To be 100% clear... Ancient Infested have the fossilised health type but they don't get any benefit from armour, right?
The reason I ask is that I'm curious to see if viral procs or 75% boosted damage is better for killing Ancients, but I guess that really depends on the weapon's status chance. The practical implications for me are a little bit further confused because I mostly use elemental damage mods, and so either I use corrosive/blast (obviously great and about a 60% damage multiplier against Fossil); or viral/heat (to avoid forming radiation) and try to get a proc instead. I guess I'll have to look around for Ancient killing guides or something.