The only known specimen of ?Oryzomys pliocaenicus is a mandible (lower jaw) found in the spring of 1935 by David Dunkle in Edson Quarry, Sherman County, Kansas. It is in the collections of the Museum of Comparative Zoology at Harvard University as specimen MCZ 6202. Edson Quarry is in the Ogallala Formation and the Hemphillian North American Land Mammal Age. Claude W. Hibbard described the mandible as ? Oryzomys pliocaenicus in a 1939 paper. Hibbard wrote that the fauna was middle Pliocene,Hibbard, 1939, p. 459 but it is now considered Miocene.Weksler, 2006, p. 87 The Edson Quarry fauna contains a diversity of other fossils, including mammals, birds, reptiles, and amphibians.Hibbard, 1939, pp. 460–461
