>>14419827coexistence is unlikely, but geologically the surface of Earth is so frequently recycled that yes, it would be possible - potentially not from our level of technology, but honestly it would be difficult to tell unless they left something like a residue of a certain isotope in a layer when they died out
fossilization is so finnicky (i.e. requires such specific conditions) that we might just never see an example of them, especially if they hadn't spread civilization across most of the planet.
as for being vastly different, yeah they definitely could be - but they would most likely be similar at a basic biological level (such as having similar base genes that occur in a lot of complex life
