Formation, and lived during the Colloncuran age of South America, when open environments predominated, which allowed more cursorial (adapted for running) and large animals to occur. The formation has provided fossils of a wide range of mammals, with a few fossils of birds, reptiles, amphibians, and fish. Taxonomy Around 2004, fossils of a phorusrhacid (or "terror bird", a group of large, predatory birds) were discovered by Argentine high school student Guillermo Aguirre-Zabala between two houses, about from the railroad of Comallo, a small village in the north-west of the Río Negro Province in the Patagonia region of Argentina (coordinates: ). The outcrops where the specimen was discovered belong to the Collón Curá Formation. Aguirre-Zabala prepared the specimen himself, and the discovery led him to shift from studying psychology to studying paleontology and Earth science. The specimen became part of the collection of the Museo Asociación Paleontológica Bariloche in Río Negro, where it was cataloged as specimen BAR 3877-11. Prior to the animal receiving a scientific name, the specimen was reported and discussed by the Argentine paleontologists Luis M. Chiappe and Sara Bertelli in a short 2006 article. In 2007, Bertelli, Chiappe, and Claudia Tambussi made the specimen the holotype of Kelenken guillermoi; the genus name refers to a spirit in the mythology of the Tehuelche people of Patagonia which is represented as a giant bird of prey, and the specific name honors its discoverer. The holotype and only known specimen consists of a nearly complete skull which is somewhat crushed from top to bottom, with most of the eye sockets, skull roof, braincase, and left quadrate bone preserved, while most of the palatal bones behind the eye sockets are missing. The specimen also includes an associated left tarsometatarsus (lower leg bone of birds), a small upper portion of a foot phalanx bone (toe bone), and some indeterminate fragments. The describers concluded these bones belonged to a single specimen due to being collected together (and with no other fossils being present), because their general preservation (such as color and texture) was similar, and because they were morphologically consistent with belonging to a large phorusrhacid. The specimen possessed the most complete skull of a large phorusrhacid known at the time. Previously, such skulls were known only from the fragmentary Devincenzia and Phorusrhacos. The skull of the latter disintegrated during collection (leaving only the tip of the beak), which hampered comparison between phorusrhacid