Cook (6 July 1964). "News Release"Archived 2021-09-01 at the Wayback Machine (PDF). Fish and Wildlife Service. United States Department of the Interior. p. I. Retrieved 26 January 2018. "Extinct Mammals of the United States: Plains wolf, Canus lupus nubilus (1926) — Great Plains"
Cook (6 July 1964). "News Release"Archived 2021-09-01 at the Wayback Machine (PDF). Fish and Wildlife Service. United States Department of the Interior. p. I. Retrieved 26 January 2018. "Extinct Mammals of the United States: Plains wolf, Canus lupus nubilus (1926) — Great Plains"