>>12708242To add, sometimes there are species whose remains are commonly found but existing only within some short, specific time frame. Plant pollen or some armoured algae (like foraminiferans) are good groups to find such species, because they easily get fossilized (which means we have a relatively good understanding of their evolution) and tend to evolve rapidly (so dating can be precise.) If you find a species of this sort in your stratum - preferably, some combination of them to make sure you are right - you can safely assume that the other fossils in said stratum also come from that specific time frame. Presumably that's what OP is referring to.
Of course, we can use those "index fossils" because we know their age from some other sources in the first place, ditto for the particular "biozones" defined by such assemblages of index fossils. The kind of vicious circle you see in OP would be too dumb to actually be used by anyone, which is why you won't see it anywhere outside creationists' imagination.
