Arnold did extensive research on the flora Paleozoic, Mesozoic and Tertiary of North America studying fossils from British Columbia[2] to Oklahoma to Greenland.[2] 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.[1] A number of fossil plants have been named in Arnold's honor including Koelruteria arnoldi[3] and Pseudolarix arnoldi.[4]