Significance Purgatorius has been considered a plausible ancestor for primates since it was discovered, but this fossil mammal has been known only from teeth and jaw fragments. We attribute to Purgatorius the first (to our knowledge) nondental remains (ankle bones) which were discovered in the same ∼65-million-year-old deposits as dentitions of this putative primate. This attribution is based mainly on size and unique anatomical specializations known among living euarchontan mammals (primates, treeshrews, colugos) and fossil plesiadapiforms. Results of phylogenetic analyses that incorporate new data from these fossils support Purgatorius as the geologically oldest known primate. These recently discovered tarsals have specialized features for mobility and provide the oldest fossil evidence that suggests arboreality played a key role in earliest primate evolution.