Biostratigraphy
Using this methodology , scientists were able to establish geological time , the boundaries of the different eras ( Paleozoic , Mesozoic , Cenozoic ) , as well as Periods ( Cambrian , Ordovician , Silurian ) through the isotopes found within fossils via radioactive decay . 
<triplet> Silurian <subj> Paleozoic <obj> part of