[1] Interaction with Loren Petry, a Cornell professor studying Devonian plants of the region, lead to Arnold shifting his focus to paleobotany.
[1] Arnold did extensive research on the flora Paleozoic, Mesozoic and Tertiary of North America studying fossils from British Columbia[2] to Oklahoma to Greenland.
[1] During his lifetime Arnold wrote approximately 121 publications, on subjects including the fossil conifers of Princeton, British Columbia, to the extinct water-fern, Azolla primaeva.
[2] He was honored with the Silver Medal from the Birbal Sahni Institute of Palaeobotany in 1972, and the Distinguished Service Award from the Paleobotanical Section of the Botanical Society of America.
[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.