Biostratigraphy
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them . 
<triplet> Biostratigraphy <subj> stratigraphy <obj> part of <triplet> stratigraphy <subj> Biostratigraphy <obj> has part <subj> strata <obj> studies <triplet> strata <subj> stratigraphy <obj> studied by