The holotype, PVL 3670-11, was found at Arroyo-Morterito in the Los Blanquitos Formation, dating to the Maastrichtian.[1] The specimen, which consists of a left pubis measuring 51.4 centimetres (1 ft 8.2 in) long, was re-studied by Fernando Novas and Federico Agnolin in 2004, who concluded that the orientation of the pubis had been misinterpreted: it pointed backwards, as was shown by the fossil still being attached to a displaced part of the pubic peduncle of the ilium.[1]
Powell originally assigned Unquillosaurus as a large theropod of uncertain taxonomic placement.[2] In 2004, Novas and Agnolín concluded from the opisthopubic pelvic anatomy that Unquillosaurus was part of the Maniraptora or at least Maniraptoriformes, and likely closely related to either the Avialae or the bird-like Alvarezsauridae; perhaps it was itself a bird, a basal member of the Metornithes.[1] In 2006, Martínez and Novas stated that Unquillosaurus probably belonged to the maniraptoran cladeDromaeosauridae.[3] In 2012, however, Carrano and colleagues considered Unquillosaurus as a member of the Carcharodontosauridae, noting that the animal has many similarities to Giganotosaurus.[4] In 2013, Agnolín and Novas interpreted Unquillosaurus as a very large averaptoran instead, suggesting that the distal pubis shows condition of a coelurosaur and different from that of a carnosaur.[5] In a 2016 study about the phylogeny of theropods found in Argentina, Unquillosaurus is considered as a maniraptoran.[6] As of 2021, no reliable records of carcharodontosaurids beyond the Turonian age were known.[7] In 2024, the describers of Diuqin considered this taxon to be a possible member of the Unenlagiinae.[8]
^R.D. Martínez and F.E. Novas, 2006, "Aniksosaurus darwini gen. et sp. nov., a new coelurosaurian theropod from the early Late Cretaceous of central Patagonia, Argentina", Revista del Museo Argentino de Ciencias Naturales, nuevo serie 8(2): 243-259
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