Propiestus archaicus, the first Mesozoic amber inclusion of piestine rove beetles and its evolutionary and biogeographical significance (Coleoptera: Staphylinidae: Piestinae)

Fossil records of piestine rove beetles are very limited, with only two definite species from Mesozoic Chinese compressions, a single taxon from mid-Eocene Baltic amber and a doubtful Oligocene compression fossil from France. Here, a remarkable new genus and species, Propiestus archaicus gen. et sp. nov., is described based on a well-preserved individual in Upper Cretaceous Burmese amber from northern Myanmar (Cenomanian, c. 99 Ma). It represents the first piestine fossil found in Mesozoic amber. The fine morphological characters preserved as an amber inclusion enable a confident systematic placement within the subfamily based on both our detailed observations and phylogenetic analyses. The resulting trees revealed a strongly supported sister-group relationship with the extant genus Piestus, comprising all currently known piestine species from the Neotropical region and one from the Nearctic. Our new discovery is congruent with a hypothesis of the Gondwanan affinity of insects found from Burmese amber. Considering the geological data as well as several previous studies, the current distribution of Piestus could possibly be explained by either a relictual Gondwanan distribution or the widespread presence of the clade Propiestus + Piestus on Pangea, rather than subsequent dispersal events during the Cenozoic. Nonetheless, more evidence is needed to corroborate the true biogeographical origin and past distribution of this clade. In agreement with strong external similarities with living Piestinae, Propiestus probably exhibits a specialized body plan, including a flattened body, long, slender antennae and short legs, suggesting that it inhabited microhabitats under the bark of rotting wood as do its modern congeners.