It is well known (since Fetis) that the first appearance of the unprepared seventh in the dominant seventh chord is in Monteverdi's madrigal "Cruda amarilli". That sets the origin of this chord as a indepent harmony.
However, I've been looking for earlier examples of the use of this chord before this piece and I cannot find many. More specifically, I'm looking for examples of this sonority whether the seventh appears prepared as a suspension or is a passing tone. I have studied thoroughly the music of the Renaissance (up to 1530 aprox.) and I have not found any example. Probably it appeared in the italian madrigal of mid XVIth century, but I'm not sure.
I know that it makes no sense calling such aggregates "dominant sevenths", and that it is also totally un-historical. However, this sonority must have had an origin before its use spread among the composers.
This question is somehow similar, but the answers did not clarify it.
Thanks a lot