The family Macrothricidae (Branchiopoda: Anomopoda) remains one of the least studied groups of water fleas (Cladocera). One of macrothricids with an unclear phylogenetic status is Lathonura Lilljeborg, 1853 comprising a single universally accepted valid species, L. rectirostris (O.F. Müller, 1785). Despite its wide distribution in the Northern Hemisphere, no studies were conducted to prove conspecificity of L. rectirostris from different regions and properly describe its gamogenetic stages. Here, the morphological and genetic diversity of Lathonura in the Holarctic is revised. Our results show that in East Eurasia and Alaska a separate species of the genus occurs, L. bekkeraesp. nov., which differs from L. rectirostris s. str. by the posteroventral valve armature, structure of antenna I, and ephippium ornamentation. Mitochondrial DNA barcoding supports separation of L. bekkeraesp. nov. and reveals a deeply divergent clade of L. rectirostris s. l. in Canada, for which parthenogenetic females are morphologically indistinct from those of L. rectirostris s. str. Lathonura distribution fits well to the known patterns of Anomopoda biogeography. Males of Lathonura possess two additional setae at antenna II basipodite, P1 with a subdistal lobe lacking setae, P1 IDL with a hook and an additional seta, and an unmodified postabdomen. As noted in some previous studies, Lathonura likely represents a phylogenetic lineage distinct from the subfamily Macrothricinae, differing from most macrothricines by the absence of Fryer’s fork at P1 inner endite anterior setae and presence of P1 accessory seta. However, the phylogenetic position of the genus and its diversity in South Eurasia, Africa, and North America requires further studies.
