Torah Temimah cites Tosefta Shevuos 1:4.
I have only the unclean — from where the clean? Scripture says “or the carcass of an unclean creeping thing” — just as you did not distinguish among creeping things between unclean and clean, so too for animal and wild beast do not distinguish.
The Tosefta extends the verse’s tumah structure to the neveilah of a pure animal by analogy to the sheretz, where no distinction is drawn between pure and impure species.