X-ray microscopy reveals endophallic structures in a new species of the ground beetle genus Trechus Clairville, 1806 from Baltic amber (Coleoptera, Carabidae, Trechini)

Abstract. The third fossil species of the genus Trechus Clairville, 1806 is described from Baltic amber: Trechus exhibitorius sp. n. Details of external and internal morphology were analysed using X-ray micro-computed tomography (micro-CT) and important diagnostic features of the internal male genital sac (endophallus) are described in detail for the first time in a fossil ground beetle. Based on these data, we could assign Trechus exhibitorius sp. n. to Trechus sensu stricto and this new fossil species seems to represent a basal branch of a lineage comprising species diverse groups of extant Trechus mainly distributed in the Caucasus and Anatolia. Thus, our results support previous studies suggesting that Trechus is a phylogenetically old lineage already present in the Eocene with numerous species.