One species gives you its range. Two overlapping give you the interval where both lived, which is shorter than either — and with enough species the intersection narrows to a slice of time far finer than any single fossil can offer.
Plot the level of each event in one section against its level in another. If both accumulated steadily the points fall on a line — and where they leave it, something happened: a flatter stretch is a slower or condensed interval, a vertical jump is a hiatus.
A good index fossil is not the prettiest or the commonest. It has to have lived briefly, widely, abundantly and independently of its surroundings — and be recognisable from a fragment, because a fragment is what you will get.
The highest fossil you find is not the last one that lived. Sampling is incomplete, so every observed range is shorter than the true one at both ends — and because rarer species are truncated more, a sudden extinction looks staggered and gradual. That is the Signor–Lipps effect, and it is why an extinction cannot be read off a range chart without accounting for it.
Pollen counting, with its own statistics and zonation, is Palynology. Hanging the sections themselves side by side is Correlation, and an absolute age to pin a zone to comes from Dating a rock.