Psilopterus

''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 in height and weighed about , while the largest members of the genus were only about . The birds resemble the modern cariama (''Cariama cristata''), except with a heavier build and considerably smaller wings. 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. In contrast to the other, larger terror birds, Tonni and Tambussi also suggested ''Psilopterus'' could use their claws to climb trees, and could even fly, but this has been rejected in more recent literature. 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.

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.

''Psilopterus bachmanni''

''Psilopterus bachmanni'' (Moreno & Mercerat, 1891) 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, and an almost complete skeleton ( PUM-15.904) 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) 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.

Synonyms:Per Alvarenga & Höfling (2003), who rely on Brodkorb (1967).
* ''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, but the incomplete foot bone (tarsometatarsus) is indistinguishable from ''P. bachmanni''.

''Psilopterus lemoinei''

''Psilopterus lemoinei'' (Moreno & Mercerat, 1891) is contemporaneous with ''P. bachmanni'' and likely filled a very similar ecological niche, though ''P. lemoinei'' is slightly larger, with an estimated weigh approaching . 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. 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). 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.

Synonyms:
* ''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) 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). Additional specimens might help clarify the taxonomy of the four apparently unrelated species. ''P. affinus'' was originally assigned to the genus ''Phororhacos'' despite the difference in size, and is distinguished from ''P. bachmanni'' by a groove on the leg bone. Bertelli et al. kept this species in ''Phororhacos''.Taxonomic opinions tied to S. Bertelli et al. 2007
at Fossilworks.org Brodkorb assigned the species to ''Andrewsornis'' in 1967, but this is no longer considered accurate.

''Psilopterus colzecus''

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).

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,'' that they named Stereornithidae. Since then, ''Psilopterus'' was considered the ancestor of larger Phorusrhacids like ''Mesembriornis'' and the modern ''Cariama.'' In 1927, ''Psilopterus'' was placed in its own family and subfamily, Psilopterinae, and later recognized as being in its own family sometimes grouped with other Phorusrhacids like '' Palaeopsilopterus'' and ''Procariama.'' However, in the phylogenetic analysis by Degrange ''et al, Psilopterus'' was found as the only Psilopterine. 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.

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.Croft, D. A. (2016). ''Horned armadillos and rafting monkeys: the fascinating fossil mammals of South America''. Indiana University Press. 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.Townsend, K. 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. Large, herbivorous, South American notoungulate mammals like the toxodontids ''Nesodon'' and '' Adinotherium'' were the large low browsers, with rabbit-like interatheriiid ''Protypotherium'' being frugivorous. 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

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