丰富多样的化石的突然出现为在世界范围内把地质年代划分为两大主要部份提供了时间界限。
The sudden appearance of abundant and varied fossils makes a worldwide time line for dividing the geological time scale into two main parts.
在整个地质年代中，浮游生物随地质时代的变化用于估计含己成为化石的浮游生物残体的岩石的相对年代很有用。
The changes in plankton over time are useful for estimation of relative ages of rocks that contain the fossilized remains of plankton.
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