New specimens of <i>Patagorhacos terrificus</i> Agnolín and Chafrat, 2015 (Aves) shed light on the phylogeny and evolution of the Phorusrhacidae

<i>Patagorhacos terrificus</i> is a phorusrhacid bird known from the early Miocene of Río Negro province, northern Patagonia. Originally based on the distal end of a quadrate bone, and a referred fragmentary ulna, its affinities were considered dubious by some authors, who also regarded it as Aves <i>incertae sedis</i>. The aim of present contribution is to report two new specimens of <i>Patagorhacos</i> that include valuable anatomical information, including the proximal end of femur, tibiotarsus and proximal and distal end of tarsometatarsus, among others. These new specimens indicate without any doubt that <i>Patagorhacos</i> belongs to phorusrhacid birds, and that it is a valid taxon that can be distinguished from other similar-sized taxa, such as <i>Patagornis</i> and <i>Hermosiornis</i>. With this new information at hand, a new phylogenetic analysis of phorusrhacid birds is performed. This analysis constitutes the most complete dataset known for phorusrhacids (25 taxa and 206 characters) and unambiguously nests <i>Patagorhacos</i> among phorusrhacids. It also lends support to the hypothesis that the loss of flight and acquisition of gigantism among phorusrhacids occurred only once. The North American and European <i>Bathornis</i> and <i>Ameghinornis</i> resulted as the successive sister taxa to Phorusrhacidae, indicating a complex biogeographical scenario.