Actually, most of the redating of these particular layers was based on microfossils, which are more global in distribution than the previously-studied macrofossils. (In turn, that means that the microfossils are also likely to occur in other parts of the world where volcanoes conveniently supplied radiometrically datable ash layers). The earlier dating was based on the percent of living species in the fossil mollusks. An added complication comes from adjustments due to a more precise drawing of the official line between age units (and occasional decisions to draw the line between age units a little higher or lower, at a point considered to be more distinctive).
Microfossils are a good example of something incompatible with young-earth and global flood claims, largely neglected by young-earth advocates because the public is ignorant about them. They do not have significant differences in escape ability or hydrodynamic properties. Various types of microfossils do reflect different habitats, but such occur at various levels through the geologic column - there is no pattern of lower elevation to higher elevation habitats. But the types of microfossils change over time, with hundreds of totally different sets of microfossils being found in different layers.