Redescription and revision of Georges Demoulin’s Eocene Baltic amber Heptageniidae (Insecta: Ephemeroptera)

Abstract The vast majority of Cenozoic fossil records of mayflies (Ephemeroptera) are represented by adult specimens, and Eocene Baltic amber is no exception. Representatives of the family Heptageniidae Needham, 1901 were described from adult males and females during the mid-twentieth century by Georges Demoulin, whose work substantially advanced the knowledge of this rheophilous component of the fossil aquatic insect fauna. Despite the generally high quality of his descriptions, the set of adult characters employed left the systematic position of several extinct taxa unresolved. Characters such as wing venation, proportions of leg segments, and linear body measurements are often of limited value for the reliable assignment to heptageniid genera. Additionally, male genitalia were in some cases documented only schematically, and species described from female imagines were not always supported by sufficient diagnostic characters. In the present contribution, we re-examine one larval and 11 adult specimens of fossil mayflies from Eocene Baltic amber originally described by Georges Demoulin between 1956 and 1970. Based on taxonomically informative characters, including newly assessed features of adult thoracic morphology, we revise and discuss the systematic placement of these taxa and provide redescriptions where appropriate. Several nomenclatural and taxonomic changes are proposed. Cinygma baltica Demoulin, 1968 is transferred to Heptagenia (? Heptagenia ) baltica (Demoulin, 1968) comb. nov. Amongst two specimens previously assigned to Rhithrogena sp., one is transferred to Heptagenia s.l., with the male imago described as Heptagenia (s.l.) demoulini sp. nov. The other specimen, i.e. Rhithrogena sepulta Demoulin, 1968, is redescribed and retained within Rhithrogena . Two species, namely Heptagenia atypica Demoulin, 1968 and H. gleissi Demoulin, 1968 are provisionally retained in Heptagenia s.l., and their possible relationship with the extant Nearctic Stenonema generic complex is discussed. Heptagenia bachofeni Demoulin, 1968 is redescribed from type material and provisionally retained in the subgenus Dacnogenia Kluge, 1988, whereas H. ligata Demoulin, 1968 is redescribed and retained in Heptagenia s.l. owing to the limited number of diagnostic characters preserved in the female holotype. Heptagenia senex Demoulin, 1968 is shortly discussed and tentatively attributed to the subgenus Kageronia Matsumura, 1931, with a brief discussion of its similarities to extant and extinct representatives of this lineage. The systematic position of Electrogenia dewalschei Demoulin, 1956 remains uncertain; possible affinities with Metretopodidae Traver, 1935 are discussed, pending the rediscovery and re-examination of the holotype. Succinogenia larssoni Demoulin, 1965 is briefly addressed, but its systematic position within Heptageniidae remains unresolved owing to the poor preservation of the larval holotype. The biogeographic implications for the taxonomic reassessments of the new findings are discussed; also discussed are the ecological implications in the light of alternative theories regarding the origin and fluvial transport of Eocene Baltic amber.