These are the least trustworthy. Its appearance and disappearance from the record has nothing to do with evolutionary events and everything to do with ecological preferences. These fossils are closely associated with particular depositional environments, which may be, in turn, indicative of particular ecological regimes. Sometimes a straight line cannot be drawn. KEY BEDS can sometimes be traced for long distances.
All carbon is in reduced form, which causes the black color.
Although the facies sequence may be different in various parts of the basin, a switch from a transgressive series to a regressive series will happen at the same time everywhere. Out in the middle of the basin a shale will be succeeded by a limestone and the limestone will be succeded by a shale. The beach deposit and the limestone record the brief period of maximum transgression at different parts of the basin. In deep-sea sediments and some marginal and epicontinental sea deposits regular changes in lithology correspond to one or more of these periodic changes in insolation. Therefore grain-size peaks at 21 kyr intervals, representing the maximum strength of the monsoon and its consequent increase in the energy of rivers.