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"
Bergman, Charles (2003). Wild echoes : encounters with the most endangered animals in North America. Urbana: University of Illinois Press. ISBN0-252-07125-5. OCLC50511171.
Mech, L. David (2012). The wolf : the ecology and behavior of an endangered species. Garden City, N.Y.: Natural History Press. ISBN978-0-307-81913-0. OCLC795713101.