Biochronology is the branch of paleontology and stratigraphy, which assigns relative ages to rock strata by exploiting the time component embedded in the fossil record (biostratigraphy is a more commonly used term, but it applies to the study of the fossil content of the sedimentary rocks in general and not necessarily to its time component; Guex 1979, 1991; Gradstein 2012). Biochronology has profound implications not only for reconstructing past geological events but also for constraining phylogenetic hypotheses. It is one of the fundamental cornerstones of Geology and the Evolution of Life. Indeed, no matter what aspect of Geology one is working on, the most common question asked by geologists is “what age is it”? Biochronology and geochronology provide the framework for answering this question.