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. 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. 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. 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. 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. References External links Genus Taxonomy Phorusrhacidae Extinct flightless birds Oligocene birds Paleogene birds