As a former anthropologist, I saw the types of taxons that you describe here all too often. It is like finding a modern skeleton mingled with mammoth bones. It had happened here in Texas and caused a bit of a stir since we knew the historical record. As it turned out, the land was filled in several years prior to reduce land erosion from nearby swamps. The soil came from land where a cemetery had been removed and resettled. Guess they left one behind but the skeleton was dumped in a layer just above the mammoth skeleton. I suppose that would be your zombie taxon.