Trapelioid fungi constitute a widespread groupof mostly crust-forming lichen mycobionts that are key tounderstanding the early evolutionary splits in theOstropomycetidae, the second-most species-rich subclassof lichenized Ascomycota. The uncertain phylogeneticresolution of the approximately 170 species referred tothis group contributes to a poorly resolved backbone forthe entire subclass. Based on a data set including 657newly generated sequences from four ribosomal and fourprotein-coding gene loci, we tested a series of a priori andnew evolutionary hypotheses regarding the relationshipsof trapelioid clades within Ostropomycetidae. We foundstrong support for a monophyletic group of nine coretrapelioid genera but no statistical support to reject thelong-standing hypothesis that trapelioid genera are sisterto Baeomycetaceae or Hymeneliaceae. However, we canreject a sister group relationship to Ostropales with highconfidence. Our data also shed light on several longstandingquestions, recovering Anamylopsoraceae nestedwithin Baeomycetaceae, elucidating two major monophyleticgroups within trapelioids (recognized here asTrapeliaceae and Xylographaceae), and rejecting themonophyly of the genus Rimularia. We transfer elevenspecies of the latter genus to Lambiella and describe thegenus Parainoa to accommodate a previously misunderstoodspecies of Trapeliopsis. Past phylogenetic studies inOstropomycetidae have invoked Bdivergence order^ fordrawing taxonomic conclusions on higher level taxa.Our data show that if backbone support is lacking, contrastingsolutions may be recovered with different oradded data. We accordingly urge caution in concludingevolutionary relationships from unresolved phylogenies.