While collecting fossils with Alonzo W. Hancock in the Clarno Formation of Oregon in 1941, Arnold and Hancock recovered the most complete Miomastodon skull known to date.[5] In 1952 Arnold was the supervisor for Herman F. Becker who extensively studied the Ruby Basin Flora of Montana.[6] Among the may correspondents of Arnold was Wesley C. Wehr, who became Affiliate curator of Paleobotany at the Burke Museum of Natural History and Culture in Seattle.[7]