described by Ameghino (1891) as Tolmodus inflatus on the basis of a premaxilla fragment found in the Santa Cruz Formation of Argentina.[24] Andrews (1899) moved it to Phororhacos and assigned several well-preserved specimens, including a complete skull, to the species.[20] Due to this, P. inflatus fossils formed the basis of many Phorusrhacos depictions, though it has since been considered a synonym of Patagornis marshi.[11][50]
  • Phororhacos pozzi was named by Kraglievich (1931) on the basis of a partial tarsometatarsus and ungual phalanx (MACN-6554 and 6681) that had been found in Pliocene-aged strata in the El Brete site in Córdoba, Argentina.[47] The Uruguayan phorusrhacid Devincenzia gallinali was later considered a synonym of P. pozzi, creating the binomial Devincenzia pozzi.[11][43] However, other studies like Agnolín (2021) have argued that the stratigraphic and geographic separation of D. gallinali and D. pozzi indicate that the latter actually belongs to the genus Onactornis.[34][51]
  • Phororhacos longissimus mendocinus was named by Kraglievich (1931) on the basis of a proximal femur fragment that had been found in Pliocene-aged rock layers of the Huayquerías Formation in the San Carlos Department of Mendoza, Argentina. In 1960, Patterson and Kraglievich moved it to the genus Onactornis,[19][48] whereas Alvarenga and Höfling (2003) synonymized it with D. pozzi.[11] However, Agnolín (2006) stated it was a nomen dubium instead and indeterminate beyond Phorusrhacinae.[29]
  • Phororhacos modicus was described by Ameghino (1895) on the basis of a humerus fragment, the proximal end of the left ulna, the distal end of the right tibia, the trochlea (a groove for articulation with the tibiotarsus) of the right tarsometatarsus, and an ungual phalanx (MLP 135-139) that had been found in the Santa Cruz Formation of Argentina. It was considered a synonym of Palaeociconia cristata, a synonym of Patagornis marshi, by Brodkorb in 1967,[19] however it is a synonym of Psilopterus lemoinei[11][49] or Psilopterus australis instead.[3]
  • Phororhacos steulleti was named by Kraglievich (1931) on the basis of an isolated pedal phalanx (toe bone) that was recovered from rock layers dating to the Miocene belonging to the Ituzaingó Formation, in the province of Entre Ríos, Argentina.[47] In their description of Andalgalornis, Patterson and Kraglievich (1960) provisionally moved P. steulleti to Andalgalornis, despite deriving from a potentially different stratigraphic layer.[34][43] Nonetheless, Alvarenga and Höfling (2003) stated that A. ferox is a synonym of A. steulleti, creating the binomial Andalgalornis steulleti. This saw widespread usage[52][53][54] however, studies like Agnolín (2013) have stated that A. ferox is valid, whereas A. steulleti is a nomen dubium.[43]

Classification

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Phorusrhacos is the type genus of the superfamily Phorusrhacoidea, family Phorusrhacidae and subfamily Phorusrhacinae.[11][18] Phorusrhacidae is an extinct group of flightless, carnivorous birds that occupied one of the dominant, large land-predator niches in South America from the lower Eocene[55] to the Pleistocene.[56] They dispersed into North America during the Great American Biotic Interchange (~3 Ma). Some remains from Africa[57] and Europe[58] and the Paleocene of Brazil have been referred to this clade or identified as phylogenetically related to the extant South American seriemas, but these assignments remain controversial.[13] Phorushracos was the first phorusrhacid to be discovered or named.[5][16] Prior to being recognized as a bird, it was placed in the group Edentata, an antique name for a clade believed to include mammals like sloths, pangolins, and armadillos. Phorusrhacidae was a clade created by Ameghino (1889), he mistakenly dubbed it Phororhacosidae and believed it was related to the anteater Myrmecophaga.[7]: 659–660  Moreno (1888) was the first to recognize the avian affinities of the phorusrhacids, referring to them as "large birds",[10][21] while Moreno and Mercerat (1891) erected the group Stereornithes to encompass them. They also stated that Stereornithes was a transitional stage between the Anatidae and the Vulturidae.[6] Furthermore, in their paper, Phorusrhacos was described as a bird for the first time in literature.[5] This clade is now considered synonymous with Phorusrhacidae due to the latter's seniority,[19][3] and it was not until Lydekker (1893) that Phorusrhacidae was moved to Neognathae.[1] In Andrews (1899) description of Patagornis fossils (then believed to come from Phorusrhacos), he assigned Stereornithes (=Phorusrhacidae) to Cariamae, the clade including the closest living relatives of the phorusrhacids, cariamas.[20]

The interrelationships of Phorusrhacidae are unclear,[11][34][3] partly due to their sparse fossil record.[59] Dolgolpol de Saez (1927), still using the term Stereornithes for Phorusrhacidae, stated that it was divided into two families: Phororhacidae (=Phorusrhacidae), which included genera like Phorusrhacos and Psilopterus, and Brontornithes, which included genera like Brontornis and Liornis.[23] Patterson and Kraglievich (1960) went on to break up Phororhacoidea (=Phorusrhacoidea)[18] into two families: Psilopteridae, which featured the subfamilies Psilopterinae and Hermosiornithinae, and Phororhacidae (=Phorusrhacidae), which featured Phororhacinae (=Phorusrhacinae) and Tolmodinae (=Patagornithinae).[48] It was not until Brodkorb (1967) that Phororhacidae was corrected to Phorusrhacidae in literature.[18] Furthermore, Brodkorb (1967) constricted Phorusrhacidae to the subfamilies Brontornithinae, Paleociconiinae, and Phorusrhacinae.[19] In 2003, Brazilian paleontologists Herculano Alvarenga and Elizabeth Höfling published a revision of the family in which they recognized six subfamilies, Brontornithinae, Mesembriornithinae, Patagornithinae, Phorusrhacinae, Physornithinae, and Psilopterinae, and defined each one.[11] In a separate study, Alvarenga and colleagues (2011) performed a phylogenetic analysis (a study of the evolutionary relationships between organisms), which recovered Patagornithinae, Phorusrhacinae, and Physornithinae in a polytomy (an unnatural grouping) (topology 1).[52] A later phylogenetic analysis by Degrange and colleagues (2015) found Phorusrhacidae to include Mesembrironithinae and Psilopterinae as distinct, while Patagornithinae, Phorusrhacinae, and Physornithinae is in polytomy (topology 2). Here, Phorusrhacos was united with genera previously included in Patagornithinae and Physornithinae,[11] while Devincenzia and Kelenken lay outside of the clade.[41]

Skulls of Phorusrhacinae and Andalgalornis (a possible phorusrhacine)

The subfamily Phorusrhacinae includes Phorusrhacos itself in addition to genera like Devincenzia (which is sometimes classified as distinct from Onactornis),[34][3] Kelenken, and Titanis.[11][34][59] Unnamed phorusrhacines have been described as well, including from the Pleistocene of Uruguay[44] and the Miocene of Colombia.[60] The name "Phorusrhacinae" was first used by Kraglievich (1931) and defined as including Phorusrhacos longissimus, Devincenzia pozzi, their descendents, and their most recent common ancestor.[47] Kraglievich characterized phorusrhacines by their large size, robust limbs, and cranial features observed in Devincenzia. Patterson and Kraglievich (1960) went on to include Phorusrhacos and Devincenzia in the clade, with Kelenken and Titanis added in later works.[11][3][61][62] Phorusrhacos is the oldest named, both historically and geologically, and sometimes the basalmost member of Phorusrhacinae,[52][34] while taxa like Devincenzia and Kelenken are typically recovered as sister taxa.[34][59] Alvarenga and Höfling (2003) limited Phorusrhacinae to Phorusrhacos, Devincenzia, Kelenken, and Titanis. They defined (the traits taxa have to be considered members) Phorusrhacinae by the presence of an overall size and gigantic build, more lithe traits than Brontornithinae (a clade which may not be phorusrhacid),[51] and a relatively elongated and slender tarsometatarsus, which is over 60% of the length of the tibiotarsus. Furthermore, a relatively long, narrow, and relatively dorsoventrally (top-bottom) short mandibular symphysis (the area where both mandibulae (lower jaws) meet) which measures over twice as long as the width of the base was considered a defining characteristic too.[11]

In contrast, Agnolín (2009) expanded Phorusrhacinae to include Phorusrhacos, Andalgalornis, Devincenzia, Kelenken, Titanis, and the tribe Physornithini, which is composed of Physornis and Paraphysornis. He diagnosed it by traits such as the presence of a tall, robust postorbital process (an extension on the quadrate that interlocked with the postorbital bone) and a subrectangular antorbital fenestra (a gap in the skull in front of the orbit). Agnolín's stance was backed by a phylogenetic analysis, which found Andalgalornis and Phorusrhacos to be sister taxa.[3][29] Similar to the results of Agnolín (2007), American paleontologists Thomas LaBarge and Chris Organ and British paleontologist Jacob Gardner (2024) recovered a broken up Phorusrhacinae, with only Titanis and Phorusrhacos being phorusrhacines, in their study. Although their analysis found Kelenken and Devincenzia as closer to Physornithi, La Barge, Organ, and Gardner (2024) stated that this may be a result of parallel evolution of robust traits, while Phorusrhacos was more gracile and cursorial. This led them to continue to include Devincenzia and Kelenken in the subfamily despite the results.[59] The cladogram below shows the position of Phorusrhacos according to an analysis by Agnolín and colleagues (2025):[34]

Description

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Size of Phorusrhacos (dark orange, furthest to the right), compared to a human and other phorusrhacids (Kelenken, Titanis, Devincenzia)

Phorusrhacids were large, flightless birds with long hind limbs, narrow pelvises, proportionally small wings, and huge skulls, with a tall, long, sideways compressed hooked beak.[13] Evidence of elongated quill-feathers is known from Patagornis and Llallawavis, with large tubercles called quill knobs present on their ulnae. These quill knobs would have supported long flight feathers.[20][41] Phorusrhacos was a large but lightly built phorusrhacid.[11][59] According to Alvarenga and Höfling (2003), Phorusrhacos was equivalent in size to Titanis,[11] which has been placed at 1.4–1.9 metres (4.6–6.2 ft) in height.[63] Meanwhile, American paleontologist Darin A. Croft gave a higher estimate of 2.4 meters (7.9 ft) tall and 130 kilograms (290 lb) in weight, around the size of a male ostrich (Struthio camelus).[64]: 120  In 2012, Degrange and colleagues estimated Phorusrhacos to have weighed 93 kilograms (205 lb) based on allometric calculations.[50]

Composite skull reconstruction including the holotype, MPEF-PV 11356, and NHMUK A529 (missing parts in grey)

Skull

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Until the description of MPEF-PV 11356, the skull of Phorusrhacos was largely known from Carlos Ameghino's sketch of the NHMUK A529. This skull differs from MPEF-PV 11356 in several areas, and it likely references a skull of Patagornis for the sketch. Otherwise, the skulls of large phorusrhacines like Phorusrhacos were based on Psilopterus and Patagornis, which are known from several, well-preserved skulls, until the discovery of Kelenken. NHMUK A529 had an estimated length of 65 centimeters (26 in) according to Ameghino (1895),[26] however the better preserved MPEF-PV 11356 has an estimated skull length of only 52 centimeters (20 in).[13]

MPEF-PV 11356 consists of much of the rostrum and part of the skull roof (top of skull) and orbit (eye cavity). Additionally, the well preserved internal structure of the cranium (posterior part of skull) bears the outline of the anteriodorsal (front top) section of the brain cavity (part of skull that stores the brain). This skull is dorsoventrally (top-down) tall, at 14.1 centimeters (5.6 in) as preserved and with a predicted peak height of 17.3 centimeters (6.8 in), and thin, at 6 centimeters (2.4 in) wide. The skull is dorsoventrally flat and wide, at 24 centimeters (9.4 in) in width, and triangular in shape from dorsal (top) view. This makes the skull proportionally shorter than that of Kelenken and more similar to those of patagornithines like Patagornis and Andalgalornis. Foramina (small pits in bone) and grooves cover the lateral faces of the rostrum. These foramina and grooves functioned as ophthalmic and nasopalatine nerve exits and are observed in many phorusrhacids. Unlike the straight premaxillary beak tips of Kelenken and psilopterines, Phorusrhacos' was slightly upturned on its ventral (bottom) margin close to the beak tip, as in patagornithines and some phorusrhacines.[13]

The skull of MPEF-PV 11356 in right lateral (side), dorsal (top) and ventral (bottom) views

The nares (nasal openings) are holorhinous (not split), as in Kelenken. These nares are overall anteroposteriorly (front-back) elongated, like in Kelenken, and curved, in contrast to the straight dorsal surfaces of Kelenken's. On the lateral faces of the premaxillae are large accessory fossae (depressions in bone), like in many phorusrhacids. Above the orbit is the supraorbital, which is large and separated from the dorsal rim, like in Devincenzia. This process (extension of bone) runs parallel to the roof of the cranium above the orbit in MPEF-PV 11356, however it is slightly deflected in the skull depicted in Ameghino (1895). Unique to Phorusrhacos, the postorbital process is slightly deflected ventrally (downward), unlike the steeply deflected processes found in Devincenzia and Kelenken. Inside the braincase are possibly impressions of the brain's vena, which are divided by a low ridge running medially (left to right). Adjacent to this ridge is an accessory fossa (a depression in bone) not found in psilopterines, mesembriornithines, or other phorusrhacines, but is preserved in Patagornis. However, this fossa is shallower in Patagornis than in Phorusrhacos.[13]

Several mandibulae, including the holotype, are known from P. longissimus. They vary in size and shape, with the longest measured (NHMUK A529) being 179 centimetres (70 in) long, 62 centimetres (24 in) tall, and 67 centimetres (26 in) wide whereas MLP 20-118 is only 160 centimetres (63 in) long, 53 centimetres (21 in) tall, and 67 centimetres (26 in) wide. Length/width measurements vary within P. longissimus specimens from 2.11 to 2.67, indicating that most specimens are shorter proportionately than phorusrhacids like Llallawavis and Patagornis. The symphysis (area where two halves of the lower jaws meet) found in MPEF-PV 11356 is robust and bears nerve openings for the rhamphotheca at its anterior tip. In contrast, NHMUK A529's symphysis bears an extremely thin border, measuring only 1 millimeter (0.039 in) thick. Its articular surface lies on a strongly upturned section of the ramus, while the section behind the ramus is deeply truncated.[26] As in all phorusrhacines and in contrast to physornithines, the mandibular symphysis is over twice as long as it is wide at its base. The mandibular fenestrae (openings in the lower jaw) of Phorusrhacos vary between individuals, with some like that of NMHUK A529 being a lot longer anteroposteriorly (front-back) than that of MPM-PV4241.[13]

Postcrania

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A dorsal vertebra (MPEF-PV 11355) possibly belonging to Phorusrhacos

In the postcrania, Phorusrhacos is distinguished from other phorusrhacids by; its tarsometatarsus length is about 70% of that of the tibiotarsus and the distal portion of the mid-trochlea of the tarsometatarsus is not as laterally grown as in Titanis. Overall, Phorusrhacos is smaller and lither than its relatives like Devincenzia and Kelenken.[11] The shaft of the tarsometatarsus is somewhat slender, with an almost rectangular mid-section, similar to Kelenken.[49] The tibiotarsus is distinguished by its narrow, deep intercondylar groove (a notch that articulates with the condyles of the tarsometatarsus). Overall, it is comparable to that of Patagornis yet differs in that it is anteroposteriorly flatter.[26] Ameghino (1895) stated that the tarsometatarsus of P. longissimus is overall extremely similar to that of Patagornis except for its size. However, the distal end of P. longissimus' is proportionally wider and flatter with a lower posterior fossa (the area where the largest pedal phalange articulates with the tarsometatarsus) than in Patagornis. Phorusrhacos is one of few phorusrhacines known from pes material,[11] which was briefly described by Ameghino (1895). The pedal ungual on the third digit is highly arched, sharp, and has an extensive basilar tubercule (a rounded bony growth for articulation with the phalanges) on its posterior side. Its entire third digit measures over 25 centimeters (9.8 in) long.[26]

Ameghino (1895) briefly described the cervical vertebrae (neck bones) of Phorusrhacos, stating that they were "larger those of a horse". One of the first cervicals measures 10 centimeters (3.9 in) long, with a peak transverse diameter of 13 centimeters (5.1 in). A dorsal vertebra he referred to P. sehuensis, a synonym of P. longissimus, was described as well. Its body is short, thick, very wide anteriorly, and truncated posteriorly. Its centrum (body of the vertebra) is 6 centimetres (2.4 in) in length with a V-shaped notch on its posterior face. Overall, it has concave lateral surfaces[26] as in other phorusrhacids[13] and an elliptical articular region for the ribs.[26]

MPEF-PV 11355, a 2nd or 3rd dorsal vertebrae potentially belonging to Phorusrhacos, was described in detail by Degrange and colleagues (2019). The vertebra overall is deeply mediolaterally compressed (thin from left and right sides), giving it a relatively tall and narrow shape when compared to those of Mesembriornis. Overall, the neural spine (a growth from the top of the vertebra) is relatively longer than that of Mesembriornis and bears extreme muscle scars on its anterior and posterior surfaces, as in Titanis and Andalgalornis. In anterior view, the anterior articular surface of the vertebra is wide and saddle-shapped with a low keel on its lateral (left side) border. Its foramen vertebrale (an opening where the neural tube travels through) is oval-shaped and topped by a high, dorsally (top)-expanded neural spine, with the vertebra measuring 17.4 centimetres (6.9 in) in overall height. This indicates that it is the second largest phorusrhacid vertebra known, second only to an undescribed dorsal vertebra of Titanis. On the ventral side of the vertebra is a long keel, the processus ventralis corpus, which runs almost the whole length of the centrum and bears a rounded distal end. Due to the lack of postcranial material from derived phorusrhacids, few comparisons could be made with MPEF-PV 11355.[13]

Paleobiology

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Life restoration

Feeding and diet

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Phorusrhacids are thought to have been ground predators or scavengers, and have often been considered apex predators that dominated Cenozoic South America in the absence of placental mammalian predators, though they did co-exist with some large, carnivorous borhyaenid mammals. Earlier hypotheses of phorusrhacid feeding ecology were mainly inferred from them having large skulls with hooked beaks rather than through detailed hypotheses and biomechanical studies, and such studies of their running and predatory adaptations were only conducted from the beginning of the 21st century.[53] In general, phorusrhacids are separated into two primary ecological stances. Smaller, lithe genera like Psilopterus and Procariama are thought to have filled a sereima-like niche, feeding on smaller prey and potentially bearing limited flight abilities. Meanwhile, Phorusrhacos and other gigantic genera are presumed to have been macropredators and apex predators. Based on its cursorial proportions, Phorusrhacos was a pursuit predator that utilized speed to capture and dispatch prey. In contrast, its relatives like Kelenken and Devincenzia were likely graviportal ambush hunters instead, relying on stealth to hunt prey.[59]

Brazilian researchers Herculano Alvarenga and Elizabeth Höfling made some general remarks about phorusrhacid habits in a 2003 article. They were flightless, as evidenced by the proportional size of their wings and body mass, and wing-size was more reduced in larger members of the group. These researchers pointed out that the narrowing of the pelvis, upper maxilla and thorax could have been adaptations to enable the birds to search for and take smaller animals in tall plant growth or broken terrain. The large expansions above the eyes formed by the lacrimal bones (similar to what is seen in modern hawks) would have protected the eyes against the sun, and enabled keen eyesight, which indicates they hunted by sight in open, sunlit areas, and not shaded forests.[11]

Leg function

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In 2005, Uruguayan paleontologists Rudemar Ernesto Blanco and Washington W. Jones examined the strength of the tibiotarsus (shin bone) of phorusrhacids to determine their speed, but conceded that such estimates can be unreliable even for extant animals. While the tibiotarsal strength of Patagornis and an indeterminate large phorusrhacine, one comparable in size to Phorusrhacos, suggested a speed of 14 m/s (50 km/h; 31 mph), and that of Mesembriornis suggested 27 m/s (97 km/h; 60 mph), the latter is greater than that of a modern ostrich, approaching that of a cheetah, 29 m/s (100 km/h; 65 mph).[40][65] They found these estimates unlikely due to the large body size of these birds, and instead suggested the strength could have been used to break the long bones of medium-sized mammals, the size, for example, of a saiga or Thomson's gazelle. This strength could be used for accessing the marrow inside the bones, or by using the legs as kicking weapons (like some modern ground birds do), consistent with the large, curved, and sideways compressed claws known in some phorusrhacids. They also suggested future studies could examine whether they could have used their beaks and claws against well-armored mammals such as armadillos and glyptodonts.[40]

According to Argentine paleontologists Luis Chiappe and Sara Bertelli in a 2006 article, the discovery of Kelenken shed doubt on the traditional idea that the size and agility of phorusrhacids correlated, with the larger members of the group being more bulky and less adapted for running. The long and slender tarsometatarsus of Kelenken, a phorusrhacid similar in size and proportions to Phorusrhacos, instead shows that this bird may have been much swifter than the smaller, more heavyset and slow Brontornis.[66] In a 2006 news article about the discovery, Chiappe stated that while Kelenken may not have been as swift as an ostrich, it could clearly run faster than had previously been assumed for large phorusrhacids, based on the long, slender leg-bones, superficially similar to those of the modern, flightless rhea. The article suggested that Kelenken would have been able to chase down small mammals and reptiles.[67] This is especially true in Phorusrhacos, which bears more cursorial and agile adaptations than other giant phorusrhacids.[59][68] In another 2006 news article, Chiappe stated that Kelenken would have been as quick as a greyhound, and that while there were other large predators in South America at the time, they were limited in numbers and not as fast and agile as the phorusrhacids, and the many grazing mammals would have provided ample prey. Chiappe stated that phorusrhacids crudely resembled earlier predatory dinosaurs like Tyrannosaurus, in having gigantic heads, very small forelimbs, and very long legs, and thereby had the same kind of meat-eater adaptations.[69]

Skull and neck function

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Stress distribution in bird skulls during various movements, including the related Andalgalornis (left, A–C, the other skulls belong to a red-legged seriema and a white-tailed eagle), and hypothetical upwards-and-downwards range of movement of the neck in the same genus (right)

In 2012, Degrange and colleagues estimated Phorusrhacos' bite force to have been 196 Newtons, which they concluded was correlated with body mass in phorusrhacids as smaller taxa like Patagornis and Psilopterus had significantly lower bite forces.[50][70] A 2010 study by Degrange and colleagues of the medium-sized phorusrhacid Andalgalornis, a close relative of Phorusrhacos,[3] based on Finite Element Analysis using CT scans, estimated its bite force and stress distribution in its skull. They found its bite force to be 133 newtons at the bill tip, and showed it had lost a large degree of intracranial immobility (mobility of skull bones in relation to each other), as was also the case for other large phorusrhacids such as Phorusrhacos. These researchers interpreted this loss as an adaptation for enhanced rigidity of the skull; compared to the modern red-legged seriema and white-tailed eagle, the skull of the phorusrhacid showed relatively high stress under sideways loadings, but low stress where force was applied up and down, and in simulations of "pullback". Due to the relative weakness of the skull at the sides and midline, these researchers considered it unlikely that Andalgalornis engaged in potentially risky behavior that involved using its beak to subdue large, struggling prey. Instead, they suggested that it either fed on smaller prey that could be killed and consumed more safely, by for example swallowing it whole, or that when targeting large prey, it used a series of well-targeted repetitive strikes with the beak, in an "attack-and-retreat" strategy. Struggling prey could also be restrained with the feet, despite the lack of sharp talons.[53]

A 2012 follow-up study by Tambussi and colleagues analyzed the flexibility of the neck of Andalgalornis, based on the morphology of its neck vertebrae, finding the neck to be divided into three sections. By manually manipulating the vertebrae, they concluded that the neck musculature and skeleton of Andalgalornis was adapted to carrying a large head, and for helping it rise from a maximum extension after a downwards strike, and the researchers assumed the same would be true for other large, big-headed phorusrhacids like Phorusrhacos.[54] A 2020 study of phorusrhacid skull morphology by Degrange found that there were two main morphotypes within the group, derived from a seriema-like ancestor. These were the "Psilopterine Skull Type", which was plesiomorphic (more similar to the ancestral type), and the "Terror Bird Skull Type", which included Phorusrhacos and other phorusrhacines, that was more specialized, with more rigid and stiff skulls. Despite the differences, studies have shown the two types handled prey similarly, while the more rigid skulls and resulting larger bite force of the "Terror Bird" type would have been an adaptation to handling larger prey.[71]

Paleoecology

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Map of the Santa Cruz Formation, where Phorusrhacos is primarily known

Phorusrhacos is known from multiple sites, mostly from outcrops of the Santa Cruz Formation along the Atlantic coast of southern Argentina.[13] Fragmentary remains have also been described from strata of the early-mid Miocene Cerro Boleadoras Formation, which dates to between 16.5 to 15.1 million years ago,[36] and the Monte León Formation, which dates to between 17.7 and 16 million years ago,[72] in Monte León.[19][73] All of these formations correspond to the Santacrucian SALMA, which dates to the early Miocene stage (18 to 14 million years ago) of the Neogene.[12] Phorusrhacos is among the most common and widely distributed phorusrhacids.[11][13]

The Santa Cruz Formation featured a heterogeneous environment composed of gallery forests, semi-arid and semi-deciduous forests, and open savannas,[74] though grasses were not as common in the Santacrucian than they are now.[75] Open, dry savanna conditions like these were suitable for pursuit predators like Phorusrhacos.[59][68] This formation also had open shrubland dotted with patches of woodlands, marshes, and seasonally flooded water bodies.[76][77] The climate was relatively warm, humid, and tropical, comparable to the Atlantic inland forests of Argentina and the dry Chaco region of Paraguay. Its forested areas were clouded by Araucaria conifers, Nothofagus beeches, and laurels among other trees, while grasses like pooideans blanketed the ground.[78] Permanent bodies of water such as lakes, ponds and streams are likely to have been present, which sported frogs like Calyptocephalella,[79] waterbirds like Ankonetta, Eutelornis, and Kaikenia,[50] and the astrapothere Astrapotherium.[80][81][82] Many Santacrucian birds were present, including at least 18 species in 15 genera and nine families.[50] This period is considered the peak of phorsurhacid diversity,[59][50] though it was nearly extinct by the Pleistocene.[56][83] Other birds known from the formation include the phorusrhacids Patagornis and Psilopterus, the enigmatic Brontornis,[23][51][42] the rhea relative Opisthodactylus, the falcon Thegornis, the piscivorous darters Liptornis and Macranhinga, and the possible spoonbill Protibis.[50][84] A menagerie of carnivorous mammals is known as well, such as the sparassodonts Borhyaena, Acrocyon, Arctodictis, and Lycopsis and the hathliacynid Cladosictis.[85][77][86] Megaherbivorous mammals are represented by ground sloths like Prepotherium and Eucholoeops,[87][88] notoungulate toxodontids Nesodon and Adinotherium, the homalodotheriid Homalodotherium, and the litopterns Diadiaphorus, Theosodon, and Tetramerorhinus. As for smaller herbivores, interatheriids like Cochilius, Interatherium, and Protypotherium, anteater Protamandua,[89] cingulates like Cochlops, Peltephilus, and Propalaehoplophorus,[90] rodents like Perimys,[91] Eocardia, and Steiromys are known.[92]

Litopterns such as Theosodon (depicted), were possible prey items for Phorusrhacos

In Santacrucian ecosystems, Phorusrhacos likely preyed upon giant (>100 kilograms (220 lb)) mammals[93] such as notoungulates and litopterns, and potentially immature astrapotheres, toxodontids, and homalodotheriids.[59][77] Smaller cingulates and unarmored xenarthrans also could have been prey for Phorusrhacos.[59][77] Sparassodonts, especially the cursorial Borhyaena, were likely a major competitor to Phorusrhacos for food. However, unlike Borhyaena, most sparassodonts were scansorial and occupied forested environments, whereas Phorusrhacos hunted in open environments, creating niche partitioning between the two groups. During the Deseadan SALMA (which corresponds to the late Oligocene-early Miocene, 29-21 mya) of Santa Cruz Province, Physornis had a graviportal macropedatory role. However, Physornis' woodland habitat transitioned to an open, savanna-like landscape towards the end of the Deseadan, leading to its extinction. As a result, the cursorial, pursuit-predator Phorusrhacos became Physornis' successor as a macropredator in Santa Cruz ecosystems.[59]

Evolution

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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.[52][94] 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 speciose group within Cariamiformes, the interrelationships between phorusrhacids are unclear due to the incompleteness of their remains.[61]

Phorusrhacids were present in South America from the Paleocene (when the continent was an isolated island) and survived until the Pleistocene. They also appeared in North America at the end of the Pliocene, during the Great American Biotic Interchange, and while fossils from Europe have been assigned to the group, their classification is disputed. It is unclear where the group originated; both cariamids and phorusrhacids may have arisen in South America, or arrived from elsewhere when southern continents were closer together or when sea levels were lower.[11][52] Phorusrhacos itself lived during the early-middle Miocene, about 18 to 14 million years ago.[29][50] Since phorusrhacids survived until the Pleistocene, they appear to have been more successful than for example the South American metatherian thylacosmilid predators (which disappeared in the Pliocene), and it is possible that they competed ecologically with placental predators that entered from North America in the Pleistocene.[44]

References

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  1. 1 2 3 4 Lydekker, Richard (1893). "On the extinct giant birds of Argentina". Ibis Series. 6 (5): 40–47.
  2. 1 2 3 4 5 6 7 8 Sinclair, William J.; Farr, Marcus S. (1932). "Aves of the Santa Cruz beds". Reports of the Princeton University Expeditions to Patagonia (1896-1899). 2. 7 (2): 157–191.
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  4. 1 2 3 Fernícola, Juan Carlos (2011). "Implicancias del conflicto Ameghino-Moreno sobre la colección de mamíferos fósiles realizada por Carlos Ameghino en su primera exploración al río Santa Cruz, Argentina". Revista del Museo Argentino de Ciencias Naturales (in Spanish). 13 (1): 41–57. ISSN 1853-0400.
  5. 1 2 3 4 5 6 7 8 9 10 11 12 Buffetaut, Eric (2013). "Who discovered the Phorusrhacidae? An episode in the history of avian palaeontology" (PDF). Paleornithological Research 2013. Proceedings of the 8th International Meeting of the Society of Avian Paleontology and Evolution: Wien, Naturhistorisches Museum. 8: 123–134.
  6. 1 2 3 4 5 6 7 8 9 10 11 12 Moreno, Francisco P.; Mercerat, Alcides (1891). "Catálogo de los pájaros fósiles de la República Argentina conservados en el Museo de La Plata". Anales del Museo de La Plata. 1: 7–71.
  7. 1 2 3 4 Ameghino, Florentino (1889). "Contribucion al conocimiento de los mamíferos fósiles de la República Argentina" [Contribution to the knowledge of mammal fossils of the Argentine Republic]. Actas de la Academia Nacional de Ciencias de Cordóba (in Spanish). 6: 1–1027.
  8. 1 2 3 4 Ameghino, Florentino (1887). "Enumeración systemática de las especies de mamíferos fósiles coleccionados por Carlos Ameghino en los terrenos eocenos de la Patagonia austral" [Systematic enumeration of the fossil mammal species collected by Carlos Ameghino in the Eocene terrains of southern Patagonia]. Boletin del Museo La Plata (in Spanish). 1: 1–26.
  9. ↑ Bond, Mariano (2025). "De Burmeister a Ameghino y el reconocimiento mundial de la Paleontología Argentina" [From Burmeister to Ameghino and the worldwide recognition of the vertebrate paleontology from Argentina]. Publicación Electrónica de la Asociación Paleontológica Argentina. 25 (1): 83–126. doi:10.5710/PEAPA.14.02.2025.520. ISSN 2469-0228.
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  13. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Federico J. Degrange, Drew Eddy, Pablo Puerta, & Julia Clarke (2019). "New skull remains of Phorusrhacos longissimus (Aves, Cariamiformes) from the Pleistocene of Argentina: implications for the morphology of Phorusrhacidae". Journal of Paleontology. 93 (6): 1221–1233. Bibcode:2019JPal...93.1221D. doi:10.1017/jpa.2019.53. hdl:11336/124874. S2CID 199094122. Retrieved 30 March 2020.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  14. 1 2 3 Creisler, Ben (2012). "Phorusrhacos "wrinkle bearer (jaw)": Etymology and Meaning". Dinosaur Mailing List. Archived from the original on 29 October 2013. Retrieved 4 July 2012.
  15. ↑ "Phororhacos". Century Dictionary. Archived from the original on 21 April 2001. Retrieved 22 April 2010.
  16. 1 2 Charles, Richmond (1902). "List of generic terms proposed for birds during the years 1890 to 1900, inclusive, to which are added names omitted by Waterhouse in his "Index Generum Avium"". Proceedings of the United States National Museum. 24 (1241–1274): 663–730. doi:10.5479/si.00963801.1267.663. ISSN 0096-3801. Archived from the original on 5 November 2014.
  17. ↑ Cost, Ian N. (22 August 2020). "Extinct Animal of the Week: The Name of Many Terrors". Extinct Animal of the Week. Retrieved 30 May 2026.
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  26. 1 2 3 4 5 6 7 8 9 10 Ameghino, Florentino (1895). "Sur les oiseaux fossils de patagonie et le faune mammalogique des couches á Pyrotherium. Premiére contribution á la connissance de la faune mammalogique des couches á Pyrotherium" [On the Fossil Birds of Patagonia and the Mammalian Fauna of the Pyrotherium Beds: First Contribution to the Knowledge of the Mammalian Fauna of the Pyrotherium Beds.]. Boletín del Instituto Geografico Argentino. 15: 603–660.
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  28. ↑ Ameghino, Carlos (1891). "Exploracion de los depósitos fosíliferos de la Patagonia austral". Revista Argentina de Historia Natural. 1: 119–121.
  29. 1 2 3 4 5 Agnolín, Federico (2006). "Posición sistemática de algunas aves fororracoideas (Ralliformes; Cariamae) Argentinas". Revista del Museo Argentino de Ciencias Naturales. 8 (1): 27–33.
  30. ↑ Vizcaíno, Sergio; Brinkman, Paul D.; Kay, Richard F. (2016). "Sobre los objetivos y resultados de la expedición paleontológica de Handel T. Martin (1903-04) a la Formación Santa Cruz en Patagonia austral" [On the objectives and results of the Handel T. Martin paleontological expedition (1903-04) to the Santa Cruz Formation in southern Patagonia]. Revista del Museo de La Plata. 1: 316–333. doi:10.24215/25456377e037. ISSN 2545-6377.
  31. ↑ Vizcaíno, Sergio F.; Iuliis, Gerardo De; Brinkman, Paul D.; Kay, Richard F.; Brinkman, Daniel L. (8 July 2017). "On an album of photographs recording fossils in the "Old collections" of the Museo de La Plata and Ameghino's private collection at the beginning of the 20th century". Publicación Electrónica de la Asociación Paleontológica Argentina. 17 (1): 14–23. doi:10.5710/PEAPA.21.06.2017.244. hdl:10161/15920. ISSN 2469-0228.
  32. ↑ Sinclair, William J. (1909). "Mammalia of the Santa Cruz beds. Part I. Typotheria". Reports of the Princeton University Expeditions to Patagonia, 1896-1899. 1 (1). Princeton University: 1–110.
  33. 1 2 Fernicola, Juan Carlos; Bargo, M. Susana; Vizcaíno, Sergio Fabián; Kay, Richard (20 December 2019). "Historical background for a revision of the paleontology of the Santa Cruz Formation (Early-Middle Miocene) along the Río Santa Cruz, Patagonia, Argentina". Publicación Electrónica de la Asociación Paleontológica Argentina. 19 (2). doi:10.5710/PEAPA.18.09.2019.300. ISSN 2469-0228.
  34. 1 2 3 4 5 6 7 8 9 10 11 Agnolin, Federico L.; Chafrat, Pablo; Álvarez-Herrera, Gerardo P. (3 July 2025). "New specimens of Patagorhacos terrificus Agnolín and Chafrat, 2015 (Aves) shed light on the phylogeny and evolution of the Phorusrhacidae". Historical Biology. 37 (7): 1744–1756. Bibcode:2025HBio...37.1744A. doi:10.1080/08912963.2025.2458127. ISSN 0891-2963.
  35. ↑ Degrange, Federico Javier (2022). "A new species of Dryornis (Aves, Cathartiformes) from the Santa Cruz Formation (lower Miocene), Patagonia, Argentina". Journal of Vertebrate Paleontology. 41 (5) e2008411: 1–6. doi:10.1080/02724634.2021.2008411. ISSN 0272-4634.
  36. 1 2 Vizcaino, Sergio F.; Bargo, M. Susana; Pérez, M. Encarnación; Aramendía, Inés; Cuitiño, José I.; Monsalvo, Eduardo S.; Vlachos, Evangelos; Noriega, Jorge I.; Kay, Richard F. (30 September 2022). "Fossil vertebrates of the early-middle Miocene Cerro Boleadoras Formation, northwestern Santa Cruz Province, Patagonia, Argentina". Andean Geology. 49 (3): 382. Bibcode:2022AndGe..49..382V. doi:10.5027/andgeoV49n3-3425. ISSN 0718-7106.
  37. ↑ Ameghino, Florentino (1901). "L'âge des formations sédimentaires de Patagonie (suite)" [The age of sedimentary formations of Patagonia]. Anales de la Sociedad Cientìfica Argentina (Suite) (in Spanish). 51–52: 65–91.
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  39. ↑ Acosta Hospitaleche, C.; Tambussi, C. P.; Reguero, M. A. (2000). "Catálogo de los tipos de aves fósiles del Museo de La Plata" (PDF). Universidad Nacional de La Plata Facultad de Ciencias Naturales y Museo. 1: 1–28. ISSN 0372-4565.
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  45. ↑ Mercerat, Alcides (1893). "Note sur la géologie de Patagonie: Buenos Aires" [Note on the geology of Patagonia: Buenos Aires]. Helvetia (in Spanish): 6p.
  46. ↑ Ameghino, Florentino (1899). "Sinopsis geológico-paleontológica. Suplemento (Adiciones y correciones)". La Plata: 13p.
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  48. 1 2 3 Patterson, Bryan; Kraglievich, Lucas (1960). "Systemática y nomenclatura de las aves fororracoideas del Plioceno argentino" [Systematics and Nomenclature of the Pliocene Phorusrhacoid Birds of Argentina.]. Publicaciones del Museo Municipal de Historia Natural y Tradicional de Mar del Plata. 1: 1–52.
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  55. ↑ Acosta Hospitaleche, C.; Jones, W. (2025). "Insights on the oldest terror bird (Aves, Phorusrhacidae) from the Eocene of Argentina". Historical Biology. 37 (2): 391–399. Bibcode:2025HBio...37..391A. doi:10.1080/08912963.2024.2304592.
  56. 1 2 Machado, Victor Hugo M.; de Vasconcelos, Marcelo F.; Vilaboim Santos, Luciano; Parisi Dutra, Rodrigo; Cartelle, Cástor; Câmara, Bruno G. O.; Dantas, Mário A. T.; Degrange, Federico J. (2026). "A new terror bird (Cariamiformes, Phorusrhacidae) from the Late Pleistocene of Brazil: insights into the last representatives of the family". Papers in Palaeontology. 12 (2) e70080. Bibcode:2026PPal...1270080M. doi:10.1002/spp2.70080. ISSN 2056-2799.
  57. ↑ Mourer-Chauviré, Cécile; Tabuce, Rodolphe; Mahboubi, M'hammed; Adaci, Mohammed; Bensalah, Mustapha (28 August 2011). "A Phororhacoid bird from the Eocene of Africa". Naturwissenschaften. 98 (10): 815–823. Bibcode:2011NW.....98..815M. doi:10.1007/s00114-011-0829-5. ISSN 1432-1904. PMID 21874523.
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  60. ↑ Degrange, Federico J.; Cooke, Siobhan B.; Ortiz-Pabon, Luis G.; Pelegrin, Jonathan S.; Perdomo, Cesar A.; Salas-Gismondi, Rodolfo; Link, Andrés (2024). "A gigantic new terror bird (Cariamiformes, Phorusrhacidae) from Middle Miocene tropical environments of La Venta in northern South America". Papers in Palaeontology. 10 (6) e1601. Bibcode:2024PPal...10E1601D. doi:10.1002/spp2.1601. hdl:11336/262433. ISSN 2056-2799.
  61. 1 2 Degrange, Federico J. (2020). "A revision of skull morphology in Phorusrhacidae (Aves, Cariamiformes)". Journal of Vertebrate Paleontology. 40 (6) e1848855. Bibcode:2020JVPal..40E8855D. doi:10.1080/02724634.2020.1848855. S2CID 234119602.
  62. ↑ Chandler, Robert; Jefferson, George; Lindsay, Lowell; Vescera, Susan (2013). "The terror bird, Titanis (Phorusrhacidae), from Pliocene Olla Formation, Anza-Borrego Desert State Park, southern California". Desert Symposium. 27: 181–183.
  63. ↑ Gould, Gina; Quitmyer, Irvy (31 December 2005). "Titanis walleri: Bones of Contention". Bulletin of the Florida Museum of Natural History. 45 (4): 201–230. Bibcode:2005BFMNH..45..201G. doi:10.58782/flmnh.xumx1681. ISSN 2373-9991.
  64. ↑ Croft, Darin A. (2016). Horned Armadillos and Rafting Monkeys: The Fascinating Fossil Mammals of South America (1st ed.). Bloomington, Indiana, USA: Indiana University Press. ISBN 978-0-253-02084-0.
  65. ↑ Hudson, Penny E.; Corr, Sandra A.; Wilson, Alan M. (2012). "High speed galloping in the cheetah (Acinonyx jubatus) and the racing greyhound (Canis familiaris): spatio-temporal and kinetic characteristics". Journal of Experimental Biology. 215 (14): 2425–2434. Bibcode:2012JExpB.215.2425H. doi:10.1242/jeb.066720. PMID 22723482. S2CID 13543638.
  66. ↑ Cite error: The named reference Nature was invoked but never defined (see the help page).
  67. ↑ Cite error: The named reference NYT was invoked but never defined (see the help page).
  68. 1 2 Tauber, Adán (1999). "Los vertebrados de la Formación Santa Cruz (Mioceno Inferior-Medio) en el extremo sureste de la Patagonia y su significado paleoecológico". Revista española de paleontología. 14 (2): 173–182. ISSN 0213-6937.
  69. ↑ Joyce, Christopher (25 October 2006). "Huge "terror bird" fossil discovered in Patagonia". NPR. Retrieved 26 May 2022.
  70. ↑ Prevosti, Francisco Juan; Forasiepi, Analia M. (2018), Prevosti, Francisco J.; Forasiepi, Analía M. (eds.), "Evolution and Biological Context of South American Mammalian Carnivores During the Cenozoic and the Biological Context", Evolution of South American Mammalian Predators During the Cenozoic: Paleobiogeographic and Paleoenvironmental Contingencies, Cham: Springer International Publishing, pp. 155–196, doi:10.1007/978-3-319-03701-1_6, ISBN 978-3-319-03701-1{{citation}}: CS1 maint: work parameter with ISBN (link)
  71. ↑ Cite error: The named reference skull morphology2 was invoked but never defined (see the help page).
  72. ↑ Parras, Ana; Guerstein, G. Raquel; Pérez Panera, Juan P.; Griffin, Miguel; Náñez, Carolina; Cusminsky, Gabriela; Quiroga, Analisa (15 October 2020). "Integrated stratigraphy and paleontology of the lower Miocene Monte León Formation, southeastern Patagonia, Argentina: Unraveling paleoenvironmental changes and factors controlling sedimentation". Palaeogeography, Palaeoclimatology, Palaeoecology. 556 109701. Bibcode:2020PPP...55609701P. doi:10.1016/j.palaeo.2020.109701. ISSN 0031-0182.
  73. ↑ Buffetaut, Eric (1 March 2014). "Tertiary ground birds from Patagonia (Argentina) in the Tournouër collection of the Muséum National d'Histoire Naturelle, Paris". Bulletin de la Société Géologique de France. 185 (3): 207–214. doi:10.2113/gssgfbull.185.3.207. ISSN 1777-5817.
  74. ↑ Kay, Richard F.; Vizcaíno, Sergio F.; Bargo, M. Susana; Spradley, Jackson P.; Cuitiño, José I. (1 August 2021). "Paleoenvironments and paleoecology of the Santa Cruz Formation (early-middle Miocene) along the Río Santa Cruz, Patagonia (Argentina)". Journal of South American Earth Sciences. 109 103296. Bibcode:2021JSAES.10903296K. doi:10.1016/j.jsames.2021.103296. ISSN 0895-9811.
  75. ↑ Barreda, Viviana; Palazzesi, Luis (1 January 2007). "Patagonian vegetation turnovers during the Paleogene-Early Neogene: Origin of arid-adapted floras". The Botanical Review. 73 (1): 31–50. doi:10.1663/0006-8101(2007)73[31:PVTDTP]2.0.CO;2. ISSN 1874-9372.
  76. ↑ Vizcaíno, Sergio F.; Bargo, M. Susana; Kay, Richard F.; Milne, Nick (4 August 2006). "The armadillos (Mammalia, Xenarthra, Dasypodidae) of the Santa Cruz Formation (early–middle Miocene): An approach to their paleobiology". Palaeogeography, Palaeoclimatology, Palaeoecology. 237 (2): 255–269. Bibcode:2006PPP...237..255V. doi:10.1016/j.palaeo.2005.12.006. ISSN 0031-0182.
  77. 1 2 3 4 Ercoli, Marcos D.; Prevosti, Francisco J.; Forasiepi, Analía M. (1 December 2014). "The Structure of the Mammalian Predator Guild in the Santa Cruz Formation (Late Early Miocene)". Journal of Mammalian Evolution. 21 (4): 369–381. doi:10.1007/s10914-013-9243-4. hdl:11336/19136. ISSN 1573-7055.
  78. ↑ Brea, Mariana; Zucol, Alejandro F.; Iglesias, Ari (2012), Bargo, M. Susana; Kay, Richard F.; Vizcaíno, Sergio F. (eds.), "Fossil plant studies from late Early Miocene of the Santa Cruz Formation: paleoecology and paleoclimatology at the passive margin of Patagonia, Argentina", Early Miocene Paleobiology in Patagonia: High-Latitude Paleocommunities of the Santa Cruz Formation, Cambridge: Cambridge University Press, pp. 104–128, doi:10.1017/cbo9780511667381.008, ISBN 978-0-521-19461-7, retrieved 14 May 2026{{citation}}: CS1 maint: work parameter with ISBN (link)
  79. ↑ Fernicola, Juan C.; Albino, Adriana (2012), Bargo, M. Susana; Kay, Richard F.; Vizcaíno, Sergio F. (eds.), "Amphibians and squamate reptiles from the Santa Cruz Formation (late Early Miocene), Santa Cruz Province, Argentina: paleoenvironmental and paleobiological considerations", Early Miocene Paleobiology in Patagonia: High-Latitude Paleocommunities of the Santa Cruz Formation, Cambridge: Cambridge University Press, pp. 129–137, doi:10.1017/cbo9780511667381.009, hdl:11336/134594, ISBN 978-0-521-19461-7, retrieved 18 May 2026{{citation}}: CS1 maint: work parameter with ISBN (link)
  80. ↑ Fernández, Mercedes; Muñoz, Nahuel Antu (20 December 2019). "Notoungulata and Astrapotheria (Mammalia, Meridiungulata) of the Santa Cruz Formation (Early-Middle Miocene) along the Río Santa Cruz, Argentina Patagonia". Publicación Electrónica de la Asociación Paleontológica Argentina. 19 (2). doi:10.5710/PEAPA.19.09.2019.288. hdl:11336/120862. ISSN 2469-0228.
  81. ↑ Vizcaíno, Sergio F.; Bargo, M. Susana; Kay, Richard F.; Fariña, Richard A.; Di Giacomo, Mariana; Perry, Jonathan M. G.; Prevosti, Francisco J.; Toledo, Néstor; Cassini, Guillermo H.; Fernicola, Juan C. (15 June 2010). "A baseline paleoecological study for the Santa Cruz Formation (late–early Miocene) at the Atlantic coast of Patagonia, Argentina". Palaeogeography, Palaeoclimatology, Palaeoecology. 292 (3): 507–519. Bibcode:2010PPP...292..507V. doi:10.1016/j.palaeo.2010.04.022. hdl:11336/135189. ISSN 0031-0182.
  82. ↑ Zurita-Altamirano, Daniel; Buffetaut, Eric; Forasiepi, Analía M.; Kramarz, Alejandro; Carrillo, Juan D.; Aguirre-Fernández, Gabriel; Carlini, Alfredo A.; Scheyer, Torsten M.; Sánchez-Villagra, Marcelo R. (1 November 2019). "The Allemann collection from the Santa Cruz Formation (late early Miocene), Argentina, in Zurich, Switzerland". Swiss Journal of Palaeontology. 138 (2): 259–275. Bibcode:2019SwJP..138..259Z. doi:10.1007/s13358-019-00185-5. hdl:11336/120785. ISSN 1664-2384.
  83. ↑ Jones, Washington; Rinderknecht, Andrés; Alvarenga, Herculano; Montenegro, Felipe; Ubilla, Martín (1 June 2018). "The last terror birds (Aves, Phorusrhacidae): new evidence from the late Pleistocene of Uruguay". PalZ. 92 (2): 365–372. Bibcode:2018PalZ...92..365J. doi:10.1007/s12542-017-0388-y. ISSN 1867-6812.
  84. ↑ Noriega, Jorge I.; Areta, Juan I.; Vizcaíno, Sergio F.; Bargo, M. Susana (2011). "Phylogeny and taxonomy of the Patagonian Miocene falcon Thegornis musculosus Ameghino, 1895 (Aves: Falconidae)". Journal of Paleontology. 85 (6): 1089–1104. Bibcode:2011JPal...85.1089N. doi:10.1666/10-122.1. hdl:11336/76576. ISSN 0022-3360.
  85. ↑ Prevosti, Francisco J.; Forasiepi, Analía M.; Ercoli, Marcos D.; Turazzini, Guillermo F. (2012), Bargo, M. Susana; Kay, Richard F.; Vizcaíno, Sergio F. (eds.), "Paleoecology of the mammalian carnivores (Metatheria, Sparassodonta) of the Santa Cruz Formation (late Early Miocene)", Early Miocene Paleobiology in Patagonia: High-Latitude Paleocommunities of the Santa Cruz Formation, Cambridge: Cambridge University Press, pp. 173–193, doi:10.1017/cbo9780511667381.012, ISBN 978-0-521-19461-7, retrieved 18 May 2026{{citation}}: CS1 maint: work parameter with ISBN (link)
  86. ↑ Argot, Christine (2004). "Evolution of South American mammalian predators (Borhyaenoidea): anatomical and palaeobiological implications". Zoological Journal of the Linnean Society. 140 (4): 487–521. doi:10.1111/j.1096-3642.2004.00110.x. ISSN 1096-3642.
  87. ↑ Toledo, Nestor (2016). "Paleobiological Integration of Santacrucian Sloths (Early Miocene of Patagonia)". Ameghiniana. 53 (2): 100. Bibcode:2016Amegh..53..100T. doi:10.5710/AMGH.07.10.2015.2891. hdl:11336/54358. ISSN 0002-7014. Archived from the original on 31 August 2025.
  88. ↑ Iuliis, Gerardo De; Pujos, François; Toledo, Nestor; Bargo, M. Susana; Vizcaíno, Sergio F. (2014). "Eucholoeops Ameghino, 1887 (Xenarthra, Tardigrada, Megalonychidae) from the Santa Cruz Formation, Argentine Patagonia: implications for the systematics of Santacrucian sloths". Geodiversitas. 36 (2): 209–255. Bibcode:2014Geodv..36..209I. doi:10.5252/g2014n2a2. ISSN 1280-9659.
  89. ↑ Hirschfeld, Sue E. (1976). "A New Fossil Anteater (Edentata, Mammalia) from Colombia, S.A. and Evolution of the Vermilingua". Journal of Paleontology. 50 (3): 419–432. ISSN 0022-3360. JSTOR 1303522.
  90. ↑ Vizcaíno, Sergio F.; Fernicola, Juan C.; Bargo, M. Susana (2012), Bargo, M. Susana; Kay, Richard F.; Vizcaíno, Sergio F. (eds.), "Paleobiology of Santacrucian glyptodonts and armadillos (Xenarthra, Cingulata)", Early Miocene Paleobiology in Patagonia: High-Latitude Paleocommunities of the Santa Cruz Formation, Cambridge: Cambridge University Press, pp. 194–215, doi:10.1017/CBO9780511667381.013, ISBN 978-0-521-19461-7, retrieved 18 May 2026{{citation}}: CS1 maint: work parameter with ISBN (link)
  91. ↑ Bostelmann, J. Enrique; Roux, Jacobous P. Le; Vásquez, Ana; Gutiérrez, Néstor M.; Oyarzún, José Luis; Carreño, Catalina; Torres, Teresa; Otero, Rodrigo; Llanos, Andrea; Fanning, C. Mark; Hervé, Francisco (25 July 2013). "Burdigalian deposits of the Santa Cruz Formation in the Sierra Baguales, Austral (Magallanes) Basin: Age, depositional environment and vertebrate fossils". Andean Geology. 40 (3): 458–489. doi:10.5027/andgeoV40n3-a04. hdl:10533/138811. ISSN 0718-7106.
  92. ↑ Arnal, Michelle; Pérez, María Encarnación; Deschamps, Cecilia Marcela (2019). "Revision of the Miocene caviomorph rodents from the Río Santa Cruz (Argentinean Patagonia)". Publicación Electrónica de la Asociación Paleontológica Argentina. 19 (2). doi:10.5710/PEAPA.25.09.2019.299. hdl:11336/175184. ISSN 2469-0228.
  93. ↑ Tambussi, Claudia P.; Degrange, Federico J. (2013), "The Dominance of Zoophagous Birds: Just a Cliché?", South American and Antarctic Continental Cenozoic Birds, Dordrecht: Springer Netherlands, pp. 87–102, doi:10.1007/978-94-007-5467-6_8, ISBN 978-94-007-5466-9, retrieved 26 May 2026{{citation}}: CS1 maint: work parameter with ISBN (link)
  94. ↑ Ksepka, Daniel T. (2014). "Flights of fancy in avian evolution". American Scientist. 102 (1): 39. doi:10.1511/2014.106.36. ISSN 0003-0996. JSTOR 43707746.
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