High level phylogeny of mammals, including all groups used in this study. Dotted lines represent possible affinities or where groups may be polyphyletic. Italicised taxonomic names are extinct groups, some of which are likely polyphyletic. Tree topology modified from Asher and Helgen , with extinct group placement based on various recent analyses or compilations [18, 21–23, 27, 29, 30, 40, 41]. This tree is intended to be illustrative of the diversity of groups covered in this analysis, and is not derived from any single phylogenetic analysis.