指准化石（Index fossils，又称为标准化石、指标化石）是指一种可以被用来推知所处地层的地质年代之化石。其运作原理基于一个前提上：虽然沉积物可能会因为沉积时所处的环境不同而有不同的形状，但它们很有可能会包含有同样一种生物的化石。假如该物种只存在于一相对短暂的时期（以地质学的角度来说，可能历时约几十万年），那么只要我们查看的地层中含有相同的化石，就足以判断出这些地层可能是在该特定时期间所沉积。被用来测定地质年代的指准化石物种存在时间越短，判定地质年代的准确度就越高。

基于以上缘故，一些演化非常快的生物之化石，会更具有地质学上的价值。除此之外，一种好的指准化石物种，还要有常见、容易辨识，与生存范围广阔等特性。尤其是最后一点：假如某种指准化石的物种分布范围很狭隘，那么在两个不同地区利用它来鉴定出同属一个时期的地质之机会，相对上就会小了许多，而减损了指准化石的应用性。

Index fossils (also known as guide fossils, indicator fossils or zone fossils) are fossils used to define and identify geologic periods (or faunal stages). They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil. If the species concerned were short-lived (in geological terms, lasting a few hundred thousand years), then it is certain that the sediments in question were deposited within that narrow time period. The shorter the lifespan of a species, the more precisely different sediments can be correlated, and so rapidly evolving types of fossils are particularly valuable. The best index fossils are common, easy-to-identify at species level, and have a broad distribution—otherwise the likelihood of finding and recognizing one in the two sediments is minor.