Abstract
Limnosida frontosa Sars, 1862 is redescribed based on the material from over its entire range, from Norway and Finland in the west to Yakutia in eastern Siberia and the Lower Amur River basin (Far East of Russia) in the east. Morphological analysis of the species has revealed a combination of advanced and evolutionary primitive features, latter characterizing Limnosida as the most primitive member of the order Ctenopoda. Investigation of intra- and interpopulational morphological variability, in particular of the post abdominal claws, led to a tentative separation of the species into two main forms, occurring regionally in the north-west and east of Northern Eurasia. Their taxonomic status as yet remains unclear. Some overlap in geographical distribution of these forms and the presence of populations with intermediate morphological features probably reflects irregularity of the diversification process in L. frontosa s.l. within its range. This diversification may be the result of both geographic disjunction and adaptation to waters of different trophic status. The low level of intraspecies differentiation and comparatively limited geographic distribution of this species imply its young age while the genus Limnosida itself is presumed ancient.