Here is the answer posted to a similar question on this board about three years ago. I can't vouch for its complete accuracy, as it doesn't give sources, but it is obvious from a tiny bit of Googling that Lomanzo is indeed an Italian surname, and that the Arabic origin is the one Wilder's family themselves believed for his name (though of course they didn't have to be right about that.)
http://www.behindthename.com/bb/arcview.php?id=80557&board=gen
"Romanzo" is the Italian word for "novel" (the type of book), just as "
Roman" is the
German word for "novel". There was an American painter named
Edwin Romanzo
Elmer who was born in 1850, but he doesn't seem to have regularly used his middle name during his lifetime:
http://www.askart.com/AskART/artists/biography.aspx?artist=26449
The -anzo ending was fashionable in that generation, just as boys' names with two syllables and ending in -n are common today. Certain sounds becoming popular at certain points in history.