This thesis compares fossil marine mammals from a poorly known bonebed in the Middle Miocene “Topanga” Formation of Orange County and compares them to those from the better known Sharktooth Hill bonebed (Round Mountain Silt) of Kern County. Both bonebeds were deposited during the middle Miocene Climatic Optimum (MMCO). Although they had been considered contemporaneous and largely taxonomically homogenous prior to this study, new chronostratigraphic analyses show that the “Topanga” bonebed is slightly older than the Sharktooth Hill bonebed, allowing for new interpretations and hypotheses. The case studies presented here show how the description of new fossils, combined with an updated chronostratigraphic framework, a consideration of different depositional environments, and an understanding of biology can provide a more finely-detailed and nuanced picture of marine mammal communities during the MMCO.