The L. adjs. were primarily f. sbs., as ōrālis ORAL, f. ōs, ōr- mouth. This type was greatly increased in medL. and modL. and consequently in the Rom. languages by formations not only on L. but on Gr. stems (after L. boreālis, hebdomadālis). In L. itself -ālis was added to existing adjs., as annuālis (f. annuus), infernālis (f. infernus); hence the suffix is added freely in the mod. langs. to many classes of formations, including those with suffixes of Gr. origin, -ac, -ic, -oid (see -ACAL, -ICAL, -OIDAL).
"-al." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. (November 12, 2018). https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/al-4
"-al." The Concise Oxford Dictionary of English Etymology. . Retrieved November 12, 2018 from Encyclopedia.com: https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/al-4