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

Abstract

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