Argentavis

Argentavis is an extinct genus of teratornithid birds, known from three sites in the Epecuén and Andalhualá Formations of central and northwestern Argentina. These fossils date to the Late Miocene (Huayquerian). The only recognized species is A. magnificens, often called the giant teratorn. It is one of the largest flying birds ever known, with a wingspan exceeded only by Pelagornis sandersi, though it remains the heaviest flying bird, weighing up to a certain maximum.

The first remains were discovered by a Museo de La Plata expedition, about 15 km south of Hidalgo station, at the Salinas Grandes de Hidalgo locality in the Epecuén (now Cerro Azul) Formation of La Pampa Province. Rosendo Pascual and Eduardo Tonni collected a partial associated skeleton, including parts of the skull, right quadrate, and portions of the legs and arms. This material, cataloged as MLP 65-VII-29-49, was brought to the Museo de La Plata and later cast at the Los Angeles County Museum. In a 1980 paper, Kenneth Campbell Jr. and Eduardo Tonni described this specimen as the holotype of the new genus and species Argentavis magnificens. The genus name combines Latin "argentum" (silver) and "avis" (bird), referencing Argentina; the species name "magnificens" means "magnificent" in Latin. The authors noted that all material was severely fractured, but most postcranial bones were not heavily crushed, though they lacked diagnostic portions. The skull and quadrate, however, clearly linked Argentavis to Teratornis, allowing its classification as a teratornithid. This made it the third described genus in the family and the first from outside North America.

In a 1995 paper, Campbell described three additional specimens. All were found during a 1983 survey of Argentine museum collections. The first, an uncatalogued ungual phalanx, came from the Museo Municipal de Ciencias Naturales in Mar del Plata. It was collected in March 1982 by Galilio Scaglia from the same Cerro Azul Formation, about 60 km west of the type locality near Carhué. It was referred to Argentavis based on the flexor muscle attachment site, a prominent groove on the bone’s lateral and medial surfaces, and its size—roughly 1.5 times larger than the largest ungual phalanges of Teratornis merriami from the La Brea Tar Pits. However, in 2012, Marcos Cenizo and colleagues reassigned this element to the family Phorusrhacidae, arguing the characters were too weak to confirm an Argentavis identity. They also reported a previously unrecognized proximal fragment of an ungual phalanx found with the holotype, which lacked the distinctive features of Campbell’s 1995 specimen and had many others, confirming it does not belong to A. magnificens.

The other two specimens came from the Andalhualá Formation in the Valle de Santa María, about 1200 km northwest of the type locality. Radioisotope dates from a 1979 publication by Marshall and colleagues gave ages of 6.02 and 6.68 million years, refining the earlier Huayquerian estimate (8–5 mya) from Campbell and Tonni. Both specimens are housed in the Paleontología Vertebrados Lillo in San Miguel de Tucumán. The first, a left coracoid (PVL 4600), was collected by G. Bossi in March 1983. The second, a tibiotarsus, was recovered by L. Peirano in October 1939. These specimens are smaller than the equivalent bones in the holotype and in slightly better condition, though few new osteological features could be distinguished.

Argentavis belongs to the family Teratornithidae, a group of large birds of prey that lived in the Americas from the Late Oligocene to the Late Pleistocene. The family currently includes seven species across six genera. A. magnificens is the second-oldest taxon, after Taubatornis campbelli. Because both oldest taxa come from South America, the group likely evolved there and later migrated to North America during the latter part of the Cenozoic.

Teratornithidae was included in a 1988 phylogenetic analysis by Steven Emslie, which used cranial characters of birds in the order Ciconiiformes (storks), focusing on Vulturidae (New World vultures). The analysis included Teratornis merriami as a representative of Teratornithidae and placed the group just outside Vulturidae. Emslie noted that the analysis found more shared characters linking Vulturidae and Teratornithidae than characters specific to either group, suggesting a close relationship. He proposed that Brodkorb’s 1964 placement of teratornithids as a subfamily within Vulturidae might be more accurate than retaining family rank, as Campbell and Tonni had done. Currently, Teratornithidae is still treated as a family.

The bones of the holotype are severely fractured, though crushing is minimal except in the preserved skull elements. As noted by Campbell, the postcranial skeleton lacks most diagnostic portions due to fracturing and other damage.