Frederik Severin Iversen/Anne Marie Berntsdatter

Frederik Severin Iversen b.16 December 1844 in Kristiania, Norway Anne Marie Berntsdatter b. 21 July 1845 in Fredrikstad, Ostfold, Norway married 22 January 1871 in Glemmen. I lose them after the 1900 census. perhaps they both died before the 1910? but I can not find any record of their deaths? He seems to use the name Severin not Frederik. She seems to use Marie not Anne. Where should I be looking? thank you for any help. David