The taxonomic placement of an enigmatic species of wasp known from two specimens in Late Cretaceous New Jersey amber is investigated through cladistic analyses of 90 morphological characters for 33 terminals ranging across non-Aculeata, non-Chrysidoidea, most subfamilies of Chrysidoidea and all genera of Plumariidae (the family to which the fossils were initially assigned), based on use of exemplars. The fossil taxon is apparently basal in Chrysidoidea, most likely sister to Plumariidae, but perhaps sister to the remaining chrysidoids, or even sister to Chrysidoidea as a whole. It is described as representing a new family, Plumalexiidaefam. n., containing a single species, Plumalexius rasnitsynigen. et sp. n. Previous estimates of relationships for the genera of Plumariidae and for the higher taxa of Chrysidoidea are mostly confirmed. The importance of outgroup choice, and additivity and weighting of characters are demonstrated.
Keywords: Plumalexiidae; Plumalexius rasnitsyni; amber; classification; fossil; new genus; new species.