化石层序律也称生物群层序律，是地质学一条普遍法则，是生物地层划分、对比的理论依据，最早由英国地质学家威廉·史密斯提出。史密斯认为：相同岩层总是以同一叠覆顺序排列着，并且每个连续出露的岩层都含有其本身特有的化石，利用这些化石可以把不同时期的岩层区分开。通俗地讲，就是“含有相同化石的地层的时代相同，不同时代的地层所含的化石不同”。利用这条规律，可以根据所含化石的面貌鉴别岩石的年龄。

The principle of faunal succession, also known as the law of faunal succession, is based on the observation that sedimentary rock strata contain fossilized flora and fauna, and that these fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances. A fossilized Neanderthal bone will never be found in the same stratum as a fossilized Megalosaurus, for example, because neanderthals and megalosaurs lived during different geological periods, separated by many millions of years. This allows for strata to be identified and dated by the fossils found within.