Evolutionary assessment of the rove beetle subfamily Proteininae Erichson, 1839 (Coleoptera: Staphylinidae) triggered by the X-ray based description of its first fossil in Baltic amber from Denmark

ABSTRACT The taxonomic composition, phylogeny, ecology, biology, behaviour, and fossil record of Proteininae Erichson, 1839 is reviewed to place the first Eocene fossil specimen from Baltic amber in the knowledge framework of this rove beetle subfamily. The Baltic amber fossil is a poorly preserved female which was examined using X-ray micro-computed tomography (μ-CT). It is assigned to the genus Proteinus Latreille, which is represented by 53 extant species in the world fauna, most of which occur in the north temperate areas of the globe. Due to a shortage of data, this newly discovered fossil could not be identified to the species level. However, it is the first known fossil of Proteinus, which indicates that the genus has been present since at least the Late Eocene. Together with other fossils, and what is so far known about the phylogeny of Proteininae and allied groups, it suggests that while the subfamily Proteininae dates back at least to the mid-Cretaceous, its species-rich lineages may be significantly younger. Open questions about the evolution of Proteininae, including reasons for the mainly bipolar geographical distribution of its extant members or the imbalanced diversity with recent Proteinus and Megarthrus from Proteinini being the only species-rich taxa with a global distribution, are discussed. Proteinus sp. from Baltic amber is a valuable piece of evidence for future research in this direction.