Mid-Cretaceous Myanmar amber fossilized the most crowded flock of rove beetles (Coleoptera: Staphylinidae)

Aggregation has been commonly reported in both living and extinct arthropod taxa, particularly among social insects. The mid-Cretaceous period is portrayed as a remarkable epoch of animal diversification and extensive resin production; therefore, the mid-Cretaceous ambers have preserved a variety of ephemeral moments in the history of arthropod evolution. Nevertheless, the aggregation of rove beetles (Coleoptera: Staphylinidae), one of the most species-rich arthropod families, has rarely been documented in mid-Cretaceous ambers, in contrast to the hyperdiversity in fossil faunas. Here, we present 2 new genera comprising 4 new species: Samavayanus azari gen. et sp. nov., Samavayanus hongzhangi sp. nov., Nartanacus hosimonis gen. et sp. nov., and Nartanacus argutus sp. nov. The new genera are classified into the tribe Thinobiini of the subfamily Oxytelinae, according to their characteristics. In addition, 51 individuals of S. hongzhangi sp. nov. have been found coexisting within 4 tiny amber pieces, with the most crammed one containing 28 individuals. This is the inaugural discovery of such substantially concentrated rove beetles in mid-Cretaceous ambers. By comparing the traits and living habits of the related extant taxa, we hypothesize that the aggregation of S. hongzhangi sp. nov. represents a form of light-induced collective activity, probably a swarming flight from 99 Ma.