At present, there are 15 species of baleen whales that belong to the monophyletic suborder Mysticeti. The family-level designations of Balaenidae (right and bowhead whales) and Neobalaenidae (pygmy right whales) are strongly supported and well resolved. However, family-level designations and inter-relationships among the remaining species and genera of Balaenopteridae (rorquals) and Eschrichtiidae (grey whales) are less well resolved. Taxonomic relationships vary between studies, reconstruction methods and marker types, despite strong morphological support for the placement of Eschrichtiidae as a sister group to the Balaenopteridae. This phylogenetic uncertainty appears to have originated from a rapid Miocene radiation of both families, which has resulted in incomplete sorting of genetic lineages and therefore conflicting phylogenetic signals among nuclear and mitochondrial loci. The earliest radiation of baleen whales (suborder Mysticeti) occurred in the Late Eocene. The fossil record of subsequent species diversifications has been strongly correlated with changes in fossil diatom diversity and isotopic oxygen levels over time.