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Psilopterus; Temporal range: Mid Oligocene-Late Pleistocene? (Deseadan-Lujanian); ~29.0–0.1 Ma PreꞒ Ꞓ O S D C P T J K Pg N
	Skull of P. lemoinei in American Museum of Natural History
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Cariamiformes
Family:	†Phorusrhacidae
Subfamily:	†Psilopterinae
Genus:	†Psilopterus; Moreno & Mercerat, 1891
Species
	P. bachmanni (Moreno & Mercerat, 1891) (type); P. lemoinei (Moreno & Mercerat, 1891); P. affinis (Ameghino, 1899); P. colzecus Tonni & Tambussi, 1988;
Synonyms
	Pelecyornis Ameghino, 1891; Staphylornis Mercerat, 1897;

Psilopterus (Greek for "bare wing") is an extinct genus of phorusrhacid ("terror bird") from the Middle Oligocene to possibly the Late Pleistocene of Argentina and Uruguay. Compared to other phorusrhacids, members of the genus are both relatively gracile and diminutive, and include the smallest known species of terror bird: with the head raised P. bachmanni was 70–80 centimeters (2.3–2.6 ft) in height^[4] and weighed about 5 kilograms (11 lb), while the largest members of the genus were only about 8 kilograms (18 lb).^[7] The birds resemble the modern cariama (Cariama cristata), except with a heavier build and considerably smaller wings.^[1] Fossil finds in Uruguay indicate the genus may have survived until 96,040 ± 6,300 years ago, millions of years after the larger phorusrhacids became extinct.^[2]

Description and taxonomy

The most recent systematic revision of Phorusrhacidae placed Psilopterus within the subfamily Psilopterinae, along with the genera Procariama and Paleopsilopterus, and divided Psilopterus into four species.^[1]

Psilopterus bachmanni

Psilopterus bachmanni (Moreno & Mercerat, 1891)^[3] is the smallest species of phorusrhacid, rivaled only by P. affinis. The species (and genera) is defined by the upper portion of a fused ankle and leg bone (the lectotype MLP-168 is a tarsometatarsus). Other material assigned the species includes additional leg bones that are probably from the same bird,^[4] and an almost complete skeleton (PUM-15.904)^[8] The material is from several sites in the Santa Cruz Formation in the Santa Cruz Province of Argentina dating to the Middle Miocene (Santacrucian). The most important diagnostic characteristics are a low skull and upper jaw (or maxilla; similar to the mesembriornithine phorusrhacids)^[1] and the extreme slant of the front edge of the hole just before the eye (rostal portion of the antorbital fenestra), though there are also differences in the rest of the skeleton.^[8]

Synonyms:^[9]

Psilopterus bachmanni (Moreno & Mercerat, 1891)
Patagornis bachmanni Moreno & Mercerat, 1891
Psilopterus communis Moreno & Mercerat, 1891
Psilopterus intermedius Moreno & Mercerat, 1891
Phororhacos delicatus Amegino, 1891

Brodkorb considered Psilopterus minutus Amerghino, 1981 a separate species,^[10] but the incomplete foot bone (tarsometatarsus) is indistinguishable from P. bachmanni.^[1]

Psilopterus lemoinei

Psilopterus lemoinei (Moreno & Mercerat, 1891)^[3] is contemporaneous with P. bachmanni and likely filled a very similar ecological niche, though P. lemoinei is slightly larger, with an estimated weigh approaching 8 kilograms (18 lb).^[7] The species is defined by part of a lower leg bone (the lectotype, MLP-162, is the distal end of a tibiotarsus), but a wide variety of material has been referred to the taxon.^[8] This material has been found at a number of sites in the Monte León and Santa Cruz Formations in the Santa Cruz Province of Argentina that are dated to the Middle Miocene (Santacrucian). Diagnostic characteristics include a higher skull and upper jaw (maxilla), and the front portion of the hole in front of the eyes (rostral edge of the antorbital fenestra) is less slanted. Additional differences in the remainder of the skeleton are noted in Sinclair and Farr (1932).^[8] A number of discrepancies between various specimens have been attributed to differences in age or sex, but material currently assigned to P. lemonei and P. bachmanni may be reclassified at the species level if reexamined in depth.^[1]

Synonyms:^[9]

Patagornis lemoinei Moreno & Mercerat, 1891
Psilopterus australis Moreno & Mercerat, 1891
Pelecyornis tubulatus Ameghino, 1895 (synonym of Psilopterus australis)
Phororhacos modicus Ameghino, 1895
Staphylornis gallardoi Mercerat, 1897 (possible synonym of Psilopterus australis)
Staphylornis erythacus Mercerat, 1897 (possible synonym of Psilopterus australis)
Pelecyornis tenuirostris Sinclair & Farr, 1932 (synonym of Psilopterus australis)

Psilopterus affinus

Psilopterus affinus (Ameghino, 1899)^[5] is the most poorly known species of terror bird, represented only by part of a leg bone (tarsometatarsus, MACN-A-52-184) which indicates the bird was very close to P. bachmanni in size. P. affinus is one of several species known from fragmentary material found in 1899 in the Chubut Province of Argentina (Patagonia), in rocks which dated to the Middle to Late Oligocene (Deseadan).^[5] Additional specimens might help clarify the taxonomy of the four apparently unrelated species.^[1] P. affinus was originally assigned to the genus Phororhacos despite the difference in size,^[5] and is distinguished from P. bachmanni by a groove on the leg bone.^[1] Bertelli et al. kept this species in Phororhacos.^[11] Brodkorb assigned the species to Andrewsornis in 1967,^[10] but this is no longer considered accurate.^[1]

Psilopterus colzecus

P. colzecus tarsometatarsus from the front, back, and below

The most recently discovered species in the genus, Psilopterus colzecus Tonni & Tambussi, 1988, is similar to P. lemoinei in size. Known only from a single incomplete skeleton that includes parts of the jaw, arm, and leg (holotype MLP-76-VI-12-2), the species is defined by a groove in the front of the thigh bone (trochlea). The elements were found in the Arroyo Chasicó Formation in Buenos Aires Province of Argentina and are dated to the Late Miocene (Chasicoan).^[6]

Classification

When P. bachmanni was originally described in 1891, few other birds now known as Phorusrhacids were described, but when Moreno & Mercerat named the taxon, they assigned Psilopterus (then Patagornis) bachmanni to a group with Phorusrhacos, Mesembriornis, and Stereornis, though the latter is now seen as a synonym of Phorusrhacos,^[12]^[1] that they named Stereornithidae.^[3] Since then, Psilopterus was considered the ancestor of larger Phorusrhacids like Mesembriornis and the modern Cariama.^[13] In 1927, Psilopterus was placed in its own family and subfamily, Psilopterinae,^[14] and later recognized as being in its own family sometimes grouped with other Phorusrhacids like Palaeopsilopterus and Procariama.^[1] However, in the phylogenetic analysis by Degrange et al. (2015), Psilopterus was found as the only psilopterine,^[15] though a 2024 study reclassified Procariama as a psilopterine.^[16] The following phylogenetic tree shows the internal relationships of Phorusrhacidae under the exclusion of Brontornis as published by Degrange and colleagues in 2015, which recovers Psilopterus as the only member of Psilopterinae as a sister clade to Mesembriornithinae.^[15]

Cariamiformes

Cariamidae

Phorusrhacidae

Mesembriornithinae

Mesembriornis incertus

Mesembriornis milneedwardsi

	Llallawavis scagliai

	Procariama simplex

Psilopterinae

	Psilopterus affinis

	Psilopterus bachmanni

	Psilopterus colzecus

	Psilopterus lemoinei

Kelenken guillermoi

Devincenzia pozzi

Titanis walleri

Paraphysornis brasiliensis

	Andrewsornis abbotti

	Andalgalornis steulleti

	Patagornis marshi

	Phorusrhacos longissimus

	Physornis fortis

Paleobiology

The strong morphological similarity between the claws of the predatory cariama and Psilopterus, both of which are sharp, curved, and laterally compressed, may indicate they were used to strike prey. Like modern seriemas, psilopterines like Psilopterus would have fed on smaller animals based on their osteological traits.^[17] It has been also suggested that, in contrast to the other larger terror birds, Psilopterus may have been able to fly,^[6] probably in a brief and clumsy manner like that of extant seriemas,^[18] with body mass estimates and hind limb proportions of psilopterines being similar to those of certain birds like Psophia and Otis which often walk but are able to run and fly.^[19] It is likely that psilopterines would have more preferred to run than fly, and that they would have utilized flight to reach the treetops for nesting and protection against predators.^[17]

Paleoenvironment

Psilopterus bachmanni & lemoinei lived during the middle Miocene in the Santa Cruz Formation, which preserves mostly a coastal environment, but also forested and grassland regions.^[20] The area had little rainfall, so forests developed around lakes and rivers, giving Santa Cruz a diverse environment. During the Miocene, the climate was similar to those of the coasts of Chile with semi-temperate forests and oceanic winds. Grasslands began spreading into Argentina during the Miocene, though much of inner Patagonia was still arid with small rainforests in between.^[21]^[20] Large, herbivorous, South American notoungulate mammals like the toxodontids Nesodon and Adinotherium were the large low browsers, with rabbit-like interatheriiid Protypotherium being frugivorous.^[21] Both mammalian and avian carnivores inhabited the area, the largest being the phorusrhacid Phorusrhacos. Marsupials also lived in the region, including the large carnivorous sparassodont Borhyaena.^[20] Psilopterus lemoinei is also known from the coastal Monte Leon Formation that was in the same region in Santa Cruz, but part of the older lower Miocene age.^[22]^[23] Monte Leon preserved more mudstone and estuarine sediments, but with a very similar fauna to the Santa Cruz Formation as the two formations had a direct transition.^[22]

References

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ Alvarenga, Herculano M. F.; Höfling, Elizabeth (2003). "Systematic Revision of the Phorusrhacidae (Aves: Ralliformes)". Papéis Avulsos de Zoologia. 43 (4): 55–91. doi:10.1590/S0031-10492003000400001. ISSN 0031-1049.
^ ^a ^b Jones, W.; Rinderknecht, A.; Alvarenga, H.; Montenegro, F.; Ubilla, M. (2017). "The last terror birds (Aves, Phorusrhacidae): new evidence from the late Pleistocene of Uruguay". Paläontologische Zeitschrift. 92 (2): 365–372. doi:10.1007/s12542-017-0388-y. S2CID 134344096. Note: their date of 96 thousand years BP is the maximum age, obtained from the bottom of the fossil-containing stratum.
^ ^a ^b ^c ^d ^e ^f 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 (in Spanish). 1: 7–71.
^ ^a ^b ^c Richmond, Charles W. (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 (1267): 663–730. doi:10.5479/si.00963801.1267.663. hdl:2027/coo.31924090189725.
^ ^a ^b ^c ^d Ameghino, Florentino (1899). "Sinopsis geológico-paleontológica, Suplemento (Adiciones y correciones)". La Plata (in Spanish): 13 pp.
^ ^a ^b ^c Tonni, Eduardo P.; Tambussi, Claudia (1988). "Un nuevo Psilopterinae (Aves: Ralliformes) del Mioceno tardio de la Provincia de Buenos Aires, Republica Argentina". Ameghiniana (in Spanish). 25: 155–160.
^ ^a ^b DEGRANGE, FEDERICO J.; TAMBUSSI, CLAUDIA P. (2011). "Re-Examination of Psilopterus Lemoinei (Aves, Phorusrhacidae), A Late Early Miocene Little Terror Bird from Patagonia (Argentina)". Journal of Vertebrate Paleontology. 31 (5): 1080–1092. Bibcode:2011JVPal..31.1080D. doi:10.1080/02724634.2011.595466. ISSN 0272-4634. JSTOR 41407663. S2CID 86790415.
^ ^a ^b ^c ^d Sinclair, W.; Farr, M. (1932). "Aves of the Santa Cruz beds". Reports of the Princeton University Expeditions to Patagonia (1896–1899). 7: 157–191.
^ ^a ^b Per Alvarenga & Höfling (2003), who rely on Brodkorb (1967).
^ ^a ^b Brodkorb, Pierce (1967). "Catalogue of fossil birds, Part III (Ralliformes, Ichthyornithiformes, Charadriiformes)". Bulletin of Florida State Museum. 2: 99–220.
^ Taxonomic opinions tied to S. Bertelli et al. 2007 at Fossilworks.org
^ Brodkorb, P. (1967). Catalogue of fossil birds: part 3 (Ralliformes, Ichthyornithiformes, Charadriiformes). University of Florida.
^ Rovereto, C. (1914). Los estratos araucanos y sus fósiles. An. del Mus. Nac. Hist. Nat. Buenos Aires, 25.
^ Dolgopol de Saez, M. (1927). Las aves corredoras fósiles del Santacrucense. In Anales de la Sociedad Científica Argentina (Vol. 103, pp. 145-64).
^ ^a ^b Degrange, Federico J.; Tambussi, Claudia P.; Taglioretti, Matías L.; Dondas, Alejandro; Scaglia, Fernando (2015-03-04). "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. ISSN 0272-4634. S2CID 85212917.
^ LaBarge, 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. PMID 38654650. Supplementary Information
^ ^a ^b Acosta 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. S2CID 267475903.
^ Degrange, F.J.; Noriega, J.I.; Areta, J.I. (2012). "9. Diversity and paleobiology of the santacrucian birds". In Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S. (eds.). Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation. Cambridge: Cambridge University Press. pp. 138–155. doi:10.1017/CBO9780511667381.010. ISBN 978-0521194617.
^ 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.
^ ^a ^b ^c Croft, D. A.; Simeonovski, V. (2016). Horned armadillos and rafting monkeys: the fascinating fossil mammals of South America. Life of the past. Bloomington (Ind.): Indiana University Press. pp. 119-121. ISBN 978-0-253-02084-0.
^ ^a ^b Townsend, K. E. B.; Croft, D. A. (2008). "Diets of notoungulates from the Santa Cruz Formation, Argentina: new evidence from enamel microwear". Journal of Vertebrate Paleontology. 28 (1): 217–230. doi:10.1671/0272-4634(2008)28[217:DONFTS]2.0.CO;2. ISSN 0272-4634. JSTOR 30126346.
^ ^a ^b Cuitiño, J. I.; Fernicola, J. C.; Raigemborn, M. S.; Krapovickas, V. (2019). "Stratigraphy and depositional environments of the Santa Cruz Formation (early–middle Miocene) along the Río Santa Cruz, southern Patagonia, Argentina". Publicación Electrónica de la Asociación Paleontológica Argentina. 19 (2): 14–33. doi:10.5710/PEAPA.27.07.2019.294. hdl:11336/110039.
^ Kay, Richard F.; Vizcaíno, Sergio F.; Bargo, M. Susana; Spradley, Jackson P.; Cuitiño, José I. (2021-08-01). "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. S2CID 233693434.