Patagorhacos is an extinct genus of phorusrhacid ("terror bird") that lived during the early Miocene epoch (corresponding to the Colhuehuapian South American Land Mammal Age) of the Neogene period in what is now Río Negro Province, Argentina. The first specimen, an isolated quadrate (the upper jaw joint bone), was discovered in rock layers of the Chichinales Formation in Paso Córdoba, a National Protection area. In 2015, the specimen became the holotype (name-bearing) specimen of the new genus and species Patagorhacos terrificus; the generic name is a combination of Patagonia, where the fossils were found, and rhacos, the suffix of Phorusrhacos, another phorusrhacid. Other specimens have been assigned to Patagorhacos, including an incomplete ulna (forearm bone) and an associated specimen that contains parts of the skull, mandible (lower jaw), and hindlimbs. However, Patagorhacos remains poorly understood. Patagorhacos was around the size of Patagornis, another phorusrhacid, which measured about 90–100 centimetres (3.0–3.3 ft) tall at the back and weighed 45–50 kilograms (99–110 lb). This makes it relatively small for a phorusrhacid. As a phorusrhacid, it would have a large, elongated head, elongated hindlimbs, and reduced forelimbs. Patagorhacos is generally comparable to the other small-medium sized phorusrhacids Andrewsornis and Patagornis, but differs from them in aspects of its quadrate and hindlimbs. For example, the quadrate bears an enlarged pterygoid condyle (a protuberance that articulates with the pterygoid bone), a characteristic absent in other phorusrhacids. The mandibular fragment is in worn condition, but bears a prominent tomial (cutting) edge on its dorsal side. Until the description of additional remains in 2025, several studies doubted the phorusrhacid affinities of Patagorhacos. A phylogenetic analysis suggests it was related to Andalgalornis, Andrewsornis, and Patagornis. Patagorhacos' fossils were discovered in rock layers of the Chichinales Formation, which is part of the wider Neuquén Group. This environment was made up of flood plains, open grasslands, and patches of woodlands, which hosted a diverse fauna of mammals. This included meridiungulates like Cochilius, Cramauchenia, and Hegetotheriopsis, xenarthrans like Stenotatus, rodents like Australoprocta, and marsupials like Cladosictis.
History and taxonomy Fossils assigned to Patagorhacos were first discovered by an expedition authorized by the Río Negro Secretariat of Culture. This venture was launched by the Museo Patagónico de Ciencias Naturales to study fossils in Paso Córdoba, a Natural Protection area in Río Negro Province, southern Argentina. The remains of Patagorhacos initially discovered included just two bones: the distal (away from body) end of a right quadrate (upper jaw joint bone; cataloged at the Paleontology of Vertebrate Collection, Museo Patagónico de Ciencias Naturales under specimen number MPCN-PV-377) and the distal end of an ulna (forearm bone, MPCN-PV-379). This makes Patagorhacos one of the most fragmentary and rare phorusrhacids known. The strata of Paso Córdoba where Patagorhacos was discovered is composed of a 73 metres (240 ft) tall area of whitish-greyish tufts and clays belonging to the Chichinales Formation. This formation dates to the early Miocene (Colhuehuapian in accordance with South American land mammal age (SALMA) classification), making it between 21 and 17.5 million years old. In 2015, Argentina paleontologists Federico Agnolín and Pablo Chafrat scientifically described the remains and assigned them to a new genus and species of phorusrhacid ("terror bird"), which they named Patagorhacos terrificus. The generic name Patagorhacos is a combination of Patagonia, the part of southern South America where its fossils were discovered, and rhacos, which was chosen for its use in the name of the type genus of the family, Phorusrhacos. The specific name "terrificus" was chosen to represent the "terrific" nature of phorusrhacids. The authors selected the quadrate as the holotype specimen (the specimen which a species is based on), while the ulna was assigned to Patagorhacos. Despite the lack of overlap, this assignment was made because both elements were found in the same locality and strata, as well as match in size. In 2025, Agnolín, Chafrat, and Argentine paleontologist Gerardo P. Álvarez-Herrera described a set of new Patagorhacos material that had been unearthed from the same site and strata as the holotype. These newly described fossils come from a single individual (MPCN-PV-1003) of an adult, consisting of: an incomplete right quadrate, dentary symphysis (the area where the dentaries meet), femur (thighbone) fragments, incomplete left tibiotarsus (shin bone), pieces of both tarsometatarsi, and an incomplete right pes (foot). MPCN-PV-1003 was found in an area covering around 1 square meter (11 sq ft). Many of these fossils are worn, cracked, or broken, giving a limited perspective on their anatomy. Another specimen (MPCN-PV-1004), the distal end of a tibiotarsus and proximal end of a tarsometatarsus, was found in articulation and is better preserved. Furthermore, the authors noted that an incomplete phorusrhacid pes from the same locale and strata as Patagorhacos may come from the genus as well. However, a 2017 conference abstract stated that this specimen comes from a hermosiornithine phorusrhacid, a group with unique tarsometatarsus anatomy compared to other phorusrhacids.
Classification and validity In a 2021 paper, Argentine paleontologist Federico Degrange questioned Patagorhacos' original classification, stating that it was too fragmentary to identify as phorurshacid. Two years later, Tambussi and colleagues followed this conclusion and stated the genus was incertae sedis (a taxon of indeterminate classification). In their 2025 description of additional material, Agnolín and colleagues refuted this, stating that anatomical features like the presence of an elongated, dense mandibular symphysis (the area where the mandibles converge) and grooves, pits, and a concave mandibular surface. This was further supported by their phylogenetic analysis (a study of the evolutionary relationships of organisms), which recovered Patagorhacos as a valid genus in a polytomy (an unnatural grouping) with Andalgalornis, Patagornis, and Andrewsornis, three genera sometimes classified in the subfamily Patagornithinae.
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