On the origin of the Theropithecus gelada specimens from the Field Museum 1926–1927 Abyssinian expedition: a case for the extended specimen approach

The historical distribution of the central gelada, Theropithecus gelada obscurus , an Ethiopian endemic primate, is generally considered to be bounded on the west by the Blue Nile Gorge. However, a small number of museum specimens reported as being collected in the areas of Gojjam and Ambo Mineral Springs (west of Lake Tana) have long represented the only physical evidence suggesting a former occurrence of geladas west of this barrier. In this short communication, we reassessed the provenance of all T. gelada specimens collected during the 1926–1927 Abyssinian Expedition of the Field Museum of Chicago by integrating archival field diaries, expedition reports, original specimen tags, catalog records, photographs, and illustrations. Our analysis demonstrates that three specimens historically attributed to Gojjam and Ambo Mineral Springs (FHMN 27038, FHMN 27039, and FHMN 27040) were in fact collected at the Muger River Canyon in Shewa in October 1926 and were subsequently mislabeled due to cataloging errors. No verified gelada specimens from Gojjam or west of Lake Tana are therefore currently housed in the Field Museum. These findings remove the only physical evidence supporting the historical presence of T. gelada west of the Blue Nile Gorge and leave as the sole remaining record of geladas in Gojjam an unconfirmed indirect report by Rüppell. This reassessment has important implications for reconstructing the historical biogeography and range limits of T. gelada and highlights the value of the “extended specimen” approach for museum collections.